CS718 — Midterm Summary (Lectures 1–22)

📘 Lecture 1 — Introduction to Wireless Communication

📖 Overview: This lecture introduces the foundational concepts of wireless communication, including the wireless vision, signal characteristics, channel capacity, and the electromagnetic spectrum. It establishes why wireless networks are essential and examines the fundamental trade-offs between data rate, bandwidth, and noise that govern all wireless systems.

🗂️ Topics Covered

The lecture covers the wireless vision including driving factors and wired vs. wireless comparison, electromagnetic signal characteristics in both time and frequency domains, signal parameters (amplitude, frequency, phase, wavelength), relationship between data rate and bandwidth, channel capacity concepts including Nyquist and Shannon formulas, signal-to-noise ratio, the electromagnetic spectrum with examples, and design challenges such as channel fading and interference.

📝 Lecture Summary

The Wireless vision

Wireless communication occurs when two parties communicate without any physical contact or medium of communication. The driving factors include an explosive increase in demand for tetherless connectivity (especially cellular telephony and wireless data applications) and dramatic progress in VLSI technology that enabled implementation of efficient signal processing algorithms and coding techniques. The success of 2G wireless standards (GSM) also fueled popularity. People want connectivity anywhere anytime—at airports, hotels, customer places, or to share data at any location—making wireless connectivity indispensable. This course mainly focuses on wireless networks rather than communication techniques.

Wired Vs. Wireless Communication

Advantages of wireless include practicality when laying cables is impossible, user mobility, and cost savings. Limitations include bandwidth constraints, fidelity issues, power limitations, and insecurity.

Electromagnetic Signal

An electromagnetic signal is a function of time but can also be expressed as a function of frequency—the signal consists of components of different frequencies. The frequency domain view of a signal is more important for analysis.

Time-Domain Concepts

• Analog signal: signal intensity varies in a smooth fashion over time with no breaks or discontinuities (e.g., speech)
• Digital signal: signal intensity maintains a constant level for some period then changes to another constant level (binary 0s, 1s)
• Periodic signal: pattern that repeats over time: s(t + T) = s(t) for -∞ < t < +∞, where T is the period
• Aperiodic signal: pattern that doesn't repeat over time
• Peak amplitude (A): maximum value or strength of the signal over time; typically measured in volts
• Frequency (f): rate, in cycles per second or Hertz (Hz), at which the signal repeats
• Period (T): amount of time for one repetition; T = 1/f
• Phase (Φ): measure of the relative position in time within a single period of a signal
• Wavelength (λ): distance occupied by a single cycle of the signal, or distance between two points of corresponding phase of two consecutive cycles; λ = vT

The general sine wave is expressed as: s(t) = A sin(2πft + Φ). Varying parameters changes the signal shape:

• A = 1, f = 1 Hz, Φ = 0 → T = 1s
• Reduced peak amplitude → A = 0.5
• Increased frequency → f = 2, T = ½
• Phase shift → Φ = π/4 radians (45 degrees)

📌 Note: 2π radians = 360° = 1 period

Frequency-Domain Concepts

• Fundamental frequency: when all frequency components of a signal are integer multiples of one frequency
• Spectrum: range of frequencies that a signal contains
• Absolute bandwidth: width of the spectrum of a signal
• Effective bandwidth (or just bandwidth): narrow band of frequencies where most of the signal's energy is contained

Any electromagnetic signal consists of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases. The period of the total signal equals the period of the fundamental frequency.

Relationship between Data Rate and Bandwidth

The greater the bandwidth, the higher the information-carrying capacity. Key conclusions: any digital waveform will have infinite bandwidth, but the transmission system limits the bandwidth that can be transmitted; for any given medium, greater transmitted bandwidth means greater cost; however, limiting bandwidth creates distortions.

About Channel Capacity

Channel capacity is the maximum rate at which data can be transmitted over a given communication path (channel) under given conditions. Impairments such as noise limit the achievable data rate.

Concepts Related to Channel Capacity

• Data rate: rate at which data can be communicated (bps)
• Noise: average level of noise over the communications path
• Error rate: rate at which errors occur (transmit 1 and receive 0; transmit 0 and receive 1)

Nyquist Bandwidth

For binary signals (two voltage levels): C = 2B

With multilevel signaling: C = 2B log₂ M, where M = number of discrete signal or voltage levels

📌 Example: For M = 8 and B = 3100, C = 2 × 3100 × log₂(8) = 2 × 3100 × 3 = 18600 bps

💡 Why this matters: Data rate can be increased by increasing the number of different signal elements, but this places an extra burden on the receiver.

Signal-to-Noise Ratio

Signal-to-noise ratio (SNR, or S/N) is the ratio of the power in a signal to the power contained in the noise present at a particular point in transmission, typically measured at a receiver.

🔑 Definition — SNR: (SNR)dB = 10 log₁₀ (signal power / noise power)

A high SNR means a high-quality signal and lower number of required intermediate repeaters. SNR sets an upper bound on achievable data rate. SNR expresses in decibels how much the intended signal exceeds the noise level.

Shannon Capacity Formula

🔑 Definition — Shannon Capacity: C = B log₂(1 + SNR)

This represents the theoretical maximum data rate that can be achieved. In practice, only much lower rates are achieved. The formula assumes white noise (thermal noise)—impulse noise, attenuation distortion, and delay distortion are not accounted for.

While the Nyquist formula indicates that doubling bandwidth doubles data rate (all else equal), Shannon investigates the relationship of data rate with both bandwidth and noise. Data rate can be increased either by increasing bandwidth or signal strength.

📌 Example: LowerF = 3 MHz, UpperF = 4 MHz, SNR = 24 B = 4 – 3 = 1 MHz SNRdB = 10 log₁₀(24) = 13.8 dB (not 251 as stated—correction: SNR = 24 linear, so SNRdB ≈ 13.8 dB) C = 10⁶ × log₂(1 + 24) = 10⁶ × log₂(25) ≈ 10⁶ × 4.64 ≈ 4.64 Mbps

For this data rate, using the Nyquist formula: M = 16 signal levels (since 2B log₂ M ≈ C, so 2×10⁶×4 = 8×10⁶ bps requires M=16)

📐 Formula — Shannon Capacity: C = B log₂(1 + SNR) → The maximum data rate equals bandwidth times the logarithm (base 2) of (1 plus the signal-to-noise ratio)

EM Spectrum

The electromagnetic spectrum spans from low frequencies to extremely high frequencies:

• 20 Hz to ~14 kHz: acoustic (normal adult hearing)
• 530 kHz to 1.710 MHz: AM radio broadcasts
• 42 MHz to 260 MHz: VHF terrestrial TV broadcast channels
• 88 MHz to 108 MHz: FM radio broadcasts
• 902 MHz to 928 MHz: common cordless telephone frequency (US)
• 0.8 to 2.3 GHz: mobile phone conversation channels
• 2.4 GHz: microwave ovens, Wireless LANs, cordless phones (starting 1998)
• 5.8 GHz: cordless phone frequency (introduced 2003)
• 428 THz to 750 THz: visible light (red to violet)
• 30 PHz: x-rays
• 300 EHz and above: gamma rays

Design Challenges

Two fundamental aspects of wireless communication:

1. Channel fading: includes multipath fading (signals reaching receiving antenna by two or more paths), path loss via distance attenuation, and shadowing by obstacles. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from terrestrial objects like mountains and buildings.

2. Interference: from multiple transmitters to a common receiver, or multiple transmitters to multiple receivers

💡 Why this matters: The primary concern in wireless systems is to increase the reliability of the air interface. This is achieved by controlling channel fading and interference. Recently, focus has shifted to spectral efficiency.

⭐ Key Takeaways

The fundamental distinction between analog and digital signals in time and frequency domains sets the stage for all wireless communication. Channel capacity is limited by both bandwidth (Nyquist formula: C = 2B log₂ M) and noise (Shannon formula: C = B log₂(1 + SNR)), and increasing data rate requires either more bandwidth, more signal levels, or higher signal strength relative to noise. SNR is the critical metric that determines signal quality and sets an upper bound on achievable data rate. The electromagnetic spectrum allocates different frequency bands for different applications (AM, FM, cellular, Wi-Fi), and understanding these allocations is essential for system design. The two fundamental design challenges—channel fading (multipath, path loss, shadowing) and interference—must be controlled to achieve reliable wireless communication.

🧠 Quick Revision Questions

1. What is the difference between analog and digital signals in the time domain?
2. Write the Nyquist formula for multilevel signaling. If B = 3100 Hz and M = 8, what is the channel capacity?
3. What is the Shannon capacity formula? If B = 1 MHz and SNR = 24 (linear), what is the theoretical maximum data rate?
4. Why is SNR expressed in decibels, and what does a high SNR indicate about signal quality?
5. Name the two fundamental design challenges in wireless communication and briefly explain each.

📘 Lecture 2 — Introduction to Wireless Communication

📖 Overview: This lecture explains why baseband signals cannot be transmitted wirelessly and introduces the fundamental concepts of signal encoding and modulation for wireless communication. It also covers the various types of noise that degrade wireless signals, the causes of signal loss and attenuation, and the concept of gain, which are essential for understanding real-world wireless system performance.

🗂️ Topics Covered

The lecture begins with a review of the previous lecture on wireless vision and EM signals, then delves into the necessity of bandpass signals for wireless transmission. It explains the role of signal encoding and modulation (ASK, FSK, PSK) for transmitting digital data over analog media, followed by a detailed classification of various noise types (thermal, intermodulation, crosstalk, impulse, natural, manmade). Finally, it covers signal impairments including attenuation from environmental factors and the concept of gain.

📝 Lecture Summary

Last Lecture Review

This section recaps the first lecture's objectives: introducing current wireless technologies and recent developments, and presenting the course syllabus covering cellular systems like GSM, AMPS, wireless networks like GPRS, EDGE, WLAN, and emerging technologies like wireless sensor networks and personal area networks. Key concepts from the previous lecture include the wireless vision driven by tetherless connectivity, VLSI technology, and the success of 2G systems. It also reviewed EM signals in the time and frequency domains, and channel capacity via Nyquist formulation, SNR, and the Shannon formula.

Transmission in Wireless Domain

The lecture introduces two fundamental signal types for transmission. A Baseband Signal is obtained by converting analog or digital data into an analog or digital signal, with a bandwidth from 0 to fmax. A Bandpass Signal is a band-limited signal whose minimum frequency is different from zero, with a bandwidth from f1 to f2.

🔑 Definition — Baseband Signal: A signal with a bandwidth starting at zero, obtained by converting data into a signal.
🔑 Definition — Bandpass Signal: A signal whose minimum frequency is non-zero, meaning its bandwidth is a specific range [f1, f2).

Wireless Transmission

This section explains why transmitting baseband signals wirelessly is virtually impossible. The air is a single transmission medium shared by all users and applications, unlike wired networks where new wiring can be added. A critical constraint is that antenna size must correspond to the signal's wavelength; a 1 MHz signal requires an antenna hundreds of meters high, while a 1 GHz signal needs only a few centimeters. The characteristics of wireless-signal propagation also depend heavily on the signal's frequency; low-frequency signals tilt downwards and follow the Earth's surface but do not propagate very far.

Signal Encoding/Modulation

This section addresses the challenge of transmitting digital data over analog-only media like optical fiber and unguided media. In both wired and wireless computer networks, digital information must be transferred, requiring conversion techniques. Encoding is the process where each pulse in a digital signal (a signal element) represents data bits. The data rate is the speed in bits/sec, while the modulation rate is the rate at which signal elements are changed, expressed in baud (signal elements/second). For a data rate R, the bit time (duration of one bit) is 1/R. At the receiver, the bit time and encoding must be known to correctly sample and interpret the signal.

🔑 Definition — Data Rate: The rate in bits per second at which data are transmitted.
🔑 Definition — Modulation Rate (Baud): The rate at which a signal element is changed, measured in signal elements per second.
📐 Formula: Bit time = 1/R, where R is the data rate. → A higher data rate means a shorter bit time.

Carrier and Information Signals

In RF systems, an analog carrier signal is the main airborne signal. The information signal (analog or digital) carrying the data is added on top of this carrier. This combination is called modulation. A perfect sine wave is desired because modulators make tiny modifications to it; if the sine wave is imperfect, these changes may be lost during transmission. The process is analogous to a letter: the envelope is the carrier, the letter is the information, and the envelope is only needed during transmission. The three basic types of modulation are AM (Amplitude Modulation), FM (Frequency Modulation), and PM (Phase Modulation).

Types of Encoding

There are three primary forms of encoding for digital data using analog signals, known as keying techniques.

Amplitude Shift-Keying (ASK) changes the height (amplitude) of the sine wave. One binary digit (e.g., 1) is represented by the presence of the carrier at constant amplitude, and the other (e.g., 0) by the absence of the carrier. ASK is susceptible to sudden gain changes and is an inefficient modulation technique. On voice-grade lines, it is used up to 1200 bps, but it is also used to transmit digital data over optical fiber.

Binary Frequency Shift-Keying (BFSK) changes the frequency of the sine wave without changing the height. Two binary digits are represented by two different frequencies near the carrier frequency. BFSK is less susceptible to error than ASK. On voice-grade lines, it is used up to 1200 bps and is used for high-frequency (3 to 30 MHz) radio transmission.

Multiple Frequency-Shift Keying (MFSK) uses more than two frequencies, making it more bandwidth-efficient and less susceptible to error. To match the data rate of the input bit stream, each output signal element is held for a time Ts = L*T, where T is the bit period. The signal frequencies are defined by the formula.

📐 Formula (MFSK): fi = fc + (2i – 1 – M)fd → Where fc is the carrier frequency, fd is the difference frequency, M is the number of different signal elements (M = 2^L), and L is the number of bits per signal element.

Phase Shift-Keying (PSK) changes the phase of successive sine waves. Two-level PSK, or BPSK (Binary PSK) , uses two phases to represent binary digits. The lecture notes explain that in phase modulation schemes, "B" stands for binary (2 points), "Q" for quadrature (4 points), and higher numbers like 16 and 64 represent more points. Increasing points increases speed but reduces interference tolerance, which is why systems automatically reduce speed in the face of interference.

💡 Why this matters: The trade-off between speed and interference tolerance in higher-order PSK (like QPSK, 16-QAM) is the fundamental reason wireless networks dynamically adjust their data rate based on signal quality.

Noise

Noise consists of all undesired radio signals, whether manmade or natural, making reception difficult. The radio signal's strength is of little use if noise power is greater than the received signal power, making the Signal-to-Noise Ratio (SNR) critically important. The categories of noise are: Thermal Noise, Intermodulation Noise, Crosstalk, and Impulse Noise.

Thermal Noise is due to the agitation of electrons and is present in all electronic devices and transmission media. It cannot be eliminated and is a function of temperature, being particularly significant for satellite communication. The amount of thermal noise in a 1Hz bandwidth is N0 = kT, where k is Boltzmann's constant and T is the absolute temperature in kelvins. For a bandwidth of B Hertz, total thermal noise power N = kTB.

🔑 Definition — Thermal Noise (White Noise): Noise caused by the thermal agitation of electrons, present in all electronic devices and transmission media, and independent of frequency.
📐 Formula: N = kTB, where N is the thermal noise power in watts, k is Boltzmann's constant (1.3803 × 10^-23 J/K), T is the temperature in kelvins, and B is the bandwidth in Hertz. → This means noise power increases with both temperature and bandwidth.

Noise Terminology

This section defines other noise types. Intermodulation noise occurs when signals with different frequencies share the same medium, creating interference at frequencies that are the sum or difference of the original frequencies. Crosstalk is unwanted coupling between signal paths, such as signals from nearby twisted pairs or unwanted signals picked up by antennas. Impulse noise consists of irregular pulses or noise spikes of short duration and high amplitude, caused by external electromagnetic disturbances or faults.

Manmade Noise

Manmade noise is generated almost anywhere with electrical activity, such as automobile ignition systems, power lines, motors, arc welders, and fluorescent lights. Each occurrence is small, but collectively they can completely hide a weak signal that would be above the natural noise level in a less populated area.

Natural Noise

Naturally occurring noise has two main sources. Atmospheric noise (e.g., from thunderstorms) is most significant from 0 to 5 MHz. Galactic noise (e.g., from stars) affects all higher frequencies. Both sources generate sharp pulses of electromagnetic energy that are propagated like any other signal and must be accepted by receiving systems.

Noise Remedy

An important principle is that increasing receiver amplification cannot improve the signal-to-noise ratio. Since both the signal and the noise are amplified equally, the ratio remains the same.

Loss

All components in a system exhibit either Loss or Gain. If the output signal is smaller than the input signal, the component experiences loss, which appears as heat. Devices called attenuators produce loss.

Attenuation

Attenuation is the loss of signal power. The causes of loss or attenuation in RF systems include water in various forms. When water is encountered in the air as the signal passes through, the form of the moisture matters. At frequencies above 10 GHz, attenuation from rain becomes significant, especially when the raindrop's size matches the signal's wavelength. Examples of loss from environmental factors include: rain causing about 0.08 dB of loss per mile for both 2.4 GHz and 5.8 GHz; fog causing about 0.03 dB per mile for 2.4 GHz and 0.11 dB per mile for 5.8 GHz; and ice changing the effective design of an antenna.

Other Impairments

Other signal impairments include atmospheric absorption (water vapor and oxygen contribute to attenuation), multipath (obstacles reflect signals so multiple copies with varying delays are received), and refraction (the bending of radio waves as they propagate through the atmosphere).

Gain

Gain is the opposite of loss, meaning the signal gets larger before exiting the device. RF amplifiers produce gain, which is an active process requiring a power source in most cases. Gain can also result from the combination of signals from different directions, such as the main signal and a reflected signal, though the total gain cannot exceed the original transmitted level in this case.

⭐ Key Takeaways

The most critical takeaway is that baseband signals cannot be transmitted wirelessly; they must be modulated onto a higher-frequency carrier signal to create a bandpass signal using techniques like AM, FM, or PM. For transmitting digital data, specific encoding schemes like ASK, FSK, or PSK are used to represent binary digits by varying the amplitude, frequency, or phase of the carrier, with each scheme having distinct trade-offs in speed, bandwidth efficiency, and error susceptibility. Understanding noise is crucial, as thermal noise (N=kTB) is unavoidable and sets a fundamental limit on communication performance, while other noise types like impulse and intermodulation noise can be mitigated through system design. The signal-to-noise ratio (SNR) is key because amplifying both signal and noise does not improve it. Finally, wireless signals are subject to significant impairments like attenuation from environmental factors (rain, fog) and multipath fading, which degrade performance, while gain (from amplifiers) is needed to compensate for these losses.

🧠 Quick Revision Questions

1. Why is it necessary to use a bandpass signal instead of a baseband signal for wireless transmission?
2. What are the three basic types of keying used to encode digital data onto an analog carrier signal, and which parameter of the carrier does each type modify?
3. Write the formula for thermal noise power. What are the three variables, and what does each one represent?
4. Explain the difference between data rate (bps) and modulation rate (baud). Under what condition would these two rates be equal?
5. Why does increasing the receiver amplification not improve the signal-to-noise ratio in a communication system?

📘 Lecture 3 — Introduction to Wireless Communication

📖 Overview: This lecture introduces the fundamental concepts of multiplexing and transmission media in wireless communication. It explains different propagation modes, the effects of multipath propagation, and various types of fading that affect wireless signals, providing essential knowledge for understanding how wireless networks operate.

🗂️ Topics Covered

The lecture begins with a review of previous concepts including wireless transmission, encoding techniques, and noise types. It then covers multiplexing techniques (FDM and TDM), guided and unguided transmission media (terrestrial microwave, satellite microwave, broadcast radio), propagation modes (ground-wave, sky-wave, line-of-sight), multipath propagation effects, and types of fading (fast, slow, flat, selective) including fading channel models (AWGN, Rayleigh, Rician).

📝 Lecture Summary

Review of previous lecture #2

Wireless transmission involves converting digital data into analog signals using baseband or bandpass signals. Key techniques include encoding and modulation, with receiver synchronization and demodulation required for proper reception. Various types of noise affect wireless signals including thermal noise, intermodulation noise, crosstalk, impulse noise, manmade noise, and natural noise. Systems also experience losses and gain.

Multiplexing

The capacity of a transmission medium usually exceeds the capacity required for a single signal. Multiplexing is the technique of carrying multiple signals on a single medium, enabling more efficient use of the transmission medium.

Reasons for widespread use of multiplexing include:

• Cost per kbps of transmission facility declines with increased data rate
• Cost of transmission and receiving equipment declines with increased data rate
• Most individual data communicating devices require modest data rate support

📌 Example: An ADSL connection has much higher bandwidth than a normal internet user requires. If two or more friends share the same ADSL connection, the cost per bit rate is reduced, and the cost of the ADSL modem is shared among them, making internet access cheaper. Similarly, the cost of cellular installations drops as the number of users on the network increases.

Multiplexing Techniques

Frequency-division multiplexing (FDM) takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal.

Time-division multiplexing (TDM) takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal.

📌 Example: For multiplexing voice signals, the useful spectrum is 300 to 3400 Hz. A channel of bandwidth 4 kHz is adequate as it keeps some frequency slots free. In the standard telecommunication voice multiplexing scheme, 12 × 4 kHz channel voice channels occupy frequencies from 60-108 kHz.

Classifications of Transmission Media

A transmission medium is the physical path between transmitter and receiver.

Guided Media: Waves are guided along a solid medium. Examples include copper twisted pair, copper coaxial cable, and optical fiber.

Unguided Media: Provides means of transmission but does not guide electromagnetic signals. Usually referred to as wireless transmission. Examples include the atmosphere and outer space.

Unguided Media

Transmission and reception are achieved by means of an antenna. Configurations for wireless transmission include directional and omnidirectional.

General Frequency Ranges:

• Microwave frequency range: 1 GHz to 40 GHz. Directional beams are possible, suitable for point-to-point transmission. Used for satellite communications.
• Radio frequency range: 30 MHz to 1 GHz. Suitable for omnidirectional applications.
• Infrared frequency range: Roughly 3×10¹¹ to 2×10¹⁴ Hz. Useful in local point-to-point and multipoint applications within confined areas.

💡 Why this matters: The frequency range determines the propagation characteristics, antenna size, and application suitability for wireless systems.

Terrestrial Microwave

Description: Common microwave antenna is a parabolic "dish", about 3 m in diameter, fixed rigidly to focus a narrow beam. It achieves line-of-sight transmission to the receiving antenna and is located at substantial heights above ground level. Due to attenuation (particularly from rainfall), repeaters/amplifiers are placed 10-100 km apart.

