MOBILE COMMUNICATION (EC-503 (B)) - Unit 2 Short Notes

1.0 CELLULAR SYSTEM FUNDAMENTALS & CAPACITY ENHANCEMENT

1.1 Cellular Concept & Frequency Reuse

• Cellular System: Divides service area into small cells, each with a base station. Reuses frequencies in non-adjacent cells to increase capacity.

• Frequency Reuse: Using the same frequency channel in geographically separated cells. Cluster of N cells uses all available channels once.

  • Reuse Factor: N = i² + ij + j² (i, j are integers).

  • Co-channel Cells: Cells using same frequency. Distance between co-channels: D = R√(3N), where R is cell radius.

  [!TIP] For N=7 (i=2,j=1), co-channels are 1st, 2nd, 3rd... tier cells. For N=19 (i=3,j=1), draw a 19-cell cluster and identify all cells with same frequency as center cell.

• Co-channel Interference (CCI): Caused by reuse of frequencies. Reduced by increasing D/R ratio (larger N), but reduces capacity.

1.2 Capacity Expansion Techniques

• Cell Splitting: Dividing a congested cell into smaller cells (microcells/picocells). Increases capacity by reducing R and allowing more clusters per area.

  • Incremental Splitting: New smaller cells overlaid on existing layout.

  • Overlay/Underlay: New cells use different frequencies (overlay) or same frequencies with reduced power (underlay).

• Cell Sectoring: Using directional antennas (e.g., 120° for 3-sector) to reduce CCI. A cell with S sectors has S times fewer co-channel cells in same cluster, effectively increasing N.

• Microcells/Picocells: Very small cells (radius < 1 km / < 100 m) for high-density areas (stadiums, malls). Require lower transmit power.

1.3 Channel Assignment Strategies

1.4 Trunking and Grade of Service (GoS)

• Trunking: Sharing a limited pool of channels among many users statistically.

• Grade of Service (GoS): Probability that a call is blocked (Erlang B) or delayed (Erlang C).

  • Erlang B Formula (no queue): P_b = \frac{A^N / N!}{\sum_{k=0}^{N} A^k / k!} where A = offered traffic (Erlangs), N = channels.

  [!TIP] Erlang B used for loss systems (FCA). Erlang C includes queuing delay.

2.0 MOBILE RADIO PROPAGATION

2.1 Large-Scale Path Loss Models

• Free Space Model: P_r(d) = P_t G_t G_r \left( \frac{\lambda}{4\pi d} \right)^2 or PL(d) = (4\pi d / \lambda)^2. Path loss exponent n=2. Valid for d >> λ, d << R_{far-field}.

• Two-Ray (Ground Reflection) Model:

$$PL(d) \propto \left( \frac{d^2}{H_t H_r} \right)^2 \quad \text{for } d \gg d_{bp}$$

**Breakpoint Distance**: `d_{bp} = \frac{4\pi H_t H_r}{\lambda}`. For `d > d_{bp}`, path loss exponent `n=4`.

• Log-Distance Path Loss Model:

$$PL(d) = PL(d_0) + 10n \log_{10}\left( \frac{d}{d_0} \right) + X_\sigma$$

`n` = path loss exponent (environment dependent, 2–6), `X_σ` = log-normal shadowing (dB).

• Okumura-Hata Model (Urban, 150–1500 MHz):

$$L_{50}(\text{urban}) = 69.55 + 26.16 \log_{10}(f_c) - 13.82 \log_{10}(H_{te}) - a(H_{mr}) + [44.9 - 6.55 \log_{10}(H_{te})] \log_{10}(d)$$

`f_c` in MHz, `d` in km, `H_te` (BS ht), `H_mr` (MS ht).

• COST-231 Hata Extension: For 1500–2000 MHz, adds C term for medium cities/suburbs.

• Foliage Loss: Empirical: L_f = 0.2 f^{0.3} d_f^{0.6} (dB), f in GHz, d_f = depth of foliage (m).

• Close-in Reference Distance Model (Practical):

$$PL(d) = PL(d_0) + 10n \log_{10}\left( \frac{d}{d_0} \right)$$

`d_0` = 1 m or 10 m (close-in). `n` derived from measurements.

2.2 Small-Scale Multipath Propagation

• Causes: Reflections, diffractions, scattering → multiple delayed copies.

