UNIT 5: MOBILE COMMUNICATION – SHORT NOTES

I. INTRODUCTION TO MOBILE COMMUNICATION SYSTEMS

Evolution of Wireless Generations (1G to 5G/6G)

Basic Cellular System Architecture

• Components:

  • Mobile Station (MS): User device (phone).

  • Base Transceiver Station (BTS): Radio transceiver, serves a cell.

  • Base Station Controller (BSC): Manages multiple BTSs, handles handoffs, channel allocation.

  • Mobile Switching Center (MSC): Core switch, routes calls, connects to PSTN/other MSCs.

  • Operation Support System (OSS): Network management.

• Performance Criteria:

  • Coverage: Geographic area served.

  • Capacity: Number of users/channels per unit area.

  • Quality: Grade of Service (GoS), blocking probability, signal quality.

[!TIP] Exam Focus: Be ready to draw a simple block diagram showing MS ↔ BTS ↔ BSC ↔ MSC ↔ PSTN. Know the function of each block.

II. CELLULAR SYSTEM DESIGN AND CAPACITY

Frequency Reuse Concept

• Definition: Reusing the same set of radio frequencies (channels) in different geographic areas (cells) separated by a sufficient distance to keep interference below a threshold.

• Need: Increases system capacity without requiring more spectrum.

• Reuse Factor (q): $$\displaystyle q = \frac{1}{N} $$, where N is the cluster size (total number of cells in a repeating pattern).

• Cluster Geometry: For a hexagonal grid, $$\displaystyle N = i^2 + ij + j^2 $$, where i, j are integer steps along two axes.

  • Common clusters: N=3 (i=1, j=1), N=4 (i=2, j=0), N=7 (i=2, j=1), N=19 (i=3, j=1).

• Reuse Distance (D): Minimum distance between co-channel cells.

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

where **R** is the cell radius.

[!TIP] Common Pitfall: Remember $$\displaystyle D/R = \sqrt{3N} $$. For N=19, $D/R ≈ 7.55$. Smaller N means higher capacity but closer co-channel cells → more interference.

Co-channel Interference (CCI)

• Sources: Use of same frequency in different cells (co-channel cells).

• Main Reason: Inadequate frequency reuse distance (D) or large cell radius (R).

• Reduction Techniques:

  1. Increase Reuse Distance (D): Larger N → reduces capacity.

  2. Power Control: Reduce transmit power of mobile/BTS.

  3. Cell Splitting: Reduce R, keep D/R constant.

  4. Sectoring: Replace omni-directional antenna with directional antennas (e.g., 120° for 3-sector).

  5. Use of Microcells/Picocells: Smaller cells, lower power, higher reuse.

Capacity Expansion Techniques

Trunking and Grade of Service (GoS)

• Trunking: Sharing a limited pool of channels among a large number of users. When all channels busy, new calls are blocked (Erlang B) or queued (Erlang C).

• Grade of Service (GoS): Probability that a call is blocked or delayed. $$\displaystyle P_{block} $$ (Erlang B) or $$\displaystyle P_{delay} $$ (Erlang C).

• Erlang B Formula (No Queue):

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

where **A** = offered traffic intensity (Erlangs), **N** = number of channels.

• Erlang C Formula (With Queue):

$$P_{delay} = \frac{\frac{A^N}{N!} \frac{N}{N-A}}{\sum_{k=0}^{N-1} \frac{A^k}{k!} + \frac{A^N}{N!} \frac{N}{N-A}}$$

Channel Assignment Strategies

III. HANDOFF AND MOBILITY MANAGEMENT

Handoff Necessity and Types

• Necessity: Maintain call continuity when MS moves from one cell's coverage to another. Without handoff, call drops.

• Types:

  1. Mobile-Assisted Handoff (MAHO): MS measures signal strengths of neighboring cells, reports to BSC. BSC decides and executes. (Used in GSM).