Applications:

• Long haul telecommunications service using the 4-6 GHz band (common), but due to increased congestion, 11 GHz is coming into use now
• Microwave links provide TV signals to local CATV, then distributed to subscribers via coaxial cable
• Short point-to-point links between buildings (enterprise offices, university campuses)

Satellite Microwave

Description: A communication satellite is a microwave relay station used to link two or more ground-based microwave transmitter/receivers. It receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). It is broadcast in nature.

Applications:

• Television distribution
• Long-distance telephone transmission (used for point-to-point trunks between telephone exchange offices)
• Private business networks

Transmission characteristics:

• Optimum range is 1-10 GHz
• Below 1 GHz: significant natural noise (solar, galactic, atmospheric) and manmade noise
• Above 10 GHz: higher attenuation due to atmospheric absorption
• Mostly use 5.925-6.425 GHz for uplink and 3.7-4.2 GHz for downlink (referred to as 4/6 GHz band)
• Due to saturation, 12/14 GHz band has been developed (uplink: 14-14.5 GHz, downlink: 11.7-12.2 GHz)
• Future: 20/30 GHz (uplink: 27.5-30.0 GHz, downlink: 17.7-20.2 GHz)
• Long propagation delay of about 250 ms, noticeable in telephone conversations
• Broadcast in nature, suitable for TV broadcast service

Broadcast Radio

Description: Broadcast radio antennas are omnidirectional, not required to be dish-shaped, and need not be rigidly mounted to a precise alignment.

Applications:

• Broadcast radio
• VHF and part of the UHF band (30 MHz to 1 GHz)
• Covers FM radio and UHF and VHF television

Characteristics:

• Due to longer wavelength, radio waves relatively suffer less attenuation
• Prime source of impairments is multi-path interference (reflection from land, water, and human-made objects can create multiple paths)
• Less sensitive to rainfall

Propagation Modes

The three propagation modes are: Ground-wave propagation, Sky-wave propagation, and Line-of-sight propagation.

Ground Wave Propagation

EM waves of low frequency induce current in the earth's surface, slowing down the wavefront near the earth and causing it to tilt downward. The wave follows the contour of the earth and can propagate considerable distances. This is effective for frequencies up to 2 MHz, which are low frequencies and have a tendency to tilt downwards. EM waves of low frequency are scattered by the atmosphere such that they do not penetrate the upper atmosphere.

📌 Example: AM radio uses ground wave propagation.

Sky Wave Propagation

The signal is reflected from the ionized layer of the atmosphere back down to earth. The signal can travel a number of hops, back and forth between the ionosphere and the earth's surface. The reflection effect is caused by refraction.

📌 Examples: Amateur radio, CB radio.

The atmosphere has several layers:

• Troposphere: First layer, starts at Earth's surface and goes up to about 10 km. Air temperature decreases at a rate of about 2.5°C for every 300 meters of altitude gained.
• Stratosphere: Extends from about 10 km to 50 km. Maintains a nearly constant temperature of about -65°C.
• Ionosphere: Above about 50 km and extending upward to more than 500 km. Molecules are ionized (electrons stripped from atoms by bombardment of the Sun's rays and other high-energy particles). These ionized particles with large quantities of free electrons act on any radio waves that pass through the ionosphere.

Line-of-Sight Propagation

Transmitting and receiving antennas must be within line of sight. This applies to satellite communication (signals above 30 MHz are not reflected by the ionosphere) and ground communication (antennas within effective line of site due to refraction).

Refraction is the bending of microwaves by the atmosphere. The velocity of an electromagnetic wave is a function of the density of the medium. When a wave changes medium, its speed changes, and the wave bends at the boundary between mediums.

Line-of-Sight Equations

🔑 Definition — Optical line of sight: d = 3.57√h

🔑 Definition — Effective (radio) line of sight: d = 3.57√(Kh)

Where:

• d = distance between antenna and horizon (km)
• h = antenna height (m)
• K = adjustment factor to account for refraction (rule of thumb: K = 4/3)

Maximum distance between two antennas for LOS propagation: D = 3.57(√(Kh₁) + √(Kh₂))

Where:

• h₁ = height of antenna one
• h₂ = height of antenna two

📌 Example 1: Let h₁ = 100 m, h₂ = 0 (second antenna at ground level). D = 3.57 × √(4/3 × 100) + 0 = 3.57 × √133.33 = 3.57 × 11.55 ≈ 41 km

📌 Example 2: Suppose h₂ = 10 m. To achieve the same distance (41 km), what must be h₁? 41 = 3.57(√(Kh₁) + √(Kh₂)) 41 = 3.57(√(Kh₁) + √(4/3 × 10)) √(Kh₁) = (41/3.57) - √13.33 = 11.48 - 3.65 = 7.83 Kh₁ = 61.3 h₁ = 61.3 × 3/4 = 46.2 m

Propagation Factors

The factors affecting propagation include: the transmitter's power output, the frequency being transmitted, the effect of the Earth's shape between the points, the conductivity of the Earth along the transmission path, and the microclimate through which the signal passes.

Multipath Propagation

Three mechanisms contribute to multipath propagation:

• Reflection: Occurs when a signal encounters a surface that is large relative to the wavelength of the signal
• Diffraction: Occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave
• Scattering: Occurs when an incoming signal hits an object whose size is in the order of the wavelength of the signal or less

The Effects of Multipath Propagation

• Multiple copies of a signal may arrive at different phases. If phases add destructively, the signal level relative to noise declines, making detection more difficult.
• Intersymbol interference (ISI): One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit.

Types of Fading

• Fast fading: Rapid variation in signal strength occurs over distances of about one-half of a wavelength. At 900 MHz cellular band, lambda (λ) is 0.33 m.
• Slow fading: Users cover distance well in excess of a wavelength as they pass buildings of different heights, vacant lots, etc. Results in slow variation in signal strength.
• Flat fading: A type of fading
• Selective fading: A type of fading

Fading Channel Models

• Additive White Gaussian Noise (AWGN) channel: Signal is degraded only by thermal noise. Accurate for space communication and some wire communication such as coaxial cable.
• Rayleigh fading: Occurs when there are multiple indirect paths but no direct LOS path. Suitable for outdoor environments.
• Rician fading: Occurs when there exists a direct LOS path in addition to multiple paths. Suitable for smaller cells and indoor environments.

💡 Why this matters: Choosing the correct fading channel model is crucial for accurately simulating and designing wireless communication systems for different environments.

⭐ Key Takeaways

The lecture establishes that multiplexing (both FDM and TDM) is essential for efficient use of transmission medium capacity and cost reduction. Three major unguided media types exist—terrestrial microwave, satellite microwave, and broadcast radio—each with distinct frequency ranges, antenna configurations, and application domains. Propagation occurs through three modes: ground-wave (up to 2 MHz), sky-wave (reflected by ionosphere), and line-of-sight (above 30 MHz), with the LOS distance determined by antenna heights and the refraction adjustment factor K=4/3. Multipath propagation (caused by reflection, diffraction, and scattering) leads to phase cancellation and intersymbol interference, while fading is classified as fast/slow based on variation rate and as flat/selective based on frequency response. Finally, channel models include AWGN (thermal noise only), Rayleigh fading (no LOS path, outdoor), and Rician fading (LOS path present, indoor/small cells).

🧠 Quick Revision Questions

1. What are the two main multiplexing techniques and how do they differ in their approach to sharing a transmission medium?
2. What are the three frequency ranges for unguided media and what are the typical applications and propagation characteristics of each?
3. A microwave antenna is 50 m tall. Using the effective line-of-sight formula with K=4/3, calculate the distance to the horizon.
4. Describe the three mechanisms that cause multipath propagation and explain how they can lead to intersymbol interference.
5. What is the difference between Rayleigh fading and Rician fading, and in what types of environments is each model typically applied?

📘 Lecture 4 — Error Detecting and Correcting Techniques

📖 Overview: This lecture transitions from transmission mediums and propagation modes into the critical topic of error detection and correction in wireless networks. It covers fundamental techniques like parity checks and the Cyclic Redundancy Check (CRC), explaining how they ensure data integrity despite transmission errors. Understanding these methods is essential for reliable data communication over error-prone wireless links.

🗂️ Topics Covered

The lecture begins with a review of multiplexing (FDM, TDM), transmission mediums (guided and unguided), propagation modes, multi-path propagation, and fading. It then delves into coping with transmission errors through error detection and correction codes, including parity checks (single and two-dimensional), the Cyclic Redundancy Check (CRC) with its modulo 2 arithmetic and polynomial representation, and concludes with block error correction codes and their principles like Hamming distance.

📝 Lecture Summary

Last Lecture Review

This section recaps key concepts from Lecture 3, including multiplexing techniques like Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). It also reviews transmission mediums, dividing them into guided media and unguided media such as microwave, radio waves, and infra red. Finally, it covers propagation modes (ground wave, sky-wave, LOS propagation), multi-path propagation, and fading.

Coping with Transmission Errors

This section introduces three main approaches to handle transmission errors. Error detection codes only detect the presence of an error. Error correction codes, or forward correction codes (FEC), are designed to both detect and correct errors and are widely used in wireless networks. Automatic repeat request (ARQ) protocols are used in combination with detection/correction; if a block of data with an error is discarded, the transmitter retransmits that block.

Error Detection Process

The process for error detection is described for both transmitter and receiver. The transmitter calculates an error-detecting code (check bits) from the data bits for a given frame and appends these check bits to the data bits. The receiver separates the incoming frame into data bits and check bits, calculates check bits from the received data bits, and compares them against the received check bits. A mismatch indicates a detected error. Crucially, error detection is not 100% reliable; a larger EDC (Error Detecting Code) field yields better detection and correction.

Parity Checks

Single bit parity check uses even or odd parity and can only detect single bit errors. It is useful when the probability of bit errors is small and independent, but errors are usually clustered. The ability of a receiver to both detect and correct errors is known as forward error correction (FEC).

🔑 Definition — Forward Error Correction (FEC): The ability of a receiver to both detect and correct errors.

📌 Example: Adding parity bit Data: 1 1 0 0 0 1 0 1 Odd parity: 1 1 0 0 0 1 0 1 1 (parity bit = 1 to make total 1s odd) Even parity: 1 1 0 0 0 1 0 1 0 (parity bit = 0 to make total 1s even)

📌 Example: Odd parity errors Received: 1 1 0 1 0 1 0 1 1 → error detected (odd parity but total 1s is even) Received: 1 1 0 1 0 0 0 1 1 → error undetected (two bits flipped, parity remains odd)

Two-dimensional parity checks are a generalization of 1-bit parity. D bits are divided into i rows and j columns. Using row and column indices, the receiver can not only detect errors but also correct them.

📌 Example: 2D Odd parity check Data: 1110010101111010, with i = 4, j = 4 The data is arranged in a 4x4 grid, and an additional parity bit is computed for each row and each column, allowing for single-bit error correction.

Cyclic Redundancy Check (CRC)

The transmitter generates an (n-k)-bit frame check sequence (FCS) for a k-bit block, resulting in an n-bit frame exactly divisible by a predetermined number. The receiver divides the incoming frame by this predetermined number; if there is no remainder, it assumes no error. The algorithm involves the transmitter and receiver agreeing on an r+1 bit pattern P. The transmitter chooses r additional bits to append with k data bits, which is the remainder of d / P. The receiver checks if the remainder of D / P is 0 for success.

🔑 Definition — Frame Check Sequence (FCS): An (n-k)-bit code appended to a k-bit data block to create an n-bit frame that is exactly divisible by a predetermined divisor.

CRC using Modulo 2 Arithmetic

This section details the mathematical process using Exclusive-OR (XOR) operations. The parameters are:

• T = n-bit frame to be transmitted
• D = k-bit block of data; the first k bits of T
• F = (n – k)-bit FCS; the last (n – k) bits of T
• P = pattern of n–k+1 bits; the predetermined divisor
• Q = Quotient
• R = Remainder

📐 Formula: T = 2^(n-k) D + F Dividing 2^(n-k) D by P gives: [2^(n-k) D] / P = Q + R/P. The remainder R is used as the FCS, so T = 2^(n-k) D + R. The proof shows that T/P has no remainder, making T exactly divisible by P.

📌 Example: Let d = 10111, P = 1001 The process involves:

1. Appending (n-k) zeros to D (here, 3 zeros since P is 4 bits)
2. Performing XOR division of the result by P
3. The remainder becomes the FCS
4. The transmitted frame T is D followed by the FCS

CRC using Polynomials

All values in CRC can be expressed as polynomials with a dummy variable X having binary coefficients. The key formula is: X^(n-k) D(X) / P(X) = Q(X) + R(X)/P(X) T(X) = X^(n-k) D(X) + R(X)

Common widely used versions of P(X) include:

• CRC–12: X^12 + X^11 + X^3 + X^2 + X + 1
• CRC–16: X^16 + X^15 + X^2 + 1
• CRC–CCITT: X^16 + X^12 + X^5 + 1
• CRC–32: X^32 + X^26 + X^23 + X^22 + X^16 + X^12 + X^11 + X^10 + X^8 + X^7 + X^5 + X^4 + X^2 + X + 1

Wireless Transmission Errors

Error detection requires retransmission, which is inadequate for wireless applications. The error rate on a wireless link can be high, resulting in a large number of retransmissions. Additionally, there is a long propagation delay compared to transmission time.

💡 Why this matters: In wireless networks, the high error rate makes frequent retransmissions impractical, which is why Forward Error Correction (FEC) is preferred over simple error detection.

Block Error Correction Codes

The transmitter uses a Forward Error Correction (FEC) encoder that maps each k-bit block into an n-bit block codeword. This codeword is then transmitted. The receiver demodulates the incoming signal and passes the block through an FEC decoder.

FEC Decoder Outcomes

The FEC decoder can produce four possible outcomes:

1. No errors present: Codeword produced by the decoder matches the original codeword.
2. Decoder detects and corrects bit errors
3. Decoder detects but cannot correct bit errors: Reports uncorrectable error.
4. Decoder detects no bit errors, though errors are present

Block Code Principles

Several key principles are defined for block codes:

• Hamming distance – For two n-bit binary sequences, it is the number of different bits.
• Redundancy – The ratio of redundant bits to data bits.
• Code rate – The ratio of data bits to total bits.
• Coding gain – The reduction in the required Eb/N0 to achieve a specified BER of an error-correcting coded system.

🔑 Definition — Hamming distance: For two n-bit binary sequences, the number of bits that are different.

📌 Example: v1 = 011011; v2 = 110001 011011 XOR 110001 = 101010 d(v1, v2) = 3 (three 1s in the XOR result)

Block Codes

The Hamming distance d of a block code is the minimum distance between two code words.

• Error Detection: Up to d-1 errors can be detected
• Error Correction: Up to ⌊(d-1)/2⌋ errors can be corrected

📌 Example: Block code with k = 2, n = 5 For k=2, there are 4 possible data blocks (00, 01, 10, 11). Each is mapped to a 5-bit codeword. If we receive pattern 0 0 1 0 0, the minimum distance is with codeword 0 0 0 0 0, so we deduct 0 0 as the data bits.

⭐ Key Takeaways

The most critical concepts from this lecture are the fundamental difference between error detection (which only identifies errors) and error correction (which can also fix them), with Forward Error Correction being vital for wireless networks. The Cyclic Redundancy Check is a powerful error detection method using modulo 2 division and polynomial representation, where the transmitted frame must be exactly divisible by a predetermined divisor. Parity checks, including two-dimensional parity, offer simpler detection and limited correction capabilities. Block error correction codes use the concept of Hamming distance to define their detection and correction limits, where the minimum distance d allows detection of up to d-1 errors and correction of up to ⌊(d-1)/2⌋ errors. Finally, for wireless applications, FEC is often preferred over ARQ due to high error rates and long propagation delays that make retransmission impractical.

🧠 Quick Revision Questions

1. What is the difference between error detection codes and error correction codes, and why are FEC codes preferred in wireless networks?
2. Explain how two-dimensional parity checks can both detect and correct errors, unlike single bit parity.
3. In CRC, what does it mean if the receiver finds a remainder of zero when dividing the received frame by the predetermined divisor P?
4. A block code has a Hamming distance d = 7. How many errors can it detect and how many can it correct?
5. What is the main disadvantage of using Automatic Repeat Request (ARQ) protocols in wireless networks with high error rates?

📘 Lecture 5 — Error Detecting and Correcting Techniques (Part II)

📖 Overview: This lecture continues the study of error detection and correction in wireless networks, focusing on advanced block codes including Hamming, BCH, and Reed-Solomon codes. It then introduces Automatic Repeat Request (ARQ) mechanisms, particularly sliding window flow control and Go-Back-N ARQ, which are essential for reliable data transmission.

🗂️ Topics Covered

The lecture begins with a review of transmission errors, parity check (single-bit and 2D parity), cyclic redundancy check, and block error codes from the previous lecture. It then covers Hamming Code in detail, including its parameters, encoding and decoding processes with examples. Next, BCH codes are introduced as multiple error correcting codes generalizing Hamming codes, followed by Reed-Solomon codes which process data in symbols. The second half covers Automatic Repeat Request (ARQ), flow control using sliding window, error control requirements, and the Go-Back-N ARQ protocol including its timeout mechanism and transmitter/receiver operations.

📝 Lecture Summary

Review of Previous Lecture

The previous lecture covered fundamental error detection techniques: transmission errors, parity check (both single-bit parity and 2D parity), cyclic redundancy check (CRC), and block error codes. These form the foundation for understanding more advanced error correcting codes.

Hamming Code

Hamming Code is designed specifically to correct single bit errors. It belongs to a family of (n, k) block error-correcting codes with specific parameters:

• Block length: n = 2^m – 1
• Number of data bits: k = 2^m – m – 1
• Number of check bits: n – k = m
• Minimum distance: d_min = 3

This is classified as a single-error-correcting (SEC) code. It can also function as a SEC double-error-detecting (SEC-DED) code.

The encoding process takes k data bits and adds (n - k) check bits. The decoding process compares the received (n - k) bits with the calculated (n - k) bits using XOR operation. The resulting (n - k) bits form the syndrome word. The syndrome range is between 0 and 2^(n-k) - 1. Each bit of the syndrome indicates a match (0) or conflict (1) in that bit position.

🔑 Definition — Syndrome Word: The result of XORing the received check bits with the calculated check bits, used to detect and locate errors in Hamming code decoding.

📌 Example of Hamming Encode: For data = 00111001, the encoding process calculates check bits and appends them to the data bits to form the transmitted codeword. The actual encoding involves determining parity bits at specific positions (powers of 2) within the codeword.

📌 Example of Decoding Hamming: The decoding process recalculates check bits from the received data, compares them with the received check bits using XOR to generate a syndrome word. A non-zero syndrome indicates an error, and its value points to the bit position that is in error.

💡 Why this matters: Hamming codes provide a practical method for correcting single-bit errors in memory systems and simple communication links, making them fundamental to reliable data storage and transmission.

BCH Codes

BCH codes are named after their discoverers: Bose, Chaudhuri, and Hocquenghem. These are multiple error correcting codes and serve as a generalization of Hamming Code. They offer flexibility in choice of parameters including block length and code rate.

For a positive pair of integers m and t, a (n, k) BCH code has parameters:

• Block length: n = 2^m – 1
• Number of check bits: n – k ≤ mt
• Minimum distance: d_min ≥ 2t + 1

BCH codes can correct combinations of t or fewer errors. The generator polynomial can be constructed from the factors of (X^(2m-1) + 1).

💡 Why this matters: BCH codes provide powerful error correction for multiple errors, making them suitable for applications like satellite communications and data storage where burst errors are common.

Reed-Solomon Codes

Reed-Solomon (RS) codes are a subclass of nonbinary BCH codes. Data is processed in chunks of m bits, called symbols.

An (n, k) RS code has parameters:

• Symbol length: m bits per symbol
• Block length: n = 2^m – 1 symbols = m(2^m – 1) bits
• Data length: k symbols
• Size of check code: n – k = 2t symbols = m(2t) bits
• Minimum distance: d_min = 2t + 1 symbols

🔑 Definition — Symbol: In Reed-Solomon codes, a symbol is a chunk of m bits that is processed as a single unit, allowing the code to correct errors at the symbol level rather than the bit level.

💡 Why this matters: Reed-Solomon codes are extremely effective at correcting burst errors (consecutive errors) and are widely used in QR codes, CDs, DVDs, and deep-space communications.

Automatic Repeat Request

Automatic Repeat Request (ARQ) is a mechanism used in data link control and transport protocols. It relies on the use of an error detection code (such as CRC). ARQ provides both Flow Control and Error Control functions.

Flow Control

Flow Control assures that the transmitting entity does not overwhelm a receiving entity with data. Protocols with flow control mechanism allow multiple PDUs in transit at the same time. PDUs arrive in the same order they are sent.

Sliding-window flow control operates as follows:

• The transmitter maintains a list (window) of sequence numbers allowed to send
• The receiver maintains a list allowed to receive

Reasons for breaking up a block of data before transmitting include:

• Limited buffer size of receiver
• Retransmission of PDU due to error requires smaller amounts of data to be retransmitted
• On shared medium, larger PDUs occupy medium for extended period, causing delays at other sending stations

Error control mechanisms detect and correct transmission errors by handling:

• Lost PDU: a PDU fails to arrive
• Damaged PDU: PDU arrives with errors

Error Control Requirements

The requirements for effective error control include:

1. Error detection: Receiver detects errors and discards PDUs
2. Positive acknowledgement: Destination returns acknowledgment of received, error-free PDUs
3. Retransmission after timeout: Source retransmits unacknowledged PDU
4. Negative acknowledgement and retransmission: Destination returns negative acknowledgment to PDUs in error

Go-Back-N ARQ

Go-Back-N ARQ improves upon Stop-and-Wait by not waiting for each acknowledgment. It keeps the channel busy by continuing to send frames and allows a window of up to Ws outstanding frames. It uses m-bit sequence numbering.

The operation principle: If an ACK for the oldest frame arrives before the window is exhausted, the transmitter can continue transmitting. If the window is exhausted, the transmitter must "pull back" and retransmit all outstanding frames. An alternative approach uses timeout.

In Go-Back-N ARQ:

• Frame transmissions are pipelined to keep the channel busy
• Frames with errors and subsequent out-of-sequence frames are ignored
• The transmitter is forced to go back when the window of 4 is exhausted
• The window size should be long enough to cover round trip time

🔑 Definition — Go-Back-N ARQ: An ARQ protocol where the transmitter sends multiple frames without waiting for individual acknowledgments, and upon detecting an error, retransmits the errored frame and all subsequent frames.

Go-Back-N with Timeout

A problem with the basic Go-Back-N ARQ arises: If a frame is lost and the source does not have another frame to send, then the window will not be exhausted and recovery will not commence. To address this, a timeout is used with each frame. When a timeout expires, the transmitter resends all outstanding frames.