• Parameters:

  • Delay Spread (τ_rms): RMS delay difference between multipaths.

  • Coherence Bandwidth (B_c): Frequency separation where channel response highly correlated.

    • Approx: B_c ≈ 1/(5τ_rms) for 50% correlation, B_c ≈ 1/(50τ_rms) for 90%.

  • Doppler Shift: f_d = \frac{v}{\lambda} \cos \theta.

  • Doppler Spread (B_D): Range of Doppler shifts.

  • Coherence Time (T_c): Time duration channel impulse response is invariant. Approx: T_c ≈ 1/(2B_D) for 50% correlation.

• Fading Types:

  | | Flat Fading | Frequency-Selective Fading | | :--- | :--- | :--- | | Condition | B_s << B_c | B_s > B_c | | Symbol Duration | T_s >> τ_rms | T_s ≈ τ_rms | | | Slow Fading | Fast Fading | | Condition | T_s << T_c | T_s > T_c | | Cause | Shadowing (large obstacles) | Mobile motion (Doppler) |

2.3 Fading Models & Statistics

• Clarke's Model (Rayleigh Fading): Assumes many scatterers, no dominant LOS. Complex baseband envelope r(t) = x(t) + jy(t) where x,y are independent Gaussian (zero mean, variance σ²).

  • Envelope |r(t)| follows Rayleigh PDF: p(r) = \frac{r}{σ^2} e^{-r²/(2σ²)}, r≥0.

  • Phase uniform [0,2π). Doppler spectrum (Jakes): S(f_d) ∝ 1/\sqrt{1-(f_d/f_{dmax})²}.

• Rician Fading: With dominant LOS component. Envelope follows Rician PDF. K = ratio of deterministic to scattered power.

• Level Crossing Rate (LCR) (N_R): Expected rate at which fading envelope crosses level R downward.

$$N_R = \sqrt{2\pi} f_d \rho e^{-\rho^2}, \quad \rho = R / \sqrt{P}$$

• Average Fade Duration (AFD) (τ̄): Mean time signal stays below a level R.

$$\bar{\tau} = \frac{e^{\rho^2} - 1}{\rho f_d \sqrt{2\pi}}$$

• Mitigation: Diversity Techniques:

  • Spatial: Multiple antennas (separation > λ/2). Selection, equal gain, maximal ratio combining.

  • Frequency: Use multiple carriers spaced > B_c.

  • Time: Repeat transmission at intervals > T_c.

  • Polarization: Orthogonal polarizations.

3.0 HANDOFF (HANDOVER)

3.1 Handoff Necessity & Types

• Necessity: Maintain call continuity when mobile moves out of current cell's coverage.

• Types:

  • Hard Handoff (Break-before-make): Release old channel before acquiring new (GSM, TDMA).

  • Soft Handoff (Make-before-make): Mobile holds both old and new channels simultaneously (CDMA). Softer handoff within same BTS sector.

  • Mobile-Assisted Handoff (MAHO): Mobile measures neighbor BS signal strengths and reports to network. Reduces network processing load.

  • Network-Controlled Handoff (NCHO): Network (BSC/MSC) makes measurements and decision.

  • Intersystem Handoff: Between different BSCs, MSCs, or technologies (e.g., GSM to UMTS).

3.2 Handoff Queuing & Prioritization

• Handoff Request Queuing: Prioritize handoff calls over new call attempts to avoid dropped calls.

• Guard Channels: Reserve N_g channels exclusively for handoff calls in a cell. Reduces total available channels but improves GoS for handoffs.

4.0 MULTIPLE ACCESS TECHNIQUES

4.1 Frequency Division Multiple Access (FDMA)

• Principle: Each user assigned a dedicated frequency band (channel) for entire call duration.

• Channel Bandwidth: B_c = B_T / N approx, where B_T = total bandwidth, N = number of channels. Guard bands B_guard between channels to prevent interference.

  [!TIP] Numerical: N = (B_T - N_g * B_guard) / B_c? Typically: N = \left\lfloor \frac{B_T - (N+1)B_{guard}}{B_c} \right\rfloor. For B_T=12.5 MHz, B_guard=10 kHz, B_c=30 kHz: N ≈ (12.5e6 - (N+1)*10e3)/30e3. Solve iteratively: N ≈ 416 channels.