  2. Network-Controlled Handoff (NCHO): BTSs measure MS signal, send to MSC. MSC decides and commands BTSs. (Used in 1G/2G analog).

  3. Mobile-Controlled Handoff (MCHO): MS independently measures, decides, and executes handoff. Requires fast processing. (Used in DECT).

MAHO Technique and Queuing Concept

• MAHO Process:

  1. MS continuously monitors serving cell and neighboring cell signal strengths (beacon frequencies).

  2. MS sends measurement reports to BSC via SACCH (slow associated control channel).

  3. BSC applies handoff algorithm (e.g., threshold, hysteresis) to decide.

  4. BSC commands target BTS to prepare, then instructs MS to switch.

• Queuing in Handoff: Handoff requests often given priority over new call attempts to reduce dropped calls. Handoff requests may be queued if no channel available in target cell, but with limited delay tolerance.

[!TIP] Exam Focus: Distinguish MAHO (MS measures, BSC decides) from NCHO (BTS measures, MSC decides). Queuing prioritizes handoffs over new calls.

Handoff Procedures in Specific Systems

• GSM:

  • Intra-cell Handoff: Changing channel within same BTS (due to interference/fading).

  • Inter-cell Handoff: MS moves between BTSs (BSC-controlled, MAHO).

  • Procedure: Measurement → BSC decision → BSC informs target BSC → channel activation → MS switch via HANDOVER COMMAND.

• CDMA (IS-95):

  • Soft Handoff: MS simultaneously connected to multiple BTSs (same frequency). BTSs in same Active Set. Combines signals (rake receiver). No break, higher reliability.

  • Softer Handoff: Soft handoff between sectors of same BTS (BSC handles).

  • Hard Handoff: Break-before-make. MS releases old channel before acquiring new (used for inter-frequency or to different systems). Similar to GSM.

IV. RADIO WAVE PROPAGATION

Large-Scale Propagation Models

• Free Space Propagation Model:

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

Path loss (dB): $$\displaystyle PL(d) = 32.4 + 20\log_{10}(f_{MHz}) + 20\log_{10}(d_{km}) $$.

• Two-Ray Ground Reflection Model:

$$P_r(d) \propto \frac{1}{d^4} \quad \text{(for large d)}$$

Includes direct and ground-reflected paths. Critical for flat terrain.

• Foliage Losses (Vegetation Attenuation): Additional loss when signal penetrates trees/vegetation. Empirical: $$\displaystyle L_f = 0.2 f^{0.5} d_f $$ (dB), where $f$ in GHz, $$\displaystyle d_f $$ depth of foliage (m).

• Near-in-Distance Propagation (Close-in Reference Distance Model):

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

where $$\displaystyle d_0 $$ is close-in reference distance (1m-1km), **n** is path loss exponent (environment dependent), $$\displaystyle X_{\sigma} $$ is log-normal shadowing (dB).

• Mobile-to-Mobile Propagation: Both Tx/Rx at low heights. Dominated by diffraction/scattering from local obstacles. Path loss exponent often > 4.

Small-Scale Multipath Propagation

• Causes: Multipath (reflection, diffraction, scattering) → multiple delayed copies. Doppler shift ($$\displaystyle f_d = \frac{v}{\lambda} \cos \theta $$) due to relative motion.

• Fading Types:

  | | Time Variation | Frequency Selectivity | | :--- | :--- | :--- | | Slow Fading | Changes over seconds/minutes (shadowing). | Flat (narrowband). | | Fast Fading | Changes over ms (multipath). | Flat or Frequency-Selective. | | Flat Fading | Bandwidth $$\displaystyle B_s << B_c $$ (coherence bandwidth). All freq components fade equally. | Non-selective. | | Frequency-Selective | $$\displaystyle B_s > B_c $$. Different freq components fade independently. | Selective. |

• Key Parameters:

  • Delay Spread ($$\displaystyle \tau_{max} $$): Max excess delay difference.