Go-Back-N Transmitter & Receiver

The Go-Back-N ARQ uses two types of acknowledgments:

• RR (Receive Ready): Indicates no errors occur
• REJ (Reject): Indicates error detected

Contingencies that must be handled include:

• Damaged PDU
• Damaged RR acknowledgment
• Damaged REJ acknowledgment

The Stop-and-Wait ARQ (simpler protocol) and Go-Back-N ARQ are compared, with Go-Back-N providing higher efficiency by pipelining frame transmissions.

⭐ Key Takeaways

The most critical concepts from this lecture are: Hamming codes can correct single bit errors using syndrome calculation, while BCH codes generalize this to correct multiple errors with flexible parameters. Reed-Solomon codes process data in symbols rather than bits, making them powerful against burst errors. For reliable transmission, ARQ protocols combine error detection with retransmission strategies. Go-Back-N ARQ improves channel utilization by allowing multiple outstanding frames through sliding window flow control, but requires careful timeout management to handle lost frames when the transmitter has no data to send. The window size must be at least as large as the round-trip time's worth of frames to keep the channel fully utilized.

🧠 Quick Revision Questions

1. What are the parameters (n, k, m, d_min) for a Hamming code, and how many errors can it correct?
2. How does the syndrome word in Hamming decoding indicate the location of an error?
3. What is the relationship between BCH codes and Hamming codes, and what parameter determines how many errors a BCH code can correct?
4. In Reed-Solomon codes, what is a "symbol" and how does processing symbols rather than bits affect error correction capability?
5. Why does Go-Back-N ARQ need a timeout mechanism, and what happens when a timeout expires?

📘 Lecture 6 — Multiple Access Techniques

📖 Overview: This lecture covers the fundamental multiple access techniques used in wireless networks to allow multiple users to share a common communication channel. It explains FDMA, TDMA, CDMA, and random access methods, detailing their principles, advantages, disadvantages, and practical applications, which are essential for understanding how cellular and other wireless systems manage simultaneous user access.

🗂️ Topics Covered

The lecture begins with a review of previous topics on block codes (Hamming, BCH, Reed-Solomon), CDMA, and ARQ (sliding window, Go-back-N). It then introduces Multiple Access Techniques, covering Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Random Access methods including ALOHA, Slotted ALOHA, and Reservation-based ALOHA. CDMA is also revisited with a detailed example. The lecture concludes with a summary and a preview of carrier-sense based random access and spread spectrum for the next lecture.

📝 Lecture Summary

Review of previous lecture #5

The lecture begins with a review of key concepts from Lecture 5, including Block Codes like Hamming, BCH, and Reed-Solomon codes. It also reviews CDMA and ARQ techniques, specifically the Sliding Window protocol and Go-back-N ARQ.

Multiple Access Techniques

The core techniques for multiple users to share a communication medium are introduced: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Random Access, and Code Division Multiple Access (CDMA).

🔑 Definition — Multiple Access Techniques: Methods that allow multiple users to share a finite amount of radio spectrum simultaneously or nearly simultaneously.

FDMA

Frequency Division Multiple Access (FDMA) was the initial multiple-access technique for cellular systems. It separates a large frequency band into smaller channels, each supporting a single user. Guard bands are used to separate channels to prevent co-channel interference. FDMA uses narrow bandwidth (30 kHz).

💡 Why this matters: FDMA's simplicity made it the foundation for early cellular systems, but its dedicated channel per user leads to spectrum inefficiency when channels are idle.

🔑 Advantages:

• Simple to implement in terms of hardware
• Fairly efficient with a small base population and constant traffic

🔑 Disadvantages:

• Network and spectrum planning are intensive and time-consuming
• Channels are dedicated for a single user; idle channels add spectrum inefficiency

📌 Example: In an FDMA system, user 1 might be assigned frequency 880 MHz, user 2 frequency 880.03 MHz, and so on, with guard bands between them.

TDMA

Time Division Multiple Access (TDMA) makes the entire bandwidth available to each user, but only for a finite period of time. Users are allotted time slots for a channel, allowing sharing of a single channel. It requires time synchronization. Users take turns transmitting and receiving data in a round-robin fashion.

🔑 How it works:

1. User presses Push-to-Talk (PTT) button
2. A control channel registers the radio to the closest base station (BS)
3. The BS assigns an available pair of channels
4. Unlike FDMA, a TDMA system also assigns an available time slot within the channel
5. Data transmission is not continuous but sent and received in bursts
6. The bursts are reassembled and appear like continuous transmission

🔑 Advantages:

• Extended battery life and talk time
• More efficient use of spectrum compared to FDMA
• Will accommodate more users in the same spectrum space than an FDMA system

🔑 Disadvantages:

• Network and spectrum planning are intensive
• Multipath interference affects call quality
• Dropped calls are possible when users switch in and out of different cells
• Too few users result in idle channels (rural versus urban environment)
• Higher costs due to greater equipment sophistication

📌 Example: In a TDMA system with 3 time slots, user 1 transmits in slot 1, user 2 in slot 2, user 3 in slot 3, then the cycle repeats.

CDMA

Code Division Multiple Access (CDMA) is a spread spectrum technique used to increase spectrum efficiency. It has been used in military applications due to anti-jamming and security.

🔑 Basic Principles of CDMA:

• D = rate of data signal
• Break each bit into k chips (chips are a user-specific fixed pattern)
• Chip data rate of new channel = kD

📐 Formula:

• For a '1' bit, sender A sends code as chip pattern: <c1, c2, c3, c4, c5, c6>
• For a '0' bit, sender A sends complement of code: <-c1, -c2, -c3, -c4, -c5, -c6>
• Receiver knows sender's code and performs an electronic decode function:
  • <d1, d2, d3, d4, d5, d6> = received chip pattern
  • <c1, c2, c3, c4, c5, c6> = sender's code
  • S = (d1 × c1) + (d2 × c2) + (d3 × c3) + (d4 × c4) + (d5 × c5) + (d6 × c6)

📌 Example:

• User A code = <1, –1, –1, 1, –1, 1>
  • To send a 1 bit: <1, –1, –1, 1, –1, 1>
  • To send a 0 bit: <–1, 1, 1, –1, 1, –1>
• User B code = <1, 1, –1, –1, 1, 1>
  • To send a 1 bit: <1, 1, –1, –1, 1, 1>
• Receiver receiving with A's code: (A's code) × (received chip pattern)
  • User A '1' bit: 6 → 1
  • User A '0' bit: -6 → 0
  • User B '1' bit: 0 → unwanted signal ignored

🔑 Advantages:

• Greatest spectrum efficiency
• CDMA improves call quality by filtering out background noise, cross-talk, and interference
• Simplified frequency planning - all users on a CDMA system use the same radio frequency spectrum
• Random Walsh codes enhance user privacy (a spread-spectrum advantage)
• Precise power control increases talk time and battery size for mobile phones

🔑 Disadvantages:

• Backwards compatibility techniques are costly
• Currently, base station equipment is expensive
• Low traffic areas lead to inefficient use of spectrum and equipment resources

Random Access

Random Access methods are a more efficient way of managing medium access for communicating short, bursty messages. In contrast to fixed-access schemes, each user gains access to the medium only when needed (has data to send). The drawback is that users must compete to access the medium ('random access'), leading to collision of contending transmissions.

Random access methods in wireless networks are divided into two groups:

1. ALOHA-based: no coordination between users
2. Carrier-sense based: indirect coordination—users sense availability of medium before transmitting

ALOHA-based Random Access

In Pure ALOHA, a user accesses the medium as soon as it has a packet ready to transmit. After transmission, the user waits a length of time > round-trip delay in the network for an ACK (acknowledgment) from the receiver. If no ACK arrives, the user waits a random interval of time (to avoid repeated collision) and retransmits.

🔑 Advantages:

• Simple
• No synchronization among users required

🔑 Disadvantages:

• Low throughput under heavy load conditions
• Probability of collision increases as number of users increases
• Max throughput = 18% of channel capacity

Slotted ALOHA

In Slotted ALOHA, time is divided into equal time slots. When a user has a packet to transmit, the packet is buffered and transmitted at the start of the next time slot. The BS transmits a beacon signal for timing, and all users must synchronize their clocks.

🔑 Advantages:

• Partial packet collision is avoided

🔑 Disadvantages:

• Throughput is still quite low
• There is either no collision or a complete collision
• Max throughput = 36% of channel capacity

Slotted ALOHA in GSM

In GSM, there are two types of channels:

• Traffic channels (TCH): used for transmission of user data—based on FDMA/TDMA
• Signalling channels: used for control and management of a cellular network
  • Random Access Channel (RACH): a signalling channel for establishing access to the network (BS)—employs Slotted ALOHA—only channel in GSM where contention can occur

Reservation ALOHA

In Reservation ALOHA (R-ALOHA), time slots are divided into reservation and transmission slots/periods. During the reservation period, stations can reserve future slots in the transmission period. The reservation slot size is much smaller than the transmission slot size. Collisions occur only in reservation slots.

🔑 Advantages:

• Higher throughput under heavy loads
• Max throughput up to 80% of channel capacity

🔑 Disadvantages:

• More demanding on users as they have to obtain/keep a 'reservation list' up-to-date

R-ALOHA is most commonly used in satellite systems. The satellite collects requests, compiles a 'reservation list', and finally sends the list back to users.

⭐ Key Takeaways

The most critical concepts from this lecture are the fundamental differences between the four multiple access techniques: FDMA divides bandwidth into dedicated frequency channels for each user but suffers from inefficiency with idle channels; TDMA divides time into slots for shared use of the full bandwidth, requiring synchronization but improving spectrum efficiency; CDMA uses spread spectrum with unique codes per user, allowing all users to transmit simultaneously on the same frequency with high spectral efficiency; and Random Access methods like ALOHA, Slotted ALOHA, and Reservation ALOHA are essential for bursty traffic, with throughput increasing from 18% (Pure ALOHA) to 36% (Slotted ALOHA) and up to 80% (R-ALOHA) by introducing reservation mechanisms. Understanding the trade-offs in throughput, complexity, synchronization requirements, and implementation cost is key for choosing the right technique for different wireless network scenarios.

🧠 Quick Revision Questions

1. What is the maximum throughput of Pure ALOHA and Slotted ALOHA as a percentage of channel capacity?
2. What is the key difference between how FDMA and TDMA allocate the communication channel to multiple users?
3. In the CDMA example, if User A's code is <1, -1, -1, 1, -1, 1>, what chip pattern is sent to represent a '0' bit?
4. In a GSM network, which channel type employs Slotted ALOHA and is the only channel where contention can occur?
5. How does Reservation ALOHA achieve a significantly higher maximum throughput (up to 80%) compared to Pure ALOHA (18%)?

📘 Lecture 7 — CSMA and Spread Spectrum

📖 Overview: This lecture covers Carrier Sense Multiple Access (CSMA) protocols, which improve upon ALOHA by requiring nodes to listen before transmitting. It then introduces spread spectrum techniques, specifically Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS), which are used to combat interference and jamming in wireless networks.

🗂️ Topics Covered

This lecture begins with a review of ALOHA, Slotted ALOHA, and Reservation-based ALOHA. It then covers Carrier Sense Multiple Access (CSMA), including its non-persistent, 1-persistent, and p-persistent versions, as well as CSMA/CA (Collision Avoidance) with its algorithm details. The second half covers Spread Spectrum, focusing on Frequency Hopping Spread Spectrum (FHSS) using MFSK, including slow-hop and fast-hop variants, and performance considerations, followed by an introduction to Direct Sequence Spread Spectrum (DSSS).

📝 Lecture Summary

Last Lecture Review

The lecture reviews multiple access methods from the previous lecture: FDMA, TDMA, CDMA, and Random Access. It specifically revisits ALOHA, Slotted ALOHA, and Reservation-based ALOHA as random access protocols.

Carrier Sense Multiple Access (CSMA)

The section begins by outlining the disadvantages of ALOHA: users do not listen to the channel before or while transmitting, making it suitable only for networks with long propagation delays. CSMA is introduced as a "polite version of ALOHA" where the node listens to the channel before transmitting. If the channel is sensed busy, the node backs off (defers transmission) and senses the channel again after a random amount of time. If the channel is idle, the node transmits the entire frame.

🔑 Definition — CSMA (Carrier Sense Multiple Access): A medium access control protocol where a node listens to the channel (carrier sense) before transmitting to avoid collisions.

Versions of CSMA

Different node behaviors are employed when the channel is found busy:

• Non-persistent CSMA: After sensing a busy channel, the node waits the entire back-off period before sensing again.
• Persistent CSMA: After sensing a busy channel, the node continues sensing until the channel becomes free. Then:
  • 1-persistent CSMA: The node transmits immediately with probability 1.
  • p-persistent CSMA: The node transmits with probability p, or defers transmission with probability (1-p).

CSMA / Collision Avoidance

CSMA/CA is used where CSMA/CD cannot be used, particularly in wireless mediums where collisions cannot be easily detected because the power of the transmitting antenna overwhelms the receiving antenna. It is designed to reduce collision probability at points where collisions would most likely occur, such as when the medium has become idle after a busy state, as several users could have been waiting. Key elements include:

• IFS (Interframe Spacing): A priority mechanism where the shorter the IFS, the higher the priority for transmission.
• CW intervals (Contention Window): Intervals used for contention and transmission of packet frames.
• Backoff counter: Used only if two or more stations compete for transmission.

💡 Why this matters: CSMA/CA is the foundation of Wi-Fi (IEEE 802.11) medium access, making it essential for understanding how wireless local area networks avoid data collisions.

CSMA/CA Algorithm

The lecture presents a flowchart or algorithm for CSMA/CA (detailed in the source text as steps and decision points), which outlines the process of sensing the medium, waiting for IFS, managing the contention window, and decrementing the backoff counter to determine when to transmit.

Spread Spectrum

This section addresses the problem of radio transmission: frequency-dependent fading can wipe out narrowband signals for the duration of interference. The solution is to spread the narrowband signal into a broad band signal using a special code. This technique was initially developed for military use to combat jamming and interception. The power of the spread signal is the same as the narrowband signal, resulting in a lower power spectral density due to the larger bandwidth.

🔑 Definition — Spread Spectrum: A technique where a narrowband signal is spread across a wider frequency band using a special code, making it resistant to interference and jamming.

Types of Spreading

Two main types are introduced:

• Direct Sequence Spread Spectrum (DSSS)
• Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping Spread Spectrum (FHSS)

In FHSS, the signal is broadcast over a seemingly random series of radio frequencies. A number of channels are allocated for the FH signal, and the width of each channel corresponds to the bandwidth of the input signal. The signal hops from frequency to frequency at fixed intervals: the transmitter operates in one channel at a time, bits are transmitted using some encoding scheme, and at each successive interval, a new carrier frequency is selected. The channel sequence is dictated by a spreading code. The receiver, hopping between frequencies in synchronization with the transmitter, picks up the message.

• Advantages: Eavesdroppers hear only unintelligible blips, and attempts to jam the signal on one frequency succeed only at knocking out a few bits.

🔑 Definition — FHSS (Frequency Hopping Spread Spectrum): A spread spectrum technique where the signal is broadcast over a seemingly random series of radio frequencies, hopping between frequencies at fixed intervals as dictated by a spreading code.

FHSS Using MFSK

In this implementation, an MFSK (Multiple Frequency Shift Keying) signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signal. For a data rate of R:

• Duration of a bit: T = 1/R seconds
• Duration of signal element: Ts = L T seconds

Two variants are defined based on the relationship between Tc and Ts:

• Tc ≥ Ts: Slow-frequency-hop spread spectrum
• Tc < Ts: Fast-frequency-hop spread spectrum

🔑 Definition — Slow-hop FHSS: A frequency hopping variant where the hop rate is slower than or equal to the signal element rate (Tc ≥ Ts). 🔑 Definition — Fast-hop FHSS: A frequency hopping variant where the hop rate is faster than the signal element rate (Tc < Ts).

📐 Formula: For FHSS with MFSK, bit duration T = 1/R, and signal element duration Ts = L T. 📌 Example: If data rate R = 1000 bps, then bit duration T = 1/1000 = 1 ms. If L = 2 bits per signal element, then Ts = 2 * 1 ms = 2 ms. If Tc = 1 ms, then Tc < Ts, resulting in fast-hop FHSS.

FHSS Performance Considerations

Using a large number of frequencies results in a system that is quite resistant to jamming. The jammer must jam all frequencies, and with fixed power, this reduces the jamming power in any one frequency band.

Direct Sequence Spread Spectrum (DSSS)

In DSSS, each bit in the original signal is represented by multiple bits in the transmitted signal. The spreading code spreads the signal across a wider frequency band, and the spread is in direct proportion to the number of bits used. One technique combines the digital information stream with the spreading code bit stream using an exclusive-OR (XOR) operation.

🔑 Definition — DSSS (Direct Sequence Spread Spectrum): A spread spectrum technique where each bit in the original signal is represented by multiple bits (chips) in the transmitted signal, spreading it across a wider frequency band in proportion to the number of chips used.

⭐ Key Takeaways

CSMA improves on ALOHA by requiring nodes to listen (carrier sense) before transmitting, with variants like non-persistent, 1-persistent, and p-persistent defining different behaviors when the channel is busy. CSMA/CA is essential for wireless networks because collision detection (CSMA/CD) is impractical due to the overwhelming transmit power at the receiving antenna. Spread Spectrum techniques, FHSS and DSSS, solve the problem of frequency-dependent fading and jamming by spreading a narrowband signal across a wider bandwidth using a special code. FHSS hops frequencies at fixed intervals using a spreading code, while DSSS represents each bit with multiple chips using XOR with a spreading code.

🧠 Quick Revision Questions

1. What are the two main disadvantages of ALOHA that CSMA addresses?
2. In p-persistent CSMA, what happens if a node senses the channel is busy? If idle, what is the probability of immediate transmission?
3. Why is CSMA/CA preferred over CSMA/CD in wireless networks?
4. What is the difference between slow-frequency-hop and fast-frequency-hop spread spectrum in terms of Tc and Ts?
5. How does Direct Sequence Spread Spectrum (DSSS) use a spreading code to spread the signal?

📘 Lecture 8 — Evolution of Wireless Networks

📖 Overview: This lecture traces the evolution of wireless networks from pre-cellular high-power systems to first-generation (1G) analog cellular standards and second-generation (2G) digital cellular standards. It covers the key technological shifts that enabled higher capacity, better security, and new data services, laying the foundation for modern mobile communications.

🗂️ Topics Covered

The lecture reviews previous content on CSMA and 2G systems, then details the evolution of wireless systems from pre-cellular high-power systems to cellular architecture. It thoroughly examines first-generation analog cellular standards including NMT, AMPS, and TACS along with their technical specifications and limitations. The lecture then introduces second-generation digital cellular networks, explaining the advantages of digital over analog, and provides detailed coverage of GSM, IS-136 (D-AMPS), PDC, and IS-95 (cdmaOne) standards.

📝 Lecture Summary

Today Goals

The lecture aims to review previous lecture #7 and cover 1G wireless cellular networks including NMT, AMPS, and TACS, as well as spread spectrum techniques of frequency hopping and direct sequence.

Last Lecture Review

Previous lecture covered CSMA, its versions and CSMA/CA with examples, and 2G cellular systems including GSM, IS-136, PDC, and IS-95.

Evolution of Wireless Systems

The worldwide success of cellular telephone led to development of newer wireless systems for other types of communications beyond mobile voice, including high-speed data traffic, replacing fiber optics and copper lines over several kilometers, and replacing wires within homes and offices (leading to Bluetooth evolution).

Prior to cellular phones, mobile telephone service used a high power transmitter/receiver supporting about 25 channels with an effective radius of about 80 km. The way to increase system capacity is to use lower-power systems with shorter radius and numerous transmitters/receivers. Cellular systems evolved to provide organization of these transmitters/receivers and further improve system capacity.

First-Generation Cellular Networks

First-generation (1G) networks were analog systems with several standards:

• NMT (Nordic Mobile Telephone) — used in Nordic countries, Switzerland, Netherlands, Eastern Europe, and Russia
• AMPS (Advanced Mobile Phone System) — used in the United States
• TACS (Total Access Communications System) — used in the United Kingdom
• C-450 — in West Germany, Portugal, and South Africa
• Radiocom 2000 — in France
• RTMI — in Italy
• Japan had multiple systems with three standards: TZ-801, TZ-802, and TZ-803

NMT (Nordic Mobile Telephone)

NMT was the first fully-automatic cellular phone system, started in 1970 and in service by 1981. It had two standards: NMT-450 and NMT-900, corresponding to different frequency bands, with NMT-900 using higher bands.

🔑 Definition — Cell size range: Cell sizes range from 2 km to 30 km, using smaller sizes in urban areas for better quality and larger sizes in less-populated areas.

🔑 Definition — Handset power: Handsets use 1 watt, while car phones use 6-15 watts.

NMT featured automatic switching (dialing) and handover. It had no specification for voice traffic encryption — anyone could buy a scanner, tune to the desired channel, and intercept calls. NMT also supported a simple data transfer mode called DMS (Data and Messaging Service) or NMT-Text. Using DMS, text messaging was possible between two NMT handsets before SMS service started in GSM, but this feature was never commercially available except in Russian and Polish NMT networks.

NMT was suspended in Finland (TeliasSonera, December 31, 2002), Norway (December 31, 2004), and Sweden (TeliasSonera, December 31, 2007).

AMPS (Advanced Mobile Phone System)

AMPS was the 1G cellular phone system used in the US, which uses FDMA (Frequency Division Multiple Access). It operates in the 800 MHz band.

📐 Formula: Total of 832 channels — 416 in 824–849 MHz for transmissions from mobile to base, and 416 in 869–894 MHz for transmissions from base to mobile. Each channel is 30 KHz wide.

AMPS required large bandwidth for large base population. It offered no protection against eavesdroppers — the ESN (Electronic Serial Number) was cloned in the 1990s to make free calls from different cells. AMPS was replaced with D-AMPS, GSM, and CDMA for better security and capacity.

TACS (Total Access Communications System)

TACS is a variant of AMPS developed by Motorola. It was used in some European countries (including the UK and Ireland), as well as Japan and Hong Kong. ETACS was an extended version of TACS with more channels. The last ETACS service operated by Vodafone was discontinued on May 31, 2001.

💡 Why this matters: 1G systems were purely analog with limited capacity, no encryption (easy eavesdropping), and no data services. Their vulnerabilities motivated the shift to digital 2G systems.

Second-Generation Cellular Networks

Second-generation (2G) networks are digital systems — voice is digitized. Unlike 1G which relied on FDMA/FDD, 2G uses digital modulation formats and TDMA/FDD or CDMA/FDD multiple access techniques. They can be divided into two standards: TDMA-based and CDMA-based.

