UNIT 3: CELLULAR MOBILE COMMUNICATION - EXAM-FOCUSED SHORT NOTES

I. FUNDAMENTALS OF CELLULAR SYSTEMS

Cellular Concept & Frequency Reuse

• Basic Principle: Replace a single high-power transmitter (large cell) with many low-power transmitters (small cells) to increase capacity. Each cell uses a subset of total channels. Frequency reuse allows same channels in non-adjacent cells separated by a cluster.

• Cluster Size (N): Number of cells in a repeating pattern. Determined by the reuse ratio $D/R$, where $D$ is the distance between co-channel cells and $R$ is the cell radius.

$$N = i^2 + ij + j^2 \quad \text{(for hexagonal grid)}$$

where $i, j$ are integers (e.g., $$\displaystyle N=3,4,7,12,... $$).

• Capacity Increase: Capacity $\propto 1/N$. Smaller $N$ → more clusters → more users per unit area, but requires higher C/I ratio.

[!TIP] Exam Focus: Deriving $N$ from $D/R$ and understanding the trade-off: Smaller N = Higher Capacity but Lower C/I.

System Elements & Architecture

[!TIP] Common Pitfall: Confusing BSC (controls multiple BSs) with MSC (core switch). BSC is part of BSS, MSC is part of NSS.

Coverage & Traffic Engineering

• Cell Coverage: Area where received signal strength exceeds a threshold. Determined by path loss, antenna height, transmit power.

• Traffic Engineering:

  • Traffic Intensity (A): $$\displaystyle A = \lambda / \mu $$ (Erlangs), where $\lambda$ = call arrival rate, $\mu$ = call completion rate ($1/\text{average holding time}$).

  • Erlang B Formula (Loss System, no queue):

$$P_b = \frac{\frac{A^c}{c!}}{\sum_{k=0}^{c} \frac{A^k}{k!}}$$

    where $c$ = number of channels per cell, $$\displaystyle P_b $$ = **Grade of Service (GoS)** = probability of call blocking.

*   **Dropped Call Rate:** Probability a call in progress gets terminated due to poor signal or handoff failure. Depends on **handoff failure probability** and **coverage margin**.

II. MOBILE RADIO PROPAGATION & PATH LOSS

Propagation Models & Path Loss

• Free-Space Path Loss:

$$P_r(dB) = P_t(dBm) + G_t(dBi) + G_r(dBi) - 20\log_{10}(4\pi d / \lambda)$$

or 

$$P_r(dB) = P_t(dBm) + G_t + G_r + 20\log_{10}(\lambda) - 20\log_{10}(4\pi d)$$

Valid for $$\displaystyle d_{min} \ll d \ll $$ **Fresnel zone** distance.

• n-th Power Law (Large-scale path loss):

$$P_r(d) \propto \frac{1}{d^n} \quad \text{or} \quad P_r(dB) = P_r(d_0) - 10n \log_{10}(d/d_0)$$

where $n$ = path loss exponent (free-space $$\displaystyle n=2 $$, urban $$\displaystyle n=4-6 $$).

• Change in dB when distance doubles:

$$\Delta P_r(dB) = -10n \log_{10}(2) \approx -3n \ \text{dB}$$

> Example: If $$\displaystyle n=4 $$, doubling distance causes **-12 dB** drop.

Large-Scale Path Loss & Shadowing

• Log-Normal Shadowing: Accounts for large-scale variations due to terrain, buildings.

$$P_r(dB) = P_r(d_0) - 10n \log_{10}(d/d_0) + X_\sigma$$

where $$\displaystyle X_\sigma \sim \mathcal{N}(0, \sigma^2) $$ (zero-mean Gaussian in dB). $\sigma$ is the **shadowing deviation** (typically 6-12 dB).

Specific Propagation Environments

III. ANTENNA PARAMETERS & CELL SITE DESIGN

Antenna Fundamentals

• Cell Site Antenna Height ($$\displaystyle h_{BS} $$):

  • Higher $$\displaystyle h_{BS} $$: Larger cell radius $R$ (horizon distance $d \propto \sqrt{h}$), better coverage, but increases interference to distant cells.

  • Lower $$\displaystyle h_{BS} $$: Smaller cell, reduces interference, but requires more sites. Trade-off in design.

• Gain & Radiation Pattern: Gain $G(\theta, \phi)$ is the directivity multiplied by efficiency. Higher gain in a direction means narrower beamwidth in that plane.

$$\text{Gain (dBi)} \approx 10\log_{10}\left(\frac{41253}{\text{Beamwidth}_\text{H} \times \text{Beamwidth}_\text{V}}\right) \quad \text{(for ideal antenna)}$$

• Classification of Patterns:

  • Omni-directional: $$\displaystyle 360^\circ $$ in azimuth (e.g., vertical dipole). Simple, but high interference.