  • Mean Excess Delay ($\bar{\tau}$): $$\displaystyle \bar{\tau} = \frac{\sum P(\tau_i) \tau_i}{\sum P(\tau_i)} $$

  • RMS Delay Spread ($$\displaystyle \sigma_\tau $$): $$\displaystyle \sigma_\tau = \sqrt{\bar{\tau^2} - (\bar{\tau})^2} $$, where $$\displaystyle \bar{\tau^2} = \frac{\sum P(\tau_i) \tau_i^2}{\sum P(\tau_i)} $$

  • Coherence Bandwidth ($$\displaystyle B_c $$): Bandwidth over which channel is flat. Approx: $$\displaystyle B_c \approx \frac{1}{5\sigma_\tau} $$ (10% variation) or $$\displaystyle B_c \approx \frac{1}{50\sigma_\tau} $$ (50% variation).

  • Doppler Spread ($$\displaystyle B_D $$): Spectrum spread due to motion. $$\displaystyle B_D \approx f_d^{max} = \frac{v}{\lambda} $$.

  • Coherence Time ($$\displaystyle T_c $$): Time duration over which channel is invariant. $$\displaystyle T_c \approx \frac{1}{2B_D} $$ (50% correlation).

Fading Models

• Clarke's Model (Rayleigh Fading): Assumes many scattered waves with uniform phase distribution, no LOS component. Envelope follows Rayleigh distribution. Phase uniform $[0,2\pi]$. Used for flat fading.

  • Level Crossing Rate (LCR): Average rate at which fading envelope crosses a given level R.

$$N_R = \sqrt{2\pi f_d} e^{-R^2/(2\sigma^2)} \quad (\sigma^2 = \text{avg power})$$

*   **Average Fade Duration (AFD):** $$\displaystyle AFD = \frac{1}{N_R} P(R) $$.

• Rician Fading: Includes a dominant Line-of-Sight (LOS) component plus scattered waves. Envelope follows Rician distribution. Parameter K = LOS power / scattered power ratio. K=0 → Rayleigh; K→∞ → AWGN.

Dispersion Parameters

• Delay Spread Parameters: $$\displaystyle \tau_{max} $$, $\bar{\tau}$, $$\displaystyle \sigma_\tau $$ (most important).

• Relation to Symbol Rate: To avoid ISI, symbol duration $$\displaystyle T_s >> \sigma_\tau $$. Or, coherence bandwidth $$\displaystyle B_c >> signal bandwidth $$B_s$.

$$T_s \geq \frac{1}{B_c} \quad \text{or} \quad B_s \leq B_c$$

V. MULTIPLE ACCESS TECHNIQUES

FDMA (Frequency Division Multiple Access)

• Principle: Divide total bandwidth $$\displaystyle B_T $$ into non-overlapping frequency channels $$\displaystyle B_c $$. Each user assigned a dedicated channel.

• Guard Bands ($$\displaystyle B_{guard} $$): Unused bands between channels to prevent interference.

• Spectral Efficiency: $$\displaystyle \eta = \frac{N}{B_T} $$ (channels/Hz), where $N$ is number of channels.

• Channel Count:

$$N = \frac{B_T - N_{gb} \cdot B_{guard}}{B_c}$$

where $$\displaystyle N_{gb} $$ = number of guard bands (typically N-1).

TDMA (Time Division Multiple Access)

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

• GSM Implementation:

  • Burst: 0.577 ms, contains 148 bits (data + tail + guard).

  • Frame: 8 TS (1 TS per user), 4.615 ms.

  • Multiframe: 26 frames (for traffic) or 51 frames (for control), ~120 ms.

  • Superframe: 1326 TDMA frames (~6.12 s).

  • Hyperframe: 2715648 TDMA frames (~3h 28m).

CDMA (Code Division Multiple Access)

• Spread Spectrum Concept: Spread signal over wide bandwidth using pseudo-noise (PN) sequences.