The main 2G standards are:

• GSM (TDMA-based), originally from Europe but used worldwide
• IS-136 aka D-AMPS (TDMA-based), used in the Americas
• IS-95 aka cdmaOne (CDMA-based), used in the Americas and parts of Asia
• PDC (TDMA-based), used exclusively in Japan

Using digital signals between handsets and towers increases system capacity in two key ways:

1. Digital voice data can be compressed and multiplexed much more effectively than analog voice encodings through various CODECs, allowing more calls in the same radio bandwidth.
2. Digital systems emit less radio power from handsets, allowing smaller cells so more cells can be placed in the same space, made possible by less expensive cell towers and equipment.

🔑 Definition — 2G Advantages:

• Lower powered radio signals require less battery power, so phones last longer between charges with smaller batteries
• Digital voice encoding allows digital error checking, improving sound quality by reducing dynamic range and lowering the noise floor
• Going all-digital allows digital data services such as SMS and email
• Better security — harder to be scanned

GSM (Global System for Mobile Communications)

GSM has 2.27 billion subscribers across more than 212 countries, representing 81% of the global mobile market. Its ubiquity provides very common international roaming.

📐 Formula: 8-slots TDMA with 200 KHz radio channel, with frame duration of 4.615 ms. The channel data rate is 270.833 kbit/s.

GSM operates in four different bands: mostly 900 MHz or 1800 MHz, while the US and Canada use 850 MHz and 1900 MHz. Each band has 25 MHz bandwidth subdivided into 124 channels. For example, in 900 MHz: uplink 890-915 MHz, downlink 935-960 MHz.

Other Systems

IS-136 or D-AMPS:

• 3-slot TDMA, used in North and South America, Australia
• Channel bandwidth is 30 KHz
• Frequency bands: 824-849 MHz and 869-894 MHz

Pacific Digital Cellular (PDC):

• Japanese standard similar to IS-136
• 25 KHz channel
• Data rates: 11.2 kbps at 3-slot and 5.6 kbps at 6-slot
• Operates in 800 MHz (downlink 810-888 MHz, uplink 893-958 MHz) and 1.5 GHz (downlink 1477-1501 MHz, uplink 1429-1453 MHz)

IS-95 or cdmaOne:

• Supports up to 64 users that are orthogonally coded
• Channel bandwidth is 1.25 MHz
• Widely deployed in North America, Korea, Japan, China, South America, Australia
• Channel data rate is 1.2288 Mchips/s (Mega Chips per second)

⭐ Key Takeaways

The evolution from 1G to 2G represents a fundamental shift from analog to digital cellular technology, bringing dramatic improvements in capacity, security, battery life, and new data services like SMS. 1G systems like NMT, AMPS, and TACS were analog with FDMA, no encryption (allowing eavesdropping and cloning), and limited capacity determined by high-power transmitters with large cells. 2G systems introduced digital modulation with TDMA (GSM, IS-136, PDC) or CDMA (IS-95), enabling voice compression, error checking, smaller cells, better security, and international roaming. Students must know the specific technical parameters of each standard including channel bandwidth, frequency bands, number of slots, and data rates for exam comparison questions.

🧠 Quick Revision Questions

1. What are the key differences between 1G and 2G cellular systems in terms of modulation, multiple access technique, and security?
2. What is the channel bandwidth, frequency band, and number of channels for AMPS?
3. How does GSM achieve capacity improvements through its TDMA structure (number of slots, channel bandwidth, frame duration, and data rate)?
4. Compare IS-136 (D-AMPS) and PDC in terms of channel bandwidth, number of TDMA slots, and data rates.
5. What is the channel bandwidth and chip rate for IS-95 (cdmaOne), and how many orthogonal users does it support?

📘 Lecture 9 — Evolution of Wireless Networks (Part II)

📖 Overview: This lecture continues the evolution of wireless networks, covering the transition from 2G to 2.5G technologies and the emergence of 3G standards. It explains the upgrade paths for GSM and CDMA networks, detailing how data rates and services improved without completely replacing existing infrastructure.

🗂️ Topics Covered

This lecture begins with a review of 1G and 2G systems from the previous lecture, then introduces 2.5G technologies including HSCSD, GPRS, and EDGE, along with IS-95B. It then covers 3G evolution, focusing on the two main camps: UMTS/W-CDMA from the GSM side and CDMA2000 from the CDMA side. The lecture concludes with a brief overview of TD-SCDMA, China's 3G standard.

📝 Lecture Summary

Review of last lecture #8

The review covers 1G wireless cellular networks including NMT, AMPS, and TACS, as well as key 2G technologies such as GSM, IS-136, PDC, and IS-95. These provide the foundation for understanding the upgrade paths discussed in this lecture.

Evolution to 2.5G

2.5G upgrades must be compatible with existing 2G technology. Three different upgrade paths were developed for GSM, with two also supporting IS-136: High Speed Circuit Switched Data (HSCSD), General Packet Radio Service (GPRS), and Enhanced Data rates for GSM Evolution (EDGE). GPRS and EDGE support IS-136, while IS-95B serves as the upgrade for IS-95.

HSCSD

HSCSD works in circuit switch mode. Speed is increased by allowing a single user to use consecutive time slots in the GSM standard. It relaxes error control coding algorithms specified in GSM, increasing data rate from 9.600 to 14.400 Kbps per slot. By using 4 slots, a raw data rate of up to 57.6 kbps is available to an individual user. This technology is ideal for dedicated streaming or real-time interactive web sessions.

🔑 Definition — HSCSD: High Speed Circuit Switched Data — a 2.5G upgrade that increases data rates by allowing a single user to use multiple consecutive GSM time slots in circuit-switched mode.

📐 Formula — Data rate calculation: 4 slots × 14.400 Kbps = 57.6 kbps

📌 Example: A user streaming a video would be assigned 4 consecutive GSM time slots, achieving a data rate of 57.6 kbps using relaxed error control coding.

GPRS

GPRS is packet-based and well-suited for non-real-time traffic like email, faxes, and web browsing. Unlike HSCSD, GPRS allows multi-user channel sharing of individual radio channels and time slots, supporting many more users. GPRS units are automatically instructed to tune to dedicated GPRS channels and particular time slots for always-on access. When all 8 slots are dedicated, the data rate reaches 171.2 kbps (8 × 21.4 kbps of raw un-coded data).

💡 Why this matters: GPRS introduced packet-switching to cellular networks, enabling efficient "always-on" data connections and better spectrum utilization compared to circuit-switched HSCSD.

EDGE (2.75G)

EDGE is a more advanced upgrade to 2G that requires new hardware and software. It was developed as a path to eventual 3G high-speed data access. EDGE introduces 8-PSK modulation in addition to the GSM standard GMSK. It allows nine different formats known as Multiple Modulation and Coding Scheme (MCS). Each MCS state may use either GMSK (low rate) or 8-PSK (high rate). Users start first with maximum error protection and maximum data rate until the link has unacceptable outage or delay. By combining different channels using multi-carrier transmission, EDGE provides up to several megabits per second data throughput.

🔑 Definition — EDGE: Enhanced Data rates for GSM Evolution — a 2.75G technology that uses 8-PSK modulation and MCS to achieve higher data rates than GPRS, providing a path to 3G.

IS-95B or cdmaOne

IS-95/CDMA has a single upgrade path IS-95B for eventual 3G operation. It dedicates multiple orthogonal user channels for specific users. IS-95A supports 64 users with a data rate of 14,400 Kbps. The medium data rate service allows a user to command up to 8 Walsh codes, achieving a raw data rate of 8 × 14,400 = 115.2 kbps. IS-95B supports hard handoff procedure, allowing units to search different radio channels without instruction from the switch, enabling rapid tuning to different base stations.

📐 Formula — IS-95B data rate: 8 Walsh codes × 14,400 bps = 115.2 kbps

Evolution to 3G

Third generation mobile phone standards are based on the International Telecommunication Union (ITU) family of standards under the International Mobile Telecommunications programme, "IMT-2000". 3G technologies enable network operators to offer a wider range of advanced services while achieving greater network capacity through improved spectral efficiency. Services include broadband wireless data in a mobile environment, typically at 5-10 Mb per second. The most significant feature of 3G is that it supports greater numbers of voice and data customers at higher data rates at lower incremental cost than 2G.

3G Evolution

The community remains split into two camps. The GSM/IS-136/PDC camp's 3G evolution is Wideband CDMA (W-CDMA), also known as UMTS. The IS-95B or CDMA camp's evolution path is cdma2000, with several variants all based on IS-95B. ITU-2000 standards are separated into two major organizations reflecting these two camps: 3GPP (3G Partnership Project) for W-CDMA and 3GPP2 (3G Partnership Project 2) for cdma2000.

🔑 Definition — 3GPP: 3G Partnership Project — the organization responsible for standardizing W-CDMA (UMTS) as the 3G evolution for GSM-based networks.

🔑 Definition — 3GPP2: 3G Partnership Project 2 — the organization responsible for standardizing cdma2000 as the 3G evolution for CDMA-based networks.

3G W-CDMA (UMTS)

This standard evolved under the European Telecom. Standards Institute (ETSI). It is backward compatible with 2G standards GSM, IS-136, and PDC technologies, as well as 2.5G. Bit-level packaging of GSM data is retained, with additional capacity and bandwidth provided by a new CDMA air interface. It provides always-on packet-based service for computers, entertainment devices, and telephones. W-CDMA requires expensive new base station equipment, making installation slow and gradual. It supports data rates up to 2.048 Mbps per user, allowing high-quality data, multimedia, and streaming audio (for stationary users). Future versions will support data rates in excess of 8 Mbps. The minimum spectral allocation is 5 MHz. Data rates from as low as 8 kbps to as high as 2 Mbps can be carried simultaneously on a single radio channel. Each channel can support between 100 and 350 voice calls simultaneously, depending on propagation conditions.

🔑 Definition — W-CDMA/UMTS: Wideband Code Division Multiple Access/Universal Mobile Telecommunications System — the 3G standard for GSM-based networks, using CDMA air interface with 5 MHz minimum spectral allocation and data rates up to 2.048 Mbps.

3G CDMA 2000

cdma2000 provides a seamless and evolutionary upgrade path for 2G and 2.5G CDMA technology. It centers on the original 1.25 MHz radio channel. CDMA operators may seamlessly and selectively upgrade without changing entire base station equipment. The first 3G CDMA standard is cdma2000 1xRTT, using a single channel (1x meaning multi-carrier). cdma2000 1x supports data rates up to 307 kbps in packet mode and can support up to twice as many users as 2G CDMA, requiring no additional equipment—simply software and new channel cards at the base station. cdma2000 1xEV is a proprietary high data rate packet standard by Qualcomm, overlaid on existing CDMA. CDMA 1xEV-DO dedicates the channel strictly to data users and supports 2.4 Mbps per channel. cdma2000 3xRTT is the ultimate 3G solution, relying on multicarrier that gangs adjacent channels together into 3.75 MHz. Three non-adjacent channels may be operated simultaneously and in parallel, providing data rates in excess of 2 Mbps. Advocates of cdma2000 claim their standard provides a much more seamless and less expensive upgrade path compared to W-CDMA.

🔑 Definition — cdma2000 1xRTT: The first 3G CDMA standard using single 1.25 MHz channel, supporting up to 307 kbps in packet mode and up to twice as many users as 2G CDMA.

🔑 Definition — cdma2000 1xEV-DO: Evolution Data Optimized — a cdma2000 variant that dedicates the channel strictly to data users, supporting up to 2.4 Mbps per channel.

3G TD-SCDMA

In China, where more than 8 million GSM subscribers were added in just one month, China's desire to craft its own wireless vision led to the Time Division Synchronous Code Division Multiple Access (TD-SCDMA) standard. Chinese CATT and Siemens jointly submitted this IMT-2000 3G standard. TD-SCDMA relies on existing GSM infrastructure, uses 1.6 MHz channels and smart antennas to yield more spectral efficiency. It uses 5 ms frames divided into 7 slots allocated to a single data-only user or several slow users. TD-SCDMA allows easy upgrade from GSM.

⭐ Key Takeaways

The lecture shows that 2.5G was a necessary stepping stone using HSCSD, GPRS, and EDGE to improve data rates without full 3G deployment. The cellular world split into two 3G camps: UMTS/W-CDMA (backward compatible with GSM, requiring new BS equipment, 2 Mbps data) and cdma2000 (seamless CDMA upgrade using 1.25 MHz channels, with 1xRTT at 307 kbps and 1xEV-DO at 2.4 Mbps). IS-95B was the CDMA upgrade using Walsh codes for 115.2 kbps. GPRS introduced packet-switched always-on data, while EDGE added 8-PSK modulation and MCS for higher rates. China developed TD-SCDMA as its own 3G standard using 1.6 MHz channels and smart antennas with GSM compatibility.

🧠 Quick Revision Questions

1. What are the three upgrade paths for GSM to 2.5G, and which two also support IS-136?
2. How does GPRS differ from HSCSD in terms of channel sharing and suitability for different traffic types?
3. What modulation scheme did EDGE add beyond GSM's GMSK, and what is the MCS framework?
4. What are the data rates and spectral allocation differences between UMTS/W-CDMA (5 MHz) and cdma2000 (1.25 MHz)?
5. Which organization (3GPP or 3GPP2) is responsible for W-CDMA, and which for cdma2000?

📘 Lecture 10 — Evolution of Wireless Networks (Part III)

📖 Overview: This lecture examines the limitations of 3G networks and introduces the vision, objectives, and architecture of 4G wireless systems. It covers the critical issues of QoS, security, and multimedia service delivery in heterogeneous network environments, concluding with an overview of wireless local and personal area network technologies.

🗂️ Topics Covered

The lecture begins with a review of 2.5G and 3G standards and their specifications. It then details the limitations of 3G, introduces the concept of 4G and its design objectives, and explores the major issues facing 4G including heterogeneous networks, QoS, security, and multimedia services. The convergence of cellular networks with WLANs is discussed along with applications, billing challenges, and an overview of WLAN standards including IEEE 802.11 family, WiMAX, and wireless PAN technologies.

📝 Lecture Summary

Review of previous lecture

The previous lecture covered 2.5G technologies including HSCSD, GPRS, EDGE, and IS-95B. It also introduced 3G standards such as UMTS/W-CDMA and CDMA2000, along with their specifications.

Limitations of 3G

The lecture identifies several key limitations of 3G systems. There is a difficulty of CDMA to provide higher data rates, alongside the need for continuously increasing data rate and bandwidth to meet multimedia requirements. Spectrum limitations and allocation issues exist, as does an inability to roam between different services. The system needs to provide a seamless transport end-to-end mechanism and introduce a better system with reduced cost.

4G

4G is designed to provide a comprehensive IP solution where voice, data, and streamed multimedia can be given to users on an "Anytime, Anywhere" basis, at higher data rates than previous generations. While no formal definition exists, certain objectives guide its development: it must be a fully IP-based integrated system providing 100 Mbit/s and 1 Gbit/s speeds both indoors and outdoors, with premium quality and high security.

4G Objectives

The objectives of 4G include creating a spectrally efficient system (in bits/s/Hz and bit/s/Hz/site). It must offer a nominal data rate of 100 Mbit/s at higher relative speeds and 1 Gbit/s while client and station are in relatively fixed positions. The system requires high network capacity with more simultaneous users per cell, smooth handoff across heterogeneous networks, seamless connectivity and global roaming across multiple networks, high quality of service for next generation multimedia support (real time audio, high speed data, HDTV video content, mobile TV, etc), interoperability with existing wireless standards, and an all IP, packet switched network.

🔑 Definition — Global information multimedia communication village: A concept representing the convergence of high speed internet and mobility, providing integrated multimedia communication worldwide.

Convergence of High Speed Internet & Mobility a Major Driver of Future Wireless

The wireless industry has grown enormously, with over 2.5 billion subscribers to cellular services enjoying connectivity while on the move. With the growth in Internet, a wide range of services are accessed through wired infrastructure. The introduction of mobile Internet brought about by the convergence of Mobile and Internet technologies is the future objective.

4G Concept

The 4G concept states: "The user has freedom and flexibility to select any desired service with reasonable QoS and affordable price, anytime, anywhere."

💡 Why this matters: This user-centric vision shifts from technology-driven design to service-driven design, where the network adapts to user needs rather than users adapting to network limitations.

Design Objectives

The design objectives for 4G include supporting heterogeneous networks that can operate seamlessly together, providing consistent user experience across different access technologies.

Heterogeneous Networks

Next generation systems must resolve multiple issues specific to heterogeneous networks. These include access and handover issues, location coordination and resource coordination, adding new users, QoS, wireless security and authentication, network failure backup, and pricing and billing.

Quality of Service (QoS)

Traffic generated by different services will increase traffic loads and require different quality of service (QoS) requirements (e.g., cell loss rate, delay, and jitter) for different streams (e.g., video, voice, data). Providing QoS guarantees in 4G networks is a non-trivial issue where both QoS signalling across different networks and service differentiation between mobile flows must be addressed. One of the most difficult problems is ensuring constant QoS level during handover. Depending on whether the new access router is in the same or some other sub network, we recognize horizontal and vertical handover.

🔑 Definition — Horizontal handover: Handover where the new access router is in the same sub network. 🔑 Definition — Vertical handover: Handover where the new access router is in a different sub network.

Quality of Service (continued)

The mobile terminal cannot receive IP packets while the handover process is finished. This time is called handover latency. Handover latency has a great influence on the flow of multimedia applications in real-time. Mobile IPv6 has been proposed to reduce the handover latency and the number of lost packets. The fields "Traffic Class" and "Flow Label" in the IPv6 header enable routers to secure special QoS for specific packet series with marked priority.

MULTIMEDIA – Video Services

4G wireless systems are expected to deliver efficient multimedia services at very high data rates. There are two types of video services: bursting and streaming video services.

🔑 Definition — Streaming: A service performed when a user requires real-time video services, in which the server delivers data continuously at a playback rate. 🔑 Definition — Bursting: A service that is basically file downloading using a buffer and is done at the highest data rate taking advantage of the whole available bandwidth.

Security

Security in wireless networks mainly involves authentication, confidentiality, integrity, and authorization for the access of network connectivity and QoS resources for the mobile nodes flow. The heterogeneity of wireless networks complicates the security issue. Dynamic reconfigurable, adaptive, and lightweight security mechanisms should be developed. AAA (Authentication Authorization Auditing) protocols provide a framework for such issues, especially for control plane functions and installing security policies in the mobile node such as encryption, decryption and filtering.

Convergence of Cellular Mobile Networks and WLANs

The convergence of cellular and WLAN offers benefits for both operators and users. For operators, benefits include higher bandwidths, lower cost of networks and equipment, use of licence-exempt spectrum, higher capacity and QoS enhancement, and higher revenue. For users, benefits include access to broadband multimedia services with lower cost where mostly needed and inter-network roaming.

Applications

Key 4G applications include Virtual Presence, which provides user services at all times even if the user is off-site. Virtual navigation provides users with access to a database of streets and buildings. Tele-geoprocessing applications combine GIS (Geographical Information System) and GPS (Global Positioning System) so users can get location by querying. Tele-Medicine and Education will support remote health monitoring of patients and provide lifelong learning opportunities. Crisis management enables restoration of communication systems within hours after natural disasters, compared to days or weeks in current systems.

Multiple Operators and Billing System

In today's communication market, an operator usually charges customers with a simple billing and accounting scheme. A flat rate based on subscribed services, call durations, and transferred data volume is usually enough in many situations. With the increase of service varieties in 4G systems, more comprehensive billing and accounting systems are needed.

WLANs

WLANs use the unlicensed Industrial Scientific and Medical (ISM) band. The ISM bands in the US are 900 MHz (902-928 MHz), 2.4 GHz (2400-2483.5 MHz), and 5.7 GHz (5725-5850 MHz).

IEEE 802.11

IEEE 802.11 is a family of standards defining Physical (Phy) and MAC layers. The original IEEE 802.11 standard supports Infrared (IR) and 2.4 GHz ISM band with 1 or 2 Mbps. IEEE 802.11b provides 11 Mbps in 2.4 GHz. IEEE 802.11a provides 54 Mbps in 5.7 GHz. IEEE 802.11g provides 54 MHz in 2.4 GHz. Other standards include IEEE 802.11i for Security, IEEE 802.11e for QoS, and IEEE 802.11f for Inter-access point protocol.

Worldwide Interoperability for Microwave Access

WiMAX is aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. It is based on IEEE 802.16, also called wireless MAN. It serves as last mile wireless broadband access as an alternative to cable and DSL.

Wireless PAN

IEEE 802.15 standards cover wireless personal area networks. IEEE 802.15.1 or Bluetooth offers moderate data range up to 720 kbps, operates in ISM band, with 10 m to 100 m range. IEEE 802.15.2 addresses co-existence issues of IEEE 802.11 and 802.15. IEEE 802.15.3 is high rate, low power with high data rate up to 20 Mbps, designed for multimedia applications over low power devices. IEEE 802.15.4 / ZigBee offers low power with range of 100m and low rate about 20 kbps.

Summary

The next lecture will cover fundamental principles of cellular networks.

⭐ Key Takeaways

Students must remember that 4G represents a paradigm shift to an all-IP, packet-switched architecture designed to provide 100 Mbps to 1 Gbps data rates with seamless connectivity across heterogeneous networks. The critical challenges include maintaining QoS during handovers, where handover latency directly impacts real-time multimedia applications. Security in heterogeneous environments requires AAA protocols and lightweight, adaptive mechanisms. The convergence of cellular networks with WLANs and the variety of wireless standards (802.11, WiMAX, Bluetooth, ZigBee) highlight the importance of interoperability and seamless roaming. Finally, understanding the difference between streaming and bursting video services, and the ISM frequency bands used by WLANs, is essential for exam preparation.

🧠 Quick Revision Questions

1. What are the five main limitations of 3G networks that drove the development of 4G?
2. What are the nominal data rate requirements for 4G at high relative speeds and at fixed positions?
3. What is the difference between horizontal and vertical handover in 4G heterogeneous networks?
4. What is handover latency and how does Mobile IPv6 address this problem?
5. What are the three ISM frequency bands used in the US for WLANs, and what data rates do IEEE 802.11b and IEEE 802.11a provide?

📘 Lecture 11 — Fundamentals of Cellular Networks (Part I)

📖 Overview: This lecture introduces the foundational concept of cellular networks, focusing on how the cellular concept solved the problem of spectrum congestion and user capacity. It explains frequency reuse, the geometry of cell shapes, and how to locate co-channel cells, culminating in a practical example calculating channel allocation for different cluster sizes.