  • Directional/Sectoral: Focused beam (e.g., $$\displaystyle 65^\circ $$, $$\displaystyle 120^\circ $$ azimuth). Used in sectorization to reduce CCI.

• Key Parameters:

  • Beamwidth: Angle between half-power (-3 dB) points.

  • Front-to-Back Ratio (F/B): Power ratio (dB) between main lobe and rear lobe. Higher F/B = less interference from behind.

  • Polarization: Linear (vertical/horizontal) or circular. Mismatch causes loss (~20 dB for cross-pol).

Cell Site Antenna Configurations

• Umbrella Pattern Effect: Using a high-gain, narrow-beam antenna tilted downward (mechanical/electrical tilt) to shrink cell radius vertically, reducing interference to distant co-channel cells while maintaining coverage near the BS. Effective in high-traffic areas.

• Sectorization: Splitting a cell into 2, 3, or 6 sectors using directional antennas. Each sector gets a unique frequency set.

  • 3-sector (120°) is most common. Increases capacity by factor ~3 (if same channels reused per sector) and reduces CCI.

• Antenna Tilting:

  • Mechanical Tilt: Physically tilting the antenna mast.

  • Electrical Tilt: Adjusting phase of radiating elements (remote control). More precise, changes both beamwidth and tilt.

IV. CO-CHANNEL INTERFERENCE & SYSTEM DESIGN

Co-Channel Interference (CCI)

• Desired C/I Ratio (Omni-directional): Minimum acceptable for acceptable voice quality. ~18 dB for analog (FM), ~14 dB for digital (TDMA). Derived from protection ratio and frequency reuse.

• C/I Ratio Derivation (for hexagonal grid, omni antennas, equal powers):

$$\left(\frac{C}{I}\right)_\text{min} = \left(\frac{D}{R}\right)^n$$

where $D$ = co-channel distance, $R$ = cell radius, $n$ = path loss exponent.

Using $$\displaystyle N = (D/R)^2 $$ for square grids (approx.), $$\displaystyle \left(\frac{C}{I}\right)_\text{min} = N^{n/2} $$.

For hexagonal, $$\displaystyle D/R = \sqrt{3N} $$ → $$\displaystyle \left(\frac{C}{I}\right)_\text{min} = 3N^{n/2} $$.

> **Key Formula:** $$\displaystyle \boxed{\left(\frac{C}{I}\right)_\text{min} = 3N^{n/2}} $$ (for first-tier interferers only, omni, equal power).

• Co-channel Interference Reduction Factor (q): Accounts for more than first-tier interferers and partial loading.

$$\left(\frac{C}{I}\right)_\text{actual} = \left(\frac{C}{I}\right)_\text{min} \times q$$

$$\displaystyle q < 1 $$ (typically 0.5-0.8). More accurate C/I calculation includes all interfering cells.

• Factors Affecting C/I:

  1. Cluster Size (N): Larger N → higher C/I.

  2. Power Control: Reduces C/I by decreasing transmit power of mobiles near BS.

  3. Antenna Patterns: Directional antennas (sectors) reduce interference from certain directions → effective $q$ increases.

  4. Cell Splitting/Sectoring: Reduces $R$ and/or uses directional antennas.

System Design for Interference Management (Step-by-Step)

1. Define Requirements: Coverage area, number of users, traffic per user (Erlangs), GoS ($$\displaystyle P_b $$).

2. Determine Cell Radius (R): From coverage (path loss, antenna height, fade margin). $$\displaystyle R = \sqrt[ n ]{\frac{P_t G_t G_r \lambda^2}{(4\pi)^2 P_{r,\min}}} $$.

3. Calculate Number of Cells: Area / (hexagon area $$\displaystyle = \frac{3\sqrt{3}}{2}R^2 $$).

4. Determine Total Traffic (A_total): Users × traffic per user.

5. Channels per Cell (c): From Erlang B table: given $$\displaystyle P_b $$ (e.g., 0.02), find $c$ for traffic per cell $$\displaystyle A_{cell} = A_{total}/\text{number of cells} $$.

6. Total Channels Needed: $c \times \text{number of cells}$.

7. Choose Cluster Size (N): From C/I requirement using $$\displaystyle \left(\frac{C}{I}\right)_\text{min} = 3N^{n/2} $$. Solve for smallest integer $N$ satisfying C/I. Also check $$\displaystyle N \geq \frac{\text{Total Channels Needed}}{\text{Available Spectrum}/B_{ch}} $$.