  • Direct Sequence SS (DS-SS): Multiply data by high-rate PN chips.

  • Frequency Hopped SS (FHSS): Rapidly change carrier frequency according to hop pattern.

• Processing Gain ($$\displaystyle G_p $$): Ratio of spread bandwidth to original data bandwidth.

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

Higher $$\displaystyle G_p $$ → better interference resistance, more users.

• Forward & Reverse Channels (IS-95):

  • Forward (BTS→MS): Pilot (coherent reference), Sync (timing), Paging (paging messages), Traffic (voice/data).

  • Reverse (MS→BTS): Access (initial call), Traffic.

  • All channels orthogonal (Walsh codes) on forward, quasi-orthogonal (PN offsets) on reverse.

• Power Control: Critical due to near-far problem (near user drowns out far user).

  • Open-loop: MS estimates forward link path loss to set reverse power.

  • Closed-loop: BTS measures $$\displaystyle E_b/N_0 $$, sends power control bits (up/down) to MS (800 bps).

• Near-Far Problem: Strong nearby signal raises interference floor for weak distant signals. Solved by tight power control.

FHSS (Frequency Hopped Spread Spectrum)

• Principle: Carrier frequency changes according to a pseudo-random hop pattern over a wide band.

• Hop Rate: Number of hops per second.

  • Fast FHSS: Hop rate > symbol rate (multiple hops per symbol).

  • Slow FHSS: Hop rate < symbol rate (multiple symbols per hop).

• Advantage: Avoids narrowband interference/jamming.

Comparison of Multiple Access Schemes

VI. GSM AND CDMA SYSTEMS

GSM Architecture

• Subsystems:

  1. Network Switching Subsystem (NSS): MSC, VLR, HLR, AUC, EIR. Call control, mobility management, databases.

  2. Base Station Subsystem (BSS): BSC, BTS. Radio resource management.

  3. Mobile Station (MS): ME + SIM.

  4. Operation Support Subsystem (OSS): Network management.

• Key Interfaces:

  • Um: Air interface (MS ↔ BTS).

  • Abis: BTS ↔ BSC.

  • A: BSC ↔ MSC (or BSC ↔ MSC via SGSN in GPRS).

  • Ater: BSC ↔ PCU (packet control unit).

  • Iu: UMTS interface (not in classic GSM).

GSM Channels and Frame Structure

• Traffic Channels (TCH): Full-rate (TCH/F, 13 kbps), Half-rate (TCH/H, 6.5 kbps).

• Control Channels:

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

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

  • Dedicated Control (DCCH): SACCH (slow associated, measurement reports), FACCH (fast associated, steals TS for urgent msg), SDCCH (stand-alone control, call setup).

• Frame Hierarchy:

  • Burst: 156.25 bits, 0.577 ms.

  • TDMA Frame: 8 bursts (one per TS), 4.615 ms.

  • Multiframe:

    • 26-frame: 26 TDMA frames (120 ms) → 26 bursts for TCH, 1 for SACCH.

    • 51-frame: 51 TDMA frames (235 ms) → 51 bursts for control channels (BCCH, CCCH, SDCCH).

  • Superframe: 1326 frames (6.12 s) = 51x26.

  • Hyperframe: 2715648 frames (~3h 28m) for encryption sequence.

CDMA System (IS-95)

• System Overview: 1.25 MHz channel, chip rate 1.2288 Mcps, data rate 13 kbps (voice). Processing Gain ≈ 94 dB.

• Call Processing Steps:

  1. Access: MS sends access probe on Access Channel (using PN code).

  2. Paging: BTS pages MS on Paging Channel.

  3. Traffic Channel Assignment: BTS assigns a traffic channel (unique PN offset + Walsh code).

  4. Handoff: MS measures Pilot Channel strengths (PN offsets) of neighbors. Reports to BTS. BSC adds/removes pilots from Active Set (soft handoff).