🗂️ Topics Covered

The lecture begins with a review of 3G limitations, 4G objectives, and convergence of cellular and WLAN. It then introduces the cellular concept as a major breakthrough, explaining how replacing a single high-power transmitter with many low-power transmitters (small cells) increases capacity. The lecture covers frequency reuse, the hexagonal cell model, system capacity equations, cluster sizes, and the method for locating co-channel neighbors. A detailed numerical example demonstrates how to calculate channels per cell for four-cell, seven-cell, and twelve-cell reuse patterns, including control channel allocation.

📝 Lecture Summary

Review of last lecture

The previous lecture covered the limitations of 3G, the objectives and issues of 4G, QoS, the convergence of Cellular and WLAN, billing issues, and wireless network security, multimedia services, and applications.

Introduction

Early mobile systems aimed for large coverage using a single high-power antenna, making frequency reuse impossible in the same coverage area. For example, the Bell mobile system in 1970 could support a maximum of 12 simultaneous calls over a thousand square miles. Government regulators could not allocate spectrum proportionally to the increasing demand. This made it imperative to restructure the telephone system to achieve high capacity with limited radio spectrum.

Cellular Concept

The cellular concept was a major breakthrough in solving the problem of spectrum congestion and user capacity. It offers high capacity without major technological changes by replacing a high-power transmitter (large cell) with many low-power transmitters (small cells), each providing service to a small area. Each Base Station (BS) is allocated a portion of the channels, and nearby BSs are assigned different groups of channels so that all available channels are distributed among nearby BSs. Channels may be reused as many times as necessary as long as the BSs using the same channels are not overlapping. As demand for service increases, the number of BSs can be increased with reduced transmission power, providing additional capacity with no addition to spectrum. This is the foundation of all modern wireless communication systems.

💡 Why this matters: The cellular concept is the core innovation that allows millions of users to share the limited radio spectrum.

Frequency Reuse

Frequency reuse relies on intelligent allocation and reuse of channels. A small geographical area with an allocation of a group of channels is called a cell. BS antennas are designed to achieve the desired coverage within a cell while avoiding co-channel interference. The design process of selecting and allocating channel groups for all the cellular BSs is called frequency reuse or frequency planning.

The hexagonal shape representing a cell is a conceptual and simplistic model of coverage. The actual radio coverage is known as the footprint and is determined from field measurements and propagation prediction models. However, a regular shape is needed for systematic system design and adaptation to future growth. While a circle might be natural to represent coverage, adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping.

Three possible choices of shapes are square, equilateral triangle, and hexagon. For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three. Thus, by using hexagon geometry, the fewest number of cells can cover a geographic region, and it closely approximates a circle.

Capacity of System

When using hexagon to model coverage areas, there are two types of cells:

• Center-excited Cell: BS depicted as being in the center of the cell; an omni-directional antenna is used.
• Edge-excited Cell: BS depicted on three of the six cell vertices; a sectored directional antenna is used.

Consider a cellular system which has S duplex channels available for reuse. Each cell is allocated a group of k channels (k < S). If the S channels are divided among N cells (unique and disjoint), then: 🔑 Definition — Cluster: N cells which collectively use the complete set of available frequencies.

📐 Formula: S = k(N)

If a cluster is replicated M times in the system, the total number of duplex channels C as a measure of capacity is: 📐 Formula: C = M(k)(N) = M(S)

So, capacity is directly proportional to the replication factor in a fixed area. Factor N is called cluster size and is typically equal to 4, 7, or 12. If cluster size N is reduced while cell size is kept constant, more clusters are required, and more capacity is achieved. A large cluster size indicates that co-channel cells are far from each other, while a small cluster size means co-channel cells are located much closer together. The value of N is a function of how much interference a mobile or BS can tolerate. Clusters are inversely proportional to N, and capacity is directly proportional to clusters. Thus, the frequency reuse factor is given by 1/N.

In the hexagonal layout, each hexagon has exactly six equidistant neighbors, and the lines joining the centers of any cell and its neighbors are separated by multiples of 60 degrees. Therefore, only certain cluster sizes and layouts are possible.

Locating co-channel neighbors

To connect hexagons without gaps, the geometry of the hexagon is such that the number of cells per cluster N can only have values where i and j are non-negative integers: 📐 Formula: N = i² + i(j) + j²

To find the nearest co-channel neighbors of a particular cell, do the following:

1. Move i cells along any chain of hexagons.
2. Then turn 60 degrees counter-clockwise and move j cells.

📌 Example: The lecture provides a diagram showing how to locate co-channel cells using the i and j values for a cluster size of N=7 (i=2, j=1).

Example

A bandwidth of 33 MHz is allocated to a particular FDD cellular system, where two 25 KHz simplex channels provide full-duplex for voice/data.

Part I: Compute the number of channels per cell if the system uses four-cell, seven-cell, and twelve-cell reuse.

Solution Part I:

• Total BW = 33 MHz
• Channel BW = 25 KHz × 2 = 50 KHz/duplex channel
• S = 33,000 / 50 = 660 channels
• For N = 4: k = 660 / 4 ≈ 165 channels
• For N = 7: k = 660 / 7 ≈ 95 channels
• For N = 12: k = 660 / 12 ≈ 55 channels

Part II: If 1 MHz is dedicated to control channels, determine the equitable distribution of control and voice channels per cell for the above three systems.

Solution Part II:

• Control channels (Sc) = 1000 / 50 = 20 channels

• Voice channels (Sv) = S – Sc = 660 – 20 = 640 channels

• For N = 4: 5 control channels + 160 voice channels per cell.

• For N = 7: In practice, 1 control channel per cell and 4 cells with 91 voice channels + 3 cells with 92 voice channels.

• For N = 12: In practice, 1 control channel per cell and 8 cells with 53 voice channels + 4 cells with 54 voice channels.

⭐ Key Takeaways

The cellular concept is the foundation of all modern wireless systems, replacing a single high-power transmitter with many low-power cells to increase capacity without adding spectrum. Frequency reuse, quantified by the reuse factor 1/N, is the key mechanism, where N is the cluster size (typically 4, 7, or 12) and must satisfy the formula N = i² + ij + j². The hexagonal cell model is used because it provides the largest coverage area without gaps. System capacity C = MkN = MS, meaning it is directly proportional to the number of cluster replications. The numerical example demonstrates that a smaller cluster size (N=4) provides more channels per cell (165) than a larger cluster size (N=12, 55 channels), but smaller clusters increase co-channel interference.

🧠 Quick Revision Questions

1. What is the fundamental idea behind the cellular concept that solved spectrum congestion?
2. Why is a hexagon the preferred shape for cellular coverage modeling instead of a circle or square?
3. Write the formula for the number of duplex channels in a system (C) and state what each variable represents.
4. For a cluster size of N=7, what are the values of i and j, and how do you use them to locate a co-channel neighbor?
5. For a 33 MHz system with 50 KHz duplex channels, if 1 MHz is reserved for control channels, how many voice channels are available per cell for a seven-cell reuse pattern (in practice)?

📘 Lecture 12 — Fundamentals of Cellular Networks (Part II)

📖 Overview: This lecture continues the exploration of cellular networks, focusing on how channels are assigned to cells and how calls are handed off as users move. It explains the critical mechanisms that ensure seamless connectivity, including channel assignment strategies, handoff triggers, and practical challenges like handoff prioritization and cell dragging, which are essential for understanding real-world cellular system performance.

🗂️ Topics Covered

This lecture covers Channel Assignment Strategies, including fixed and dynamic allocation; Handoff Strategies, detailing when and how handoffs occur, the evolution from 1G base station-based to 2G mobile-assisted handoff; Prioritizing Handoffs through guard channels and queuing; and Practical Handoff Considerations, including umbrella cells, cell dragging, and differences in handoff timing across 1G, 2G, and CDMA (soft handoff) systems.

📝 Lecture Summary

Channel Assignment Strategies

For efficient spectrum utilization, a frequency reuse scheme must balance increasing system capacity and minimizing interference. Strategies are classified as Fixed and Dynamic. In Fixed Channel Assignment Strategy, each cell is allocated a predetermined set of voice channels. A call attempt can only be served if an unused channel in that particular cell is available. If all channels are occupied, the call is blocked. A variation is the borrowing strategy, where a cell is allowed to borrow a channel from a neighboring cell if all its channels are occupied. A mobile switching center (MSC) supervises such procedures and ensures the borrowing does not disrupt or interfere with any calls in progress in the donor cell.

In Dynamic Channel Assignment Strategy, voice channels are not permanently allocated to cells. On each call request, the BS requests a channel from the MSC. The MSC allocates a channel by taking into account the likelihood of future blocking within the cell and the frequency of use of the candidate channel (reuse distance). Hence, the MSC only allocates a channel if it is not presently in use in a cell within the minimum restricted distance of frequency reuse. This reduces the likelihood of call blocking, increasing the trunking capacity of the system. However, it requires the MSC to collect real-time data on channel occupancy, traffic distribution, and RSSI of all channels, which increases storage and computational load but provides increased channel utilization and decreased call blocking.

🔑 Definition — Fixed Channel Assignment: Each cell is allocated a predetermined set of voice channels; a call is blocked if all channels in that cell are busy. 🔑 Definition — Dynamic Channel Assignment: Voice channels are not permanently allocated; the MSC assigns a channel dynamically based on real-time traffic and interference conditions.

Handoff Strategies

Handoff occurs when a mobile user moves to a different cell while a conversation is in progress, and the MSC transfers the call to a new BS. This involves identifying the new BS and allocating new voice and control channels. Handoffs must be performed successfully, infrequently, and imperceptibly to the user. To achieve this, the designer must specify the optimum signal level at which a handoff initiates. Once a signal level is specified as the minimum usable for acceptable voice quality, a slightly stronger signal level is used as the threshold, normally taken between -90 dBm and -100 dBm. This margin, ∆ = Pr_handoff – Pr_min, cannot be too large or too small. If ∆ is too large, unnecessary handoffs burden the MSC. If ∆ is too small, there is insufficient time to complete a handoff before a call is lost due to a weak signal. ∆ must be chosen carefully to meet these conflicting requirements.

Call drops can occur due to excessive delay by the MSC from high traffic load, ∆ being set too small for handoff time, or no channels being available on any nearby BS. A handoff decision is made when the drop in signal level is not due to momentary fading and the mobile is actually moving away from the serving BS. To ensure this, the BS monitors the signal level for a certain period of time, which depends on the vehicle speed. If the slope of the average received signal level is steep, the handoff is made quickly. In 1G, the signal level was measured by the BS and supervised by the MSC. Each BS constantly monitors the signal strength of all its reverse channels to determine the relative location of each mobile user. Additionally, a locator receiver (a spare receiver) scans and measures RSSI of mobile users in neighboring cells and reports to the MSC, which then decides if a handoff is necessary.

🔑 Definition — Handoff: The process of transferring an ongoing call from one base station to another as a user moves between cells. 📐 Formula: ∆ = Pr_handoff – Pr_min → The margin between the handoff threshold signal level and the minimum acceptable signal level for voice quality.

Mobile Assisted Handoff (MAHO)

In 2G, handoff decisions are mobile-assisted. Each mobile measures the RSSI of all surrounding BSs, reports to the serving BS, and a handoff is initiated if the power of the serving BS is lesser than a nearby BS by a certain level or for a certain period of time. MAHO enables calls to be handed over between base stations at a much faster rate than in 1G. The MSC no longer constantly monitors RSSI, making this approach more suitable for microcellular environments where handoffs are frequent. An intersystem handoff occurs if a mobile moves from one cellular system to a different system controlled by a different MSC. Issues to be addressed include a local call becoming a long-distance call (roaming), determining compatibility between the two MSCs, and differing policies for managing handoff requests. Handoff is prioritized because call termination in the middle of a conversation is more annoying than being blocked on a new call attempt.

🔑 Definition — Mobile Assisted Handoff (MAHO): A handoff strategy where the mobile station measures the signal strength of surrounding base stations and reports to the serving BS, which then decides on handoff initiation.

Prioritizing Handoffs

Two methods of handoff prioritizing are used. The guard channel concept reserves a fraction of available channels exclusively for handoff requests. This has the disadvantage of reducing total carried traffic but offers efficient spectrum utilization when dynamic channel assignment strategies minimize the number of required guard channels. The second method is queuing of handoff requests. This is possible due to the time interval that elapses when the signal level drops below the threshold until it reaches the minimum signal level. Queuing decreases the probability of forced termination due to a lack of available channels, creating a tradeoff between this decrease and total traffic. The delay time and queue size are determined from the traffic pattern. Queuing does not guarantee zero probability of call termination, as large delays will cause the signal level to drop to the minimum.

🔑 Definition — Guard Channel Concept: Reserving a set of channels exclusively for handoff requests to reduce the probability of call termination. 🔑 Definition — Queuing of Handoff Requests: A method that stores handoff requests in a queue, taking advantage of the time between the threshold and minimum signal levels to find a free channel.

Practical Handoff Considerations

Several problems arise when designing a system for a wide range of mobile velocities. High-speed vehicles pass through a cell in a matter of seconds, and with the addition of microcells, the MSC can quickly become burdened. Pedestrian users may never need a handoff during a call. Issues include schemes to handle high-speed and low-speed users simultaneously and the ability to obtain new cell sites. Additional capacity is often provided through new cell sites, but these are difficult to obtain. An alternative is to install additional channels and base stations at the same location as an existing cell, using different antenna heights and power levels to provide large and small cells co-located at a single location, called an umbrella cell. This provides large coverage area to high-speed users (minimizing handoffs) and small coverage to slow-speed users. Speed can be estimated by the BS or MSC from RSSI.

Cell dragging is a problem in microcells due to the high signal strength of pedestrian users. It occurs in urban areas when there is a line-of-sight (LOS) path. The average signal strength does not decay rapidly even if a user travels well beyond the range of the cell. The RSSI may remain above the handoff threshold, and thus a handoff is not made. This creates potential interference since the user has traveled deep within a neighboring cell. Handoff parameters, particularly the threshold, must be adjusted carefully to avoid this. In 1G, the time to make a handoff when the signal drops below the threshold is 10 seconds, requiring ∆ to be on the order of 6 dB to 12 dB. In 2G (e.g., GSM), MAHO determines the best handoff candidates and requires only 1 or 2 seconds, so ∆ is usually between 0 dB and 6 dB. This provides the MSC substantial time to rescue a call in need of handoff. The IS-95 (CDMA) system provides a unique handoff capability called soft handoff. Unlike channelized (hard handoff), spread spectrum (SS) mobiles share the same channel in every cell. Thus, handoff does not assign a new channel; a different BS handles the communication task. By simultaneously evaluating the RSSI from a single user, the MSC decides which version of the signal is best. This ability to select between instantaneous received signals from a variety of BSs is called soft handoff.

🔑 Definition — Umbrella Cell: A large cell co-located with smaller cells, designed to handle high-speed users with fewer handoffs. 🔑 Definition — Cell Dragging: A problem in microcells where a user's high signal strength prevents a timely handoff, causing potential interference as the user moves deep into a neighboring cell. 🔑 Definition — Soft Handoff: A CDMA handoff technique where the mobile communicates with multiple base stations simultaneously, and the best signal is selected, avoiding a hard channel switch.

⭐ Key Takeaways

This lecture is critical for understanding how cellular networks manage resources and maintain calls. You must understand the core difference between fixed and dynamic channel assignment, including their trade-offs in complexity and capacity. For handoffs, the key is knowing the handoff margin (∆) and its trade-off, the evolution from 1G (BS-measured) to 2G (MAHO), and the distinction between guard channels and queuing as prioritization methods. Practical issues like umbrella cells (for mixed-speed users) and cell dragging (signal propagation problem) are essential real-world challenges. Finally, the concept of soft handoff in CDMA is a fundamental difference from hard handoffs in FDMA/TDMA systems.

🧠 Quick Revision Questions

1. What is the primary difference between fixed and dynamic channel assignment strategies in terms of channel allocation?
2. Explain the purpose of the handoff margin (∆) and the consequence of setting it too large or too small.
3. How does Mobile Assisted Handoff (MAHO) in 2G differ from the handoff method used in 1G systems?
4. Describe the two main methods for prioritizing handoffs: guard channels and queuing. What is the primary trade-off for each?
5. What are the causes and consequences of "cell dragging" in a microcellular environment?

📘 Lecture 13 — Fundamentals of Cellular Networks (Part III)

📖 Overview: This lecture covers interference and system capacity in cellular networks, focusing on co-channel and adjacent channel interference. It explains how system capacity is limited by interference and describes channel planning strategies for wireless systems, including trade-offs between capacity and transmission quality.

🗂️ Topics Covered

The lecture begins with a review of previous topics including channel assignment strategies, handoff strategies, and practical handoff considerations. It then covers interference and system capacity in detail, with major sections on co-channel interference and system capacity, adjacent channel interference and capacity, and channel planning for wireless systems including CDMA considerations and breathing cell phenomena.

📝 Lecture Summary

Last lecture review

The lecture reviews channel assignment strategies, handoff strategies including when to handoff, 1G BS-based handoff, and 2G Mobile-Assisted Handoff. Prioritizing handoff using guard channels and queuing handoff requests is reviewed. Practical handoff considerations include umbrella cell and cell dragging concepts.

Interference and system capacity

Interference sources include another mobile in the same cell, a call in progress in a neighboring cell, other BS operating in the same frequency, and another non-cellular system leaking energy into the cellular frequency band. Interference on voice channels causes cross-talk, while on control channels it leads to missed and blocked calls. Interference is a major bottleneck in increasing capacity. Two major types are co-channel and adjacent channel interference.

Co-channel interference and system capacity

Co-channel cells are cells that use the same set of frequencies, and their interference is called co-channel interference. Unlike noise, co-channel interference cannot be combated by increasing SNR. To reduce it, co-channel cells must be separated by a minimum distance. When cell size is approximately the same and BS transmit at the same power, the co-channel interference ratio is independent of transmission power and is a function of radius of cell (R) and distance between centers of nearest co-channel cell (D).

By increasing the ratio of D/R, separation between co-channel cells relative to coverage distance is increased, thus interference is reduced. The parameter Q (co-channel reuse ratio) is related to cluster size. For hexagonal geometry: Q = D/R = √(3N), where N is cluster size. A small value of Q provides larger capacity since N is smaller, while a large value of Q improves transmission quality due to smaller co-channel interference. A trade-off must be made between these two objectives.

Let i₀ be the number of co-channel interfering cells. The signal-to-interference ratio (SIR) for a mobile receiver monitoring a forward channel is: S/I = S / ∑ᵢ₌₁ⁱ⁰ Iᵢ, where S is the desired signal power and Iᵢ is the interference power from the i-th interfering co-channel cell.

Average received signal strength at any point decays as a power law of distance. Average received power Pᵣ at distance d is: Pᵣ = P₀(d/d₀)⁻ⁿ where P₀ is power received at close-in reference point d₀, and n is the path loss exponent (2 to 4).

For co-channel cell interference, if Dᵢ is the distance of the i-th interferer from the mobile, received power is proportional to (Dᵢ)⁻ⁿ. When transmit power of each BS is equal and path loss exponent is the same throughout coverage: S/I ≈ (D/R)ⁿ / i₀ = (√(3N))ⁿ / i₀

Considering only the first layer of interfering cells, which are equidistant D from the desired BS: S/I = (√(3N))ⁿ / i₀ This relates S/I to cluster size N, which determines overall system capacity.

For the US AMPS system, tests indicate S/I ≥ 18 dB for sufficient voice quality. Using the formula with n=4, N must be at least 6.49, so a minimum cluster size of 7 is required.

For a 7-cell cluster with hexagonal geometry, the mobile is at the boundary of the cell. The worst case S/I ratio can be approximated. In terms of co-channel reuse ratio Q: S/I = (Q - 1)⁻ⁿ + (Q + 0.7)⁻ⁿ For N=7, Q=4.6, and worst case S/I ≈ 49.56 (17 dB), while the exact solution is 17.8 dB.

🔑 Definition — Co-channel cells: Cells that use the same set of frequencies, causing co-channel interference. 🔑 Definition — Co-channel reuse ratio (Q): Q = D/R, where D is distance between centers of nearest co-channel cells and R is cell radius. Q = √(3N). 📐 Formula: S/I = (√(3N))ⁿ / i₀ → The signal-to-interference ratio depends on cluster size N raised to the power n/2, divided by the number of interfering cells. 📌 Example: If S/I is required 15 dB, n=4: For 7-cell reuse, Q=4.583, S/I = 1/6 × (4.583)⁴ = 75.3 = 18.66 dB. Since this exceeds minimum, N=7 can be used. For n=3: 7-cell reuse gives S/I = 12.05 dB (too low). Next N=12 with Q=6.0 gives S/I = 15.56 dB (acceptable). So N=12 is used.

Channel Planning for Wireless Systems

Generally, available spectrum is divided into control channels (5% of total for initiating, requesting, or paging calls) and voice channels (95% for revenue-generating traffic). Control channels are not allowed to be used as voice channels or vice versa. Control channels use more conservative frequency reuse (21-cell reuse), while voice channels use 7-cell reuse.

A key feature of CDMA systems is that cluster size N=1, so frequency reuse planning is simpler than for TDMA or 1G systems. However, most practical CDMA systems use some limited frequency reuse due to ill-behaved propagation conditions. Interfering channels on the same channel can create interference overload exceeding the dynamic range of CDMA power control, leading to dropped calls.

The most popular approach is f1/f2 cell planning, where nearest neighbor cells use different channels. This requires mobiles to make hard handoff. In CDMA, a single 1.25 MHz channel carries 64 simultaneous voice channels. CDMA has dynamic time-varying coverage depending on instantaneous number of users, known as breathing cell. The wireless engineer must carefully plan coverage and signal levels for best and worst cases. Breathing cell can lead to abrupt dropped calls.

🔑 Definition — Breathing cell: A phenomenon in CDMA where coverage region dynamically changes depending on instantaneous number of users.

Adjacent channel interference

Adjacent channel interference results from signals adjacent in frequency, caused by imperfect receiver filters allowing nearby frequencies to leak into the passband. It is more serious if the transmitter is close to the user's receiver listening to the desired channel, called the near-far effect.

The near-far effect occurs when a nearby transmitter captures the receiver of a subscriber, or when a mobile close to the BS transmits on an adjacent channel to one being used by a weak mobile. This interference can be minimized by careful filtering and channel assignment. A cell need not be assigned channels adjacent in frequency. By keeping frequency separation large in a given cell, interference is minimized.

Channel allocation schemes sequentially assign successive channels to different cells, separating channels by as many as N. Some strategies avoid adjacent channels in neighboring cells. If reuse factor (1/N) is large (N is small), separation may not keep interference within tolerable limits.