• Advantages: Simple, low latency. Disadvantages: Inflexible, prone to jamming, inefficient spectrum use.

4.2 Time Division Multiple Access (TDMA)

• Principle: Users share same frequency but transmit in assigned time slots (TS) within a repeating frame.

• Frame Structure: Frame → N time slots. Group of frames → Multiframe (e.g., GSM: 26-frame for TCH, 51-frame for control). Group of multiframes → Superframe (e.g., 1326 TDMA frames), Hyperframe (e.g., 3,240,000 TDMA frames for encryption).

• Guard Times: Small guard intervals between TS to prevent overlap due to timing errors.

• Advantages over FDMA: Higher capacity (multiple users per carrier), flexible allocation, lower power consumption (mobile transmits only in its TS).

4.3 Code Division Multiple Access (CDMA)

• Principle: Spread Spectrum. All users transmit simultaneously in same wide band. Each user assigned a unique, orthogonal pseudo-noise (PN) code to spread data.

• Forward Channel (BS→MS):

  • Pilot: Unmodulated PN sequence for synchronization & coherent demodulation.

  • Sync: System time, PN code phase.

  • Paging: Page messages for mobiles.

  • Traffic: Voice/data to specific mobile (orthogonal Walsh codes).

• Reverse Channel (MS→BS):

  • Access: Random access for initial call setup (slotted ALOHA).

  • Traffic: Data spread by user-specific PN code (not orthogonal due to asynchronous transmission).

• Processing Gain (G_p): Ratio of spread bandwidth to data bandwidth.

$$G_p = \frac{W}{R_b} \quad (\text{linear}) \quad \text{or} \quad 10\log_{10}(W/R_b) \text{ (dB)}$$

Measures resistance to interference & jamming.

• Near-Far Problem: Strong nearby signal overwhelms weak distant signal at receiver. Requires tight power control.

  • Open-loop: MS estimates path loss from forward pilot and adjusts power.

  • Closed-loop: BS measures SIR and sends power adjustment commands to MS (every 1.25 ms in IS-95).

• Power Limit & Interference-Limited Capacity: Capacity limited by total interference (I_total). Each user's power must be controlled so that SIR at BS is sufficient. Max users K_max when E_b/I_0 threshold met.

• Call Processing & Soft Handoff:

  1. Mobile measures pilot strengths from multiple BSs.

  2. Reports to current BS.

  3. MSC/BSC adds new BS to active set.

  4. Traffic channels from all active BSs are macrodiversed (combined) at mobile.

  5. When old BS signal weakens, it's dropped from active set.

• BER Performance (for K users, AWGN, perfect power control):

$$P_b \approx Q\left( \sqrt{ \frac{2E_b}{N_0 + \frac{2(K-1)E_b}{3G_p} } } \right)$$

> [!TIP] **Numerical (IS-95)**: `K=20`, `W=1.2288 Mcps`, `R_b=13 kbps` → `G_p = 1.2288e6 / 13e3 ≈ 94.53` (≈ 19.8 dB). `E_b/N_0 = 7.8 dB = 6.03`. BER ≈ `Q(√(2*6.03 / (1 + (19*6.03)/(3*94.53))))`.

4.4 Frequency Hopped Multiple Access (FHMA)

• Principle: Carrier frequency changes rapidly according to a pseudorandom hopping pattern known to transmitter/receiver.

• Slow FH: T_hop >> T_sym (symbol duration). Frequency changes per symbol block.

• Fast FH: T_hop < T_sym. Frequency changes within symbol duration.

• FHSS: Basis for FHMA. Provides frequency diversity against fading and interference.

4.5 Spread Spectrum Multiple Access (SSMA)

• Encompasses CDMA (DSSS) and FHMA.

• Direct Sequence Spread Spectrum (DSSS): Data multiplied by high-rate PN code (chip rate W). Bandwidth ≈ W. Basis for CDMA.

5.0 GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS)

5.1 GSM Architecture & Subsystems

[MS] --Um (Air)--> [BTS] --A-bis--> [BSC] --A--> [MSC]

                                     |--> [HLR] [VLR] [AUC] [EIR]

                                     |--> [OSS]

• Mobile Station (MS): ME + SIM.