• Handoff Procedures:

  • Soft Handoff: MS connected to ≥2 BTSs simultaneously (same freq, different PN offsets). BTSs in same Active Set. Selection Diversity at BSC.

  • Softer Handoff: Soft handoff between sectors of same BTS (handled by BSC, same BTS).

  • Hard Handoff: Break-before-make. Used for inter-frequency or to other systems (e.g., GSM). MS releases old channel before acquiring new.

VII. ADVANCED ANTENNA AND MIMO TECHNOLOGIES

Antennas in Mobile Systems

• Cell Site Antennas:

  • Types: Omni-directional, directional (sector: 65°, 90°, 120°, 180°).

  • Placement: Height critical for coverage. Often on towers/buildings.

  • Unique Situations: Tilt (electrical/mechanical) to control cell size/overlap; mounting height to avoid obstacles.

• Mobile Antennas:

  • Types: Whip, patch, internal.

  • Diversity: Often implemented as space diversity (two antennas, ~λ/2 apart).

Diversity Techniques

MIMO (Multiple Input Multiple Output)

• Concept: Use multiple antennas at both Tx and Rx.

• Benefits:

  • Capacity Increase: Spatial multiplexing → multiple data streams → $$\displaystyle C \approx \min(N_t, N_r) \times \text{SISO capacity} $$ (under rich scattering).

  • Diversity Gain: Improved reliability (space-time coding).

  • Array Gain: Beamforming → directivity, increased SNR.

• Configurations:

  • SISO: Single-In/Single-Out.

  • SIMO: Single-In/Multi-Out (Rx diversity).

  • MISO: Multi-In/Single-Out (Tx diversity/beamforming).

  • MIMO: Multi-In/Multi-Out (both multiplexing & diversity).

OFDM (Orthogonal Frequency Division Multiplexing)

• Principle: High-rate data stream split into N parallel low-rate streams, modulated on N orthogonal subcarriers.

  • Orthogonality: Subcarrier spacing $$\displaystyle \Delta f = 1/T_s $$, where $$\displaystyle T_s $$ = symbol duration. No ICI.

  • Implementation: IFFT at Tx, FFT at Rx.

• Advantages in Mobile Comm:

  • Robust to Multipath: Long $$\displaystyle T_s $$ >> delay spread → flat fading per subcarrier, simple single-tap equalization.

  • High Spectral Efficiency: Tightly spaced orthogonal subcarriers (no guard bands).

  • Flexible Bandwidth: Allocate subcarriers dynamically (OFDMA).

  • Simplifies Equalization: Frequency-domain equalization (1-tap per subcarrier).

VIII. PROBLEM-SOLVING AND CALCULATIONS

1. Symbol Rate from Coherence Bandwidth

• Concept: To avoid ISI, channel must be flat fading. Condition: Signal bandwidth $$\displaystyle B_s < B_c $$.

• For a baseband signal with symbol rate $$\displaystyle R_s $$ (symbols/sec), approximate bandwidth $$\displaystyle B_s \approx R_s $$ (for rectangular pulses).

• Formula:

$$R_s \leq B_c$$

• Example (Jun 2025): $$\displaystyle f_c = 900 $$ MHz, $$\displaystyle B_c = 100 $$ kHz.

  Max symbol rate $$\displaystyle R_s \leq 100 $$ kbaud (or 100 kbps for binary modulation).

2. FDMA Channel Count Calculation

• Formula:

$$N = \frac{B_T - (N_{gb} \cdot B_{guard})}{B_c}$$

where $$\displaystyle N_{gb} = N - 1 $$ (guard bands between N channels).

• Example (Nov 2023): $$\displaystyle B_T = 12.5 $$ MHz, $$\displaystyle B_{guard} = 10 $$ kHz, $$\displaystyle B_c = 30 $$ kHz.

  • Assume N channels → $$\displaystyle N_{gb} = N-1 $$.