For example, if a close-in mobile is 20 times closer to BS than another mobile and energy has leaked to the passband, S/I at BS for the weak mobile is approximately S/I = (20)⁻ⁿ. For n=4, this is -52 dB. If the filter has a slope of 20 dB/octave, interference must be displaced 6 times the passband bandwidth from the center to achieve 52 dB attenuation, meaning more than 6 channels separation are needed for acceptable S/I.

🔑 Definition — Near-far effect: A condition where a nearby transmitter captures the receiver of a subscriber, or a mobile close to the BS transmits on an adjacent channel to one used by a weak mobile, causing adjacent channel interference.

⭐ Key Takeaways

The critical relationship between co-channel reuse ratio Q = √(3N) and cluster size N determines both system capacity and signal quality, requiring careful trade-offs. The signal-to-interference ratio formula S/I = (√(3N))ⁿ/i₀ is essential for calculating minimum cluster sizes to meet quality requirements, with the US AMPS example showing N=7 is needed for 18 dB S/I with n=4. Adjacent channel interference from the near-far effect can cause severe degradation, requiring proper filtering and channel separation strategies. Channel planning must differentiate between control channels (conservative 21-cell reuse) and voice channels (7-cell reuse), while CDMA systems face unique challenges from breathing cell phenomena and dynamic coverage regions. The trade-off between capacity (small N) and quality (large N) is fundamental to cellular system design.

🧠 Quick Revision Questions

1. What is the formula relating co-channel reuse ratio Q to cluster size N in hexagonal cellular systems?
2. Why can co-channel interference not be combated by increasing SNR, and how is it reduced?
3. What is the minimum cluster size required for the US AMPS system to achieve 18 dB S/I with path loss exponent n=4?
4. What is the near-far effect and how does it cause adjacent channel interference?
5. What is the breathing cell phenomenon in CDMA systems and why does it cause abrupt dropped calls?

📘 Lecture 14 — Fundamentals of Cellular Networks (Part IV)

📖 Overview: This lecture explores trunking theory and grade of service (GOS) — the mathematical foundation for determining how many channels are needed to serve a given number of users without excessive blocking. It then covers three key techniques for improving cellular network coverage and capacity: cell splitting, sectoring, and the microcell zone concept.

🗂️ Topics Covered

Trunking and Grade of Service: measuring traffic intensity (Erlangs), types of trunked systems (blocked calls cleared vs. blocked calls delayed), and Erlang B/C formulas. Improving Coverage and Capacity: cell splitting (reducing cell radius), sectoring (using directional antennas to improve S/I and reduce cluster size), repeaters for range extension, and the microcell zone concept (distributed antennas sharing a single BS).

📝 Lecture Summary

Trunking and Grade of Service

Trunking allows a large user community to share a small number of channels on a per-call basis, relying on statistical behavior. Trunking theory, developed by Erlang, determines how many channels are needed for a given area and user load. There is a tradeoff between the number of available channels and the likelihood of call blocking during peak hours.

🔑 Definition — 1 Erlang: the amount of traffic intensity carried by a completely occupied channel (e.g., one call-hour per hour, or 0.5 Erlang if occupied 30 minutes in an hour).

🔑 Definition — Grade of Service (GOS): a benchmark measuring the ability of a user to access a trunked system during the busiest hour (typically 4-6 PM rush hours), usually given as the probability of call blocking or delay exceeding a certain queue time.

Measuring Traffic Intensity

Traffic intensity is calculated as the call request rate multiplied by the call holding time. 📐 Formula: User traffic intensity (A_u = \lambda H) (where (\lambda) = average call request rate, (H) = average call duration or holding time). 📐 Formula: Total offered traffic intensity for (U) users: (A = U A_u). 📐 Formula: Traffic per channel in a C-channel system: (A_c = UA_u / C). 💡 Why this matters: Offered traffic is not necessarily carried traffic — if offered load exceeds system capacity, the carried traffic becomes limited. The maximum possible carried traffic (in Erlangs) equals the number of channels (C). AMPS is designed for a GOS of 2% blocking (2 out of 100 calls blocked).

Trunked Systems: Blocked Calls Cleared

In this system, if no channel is available, the requesting user is immediately blocked and must try later. The Erlang B formula determines the probability of call blocking (GOS) assuming Poisson-distributed call arrivals and no queuing. 📐 Formula: Erlang B Trunking GOS — (P[blocking] = \frac{A^C / C!}{\sum_{k=0}^{C} A^k / k!}) (where (A) = total offered traffic in Erlangs, (C) = number of channels). This formula is widely provided in tabular form (Erlang B tables).

Trunked Systems: Blocked Calls Delayed

Here, a queue holds blocked calls until a channel becomes available. GOS is defined as the probability that a call is delayed longer than a specific time (t). The Erlang C formula gives the probability that a call is not immediately served. 📐 Formula: Erlang C — (P[delay > 0] = \frac{A^C}{A^C + C! (1 - A/C) \sum_{k=0}^{C-1} A^k/k!}). 📐 Formula: Probability of delay > t seconds: (P[delay > t] = P[delay > 0] \cdot e^{-(C - A)t / H}) (where (H) = average call holding time). 📐 Formula: Average delay D for all calls in a queued system: (D = P[delay > 0] \cdot \frac{H}{C - A}). The average delay for a queued (delayed) call is (H / (C - A)).

Trunking Efficiency

Trunking efficiency measures how many users can be served at a given GOS with a given channel configuration. Grouping channels differently alters capacity. For example: at GOS = 0.01, 10 trunked channels can support 4.46 Erlangs, while two separate groups of 5 channels can only support 2 × 1.36 = 2.72 Erlangs — about 60% less traffic.

Improving Coverage and Capacity: Cell Splitting

Cell splitting increases capacity by decreasing the cell radius (R) while keeping the cell reuse ratio D/R unchanged. Congested cells are divided into smaller micro cells. Halving the radius requires approximately 4 smaller cells to replace one larger cell, increasing the number of clusters and thus capacity. 📌 Example: To maintain equal received power at the new cell boundary, transmit power must be reduced. If path loss exponent (n = 4), the new transmitter power (P_{t2} = P_{t1}/16) (since (P_{t2}/P_{t1} = (R_2/R_1)^n = (1/2)^4)). This corresponds to a 12 dB reduction to maintain S/I requirements. Initially, fewer channels are assigned to the smaller power groups; as demand grows, more micro cells replace the old cells entirely.

Improving Coverage and Capacity: Sectoring

Sectoring keeps the cell radius unchanged but decreases the D/R ratio by using directional antennas to improve S/I, allowing a smaller cluster size (N) and thus higher capacity. 🔑 Definition — Sectoring: partitioning a cell into 3 sectors of 120° or 6 sectors of 60°, using directional antennas. Instead of interference from 6 co-channel cells (omnidirectional), only 2 sectors cause interference. 📌 Example: With 120° sectoring, the worst-case S/I improves from about 17 dB to 24.2 dB. This improvement allows designers to reduce cluster size (N), thereby increasing capacity. ⚠️ Drawback: Increased number of handoffs as mobiles move between sectors.

Improving Coverage and Capacity: Microcell Zone Concept

In the microcell zone concept, a single base station (BS) divides its cell into several zones, each with its own antenna (placed at the cell edges). Zones are connected to the BS via coaxial cable, fiber optics, or microwave links. The BS can assign any channel to any zone. 🔑 Key advantage: As a mobile travels from one zone to another, it retains the same channel — the BS simply switches the channel to the new zone. This eliminates the need for a new handoff. 💡 Why this matters: Co-channel interference is minimized because the large BS transmitter is replaced by several low-power zone transmitters, significantly improving S/I without increasing the number of handoffs.

⭐ Key Takeaways

Traffic intensity is measured in Erlangs, with 1 Erlang representing a fully occupied channel. GOS for a trunked system is determined using either the Erlang B formula (for blocked calls cleared — no queue) or the Erlang C formula (for blocked calls delayed — queue provided). Grouping more channels together improves trunking efficiency. To expand system capacity, cell splitting reduces cell radius and transmitter power; sectoring uses directional antennas to improve S/I and allows a smaller cluster; and the microcell zone concept uses distributed low-power antennas to reduce interference while eliminating intra-cell handoffs. Always remember the tradeoff: sectoring increases handoffs, while microcell zones avoid them.

🧠 Quick Revision Questions

1. Define 1 Erlang and explain how it is related to channel occupancy.
2. What is the fundamental difference between a "Blocked Calls Cleared" system and a "Blocked Calls Delayed" system?
3. In cell splitting, if the cell radius is halved and the path loss exponent n = 4, by how many dB must the transmitter power be reduced to maintain the same received power at the new cell boundary?
4. Explain the key advantage of the microcell zone concept over sectoring in terms of handoffs and co-channel interference.
5. If two separate groups of 5 channels can support 2.72 Erlangs at GOS = 0.01, why can a single group of 10 channels support 4.46 Erlangs at the same GOS?

📘 Lecture 15 — Analog Mobile Phone System

📖 Overview: This lecture introduces the Analog Mobile Phone System (AMPS), the first-generation cellular standard deployed in the US in 1983. It covers the system architecture, call handling procedures, air interface specifications, supervisory signaling, and the narrowband evolution N-AMPS, providing foundational knowledge for understanding how early cellular networks operated.

🗂️ Topics Covered

The lecture covers AMPS introduction with its history and frequency allocation, system architecture including forward and reverse channels, call handling procedures for both landline-to-mobile and mobile-to-landline calls, AMPS and ETACS air interface parameters, supervisory signals (SAT and ST tones), wideband blank-and-burst encoding, and the Narrowband AMPS (N-AMPS) system.

📝 Lecture Summary

AMPS Introduction

AMPS (Advanced Mobile Phone System) was first deployed in late 1983 in Chicago, covering approximately 2100 square miles. The FCC allocated a total of 40 MHz in the 800 MHz band, with an additional 10 MHz added later due to increased demand. Early systems used large cells and omni-directional antennas to minimize initial equipment costs.

AMPS uses a 7-cell reuse pattern with provision for sectoring and cell splitting to increase capacity. Through extensive testing, it was determined that a 30 KHz channel requires a Signal-to-Interference Ratio (SIR) of 18 dB. The smallest reuse factor satisfying this requirement with 120-degree directional antennas is N = 7.

🔑 Definition — ETACS (European Total Access Communication System): Identical to AMPS but scaled to 25 KHz channels instead of 30 KHz, with a different mobile identification number (MIN) format to accommodate different country codes in Europe versus area codes in the US.

System Overview

AMPS and ETACS both use FM (Frequency Modulation) and FDD (Frequency Division Duplex) for radio transmission. In the US, transmissions from mobiles to base stations (reverse link) use frequencies between 824-849 MHz, while base stations transmit to mobiles (forward link) using frequencies between 869-894 MHz. A separation of 45 MHz between forward and reverse channels allows for inexpensive duplexers in mobile units.

The control channel and blank-and-burst data streams are transmitted at 10 kbps in AMPS and 8 kbps in ETACS, with maximum frequency deviations of ±8 KHz and ±6.4 KHz respectively. Each base station has:

• One control channel transmitter for the Forward Control Channel (FCC)
• One control channel receiver for the Reverse Control Channel (RCC)
• 8 or more duplex voice channels (commercial BS supports up to 57 voice channels)

The Forward Voice Channel (FVC) carries conversation from landline caller to cellular subscriber, while the Reverse Voice Channel (RVC) carries it in the opposite direction. Each BS continuously transmits digital FSK data on FCC so idle subscriber units can lock onto the strongest signal. All users must be locked onto an FCC to originate or receive calls.

In US AMPS, there are 21 control channels per provider, while ETACS supports 42 control channels. The nonwireline service provider ("A" provider) is assigned odd System Identification Numbers (SID), and wireline provider ("B" provider) gets even SID. SID is transmitted every 0.8 seconds on each FCC. ETACS uses Area Identification Numbers (AID) instead.

Call Handling

Landline to Cellular Subscriber Call:

1. Call arrives from PSTN at the MSC (Mobile Switching Center)
2. A paging request with the subscriber's MIN (Mobile Identification Number) is sent simultaneously on every BS's FCC
3. The subscriber responds with an acknowledgment on RCC
4. MSC directs the BS to assign an FVC and RVC pair
5. BS assigns a Supervisory Audio Tone (SAT) and a Voice Mobile Attenuation Code (VMAC)

🔑 Definition — SAT (Supervisory Audio Tone): Allows user and BS to distinguish each other from co-channel users in different cells; transmitted continuously on both FVC and RVC at three frequencies: 5970 Hz, 6000 Hz, or 6030 Hz.

🔑 Definition — VMAC (Voice Mobile Attenuation Code): Instructs the user to transmit at a specific power level.

🔑 Definition — Blank-and-burst signaling: Allows the MSC to send bursty data on voice channels by temporarily omitting speech and SAT and replacing with data.

Mobile to Landline Call:

1. Subscriber transmits request (MIN, electronic serial number, station class mark, destination number) on RCC
2. BS forwards request to MSC
3. MSC checks user registration and connects to PSTN
4. Assigns FVC and RVC with SAT and VMAC

During calls, MSC issues blank-and-burst commands to switch between different voice channels on different BS as the user travels. The MSC uses a scanning receiver called a locator in nearby BS to determine RSSI for handoff decisions.

When all voice channels are occupied, the MSC may issue a directed retry to the subscriber on FCC, forcing them to switch to a different control channel or BS, though this may or may not succeed.

AMPS and ETACS Air Interface

The air interface parameters are summarized in the table:

Supervisory Signals (SAT and ST Tones)

Supervisory signals allow each user and BS to confirm connection during a call. SAT always exists during use of any voice channel. AMPS and ETACS use three SAT signals at 5970 Hz, 6000 Hz, or 6030 Hz. The BS constantly transmits one of three SAT tones on each voice channel when in use.

When a call is set up:

1. SAT is transmitted immediately on FVC
2. Subscriber must detect, filter, and demodulate SAT from FVC
3. Subscriber reproduces SAT on RVC
4. If SAT is not properly detected within one second, both BS and subscriber cease transmission

💡 Why this matters: SAT ensures that calls are not maintained on incorrect channels or with interfering users, preventing co-channel interference.

🔑 Definition — ST (Signaling Tone): A 10 kbps data burst signaling call termination by the subscriber; contains alternating 1s and 0s sent on RVC for 200 ms. Unlike blank-and-burst messages, ST tone is sent simultaneously with SAT, alerting the system that the user deliberately terminated the call.

Wideband Blank-and-burst Encoding

AMPS voice channels carry wideband (10 kbps) data streams for blank-and-burst signaling. The wideband data stream is encoded using Manchester coding. The advantage is that the energy of the Manchester coded signal is concentrated at the transmission rate frequency of 10 KHz, with little energy leaking into the audio band below 4 KHz.

Narrowband AMPS (N-AMPS)

N-AMPS (Narrowband AMPS) uses 10 KHz channels, allowing three times more users compared to standard AMPS. It uses the same SAT, ST, and blank-and-burst signaling, but with signaling done using sub-audible data streams.

⭐ Key Takeaways

AMPS was the first commercial cellular system using 30 KHz channels in the 800 MHz band with 7-cell reuse pattern requiring 18 dB SIR. The system architecture uses 21 control channels per provider with FDD and FM modulation, while call handling involves paging on FCC, acknowledgments on RCC, and voice channel assignment with SAT and VMAC parameters. Supervisory signals (SAT at three frequencies and ST at 10 kbps) are critical for maintaining call integrity and detecting user-initiated call termination versus system drops. Blank-and-burst signaling using Manchester coding enables handoffs and power control during active calls. N-AMPS evolved the system to 10 KHz channels for threefold capacity increase using sub-audible signaling.

🧠 Quick Revision Questions

1. What is the frequency separation between forward and reverse channels in AMPS, and why was this specific value chosen?
2. How does the call setup procedure differ between a landline-to-mobile call and a mobile-to-landline call in AMPS?
3. What are the three SAT frequencies used in AMPS, and what happens if SAT is not detected within one second?
4. What is the difference between Signaling Tone (ST) and blank-and-burst signaling in terms of SAT transmission?
5. How does N-AMPS achieve three times more capacity than standard AMPS, and what signaling method does it use?

📘 Lecture 16 — GSM: Global System for Mobile Communication

📖 Overview: This lecture introduces the Global System for Mobile Communications (GSM), a digital cellular standard that addressed the limitations of analog systems. It covers GSM system architecture, network areas, technical specifications, subscriber services, mobility support, and the identifiers used for routing and subscriber management.

🗂️ Topics Covered

The lecture begins by contrasting analog and digital systems, then introduces GSM's three major subsystems: Switching System (SS), Base Station System (BSS), and Operation and Support System (OSS). It details key network components including MSC, BSC, BTS, HLR, VLR, AUC, EIR, MXE, GMSC, and GIWU, followed by network areas (cell, location area, MSC, PLMN), technical specifications, subscriber services like DTMF, fax, SMS, and voice mail, mobility concepts, the mobile station components (ME and SIM), and the identifiers used in GSM networks.

📝 Lecture Summary

Review of Last Lecture

The previous lecture covered AMPS (Advanced Mobile Phone System) introduction, system overview, call handling, air interface, supervisory signals, and N-AMPS.

GSM Introduction

Analog cellular systems faced inability to handle growing capacity needs cost-efficiently and were developed without standardized specifications. Digital systems offer ease of signaling, lower interference, integration of transmission and switching, and increased capacity. GSM provides recommendations, not requirements, defining functions and interface requirements in detail without addressing hardware to limit designers.

The GSM network is divided into three major systems:

• Switching System (SS): Responsible for call processing and subscriber-related functions.
• Base Station System (BSS): Performs all radio-related functions, consisting of base station controllers (BSCs) and base transceiver stations (BTSs).
• Operation and Support System (OSS): The functional entity from which the network operator monitors and controls the system, providing cost-effective support for centralized, regional, and local operational and maintenance activities.

GSM System Architecture

Mobile Switching Centre (MSC) The core switching entity in the network, connected to the radio access network (RAN) formed by BSCs and BTSs within the Public Land Mobile Network (PLMN). All calls to and from the user are controlled by the MSC. A GSM network has one or more MSCs, geographically distributed.

Base Station Controller (BSC) Provides all control functions and physical links between the MSC and BTS. It is a high-capacity switch that handles handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

Base Transceiver Station (BTS) Handles the radio interface to the mobile station. The BTS contains the radio equipment (transceivers and antennas) needed to service each cell. A group of BTSs are controlled by a BSC.

Home Location Register (HLR) A database used for storage and management of subscriptions, containing a subscriber's service profile, location information, and activity status. When an individual buys a subscription, they are registered in the HLR of that operator.

Visitor Location Register (VLR) A database containing temporary information about subscribers needed by the MSC to service visiting subscribers. The VLR is always integrated with the MSC. For a roaming user, the VLR connected to that MSC requests data about the mobile station from the HLR through the MSC.

Authentication Centre (AUC) Provides authentication and encryption parameters that verify the user's identity and ensure call confidentiality. The AUC protects network operators from fraud.

Equipment Identity Register (EIR) A database containing information about mobile equipment identity that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR can be implemented as stand-alone nodes or combined.

Message Centre (MXE) Provides integrated voice, fax, and data messaging. Specifically handles short message service (SMS), cell broadcast, voice mail, fax mail, email, and notification.

Gateway Mobile Services Switching Centre (GMSC) A node used to interconnect two networks. The gateway is often implemented in an MSC, which is then referred to as the GMSC.

GSM Inter-working Unit (GIWU) Consists of hardware and software providing an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware is physically located at the MSC/VLR.

GSM Network Areas

• Cell: Identified by cell global identity (CGI)
• Location Area (LA): Group of cells, identified by LAI
• MSC: Service area of one MSC
• Public Land Mobile Network (PLMN): Service area of one operator

Specifications

• Frequency band: 1,850 to 1,990 MHz (mobile station to base station)
• Duplex distance: 80 MHz
• Channel bandwidth: 200 kHz
• Modulation: Gaussian minimum shift keying (GMSK)
• Transmission rate: Over-the-air bit rate of 270 kbps
• Access method: Time division multiple access (TDMA)
• Speech coder: Uses linear predictive coding (LPC), speech encoded at 13 kbps

Subscriber Services

Two basic types of services: Telephony (tele-services) for voice communication, and Data (bearer services) for transmitting data signals between access points.

Dual-tone multi-frequency (DTMF): A tone signalling scheme used for various control purposes via the telephone network, such as remote control of an answering machine.

Facsimile group III: GSM supports CCITT Group 3 facsimile. Special fax converters connected to the exchange allow GSM-connected faxes to communicate with any analog fax in the network.

Short message services (SMS): A message of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. If the subscriber's mobile unit is powered off or out of coverage, the message is stored and delivered when the mobile is powered on or re-enters coverage.

Cell broadcast: A variation of SMS, broadcasting a message of a maximum of 93 characters to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and accident reports.

Voice mail: An answering machine within the network controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box, and the subscriber checks messages via a personal security code.

Fax mail: The subscriber can receive fax messages at any fax machine. Messages are stored in a service centre and retrieved via a personal security code to the desired fax number.

GSM Mobility

Roaming in GSM is made possible through the separation of switching capability and subscription data. A subscriber's data is permanently registered in the HLR in their Home PLMN (HPLMN). The MSC and GMSC are not specific to one subscriber group.

💡 Why this matters: This separation allows subscribers to use their mobile service in networks other than their home network, which is the foundation of international roaming.

Mobile Station

Consists of two components:

• Mobile Equipment (ME): The physical device
• Subscriber Identification Module (SIM): A chip in the SIM card that identifies a subscriber of a GSM network. When inserted in the ME, the subscriber may register with a GSM network, personalizing the ME. The SIM card contains information such as IMSI, advice of charge parameters, and operator-specific emergency numbers.

Identifiers in the GSM Network

GSM uses several identifiers for routing calls, identifying subscribers (e.g., for charging), locating the HLR, and identifying equipment.

International Mobile Subscriber Identity (IMSI) 🔑 Definition — IMSI: A unique identifier embedded on the SIM card used to identify a subscriber. It is also contained in the subscription data in the HLR. For roaming charging, a VPLMN (Visited PLMN) uses the IMSI to send billing records to the HPLMN of a subscriber.

⭐ Key Takeaways

GSM is a digital cellular standard that overcame analog system limitations by using digital signaling, lower interference, and standardized specifications. The network architecture consists of three main systems: SS, BSS, and OSS, with key components including MSC, BSC, BTS, HLR, VLR, AUC, EIR, MXE, GMSC, and GIWU. GSM uses TDMA with GMSK modulation, 200 kHz channels, and a transmission rate of 270 kbps. Important subscriber services include SMS (160 characters), cell broadcast (93 characters), DTMF, fax, and voice/fax mail. Mobility is enabled through the separation of switching and subscription data, with the SIM card storing the IMSI for subscriber identification and roaming charging.

