Cellular Mobile Communication (EC-703 (A)) - Unit 4 Short Notes

I. FUNDAMENTALS OF CELLULAR SYSTEMS

Basic Principles and Operation

• Cellular Architecture: Geographic area divided into small regions called cells, each served by a base station (BS). Cells are typically hexagonal for theoretical analysis.

• Frequency Reuse: Same frequency channels reused in non-adjacent cells to increase capacity. Cluster is a group of cells using all available channels once. Reuse factor $$\displaystyle Q = \sqrt{3N} $$, where $N$ = cluster size (1, 3, 4, 7, 9, 12, 13, 19,...).

• Cell Splitting: Subdividing a congested cell into smaller cells to increase capacity. New cell radius $$\displaystyle R_{new} = R_{old}/\sqrt{m} $$, where $m$ = splitting ratio.

• Sectoring: Using directional antennas (e.g., 120° for 3-sector) to reduce co-channel interference, effectively increasing $N$.

System Elements

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Exam Focus: Distinguish between VLR (Visitor Location Register, temporary data at MSC) and HLR (permanent data). MSC controls BS via BSC (Base Station Controller) in GSM.

Frequency Reuse Concept

• Cluster Size $N$: Determined by $$\displaystyle N = i^2 + ij + j^2 $$, where $i,j$ are integers in hexagonal layout.

• Capacity: Total channels $$\displaystyle C = M \times N_{cell} $$, where $M$ = channels per cell, $$\displaystyle N_{cell} $$ = number of cells in system. Reuse factor $$\displaystyle Q = 1/\sqrt{N} $$; smaller $N$ → higher capacity but lower C/I.

• Reuse Distance $D$: Minimum distance between co-channel cells, $$\displaystyle D = R \sqrt{3N} $$ ($R$ = cell radius).

II. MOBILE RADIO PROPAGATION CHARACTERISTICS

Path Loss and Signal Strength Prediction

• Free-Space Propagation:

$$P_r(d) = P_t G_t G_r \left( \frac{\lambda}{4\pi d} \right)^2$$

In dB: $$\displaystyle P_r(d)[dBm] = P_t[dBm] + G_t[dB] + G_r[dB] - 20\log_{10}(f) - 20\log_{10}(d) - 32.44 $$ (for $d$ in km, $f$ in MHz).

• Log-Distance Path Loss Model:

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

where $n$ = path loss exponent (2–6), $$\displaystyle X_\sigma $$ = log-normal fading (0 dB mean).

• Received Power (dB):

$$P_r(d)[dBm] = P_t[dBm] - PL(d)[dB]$$

• Path Loss Exponent $n$: Determined from measurements; higher $n$ indicates more obstruction (urban $n \approx 4$, open area $n \approx 2$).

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Common Pitfall: Forgetting to convert units in free-space formula. Use $f$ in MHz, $d$ in km for the 32.44 constant.

Propagation Over Special Terrains

• Over Water/Flat Open Areas:

  • Surface wave follows Earth curvature (ground wave propagation).

  • Ducting: Trapping of radio waves in low-level atmospheric layers, causing long-distance propagation (VHF/UHF).

  • Challenges:

    • Multipath: Reflections from water surface cause fading.

    • Doppler shift from moving objects.

    • Fast fading due to constructive/destructive interference.

Mobile-to-Mobile Propagation Model

• Two-Ray Ground Reflection Model:

$$P_r \propto \frac{h_t^2 h_r^2}{d^4} \quad \text{for } d \gg h_t, h_r$$

• Critical Distance $$\displaystyle d_c = \frac{4\pi h_t h_r}{\lambda} $$: beyond $$\displaystyle d_c $$, path loss exponent ≈ 4 (vs. 2 for free-space).

• Difference from Mobile-to-Base: Mobile-to-mobile has both antennas near ground; mobile-to-base often has BS antenna elevated, reducing ground reflection impact.

Point-to-Point Propagation Model

• Assumes direct LOS between fixed antennas; ignores multipath and fading. Used for microwave links.

• Limitations: Not valid for mobile environments with moving scatterers.

III. ANTENNA SYSTEMS IN CELLULAR NETWORKS

Cell Site Antenna Height and Coverage

• Coverage Radius $R$:

$$R \approx \frac{h \cdot \tan\theta}{\sqrt{2}}$$

for flat Earth, where $h$ = antenna height, $\theta$ = downtilt angle.

• Fresnel Zone: Ellipsoidal region around LOS; first Fresnel zone radius at midpoint:

$$r_1 = \sqrt{\frac{\lambda d_1 d_2}{d_1 + d_2}}$$

Obstructions within 60% of $$\displaystyle r_1 $$ cause diffraction loss.