  • $$\displaystyle 12.5 \times 10^6 = N \times 30 \times 10^3 + (N-1) \times 10 \times 10^3 $$

  • $$\displaystyle 12500 = 30N + 10N - 10 $$

  • $$\displaystyle 12510 = 40N $$

  • $$\displaystyle N = 312.75 \approx \boxed{312 \text{ channels}} $$

3. CDMA BER and Processing Gain Calculation (IS-95)

• Given (Nov 2023): K=20 users, $$\displaystyle W = 1.25 $$ MHz, $$\displaystyle R_b = 13 $$ kbps, $$\displaystyle E_b/N_0 = 7.8 $$ dB, PN code length $$\displaystyle L=32768 $$ ($$\displaystyle 2^{15} $$).

• Processing Gain ($$\displaystyle G_p $$):

$$G_p = \frac{W}{R_b} = \frac{1.2288 \text{ Mcps}}{13 \text{ kbps}} = \frac{1228800}{13000} \approx 94.53 \quad (\approx 39.5 \text{ dB})$$

*Note: Chip rate 1.2288 Mcps, not 1.25 MHz exactly, but often approximated.*

• BER for BPSK in AWGN: $$\displaystyle P_b = Q\left(\sqrt{2E_b/N_0}\right) $$.

  • $$\displaystyle E_b/N_0 = 7.8 $$ dB = $$\displaystyle 10^{0.78} \approx 6.03 $$ (linear).

  • $$\displaystyle P_b = Q\left(\sqrt{2 \times 6.03}\right) = Q(3.47) \approx 0.00026 $$ (from Q-table).

• In CDMA with MAI: Approx BER for large K:

$$P_b \approx Q\left( \sqrt{ \frac{G_p \cdot E_b/N_0}{K} } \right) \quad \text{(if power controlled perfectly)}$$

But in IS-95, power control keeps $$\displaystyle E_b/N_0 $$ roughly constant at receiver, so BER ≈ Q-function value above if near-far problem solved. **Exact formula depends on model.** Often, with perfect power control, BER ≈ $$\displaystyle Q(\sqrt{2E_b/N_0}) $$.

4. Propagation: Incident Angle & Slope Angle (Hilly Terrain)

• Given (Nov 2023): Hill height $$\displaystyle H = 100 $$ m, $$\displaystyle h_{ts} = 50 $$ m (Tx), $$\displaystyle h_{rs} = 3 $$ m (Rx), path length $$\displaystyle d = 5 $$ km.

• Slope Angle ($$\displaystyle \theta_s $$): Angle of terrain slope from Tx to hilltop.

$$\tan \theta_s = \frac{H - h_{ts}}{d_1} \approx \frac{H}{d} \quad (\text{if } h_{ts} \ll H)$$

$$\displaystyle \theta_s = \tan^{-1}(100/5000) = \tan^{-1}(0.02) \approx 1.15^\circ $$.

• Incident Angle ($$\displaystyle \theta_i $$): Angle at which signal hits hilltop from Tx.

$$\tan \theta_i = \frac{H - h_{ts}}{d_1} \approx \theta_s \quad (\text{small angle})$$

$$\displaystyle \theta_i \approx 1.15^\circ $$.

*Note: Precise calculation requires $$\displaystyle d_1 $$ (Tx to hilltop). If hill midpoint, $$\displaystyle d_1 = d/2 = 2.5 $$ km, then $$\displaystyle \tan \theta_i = 100/2500 = 0.04 $$, $$\displaystyle \theta_i \approx 2.29^\circ $$.*

**Assumption:** Hill at midpoint unless specified. Clarify in exam.

QUICK RECAP: MUST-KNOW FORMULAS

[!TIP] Final Exam Strategy: For 7-mark questions, always start with a clear definition, then explain with a diagram/schematic, followed by key formulas and advantages/disadvantages where applicable. For numerical problems, show step-by-step substitution. For short notes (4-5 marks), focus on core concept + 2-3 key points.