• Base Station Subsystem (BSS): BTS (radio transceiver), BSC (controls BTSs, handoff, frequency hopping).

• Network Switching Subsystem (NSS):

  • MSC: Call switching, mobility management.

  • HLR: Permanent DB (subscriber info).

  • VLR: Temporary DB (visiting subscribers).

  • AUC: Authentication.

  • EIR: Equipment identity register.

• OSS: Operation & maintenance.

• Key Interfaces: Um (air), A-bis (BTS-BSC), A (BSC-MSC), D (MSC-HLR/VLR).

5.2 GSM Logical Channels & Frame Structure

• Traffic Channels (TCH):

  • TCH/F: Full-rate (13 kbps speech, 22.8 kbps data).

  • TCH/H: Half-rate (6.5 kbps speech).

• Control Channels (CCH):

  • Broadcast (BCH): BCCH (system info), FCCH (frequency correction), SCH (synchronization).

  • Common Control (CCCH): PCH (paging), AGCH (grant), RACH (random access).

  • Dedicated Control (DCCH): SDCCH (stand-alone control), SACCH (slow associated control, e.g., measurement reports), FACCH (fast associated control, steals TCH frame for urgent messages).

• Frame Hierarchy:

  • TDMA Frame: 8 TS (each 156.25 bit duration ≈ 577 µs).

  • Multiframe: 26 frames (TCH) or 51 frames (control).

  • Superframe: 1326 TDMA frames (26-multiframe × 51 or vice versa).

  • Hyperframe: 3,240,000 TDMA frames (~3.5 hrs). Repeats encryption sequence.

• Burst Types:

  • Normal Burst: TCH/SDCCH/FACCH. 148 bits data + 3 tail + 64.25 guard.

  • Frequency Correction Burst: All-zero sequence for FCCH.

  • Synchronization Burst: 64-bit sync sequence for SCH.

  • Access Burst: Shorter (88 bits), large guard (68.25) for random access (RACH).

5.3 GSM Radio Subsystem

• Components: MS, BTS, BSC.

• Functions:

  • BTS: RF transmission/reception, channel coding/decoding, encryption.

  • BSC: Radio resource management (channel allocation, handoff control), frequency hopping management, power control.

  • MS: Measurement of neighbor cell signals (for MAHO).

6.0 ADVANCED & EMERGING TOPICS (SHORT NOTES)

6.1 Multiple Input Multiple Output (MIMO)

• Concept: Use N_t transmit and N_r receive antennas (N_t, N_r > 1).

• Benefits:

  • Spatial Multiplexing: Transmit independent data streams → capacity increase ∝ min(N_t, N_r).

  • Diversity: Multiple paths → link reliability (array gain, diversity gain).

  • Beamforming: Directional transmission to desired user.

• Used in 4G LTE, 5G NR, Wi-Fi 6/7.

6.2 Orthogonal Frequency Division Multiplexing (OFDM)

• Principle: High-rate data stream split into N parallel low-rate subcarriers. Subcarriers are orthogonal (Δf = 1/T_sym), so spectra overlap without ICI.

• Key Feature: Long symbol duration T_sym → robust against frequency-selective fading (flat fading per subcarrier). Guard interval (cyclic prefix) combats ISI.

• Advantages: High spectral efficiency, simple equalization (one-tap per subcarrier), flexible bandwidth allocation.

• Disadvantages: High PAPR (peak-to-average power ratio), sensitive to frequency offset.

• Use: 4G LTE downlink, 5G NR (both downlink/uplink), WiMAX, Wi-Fi (802.11a/g/n/ac/ax).

6.3 Diversity Techniques (Spatial Focus)

• Purpose: Mitigate fading by providing multiple independent signal replicas.

• Spatial Diversity:

  • Antenna Separation: > λ/2 for uncorrelated fading.

  • Combining Methods:

    1. Selection Diversity: Choose antenna with highest SNR.

    2. Equal Gain Combining: Coherent sum with equal weights.

    3. Maximal Ratio Combining (MRC): Weighted sum proportional to SNR → optimal.

• Other Types:

  • Frequency Diversity: Spread signal over bandwidth > B_c.

  • Time Diversity: Repeat transmission at intervals > T_c (e.g., interleaving, ARQ).

  • Polarization Diversity: Orthogonal polarizations (vertical/horizontal).