• Cell Shape: Hexagonal approximation; actual coverage irregular due to terrain and antenna pattern.

Antenna Parameters

Umbrella Pattern Effect

• Concept: Radiation pattern with a dip (null) in the horizontal plane at higher elevation angles.

• Application: Reduces interference to distant co-channel cells by suppressing energy at high angles; improves frequency reuse.

• Pattern Shaping: Achieved via vertical array phasing.

Gain-Pattern Relationship

• Trade-off: Higher gain → narrower beamwidth (approximate: $$\displaystyle G \approx \frac{41250}{\theta_E \theta_H} $$ for $\theta$ in degrees).

• Pattern Classification:

  • Sectoral: 60°–120° beamwidth (e.g., 3-sector cells).

  • Hemispherical: Omnidirectional in azimuth, shaped in elevation.

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Exam Derivation: Be ready to derive $$\displaystyle G \approx \frac{41250}{\theta_E \theta_H} $$ from directivity formula $$\displaystyle D = \frac{4\pi}{\Omega_A} $$, where $$\displaystyle \Omega_A \approx \theta_E \theta_H \pi/180^2 $$.

IV. INTERFERENCE IN CELLULAR SYSTEMS

Co-Channel Interference (CCI)

• Desired C/I Ratio: Minimum acceptable carrier-to-interference ratio for acceptable voice quality (typically 18 dB for analog, 6–12 dB for digital).

• SIR in Omnidirectional System:

$$SIR = \frac{D^{-n}}{\sum_{i=0}^{i_0} (R_i)^{-n}}$$

where $D$ = distance to desired co-channel cell, $$\displaystyle R_i $$ = distances to $$\displaystyle i_0 $$ interfering co-channel cells.

• Worst-case SIR (for 1st-tier interferers only, $$\displaystyle i_0=6 $$):

$$\boxed{SIR = \frac{1}{6} \left( \frac{D}{R} \right)^n = \frac{N}{6}}$$

since $$\displaystyle D/R = \sqrt{3N} $$.

Interference Reduction Techniques

• Co-Channel Interference Reduction Factor $Q$:

$$Q = \frac{D}{R} = \sqrt{3N}$$

Larger $Q$ (larger $N$) reduces CCI but lowers capacity.

• Sectorization: 120° antennas reduce $$\displaystyle i_0 $$ from 6 to 2 (3-sector), improving SIR by factor 3.

• Beamforming: Adaptive antennas steer nulls toward interferers.

Adjacent Channel Interference (ACI)

• UHF-TV Interference: TV broadcasters in UHF band (470–890 MHz) adjacent to cellular bands (e.g., 824–894 MHz for GSM 850). Causes near-far problem.

• Mitigation:

  • Guard bands between channels.

  • High-quality receiver filters (steep roll-off).

  • Increase frequency separation between adjacent channels.

System Design for Interference-Limited Areas

• Design Approach:

  1. Determine required C/I for service.

  2. Choose $N$ based on $$\displaystyle SIR = N/6 \geq \text{required C/I} $$.

  3. Calculate $$\displaystyle D = R\sqrt{3N} $$; ensure minimum co-channel distance.

• Trade-off: Larger $N$ reduces interference but decreases capacity per area.

V. CHANNEL ASSIGNMENT AND TRAFFIC MANAGEMENT

Channel Assignment Algorithms

Traffic Engineering

• Erlang B Formula (loss system, no queue):

$$\boxed{B(E, N) = \frac{\frac{E^N}{N!}}{\sum_{k=0}^{N} \frac{E^k}{k!}}}$$

where $E$ = offered traffic (Erlangs), $N$ = number of channels.

• Call Blocking Structure:

  • Subscriber Perspective: Call blocked if no channel available at call attempt.

  • Practical Solutions:

    • Queuing: Delay call until channel free (not typical in cellular).

    • Overflow: Redirect to neighboring cells.

    • Channel Borrowing: Lend channels to congested cells with return promise.

Dropped Call Rate

• Definition: Probability that a call in progress is terminated due to handoff failure.

• Derivation:

  Let $$\displaystyle P_b $$ = new call blocking, $$\displaystyle P_h $$ = handoff blocking.

  Dropped call rate $$\displaystyle P_{drop} = P_h $$ (since handoff failure drops call).

• Factors: Insufficient channels in target cell, delayed handoff decision, high mobility.

Power Control

• Effects:

  • Coverage: Reduced transmit power shrinks cell radius.

  • Interference: Lower power reduces CCI, improves SIR.