8. Frequency Planning: Assign channel groups to cells in a cluster (e.g., $1,2,...,N$).

9. Apply Interference Reduction: Use sectorization (3x 120°), antenna tilting, power control to improve effective C/I, potentially allowing smaller $N$.

[!TIP] Design Trade-off: Small R (many cells) → high capacity (more cells × channels/cell) but high infrastructure cost. Large N → good C/I but low capacity. Sectorization breaks this trade-off by increasing capacity and C/I.

Effects of Power Decrease

• Coverage: Cell radius $R$ decreases ($$\displaystyle R \propto P_t^{1/n} $$). Creates coverage holes at cell edge.

• Interference: Decreases for all co-channel cells (both desired and interfering signals drop). Net effect on C/I depends on relative positions. If mobile near its BS lowers power, C/I improves for that mobile but may worsen for mobiles at edge of other cells.

Adjacent Channel Interference (ACI)

• Caused by imperfect receiver filters allowing nearby channel leakage. More severe than CCI because adjacent channel power is much higher than co-channel (due to near-far effect). Mitigated by increasing channel spacing, better filters, power control.

V. CHANNEL ASSIGNMENT & HANDOFF

Channel Assignment Strategies

Handoff (Handover)

• Classification based on Nature:

  • Mobile-Assisted Handoff (MAHO): Mobile measures BS signals and reports to network. Network decides. (e.g., GSM).

  • Network-Controlled Handoff (NCHO): Network (BSs/MSC) measures mobile signal, makes decision. (e.g., 1G analog).

  • Hard Handoff: Break-before-make. Connection with old BS released before connecting to new BS. Used in FDMA/TDMA (GSM). Short interruption (~200ms).

  • Soft Handoff: Make-before-break. Mobile connects to multiple BSs simultaneously (same frequency in CDMA). Seamless, lower drop probability. Unique to CDMA.

  • Intersystem Handoff: Between different MSCs/PLMNs (e.g., GSM to GSM in different region, or GSM to UMTS). Requires inter-MSC signaling.

  • Cell Site Handoff: Within same MSC (between BSs). Most common.

• Handoff Parameters:

  • Threshold ($T$): Signal level at which handoff is considered.

  • Hysteresis ($H$): Prevents "ping-pong" effect. Handoff only if new BS signal > current BS signal + $H$.

  • Time Delay ($t$): Minimum time signal must be above threshold to avoid false handoffs.

  • Procedure: 1. Measurement → 2. Reporting (MAHO) → 3. Decision (MSC) → 4. Execution (new channel allocation).

• Handoff in CDMA:

  1. Mobile measures pilot signal strengths from all BSs.

  2. Active set: BSs currently connected. Candidate set: BSs above threshold but not in active set.

  3. When candidate pilot > active pilot - add threshold, MSC adds it to active set (soft handoff).

  4. When active pilot < threshold - drop timer, it's removed.

  5. Call Processing: Same frequency, different PN codes. Mobile uses RAKE receiver to combine signals from multiple BSs.

• Dropped Call Rate & Handoff Failure: Probability handoff fails due to lack of resources in target cell or late decision. Reduced by channel reservation (guard channels) for handoffs.

VI. MULTIPLE ACCESS & SPECIFIC SYSTEM ARCHITECTURES

Multiple Access Techniques

GSM (Global System for Mobile)

• Architecture:

  • MS (Mobile Station): ME (Mobile Equipment) + SIM.

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

  • NSS (Network Switching Subsystem): MSC (core switch), HLR (home location register), VLR (visitor location register), AuC (authentication), EIR (equipment identity).

  • OSS (Operation Support Subsystem): Network management.

• GSM Channels:

  • Physical Channels: Defined by frequency and time slot (in TDMA frame).

  • Logical Channels: Mapped to physical channels.

    • Traffic Channels (TCH): Voice/data (TCH/F, TCH/H).

    • Control Channels:

      • Broadcast (BCH): FCCH, SCH, BCCH (broadcast system info).

      • Common Control (CCCH): RACH (uplink access), AGCH, PCH, CBCH.

      • Dedicated Control (DCCH): SDCCH, SACCH, FACCH.

• Frame & Multiframe Structure:

  • TDMA Frame: 8 time slots (TS0-TS7), each 156.25 bits, duration 4.615 ms.

  • Multiframe (26-frame): 26 TDMA frames for traffic (TCH) + control (SACCH). 120 ms.