🧠 Quick Revision Questions

1. What are the three major systems in the GSM network architecture, and what is the primary function of each?
2. What is the difference between an HLR and a VLR, and how do they interact during roaming?
3. What are the technical specifications of GSM including frequency band, channel bandwidth, modulation, transmission rate, and access method?
4. How does SMS work when a mobile subscriber is powered off or out of coverage?
5. What is the IMSI and how is it used for roaming charging between VPLMN and HPLMN?

📘 Lecture 17 — GPRS: General Packet Radio Service (Part I)

📖 Overview: This lecture introduces GPRS (General Packet Radio Service), a major enhancement over GSM that enables packet-switched data services. It covers GPRS architecture, the new network nodes called GSNs, how mobile devices register and manage sessions, and the classification of channels used for data transmission. Understanding GPRS is essential because it bridges circuit-switched cellular networks to the packet-switched Internet.

🗂️ Topics Covered

The lecture begins with a review of GSM identifiers and call routing, then introduces GPRS as a packet-switched enhancement. It details the GPRS architecture including A/Gb and Iu modes, BSS upgrades with PCU, mobile registration (attach/detach), session management via PDP context activation, physical channel classification into shared/dedicated/packet data channels, and logical channel types for control and user data.

📝 Lecture Summary

Review of last lecture

The previous lecture covered GSM system architecture, network areas, subscriber services (DTMF, fax, SMS, cell broadcast, voice/fax mail), mobility, and key identifiers. GSM uses several identifiers for routing calls, identifying subscribers, locating the HLR, and identifying equipment.

Identifiers in GSM Network and Call Routing

International Mobile Subscriber Identity (IMSI) is embedded on the SIM card and used to identify a subscriber. It is also contained in the subscription data in the HLR.

International Mobile Equipment Identifier (IMEI) is a unique number hard-coded in the ME that cannot be modified. It is used to identify the mobile equipment.

Mobile Station Roaming Number (MSRN) is used in the GSM network for routing a call to an MS. The MSRN is allocated to a subscriber during MT call handling and is released when the call is established. Each MSC in a PLMN has a limited range of MSRNs allocated to it.

Mobile Station Integrated Services Digital Network Number (MSISDN) is used to identify the subscriber when establishing a call or sending an SMS. The MSISDN is not stored on the subscriber's SIM card and is normally not available in the MS. It is provisioned in the HLR as part of the subscriber's profile and sent to MSC during registration.

🔑 Definition — IMSI: A unique identifier embedded on the SIM card used to identify a subscriber in the GSM network.

🔑 Definition — IMEI: A unique, non-modifiable identifier hard-coded in the mobile equipment used to identify the device itself.

🔑 Definition — MSRN: A temporary number allocated to a subscriber during mobile-terminated call handling for routing the call to the MS.

🔑 Definition — MSISDN: The telephone number used to identify a subscriber for call establishment and SMS delivery, provisioned in the HLR.

Introduction to GPRS

GPRS is an enhancement over GSM that adds nodes called GSNs (GPRS Support Nodes) to provide packet-switched services. These nodes are responsible for routing and delivery of data packets to and from the MS and external packet data networks (PDN) .

The goals of GPRS are: efficient bandwidth usage for bursty data traffic (e.g., Internet), higher data rates, and new charging models. GPRS was initially specified by ETSI with multiple releases (R97, R98, R99, R4), then handed over to 3GPP. The lecture considers Release 5 and Release 6.

GPRS Release 5/6 determines two modes based on core network generation: 2G core uses A/Gb mode, and 3G core uses Iu mode. The Iu interface was added in Release 5 to align with UMTS.

GPRS Architecture

The GERAN Reference Architecture shows the network elements. In A/Gb mode, there are three classes of mobile stations: Class A can operate simultaneous packet switched and circuit switched services; Class B can operate either one at a time (most common for handsets today); Class C can operate only packet switched services (e.g., expansion cards for laptops).

In Iu mode, the modes are: CS/PS mode (same as Class A in A/Gb mode), PS mode (MS can only operate packet switched services), and CS mode (MS can only operate circuit switched services).

Service types include Point-to-Point (e.g., Internet access by user) and Point-to-Multipoint (e.g., delivery of information like news to multiple locations or interactive conference applications).

🔑 Definition — Class A (A/Gb): A mobile station capable of simultaneous packet-switched and circuit-switched services.

🔑 Definition — Class B (A/Gb): A mobile station that can operate either packet-switched or circuit-switched services, but not simultaneously.

🔑 Definition — Class C (A/Gb): A mobile station that can only operate packet-switched services.

Registration and Session Management

For GPRS BSS, a software upgrade is required in the existing Base Transceiver Site (BTS) . The Base Station Controller (BSC) also requires a software upgrade and installation of a new hardware called packet control unit (PCU) . The PCU directs data traffic to the GPRS network and provides a physical and logical data interface out of BSS for packet data traffic.

Registration of a Mobile Node requires the MS to register with the GPRS network through GPRS attach. The device sends a message to the new SGSN containing the last assigned Temporary Mobile Subscriber Id (TMSI) and location area information. The new SGSN queries the old SGSN for the identity of this mobile device, then requests more information for authentication. GPRS detach can be initiated by the MS or the network.

Session Management begins after successful attach. To start packet data, the MS must activate a Packet Data Protocol (PDP) address. This address is unique only for a particular session and consists of: PDP type, PDP address assigned to MS, and requested QoS. Once PDP Context is activated, a two-way tunnel is established between the device's current SGSN and the corresponding GGSN. The GGSN hides the mobility from onward networks.

PDP-Address allocation can be static (assigned by network operator of user's home PLMN) or dynamic (assigned by the corresponding GGSN).

🔑 Definition — PCU: A hardware element associated with BSC that directs data traffic to the GPRS network.

🔑 Definition — PDP Context: An active session between the MS and GGSN that defines the PDP type, address, and QoS for packet data transfer.

🔑 Definition — PDP Address: A unique address assigned for a particular packet data session, which can be static or dynamic.

📌 Example: When a mobile user wants to browse the Internet, the MS first performs GPRS attach to register with the SGSN. Then it initiates PDP Context Activation with parameters like PDP type = IP, requested QoS = best effort. The GGSN assigns a dynamic IP address, and a tunnel is established between SGSN and GGSN for the session.

Routing Scenario in GPRS

[Note: The lecture text indicates this topic is listed in the outline but the detailed routing scenario content is not provided in the transcript. Only the diagram reference and citation were given.]

Channels Classification

Physical Channels are defined by timeslot (0-7) and radio frequency channel. There are Shared Basic Physical Sub Channels (shared among up to 8 users) and Dedicated Basic Physical Sub Channels (one user). A Packet Data Channel (PDCH) is dedicated to packet data traffic from logical channels for both control and user data.

🔑 Definition — PDCH: A physical channel dedicated to carrying packet data traffic from logical control and user data channels.

⭐ Key Takeaways

The most critical concepts from this lecture are the GSM identifiers (IMSI, IMEI, MSRN, MSISDN) and their distinct roles in subscriber identification, equipment identification, and call routing. GPRS architecture introduces new nodes called GSNs (SGSN and GGSN) with the PCU at the BSS level to enable packet switching. Mobile registration involves GPRS attach with TMSI verification, while session management requires PDP Context Activation to establish tunnels between SGSN and GGSN. MS classes in A/Gb mode (A, B, C) define whether devices can handle simultaneous circuit and packet services. Finally, physical channels are divided into shared/dedicated sub-channels, with PDCH being exclusively for packet data traffic.

🧠 Quick Revision Questions

1. What are the four main identifiers used in GSM networks, and what is the specific purpose of each?
2. During GPRS attach, what information does the MS send to the new SGSN, and how does the new SGSN verify the device identity?
3. What are the three components that make up a PDP address during session management?
4. In A/Gb mode, what is the difference between Class A and Class B mobile stations?
5. What is a PDCH, and how does it differ from a shared basic physical sub-channel?

📘 Lecture 18 — GPRS: General Packet Radio Service (Part II)

📖 Overview: This lecture continues the study of GPRS by diving into its detailed protocol architecture, air interface design, and mobility management. It explains how data is routed and transferred in both uplink and downlink directions, and how quality of service (QoS) is supported for packet-switched services. This matters because GPRS was a foundational technology for mobile data, bridging GSM circuit-switched voice with IP-based packet data.

🗂️ Topics Covered

This lecture covers the full GPRS protocol stack from the transmission and signaling planes, including Radio Link Control (RLC) and Media Access Control (MAC) sublayers. It explains the GPRS air interface using a master-slave PDCH concept with capacity on demand, details the procedures for uplink and downlink data transfer, describes the three mobility states (Idle, Standby, Active), and outlines QoS profiles with service precedence, reliability, delay, and throughput parameters.

📝 Lecture Summary

GPRS Protocol Architecture

The GPRS protocol stack is divided into two planes. The Transmission Plane provides protocols for the transmission of user data and its associated signaling. The Signaling Plane comprises protocols for controlling and supporting the functions of the transmission plane.

🔑 Transmission Plane: The protocols used for actually moving user data packets.

• GPRS Backbone (SGSN – GGSN): Uses GTP (GPRS Tunneling Protocol) to tunnel user packets and related signaling information between GPRS support nodes.
• Sub-network dependent convergence protocol (SNDCP): Used to transfer packets between SGSN and MS.
• Data link layer: Consists of LLC (Logical Link Control) between MS and SGSN, and RLC/MAC between MS and BSS.
• Physical layer:
  • PLL (Physical Link Layer): responsible for channel coding, detection of errors, forward error correction, interleaving, and detection of physical link congestion.
  • RFL (Radio Frequency Layer): handles modulation and demodulation.

Radio Link Control (RLC)

The RLC sublayer provides reliability for MAC transmissions and operates in three modes.

• Transparent mode: No functionality.
• Acknowledged mode: Uses Selective Repeat ARQ, with a sender window at the transmitter and Uplink ACK/NACK or Downlink ACK/NACK at the receiver.
• Unacknowledged mode: Controlled by numbering within a TBF (Temporary Block Flow). No retransmissions are performed; missing packets are replaced with dummy information bits.

Media Access Control (MAC)

The MAC sublayer performs contention resolution between channel access attempts and is connection-oriented.

• Temporary Block Flow (TBF): A logical unidirectional connection between two MAC entities that is allocated resources on PDCH(s) (Packet Data Channels). The Temporary Flow Identity (TFI) is unique among concurrent TBFs in the same direction.

MAC: Channel Access & Resource Allocation

• Slotted Aloha: Used in the PRACH channel. MSs send packets in the uplink direction at the beginning of a time slot. If a collision occurs, the MS backs off using an arbitrary timer and then re-transmits.
• Time Division Multiple Access (TDMA): Predefined slots are allocated by the BSS, providing contention-free channel access.

GPRS Air Interface

The air interface uses a master-slave concept. One PDCH acts as the Master and holds all PCCCH (Packet Common Control Channel) channels. The rest of the channels act as Slaves. This design enables capacity on demand: PDCH(s) are increased or decreased according to demand, with load supervision performed in the MAC Layer.

🔑 PCCCH (Packet Common Control Channel): A set of control channels that GPRS mobiles camp on when allocated in a cell. It includes PRACH (Random Access), PPCH (Paging), PAGCH (Access Grant), and PNCH (Packet Notification for multicast). 🔑 PDCCH (Packet Dedicated Control Channel): Includes SACCH (Slow Associated Control Channel) for radio measurements and power control, FACCH (Fast Associated Control Channel) for acknowledgment on a TCH, and SDCCH (Stand-alone Dedicated Control Channel) for reliable signaling and SMS. 🔑 PBCCH (Packet Broadcast Control Channel): Contains frequency correction channels for synchronizing the MS's Local Oscillator to the BS, a synchronization channel, and a broadcast control channel for general base station information.

💡 Why this matters: The master-slave PDCH concept was a key innovation allowing GPRS to dynamically share radio resources between voice and data traffic without requiring dedicated data channels.

Uplink Data Transfer

[The lecture text contains a diagram reference but no textual explanation for the uplink procedure. The summary is derived from context and the standard GPRS procedure.] In uplink data transfer, the MS initiates a packet transfer request on the PRACH using Slotted Aloha. The BSS responds with resource assignment on the PAGCH, allocating a TBF and TFI. The MS then transmits data blocks on the allocated uplink PDCHs using the assigned TBF. The BSS can send uplink ACK/NACK messages to the MS for error recovery.

Downlink Data Transfer

[The lecture text contains a diagram reference but no textual explanation for the downlink procedure. The summary is derived from context and the standard GPRS procedure.] For downlink data transfer, the SGSN receives packets from the GGSN and forwards them to the BSS. The BSS pages the MS on the PPCH if the MS is in standby state. Once the MS responds, the BSS assigns a downlink TBF on the PAGCH and transmits data blocks. The MS sends downlink ACK/NACK to the BSS for selective retransmission.

Mobility

A GPRS mobile station operates in three states:

• Idle: The MS does not have a logical GPRS context activated or any PSPDN (Packet-Switched Public Data Network) addresses allocated. It can only receive multicast messages available to any GPRS mobile.
• Standby: The location of the MS is known only to which routing area it belongs.
• Active: The SGSN knows the cell-level location of the mobile station. Data is transmitted between the MS and the GPRS network only in this state.

💡 Why this matters: These states allow GPRS to conserve radio and network resources by tracking mobile stations with different granularity based on their activity level.

QoS in GPRS

GPRS assumes that IP multimedia applications can define their requirements, negotiate their capabilities, and identify and select available media components. GPRS specifies signaling that enables support for various traffic streams, including constant/variable bit rate and connection oriented/connectionless services.

QoS Profile for GPRS Bearers The QoS profile is defined by four parameters:

• Service precedence: 3 classes (high, normal, low).
• Reliability parameter: 3 classes.
• Delay parameters: 4 classes.
• Throughput parameter: maximum and mean bit rates.

The QoS profile is included in the Packet Data Protocol (PDP) context, and negotiation is managed through PDP procedures (activation, modification, and deactivation).

🔑 PDP Context: A data structure that contains the subscriber's session information, including the IP address, QoS profile, and connection parameters, when the MS is attached to the GPRS network.

⭐ Key Takeaways

GPRS uses a layered protocol architecture with a transmission plane (GTP, SNDCP, LLC, RLC/MAC, physical) and a signaling plane. The air interface employs a master-slave PDCH concept with capacity on demand, using Slotted Aloha for contention-based uplink access on PRACH and TDMA for contention-free allocated slots. RLC provides reliability through Selective Repeat ARQ in acknowledged mode, while MAC manages TBFs as logical unidirectional connections. Mobility is managed through three states (Idle, Standby, Active) that determine tracking granularity and data transmission capability. Finally, GPRS supports four QoS parameters (precedence, reliability, delay, throughput) negotiated through PDP context procedures, enabling support for diverse traffic streams.

🧠 Quick Revision Questions

1. What are the two planes in the GPRS protocol architecture, and what is the primary function of each?
2. What are the three modes of RLC operation, and how does acknowledged mode handle errors?
3. Explain the master-slave concept in the GPRS air interface and what channel each uses.
4. In which GPRS mobility state does the SGSN know the cell location of the MS?
5. What are the four parameters of a GPRS QoS profile, and how are they negotiated?

📘 Lecture 19 — cdmaOne/IS-95

📖 Overview: This lecture covers the transition from TDMA-based IS-136 to the CDMA-based IS-95 (cdmaOne) standard, a key 2G wireless technology. It explains the fundamental principles of CDMA, its advantages and drawbacks, and provides a detailed breakdown of both forward and reverse channel structures in IS-95, which laid the groundwork for 3G systems.

🗂️ Topics Covered

The lecture begins with a recap of IS-136 (TDMA-based) and its specifications. It then defines CDMA as both an access method and air interface, outlining its three main systems (IS-95, W-CDMA, CDMA2000). The advantages and drawbacks of CDMA cellular systems are discussed, followed by a detailed examination of the IS-95 forward channels (Pilot, Sync, Paging, Traffic) and their specific roles. Finally, the forward link transmission process for voice traffic is explained.

📝 Lecture Summary

Last Lecture

The previous lecture covered the GPRS Protocol Architecture including interfaces between MS-BSS, BSS-SGSN, SGSN-GGSN, and GGSN-PDN. It also discussed the GPRS Air Interface, data routing and mobility management for both uplink and downlink data transfer, and Quality of Service (QoS) in GPRS.

IS-136

IS-136 is an evolution of the AMPS standard, based on TDMA (Time Division Multiple Access) technology. It operates in the 800 / 1900 MHz band and uses TDMA frames of 6 time slots, each 40 ms in length. The system supports a half-rate channel using 1 slot and a double-rate channel using 4 slots.

IS-136 Channels include the Digital Control Channel (DCCH) which occupies a full rate channel (2 time slots) and is divided into logical channels: SPACH (SMS point-to-point, paging and access response channel), BCCH (Broadcast Control Channel), SCF (Shared Channel Feedback), and RACCH (Random Access Control Channel).

🔑 Definition — IS-136: A 2G digital cellular standard based on TDMA/FDD, operating in the 800/1900 MHz bands with a channel bandwidth of 30 kHz and a data rate of 48.6 kb/s.

📐 Formula: Spectrum Efficiency = 48.6 kb/s / 30 kHz = 1.62 b/s/Hz

📌 Example: In IS-136, the forward channel frequency band is 869 – 894 MHz, the reverse channel is 824 – 849 MHz, and each channel supports 3 or 6 users per channel using 6 time slots.

What is CDMA

CDMA (Code Division Multiple Access) is both an access method and an air-interface. The rest of the network (radio resource management, mobility management, security) is very similar to other systems, but power control and handoffs are different. CDMA uses DSSS (Direct Sequence Spread Spectrum) and has a frequency reuse factor of 1, meaning all cells use the same frequency. The three main CDMA systems are IS-95 (2G), W-CDMA, and CDMA2000. 💡 Why this matters: The frequency reuse factor of 1 is a major advantage over TDMA/FDMA systems, dramatically increasing spectral efficiency.

Advantages of CDMA Cellular

CDMA offers higher capacity, improved voice quality (using a new coder), and less power consumption (6-7 mW). It was the choice for 3G systems because it provides frequency diversity (frequency-dependent transmission impairments have less effect on the signal) and multipath resistance (chipping codes exhibit low cross-correlation and low autocorrelation). CDMA also provides inherent privacy (spread spectrum uses noise-like signals) and graceful degradation (the system only gradually degrades as more users access the system).

Drawbacks of CDMA Cellular

Key drawbacks include self-jamming (arriving transmissions from multiple users not aligned on chip boundaries unless perfectly synchronized), the near-far problem (signals closer to the receiver are received with less attenuation than signals farther away), and soft handoff (the mobile must acquire the new cell before relinquishing the old one, which is more complex than hard handoff). Additionally, the CDMA air-interface is the most complex.

Mobile Wireless CDMA Design Considerations

The RAKE receiver is a critical design component. When multiple versions of a signal arrive more than one chip interval apart, the RAKE receiver attempts to recover signals from multiple paths and combine them. This method achieves better performance than simply recovering the dominant signal and treating remaining signals as noise.

IS-95 CDMA Forward Channel

The forward link uses the same frequency spectrum as AMPS (824-849 MHz). There are 4 types of logical channels: one pilot, one synchronization, seven paging, and 55 traffic channels. The modulation scheme is QPSK. Orthogonal Walsh codes (64 total, 64 chips per bit) are used for channel separation. After orthogonal coding, signals are further spread by short PN spreading codes. 💡 Why this matters: The 64 Walsh codes allow 64 simultaneous logical channels on the same frequency, enabling the high capacity of CDMA.

Forward Channels

The Pilot Channel

The pilot channel is a continuous signal on a single channel. It allows the MS (Mobile Station) to acquire timing information, provides a phase reference for demodulation, and a means for signal strength comparison. It is 4-6 dB stronger than all other channels. The pilot channel is obtained using the all-zero Walsh code (W0) and contains no information except the RF carrier. There is no power control on the pilot channel.

Sync Channel

The sync channel is used to acquire initial time synchronization. The sync message includes the system ID (SID), network ID (NID), the offset of the PN short code, the state of the PN-long code, and the paging channel data rate (4.8 or 9.6 Kbps). It uses W32 for spreading and operates at 1200 bps.

Paging Channel

The paging channel uses W1-W7 and has no power control. It is additionally scrambled by the PN long code, which is generated by an LFSR (Linear Feedback Shift Register) of length 42. The data rate is either 4.8 Kbps or 9.6 Kbps.

Traffic Channels

Traffic channels carry user information. There are two possible data rate sets: RS1 = {9.6, 4.8, 2.4, 1.2 Kbps} and RS2 = {14.4, 7.2, 3.6, 1.8 Kbps}. RS1 is mandatory for IS-95, while RS2 support is optional. Traffic channels also carry power control bits for the reverse channel.

Forward Link Transmission

For voice traffic, speech is encoded at 8550 bps, and after adding error detection bits, it becomes 9600 bps. The full channel capacity is not used when the user is not speaking: during quiet periods, the data rate drops to 1200 bps; 2400 bps is used for background noise transients; 4800 bps mixes digitized speech and signaling data.

📌 Example: Digitized speech is transmitted in 20 ms blocks with FEC rate ½, making the effective data rate a maximum of 19.2 kbps. This resulting stream is then XORed with the Walsh code, generating data at 1.2288 Mbps (19.2 kbps × 64 chips/bit = 1.2288 Mcps, the chip rate).

⭐ Key Takeaways

The most critical concepts from this lecture are the fundamental difference between IS-136 (TDMA/FDD) and IS-95 (CDMA with reuse factor 1), and the detailed structure of IS-95's forward channels. You must remember the four forward channel types (Pilot, Sync, Paging, Traffic), their specific Walsh codes, data rates, and functions. The pilot channel provides synchronization without data; the sync channel uses W32 at 1200 bps for initial timing; paging uses W1-W7 for overhead; and traffic channels support mandatory RS1 data rates. The RAKE receiver's role in combating multipath fading is essential, as is understanding the voice transmission process from 8550 bps speech coding to 1.2288 Mbps chip rate after Walsh spreading.

🧠 Quick Revision Questions

1. What is the frequency reuse factor in IS-95 CDMA, and why is it significant?
2. Name the four types of logical channels in the IS-95 forward link and their respective Walsh code assignments.
3. What information is contained in the sync channel message?
4. Explain how the RAKE receiver improves performance in a CDMA system.
5. Describe the step-by-step process of how voice data at 8550 bps becomes a 1.2288 Mbps signal in the forward link transmission.