• Types:

  • Open-loop: MS adjusts power based on received downlink signal (fast, but inaccurate).

  • Closed-loop: MSC sends power commands to MS (slow, precise; used in CDMA).

VI. HANDOFF IN CELLULAR SYSTEMS

Classification of Handoff

Handoff Procedures

• GSM (Mobile-Assisted):

  1. MS continuously measures BA list (Broadcast Assist) neighbors.

  2. Reports via SACCH (Slow Associated Control Channel).

  3. BSC evaluates; if threshold crossed, initiates handoff via handover request to target BTS.

  4. Hard handoff: old channel released after new channel assigned.

• CDMA:

  • Soft Handoff: MS searches multiple pilots; active set maintained. Rake receiver combines signals.

  • Hard Handoff: Used for inter-frequency or inter-system.

  • Power Control: During handoff, closed-loop power control adjusts to maintain link quality.

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Key Difference: GSM uses hard handoff (time-division), CDMA uses soft handoff (code-division, same frequency).

VII. MULTIPLE ACCESS TECHNOLOGIES

GSM System

• Architecture:

  
  MS → BTS → BSC → MSC → HLR/VLR → PSTN
  
  

  • BSC: Manages radio resources, handoff.

  • MSC: Switching, mobility management.

  • HLR/VLR: Location databases.

• Channels:

  | Traffic Channels (TCH) | Control Channels | |---------------------------|---------------------| | TCH/F (full-rate) | BCCH (Broadcast Control) | | TCH/H (half-rate) | CCCH (Common Control: RACH, AGCH, PCH) | | | SDCCH (Stand-alone Dedicated Control) | | | SACCH (Slow Associated Control) |

• Frame Structure: 8 time slots per 4.615 ms frame; 26-frame (120 ms) multiframe for TCH, 51-frame for control.

CDMA System

• Call Processing:

  1. Access Channel: MS sends access probe with PN code.

  2. Pilot Channel: BS broadcasts pilot for synchronization.

  3. Sync Channel: Transmits system parameters.

  4. Traffic Channel: Assigned after paging and authentication.

• Handoff:

  • Soft Handoff: MS maintains multiple active set pilots; softer handoff within same BS sector.

  • Hard Handoff: For different frequencies or systems.

• Power Control:

  • Open-loop: Initial power based on downlink.

  • Closed-loop: 800 bps power control bits on forward channel.

Other Multiple Access Schemes

• TDD (Time Division Duplexing): Uplink/downlink share same frequency, separated in time (e.g., TD-SCDMA, LTE-TDD). Advantage: asymmetric traffic support.

• Non-Cellular Systems: Wi-Fi (802.11), Bluetooth – limited coverage, no frequency reuse across cells, typically unlicensed bands.

VIII. SYSTEM CAPACITY AND EXPANSION TECHNIQUES

Cell Splitting

• Concept: Divide congested cells into smaller cells (smaller $R$) while reusing frequencies.

• Implementation:

  • New BSs installed at existing sites (lower antenna height) or new sites.

  • Splitting Ratio $m$: $$\displaystyle R_{new} = R_{old}/\sqrt{m} $$; capacity increases by factor $m$.

• Challenge: Sudden increase in number of BSs; requires careful planning to avoid interference.

Frequency Spectrum Utilization

• Efficiency Metric: $$\displaystyle \eta = \frac{\text{Total throughput (bps)}}{\text{Total bandwidth (Hz)} \times \text{Area (m}^2\text{)}} $$ (bits/sec/Hz/cell).

• Improvement Techniques:

  • Sectorization: Reduces $$\displaystyle i_0 $$ in SIR, allows smaller $N$.

  • Tighter Frequency Reuse: $$\displaystyle N=3 $$ or 4 with interference mitigation.

  • Digital Modulation: Higher bits/symbol (e.g., 64-QAM).

Sectorization

• 3-Sector: 120° antennas; $$\displaystyle i_0=2 $$ (instead of 6), SIR improves by 3×.

• 6-Sector: 60° antennas; further reduces interference but increases infrastructure.

• Impact:

  • Capacity: Increases by factor ≈ $$\displaystyle 6/i_0 $$ (e.g., 3× for 3-sector).

  • Interference: Lower CCI, but more handoffs.

IX. SYSTEM DESIGN AND OPTIMIZATION

Coverage-Capacity Trade-offs

• High-Traffic Areas (urban):

  • Small cells (low $R$), sectorization, small $N$ (e.g., $$\displaystyle N=3 $$).

  • Focus on capacity, accept lower C/I.