  • 51-frame Multiframe: For control channels (BCCH, CCCH, SDCCH). 235.4 ms.

CDMA (Code Division Multiple Access)

• Principles:

  • Spread Spectrum: User data multiplied by high-rate PN sequence (chip rate >> data rate). Occupies wide bandwidth.

  • Orthogonality: Different users have different PN codes (e.g., Walsh codes for forward link). Ideally zero cross-correlation.

  • Soft Handoff Advantage: Mobile can combine signals from multiple BSs (macro-diversity), improving signal quality and reducing drop rate. No frequency planning required (all cells use same frequency).

• Call Processing & Handoff Procedure (IS-95/3G):

  1. Acquisition: Mobile searches for strongest pilot (synchronization channel).

  2. Registration: Mobile sends access request on reverse access channel.

  3. Traffic Channel Assignment: MSC assigns forward/reverse traffic channels with unique PN offsets.

  4. Handoff (Soft/Softer):

     • Mobile continuously measures pilot Ec/Io from all BSs.

     • Active Set: BSs currently serving the mobile.

     • When a neighbor pilot exceeds T_ADD threshold, it's added to active set (soft handoff).

     • When an active pilot drops below T_DROP for T_DROP_TIMER, it's removed.

     • Softer Handoff: Between sectors of same BS (different antenna).

VII. ADVANCED TOPICS & SYSTEM EXPANSION

Cell Splitting & Sectoring

• Cell Splitting: Divide congested cells into smaller microcells. Original cell radius $R$ halved → area $$\displaystyle \propto R^2 $$ → 4× capacity. Requires:

  • Reducing transmit power.

  • Adding new BSs at existing sites or new locations.

  • Challenge: Splitting may not be uniform; requires careful planning to avoid interference.

• Sectorization: As in Section III. Capacity increase by factor of number of sectors (if same channel set reused per sector). Reduces CCI by narrowing antenna beamwidth.

Frequency Spectrum Utilization

• Efficient Techniques:

  • Tight Frequency Reuse: $$\displaystyle N=1 $$ or $$\displaystyle N=3 $$ (with interference cancellation).

  • Dynamic Channel Allocation (DCA): See Section V.

  • Spread Spectrum (CDMA): All users share band; capacity interference-limited.

  • Smart Antennas (MIMO): Spatial multiplexing, beamforming.

• Duplexing Methods:

  • FDD (Frequency Division Duplexing): Separate frequency bands for uplink/downlink. Requires frequency guard band. Used in GSM, CDMA2000, WCDMA.

  • TDD (Time Division Duplexing): Same frequency, separate time slots. Flexible for asymmetric traffic, but requires guard time. Used in TD-SCDMA, Wi-Fi, 4G TDD-LTE.

Non-Cellular Systems (Brief)

VIII. DESIGN & INTEGRATED PROBLEM-SOLVING

Comprehensive System Design Approach

1. Coverage Planning:

   • Compute cell radius $R$ from path loss model, transmit power, receiver sensitivity, fade margin.

   • Account for terrain (Okumura-Hata model for urban/suburban).

   • Use link budget: $$\displaystyle P_{r,\min} = P_t + G_t + G_r - L_{path} - L_{misc} $$.

2. Capacity Planning:

   • Estimate number of subscribers in area, traffic per subscriber (Erlangs).

   • Compute total traffic $$\displaystyle A_{total} $$.

   • From Erlang B formula, find channels per cell $c$ for given GoS ($$\displaystyle P_b $$).

   • Total channels needed = $c \times \text{number of cells}$.

3. Frequency Planning:

   • Determine cluster size $N$ from C/I requirement ($$\displaystyle \left(\frac{C}{I}\right)_\text{min} = 3N^{n/2} $$).

   • Check if available spectrum supports required channels: $$\displaystyle \text{Total Channels} = \frac{\text{Bandwidth}}{B_{ch}} \geq c \times \text{number of cells} $$.

   • If not, increase sectorization (e.g., 3-sector → effective $N$ reduced by 3) or use DCA.

4. Interference Management:

   • Apply sectorization, antenna tilting, power control.

   • Recalculate effective C/I with $q$ factor.

5. Trade-off Analysis:

   • Small R → more cells → higher capacity (more sites × channels/site) but higher cost.

   • Small N → higher capacity but lower C/I → may require better antennas/power control.

   • Sectorization increases capacity and C/I but adds complexity.

[!TIP] Exam Problem Strategy: Always start with given parameters (area, users, traffic, spectrum). Compute R from coverage, cells from area, c from Erlang B, N from C/I. Then check consistency and suggest sectorization if needed.