📘 Lecture 20 — EDGE

📖 Overview: This lecture covers Enhanced Data rates for GSM Evolution (EDGE), a 2.5G technology bridging GPRS and 3G. It explains EDGE's modulation and coding schemes, link adaptation techniques, capacity planning strategies, and the Dynamic Abis pool concept for efficient resource allocation.

🗂️ Topics Covered

The lecture begins with Walsh Codes and the IS-95 Reverse Link as review, then introduces EDGE as an evolution of GPRS. It covers Modulation and Coding Schemes (MCS) with 8-PSK modulation, Link Adaptation and Incremental Redundancy for optimizing throughput, Capacity Planning with frequency reuse patterns, and the Dynamic Abis pool concept for efficient backhaul.

📝 Lecture Summary

Last Lecture Review

The review covers IS-136 and CDMA/IS-95 advantages including frequency diversity, multipath resistance, privacy, and graceful degradation. Drawbacks include self-jamming, the near-far problem, and soft handoff. The IS-95 forward channels are Pilot Channel, Sync Channel, Paging, and Traffic, while reverse channels are Access Channels and Traffic.

Walsh Codes

Walsh codes are orthogonal codes used in CDMA systems for channel separation. A 2x2 Walsh Matrix assigns User 1 the code (1, 1) and User 2 the code (1, -1). The lecture also references a 4x4 Walsh matrix for larger systems.

IS-95 Reverse Link

The reverse link consists of up to 94 logical channels each occupying the same bandwidth of 1228 KHz. It supports 32 access channels and 62 traffic channels. The access channel is used to initiate a call, to respond to the paging channel, and for location update. In the reverse link, the convolutional encoder has a rate of 1/3, thus tripling the effective rate to a maximum of 28.8 kbps.

The IS-95 CDMA Reverse Channel uses OQPSK for power efficiency while QPSK demodulation remains easy, operating in the 869-894 MHz range. There is no spreading of data using orthogonal codes; instead, data from the block interleaver is grouped in units of 6 bits that serve as an index to select a row of the 64x64 Walsh matrix, and that row substitutes for the input. This expands the data rate by a factor of 64/6 to 307.2 kbps.

Enhanced Data rates for GSM Evolution

GPRS data rates still fall short compared to 3G. The delay in 3G deployment led to EDGE. Phase 1 (Release ’99 & 2002 deployment) supports best-effort packet data at speeds up to about 384 kbps. Phase 2 (Release ’2000 & 2003 deployment) adds Voice over IP capability.

🔑 Definition — EDGE: Enhanced Data rates for GSM Evolution, a 2.5G technology that provides higher data rates than GPRS using 8-PSK modulation.

GPRS Architecture

EDGE architecture is similar to GPRS, but with important changes for higher data rates, mainly the modulation scheme. GMSK is used in GPRS, transmitting only one bit per symbol. In EDGE, Octogonal PSK (8-PSK) is used, enabling a threefold higher data rate of 59.2 kbps per radio time slot, achieved by transmitting 3 bits per symbol. GMSK has constant amplitude modulation while 8-PSK has variations in amplitude, changing radio frequency characteristics and requiring changes in the Base Station. This leads to minor hardware and software changes but major network performance improvements.

📐 Formula: Peak data rate = 8 time slots × 59.2 kbps per slot = 473 kbps (theoretical maximum)

The lecture presents nine modulation and coding schemes (MCS) providing different throughput levels. Though GMSK is more robust, 8-PSK gives higher throughput. However, increased data rate comes at the price of decreased system sensitivity, impacting coverage and network planning. A key advantage in EDGE is that switching between different coding schemes happens easily — a data block can be sent with better protection on failure, unlike GPRS where retransmission uses the same protection as the initial transmission.

💡 Why this matters: The ability to switch coding schemes dynamically means EDGE can adapt to changing radio conditions, maintaining throughput where GPRS would fail.

Link Adaptation and Incremental Redundancy

Link Adaptation (LA) adjusts the MCS as propagation conditions change, maximizing throughput per channel based on bit error probability (BEP) measurements. Incremental Redundancy (IR) improves throughput by automatically adapting transmitted redundancy to channel conditions, achieved through ARQ and FEC.

🔑 Definition — Incremental Redundancy: A technique that sends redundancy only when necessary, using Generalized Type-II ARQ with finer granularity of code rate. If the first transmission fails, additional redundant bits are sent rather than the entire packet.

Capacity Planning in EDGE

Capacity planning is similar to GPRS, but higher throughput per radio time slot changes some aspects. The reuse pattern defines the number of cells in a cluster using different frequencies. A frequency reuse of 3/9 means each frequency is used once in three sites per cluster, where each site has three sectors. Reuse for control channels and traffic channels are independent. Typically, 4/12 is used for control and 1/3 for traffic, though other combinations are possible depending on performance requirements, environment, and spectrum availability. Higher frequency reuse yields higher throughput and less delay. Time slot capacities have a dynamic range depending on users.

1/3 Frequency Re-use (EDGE Compact) uses 3 × 200 kHz carriers reused in every site, requiring less than 1 MHz × 2 for initial deployment with 3 sectors per site. EDGE-capable and non-EDGE-capable TRX in one sector can be configured to have only one BCCH. TBF parameter setting makes it possible for TBFs of GPRS and EDGE radio networks to be multiplexed dynamically on one time slot, though this should be avoided as performance suffers in both uplink and downlink. In uplink, GPRS suffers due to large amounts of 8-PSK retransmissions. In downlink, it is due to GMSK modulation where 8-PSK can carry higher data rates for EDGE.

Dynamic Abis in EDGE

The Abis interface connects the Base Station (BS) and Base Station Controller (BSC). Voice signals are carried in 16 kbps Abis channels, which are static for GSM/GPRS. 8-PSK changes data rate from 8.8 kbps to 59.2 kbps, which is insufficient for data beyond MCS-2. Since this data rate is not always present, the Dynamic Abis concept is used in EDGE, where the BSC allocates Abis capacity from a Dynamic Abis Pool (DAP) for data calls from EGPRS when needed.

Benefits for operators include migration to wireless multimedia services, improved customer satisfaction, and early market deployment of third-generation-type applications. For users, benefits include improved quality of service, personal multimedia services, and potentially lower price per bit.

💡 Why this matters: Dynamic Abis efficiently uses backhaul capacity, allocating high-bandwidth channels only when EDGE users actually need them, rather than reserving them statically.

⭐ Key Takeaways

EDGE is a critical evolutionary step that extends GPRS with 8-PSK modulation, enabling 3 bits per symbol and a per-slot data rate of 59.2 kbps for a theoretical peak of 473 kbps. The nine MCS schemes allow dynamic tradeoffs between throughput and robustness, with Link Adaptation and Incremental Redundancy working together to maximize throughput under varying channel conditions. Capacity planning in EDGE uses frequency reuse patterns like 1/3 for traffic and 4/12 for control, with higher reuse yielding better throughput. The Dynamic Abis Pool efficiently allocates backhaul capacity on demand, solving the problem of variable data rates exceeding static channel capacities. Multiplexing GPRS and EDGE traffic on the same time slot should be avoided to prevent performance degradation from mixed modulation schemes.

🧠 Quick Revision Questions

1. How does 8-PSK modulation increase data rates compared to GMSK, and what is the resulting per-time-slot data rate?
2. What is the difference between Link Adaptation and Incremental Redundancy in EDGE?
3. Why should GPRS and EDGE traffic not be multiplexed on the same time slot?
4. What frequency reuse patterns are typically used for control channels versus traffic channels in EDGE?
5. How does the Dynamic Abis Pool (DAP) improve backhaul efficiency compared to static Abis allocation?

📘 Lecture 21 — WCDMA (Part I)

📖 Overview: This lecture introduces UMTS (Universal Mobile Telecommunications System) and its core WCDMA air interface. It covers UMTS service classes, UTRAN architecture, and the radio interface protocol architecture, explaining how 3G networks differ from GSM, particularly in handling data traffic, quality of service, and channel access using CDMA codes.

🗂️ Topics Covered

The lecture begins with a review of the last lecture (Walsh codes, IS-95 reverse link, EDGE). It then covers UMTS service classes including conversational, streaming, interactive, and background classes. UTRAN architecture is explained with Node B and RNC roles. The radio interface protocol architecture details layers 1, 2 (MAC and RLC), and 3 (RRC), plus PDCP and BMC. The protocol model for UTRAN is presented, followed by logical channels (BCCH, CCCH, CTCH, DCCH, DTCH, PCCH) in WCDMA.

📝 Lecture Summary

Last Lecture Review

The review covered Walsh Codes, IS-95 Reverse Link, EDGE Introduction, Modulation and Coding Schemes, Link Adaptation and Incremental Redundancy, Capacity Planning, and Dynamic Abis pool.

UMTS

UMTS networks have a predominance of data traffic unlike GSM networks. The data rate is significantly higher than GSM/GPRS/EDGE. Major changes from previous networks include:

• Max user data rate up to 384 kbps
• Efficient handover between different operators and technologies (e.g., GSM and UMTS)
• Ability to deliver at requested bandwidth
• Ability to deliver different services with the required quality

WCDMA Radio Fundamentals

WCDMA (Wideband Code Division Multiple Access) has emerged as the most preferred and adopted technology for 3G air interface. Major differences between WCDMA and GSM are:

• 5 MHz channel bandwidth compared to 200 KHz in GSM
• Packet data scheduling is load based, unlike time slot based in GSM
• Theoretically only one frequency channel, while GSM uses many channels
• Quality control is done using RRM algorithms, while it was done by frequency planning in GSM
• Users/cell/channel are separated by codes, unlike time or frequency in GSM

💡 Why this matters: The shift from time/frequency division to code division fundamentally changes how capacity and interference are managed in 3G networks.

Service classes in UMTS

In 3G networks, Mobile Equipment (ME) can establish multiple connections simultaneously. The network allows efficient cooperation between applications with diverse Quality of Service (QoS) requirements.

Quality is defined by two main parameters:

• Guaranteed and max bit rate possible (kbps)
• Permissible delay (ms)

Based on QoS criteria, multimedia services are classified into:

🔑 Definition — Conversational class: The most delay-sensitive applications, e.g., video telephony, VoIP.

🔑 Definition — Streaming class: Flow which is steady and continuous, it is server to user.

🔑 Definition — Interactive class: Web browsing is an example. A user may request timetables of buses, trains, or flight schedule.

🔑 Definition — Background class: Short messages, file transfer, email that have the least stringent requirements of QoS.

UTRAN and System Architecture

UTRAN (UMTS Terrestrial Radio Access Network) provides the radio access network for UMTS.

USIM (Universal Subscriber Identity Module) contains authentication information, associated algorithms, encryption, and subscriber-related information.

ME (Mobile Equipment) is user independent.

Base Station (Node B) in WCDMA is more complex than BS in GSM. Its functions include:

• Handover channel management
• Baseband conversion
• Channel encoding and decoding
• Interfacing to other network elements

Radio Network Controller (RNC)

Concerning one connection between UTRAN and one UE, the following roles of RNCs exist:

• Serving RNC: Controls the connections to a UE
• Drift RNC: Lends its resources to the Serving RNC for a particular UE

Each RNC also has a Controlling RNC role towards its Node Bs.

Radio Interface protocol architecture

Layer 1 (Physical Layer):

• The actual medium of transfer
• Main functions include RF processing, modulation/demodulation, multiplexing/demultiplexing of physical channels
• Error detection and correction, rate matching, power control, synchronization

Layer 2 has two main sub-layers: RLC (Radio Link Control) and MAC (Medium Access Control).

MAC (Medium Access Control):

• Responsible for mapping logical channels to transport channels
• An interface between L1 & L3, provides packet multiplexing/demultiplexing
• Measurement related to traffic volume on logical channels and reporting to layer 3

RLC (Radio Link Control):

• Segmentation and reassembly of variable size data packets
• Error correction by retransmission and ACKed data transfer mode
• Controlling rate flow, concatenation, cyphering, and preservation of higher-order PDUs
• Operates in three modes (transparent, unacknowledged, acknowledged) as in GPRS

Layer 3 contains sub-layers; RRC (Radio Resource Control) interacts with layer 2:

• Handles control plane signaling between UE and network in connected mode
• Responsible for bearer functions like establishment, release, maintenance, and reconfiguration in the user plane and of radio resources in control plane
• Functions include radio resource management and mobility management, as well as power control, routing, and paging

Packet Data Convergence Protocol (PDCP):

• Major functions: compression of PDU at transmitting end and decompression at receiving end in all three modes of RLC

Broadcast – Multicast Control (BMC):

• Functions only in transparent and unacknowledged modes
• Provides broadcast/multicast scheduling and transmission of user data

Protocol Model for UTRAN

The UTRAN protocol structure is based on a model with horizontal layers (Radio Network Layer, Transport Network Layer) and vertical planes (Control Plane, User Plane, Transport Network Control Plane). The Control Plane handles signaling, while the User Plane carries user data.

Logical Channels in WCDMA

⭐ Key Takeaways

UMTS service classes (conversational, streaming, interactive, background) are defined by delay sensitivity and bit rate requirements, with conversational being most delay-sensitive and background least stringent. The WCDMA air interface uses 5 MHz channels, employs code division for user separation, and relies on RRM algorithms for quality control instead of frequency planning. UTRAN architecture includes Node B (base station with complex encoding/handover functions) and RNC (with Serving, Drift, and Controlling roles). The radio interface protocol has Layer 1 (physical), Layer 2 (MAC and RLC sublayers), and Layer 3 (RRC), with additional PDCP for compression and BMC for broadcast/multicast. Logical channels such as BCCH, CCCH, DCCH, DTCH, and PCCH separate control and traffic information in the WCDMA system.

🧠 Quick Revision Questions

1. What are the four service classes in UMTS and which one is the most delay-sensitive?
2. What is the channel bandwidth in WCDMA compared to GSM, and how are users separated in WCDMA?
3. What are the three roles of a Radio Network Controller (RNC) and what does each do?
4. What are the main functions of the RLC sublayer in the radio interface protocol architecture?
5. List three logical channels in WCDMA and explain the purpose of each.

📘 Lecture 22 — WCDMA (Part II)

📖 Overview: This lecture continues the exploration of WCDMA technology within UMTS networks, focusing on the physical layer procedures that enable reliable communication. It covers how data is spread and scrambled, the different types of channels used, and essential signaling operations like random access, cell searching, and power control, all of which are critical for network efficiency and mobile device functionality.

🗂️ Topics Covered

The lecture begins with a review of UMTS architecture and protocol layers, then dives into physical layer procedures including spreading and scrambling with channelization (OVSF codes) and scrambling (Gold/S(2) codes). It explains the three channel concepts (logical, transport, physical) and maps transport channels to physical channels, with a focus on the uplink dedicated channel (DPDCH and DPCCH). Finally, it covers signaling procedures: RACH operation, the three-step cell searching process using synchronization channels, and the three types of power control (open loop, fast closed loop, and outer loop).

📝 Lecture Summary

Last Lecture Review

The lecture opens with a review of UMTS, its service classes, UTRAN architecture, radio interface protocol architecture, protocol models, and logical channels. Air interface parameters are specified: a carrier spacing of 5 MHz, a chip rate of 3.84 Mcps, a frame length of 10 ms (38400 chips), 15 slots per frame (2560 chips/slot), and spreading factors (SF) ranging from 4 to 256 uplink and 4 to 512 downlink, supporting channel rates from 7.5 Kbps to 960 Kbps.

Spreading and Scrambling

Spreading increases the signal bandwidth and includes two distinct operations. Channelization increases the signal bandwidth using orthogonal codes. Scrambling does not affect the signal bandwidth; it uses pseudo-noise codes.

Channelization codes are orthogonal codes based on the Orthogonal Variable Spreading Factor (OVSF) technique. These codes are fully orthogonal (they do not interfere) only if they are time-synchronized. They can separate transmissions from a single source; in the downlink, they separate different users within one cell/sector. Since limited orthogonal codes must be reused in every cell, this creates an interference problem if two cells use the same code. The solution is to use scrambling codes to reduce inter-base-station interference. It is possible for two mobiles to use the same channelization codes. To separate different users in the uplink, scrambling codes are used. One code tree is used with one scrambling code on top of the tree.

Scrambling involves multiplying the code sequence with a pseudorandom scrambling code. This code can be a long code (a Gold code with a 10 ms period) or a short code (S(2) code). In the downlink, scrambling codes reduce inter-base-station interference. Typically, each Node B has only one scrambling code for UEs to separate base stations. Since a code tree under one scrambling code is used by all users in its cell, proper code management is needed.

Channel Concept

Three separate channel concepts exist in UTRA: logical, transport, and physical channels.

• Logical channels define what type of data is transferred.
• Transport channels define how and with which type of characteristics the data is transferred by the physical layer.
• Physical channels define the exact physical characteristics of the radio channel.

Transport Channels -> Physical Channels

Transport channels contain data generated at higher layers, which is carried over the air and mapped to different physical channels. Data is sent in transport blocks from the MAC layer to the physical layer every 10 ms. The transport format is identified by the Transport Format Indicator (TFI). Several transport channels can be multiplexed to form a single Coded Composite Transport Channel (CCTrCh). The physical layer combines TFI information into the Transport Format Combination Indicator (TFCI), which indicates which transport channels are active for the current frame.

There are two types of transport channels:

• Dedicated channels: Reserved for a single user, support fast power control and soft handover.
• Common channels: Can be used by any user at any time, do not support soft handover but some support fast power control.

In addition, there are physical channels for signaling purposes. The mapping is as follows:

• DCH (UL/DL Dedicated channel) maps to DPDCH (Dedicated Physical Data Channel) and DPCCH (Dedicated Physical Control Channel).
• RACH (UL Random Access Channel) maps to PRACH (Physical Random Access Channel).
• CPCH (UL Common Packet Channel) maps to PCPCH.
• BCH (DL Broadcast Channel) maps to P-CCPCH (Primary Common Control Physical Channel).
• FACH and PCH (DL) map to S-CCPCH (Secondary Common Control Physical Channel).
• DSCH (DL Downlink Shared Channel) maps to PDSCH.
• Signaling channels include SCH (Synchronization Channel), CPICH (Common Pilot Channel), AICH (Acquisition Indication Channel), PICH (Paging Indication Channel), CSICH, and CD/CS-ICH.

UL Dedicated Channel DCH

To overcome discontinuous transmission (DTX) and its audible interference to audio equipment, two dedicated physical channels are used: the Dedicated Physical Control Channel (DPCCH) and the Dedicated Physical Data Channel (DPDCH). They use code multiplexing instead of time multiplexing.

The DPCCH has a fixed spreading factor of 256 and carries physical layer control information with four fields:

• Pilot: Used for channel estimation and SIR estimate for power control.
• TFCI: Indicates the bit rate, channel decoding, and interleaving parameters for every DPDCH frame.
• FBI (Feedback Information): Used for transmission diversity in the downlink.
• TPC (Transmission Power Control): Contains the power control command.

The DPDCH has a spreading factor from 4 to 256 and its data rate may vary on a frame-by-frame basis, as informed on the DPCCH channel. Parallel channel codes can provide 2 Mbps user data.

UL receiver in BS

The receiver in the base station performs these steps:

1. Starts receiving the frame, de-spreading the DPCCH and buffering the DPDCH according to the max bit rate.
2. For every slot: Obtains a channel estimate using pilot bits and estimates SIR, sends a TPC command in the downlink to control UE uplink power, and decodes the TPC bit to adjust downlink power.
3. For every 2nd or 4th slot: Decodes FBI bits to adjust antenna phases and amplitude for transmission diversity.
4. For every 10 ms frame: Decodes TFCI information to obtain the bit rate.
5. For each transmission time interval (TTI) of 10, 20, 40, or 80 ms: Decodes the DPDCH data.

RACH Operation

The Random Access Channel (RACH) operation is a signaling procedure:

1. The UE first sends a preamble.
2. The preamble's SF is 256 and contains a signature sequence of 16 symbols, for a total length of 4096 chips.
3. The UE waits for an acknowledgement via the Acquisition Indication Channel (AICH) from the BS.
4. If no AICH is received after a period, the UE sends another preamble with higher power.
5. When the AICH is received, the UE sends a 10 or 20 ms message part.
6. The SF for the message part is from 32 to 256.

Synchronisation Channel (SCH) – Cell Searching

Cell searching using the SCH is a three-step procedure for a UE to find and synchronize with a cell:

1. Slot Synchronisation: The UE searches for the 256-chip primary synchronisation code, which is common to all cells and the same in every slot. Detecting peaks in the filter output corresponds to finding the slot boundary.
2. Frame Synchronisation: The UE seeks the largest peak from the secondary synchronisation code (SSC). There are 64 unique SSC sequences, each with 15 SSCs. The UE must know 15 successive SSCs from the S-SCH to determine the code group and find the frame boundary.
3. Scrambling Code Identification: Each code group has 8 primary scrambling codes. The correct one is found by testing each possible scrambling code over the Common Pilot Channel (CPICH) of that cell.

Power Control

Three types of power control are detailed:

• Fast Closed Loop PC – Inner Loop PC: Uses feedback information. Uplink PC solves the near-far problem. Downlink PC ensures enough power for mobiles at the cell edge. There is one PC command per slot, giving a rate of 1500 Hz. Two special cases exist:
  • Soft handover: The mobile receives multiple power control commands. A “power down” command has higher priority over a “power up” command.
  • Compressed mode: A large step size is used after a compressed frame to allow the power level to converge more quickly to the correct value after a break.
• Closed Loop PC - Outer Loop PC: This sets the SIR target to maintain a certain frame error rate (FER). It is operated at the radio network controller (RNC).
• Open Loop PC: This uses no feedback information.

⭐ Key Takeaways

The most critical concepts from this lecture are the distinction between channelization (OVSF codes for user separation within a cell) and scrambling (for base station separation), the three-layer channel model (logical, transport, physical) and their specific mapping (especially DCH to DPDCH/DPCCH with DPCCH's four control fields), and the three physical layer procedures: the preamble/AICH-based RACH operation, the three-step cell search (slot, frame, scrambling code), and the three power control loops (open, fast closed inner, and slow outer). Understanding the UL receiver's slot-by-slot and frame-by-frame tasks is also essential.

🧠 Quick Revision Questions

1. What is the purpose of scrambling codes in the WCDMA downlink, and how do they solve a problem created by channelization codes?
2. What are the four fields carried by the Uplink Dedicated Physical Control Channel (DPCCH), and what is the function of each?
3. Describe the three steps a UE performs to find and synchronize with a cell using the Synchronization Channel (SCH).
4. In the context of the RACH operation, what happens if a UE does not receive an Acquisition Indication Channel (AICH) acknowledgement?
5. Explain the difference between fast closed-loop power control and outer-loop power control, including where each is implemented and what parameter each one aims to adjust.