• Wide-Area Coverage (rural):

  • Large cells (high $R$), omnidirectional, large $N$ (e.g., $$\displaystyle N=7 $$ or 12).

  • Focus on coverage, higher C/I.

• Design Equation: $$\displaystyle C = \frac{A \cdot \eta}{R^2} $$, where $A$ = total area, $\eta$ = spectrum efficiency.

Design for Specific Conditions

• Interference-Prone Areas:

  • Use larger $N$, sectorization, umbrella patterns.

  • Increase antenna height to reduce path loss exponent $n$? Actually, higher antenna may increase interference to distant cells; optimize height for Fresnel zone clearance.

• Terrain-Specific:

  • Urban: High $n$ (4–5), use microcells/picocells.

  • Suburban: Medium $n$ (3–4), macrocells.

  • Rural/Water: Low $n$ (2–3), long-distance propagation possible.

Practical Deployment Challenges

• Antenna Site Selection:

  • Height: Balance coverage vs. interference (higher → larger cell but more CCI).

  • Location: Avoid obstructions; ensure Fresnel zone clearance.

• Signal Maintenance:

  • Fading margins (3–10 dB).

  • Power control to combat near-far effect.

  • Regular drive tests for optimization.

X. SHORT NOTE TOPICS (From Exam Patterns)

TDD Systems

• Principle: Uplink and downlink use same frequency, separated by time slots. Requires guard time to avoid collision.

• Applications: TD-SCDMA, LTE-TDD (TDD-LTE), 5G NR-U.

• Advantages: Flexible asymmetric traffic, no duplexer needed, easier MIMO.

• Disadvantages: Timing synchronization critical, guard time reduces efficiency.

Cell Splitting

• Methodology:

  1. Identify congested cell.

  2. Reduce cell radius by $\sqrt{m}$ (e.g., $$\displaystyle m=4 $$ → radius halved).

  3. Increase number of cells per cluster: new $$\displaystyle N' = N/m $$.

  4. Reassign frequencies using same reuse pattern.

  5. Install new BSs at existing sites (lower height) or new sites.

• Capacity Increase: $$\displaystyle C_{new} = m \cdot C_{old} $$ (if spectrum unchanged).

• Challenges: Sudden increase in handoff rate, need for more BSs, potential interference if not planned.

UHF-TV Interference

• Sources: TV broadcasters in UHF band (channels 14–83, 470–890 MHz) adjacent to cellular bands (e.g., GSM 850: 824–894 MHz).

• Mitigation:

  • Guard bands: e.g., 10 MHz between TV channel 83 and cellular.

  • Receiver filters: SAW filters with steep roll-off (e.g., 40 dB/octave).

  • Frequency planning: Avoid cellular channels adjacent to strong TV transmitters.

  • Increase antenna height to reduce TV signal strength at receiver.

GSM Channels

Non-Cellular Systems

• Examples: Wi-Fi (802.11), Bluetooth, ZigBee.

• Comparison:

  | Aspect | Cellular | Non-Cellular | |------------|--------------|------------------| | Coverage | Wide (km), frequency reuse | Limited (10s–100s m), single cell | | Mobility | Designed for high mobility | Typically stationary or low mobility | | Infrastructure | Centralized (MSC, BS) | Ad-hoc or access point | | Frequency | Licensed bands, planned reuse | Unlicensed ISM bands, contention-based | | Handoff | Complex, network-controlled | Simple or none (Wi-Fi: 802.11r) |

Layer Modelling Classification

• Protocol Layers (OSI-like in cellular):

  1. Physical Layer: Modulation, coding, RF (e.g., GSM: GMSK, CDMA: QPSK).

  2. Data Link Layer: MAC (TDMA, CDMA), ARQ (GSM: SACCH).

  3. Network Layer: Routing, mobility management (MSC, HLR).

  4. Transport Layer: TCP/UDP for data services.

  5. Application Layer: SMS, USSD, mobile internet.

• Applications:

  • Physical: Power control, handoff measurement.

  • Network: Location update, call routing.

  • Application: MMS, web browsing.

Frequency Spectrum Utilization

• Efficiency Metrics:

  • Spectral Efficiency: bits/sec/Hz/cell.

  • Trunking Efficiency: Erlangs per channel (Erlang B).

• Allocation Strategies:

  • Regulatory: Government assigns bands (e.g., FCC, TRAI).

  • Dynamic Spectrum Access: Cognitive radio, spectrum sharing (e.g., LTE-U, 5G NR-U).

  • Carrier Aggregation: Combine non-contiguous bands (LTE-A).

• Improvement: Higher-order modulation (64-QAM), MIMO, small cells.