Cellular Mobile Communication (EC-703 (A)) - Unit 5 Short Notes

1.0 FUNDAMENTALS OF CELLULAR SYSTEMS

1.1 Basic Principle of Operation & System Concept

• Core Idea: Replace a single high-power transmitter (large cell) with many low-power transmitters (small cells) to increase capacity.

• Key Concepts:

  • Cell: Geographic area covered by a Base Station (BS).

  • Cluster: A group of N cells that use all available channels. N is the cluster size.

  • Frequency Reuse: The same set of frequencies (channels) can be reused in non-adjacent cells (separated by a distance D). The reuse ratio is D/R, where R is the cell radius.

  • Handoff (Handover): The process of transferring an ongoing call from one cell to another as the user moves.

1.2 Key Elements for Establishing a Cellular Radio System

1.3 Frequency Spectrum Utilization in Cellular Systems

• Allocation: Regulatory bodies (e.g., TRAI in India, FCC in USA) assign specific frequency bands (e.g., 900 MHz, 1800 MHz, 2100 MHz) to operators.

• Efficiency: Measured in Erlangs per square kilometer per MHz. Improved by techniques like cell splitting, sectoring, and using digital modulation.

• Aspects: Licensing, spectrum pricing, and coordination to avoid interference between operators.

[!TIP] Exam Focus: Be prepared to draw a simple block diagram showing MS, BS, MSC, and databases (HLR/VLR) and state their functions. This is a frequent 7-mark question.

2.0 MOBILE RADIO PROPAGATION & PATH LOSS MODELS

2.1 Mobile Point-to-Point Propagation Model

• A statistical model for a specific mobile radio link between a fixed BS and a moving MS.

• Considers large-scale path loss (average signal strength over long distances) and small-scale fading (rapid fluctuations over short distances).

• The received power P_r at distance d is given by the log-distance path loss model:

2.2 Path Loss and Received Signal Power Calculation

Log-Distance Path Loss Model:

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

Where:

• PL(d) = Total path loss at distance d (dB).

• PL(d_0) = Path loss at a known close-in reference distance d_0 (usually 1m or 100m).

• n = Path Loss Exponent (PLE). Indicates the rate of attenuation. n=2 for free space, n=4-6 for dense urban areas.

• X_σ = Zero-mean Gaussian random variable (dB) representing shadowing (slow fading).

Change in Received Power when distance doubles:

If d_2 = 2d_1, then:

$$ \Delta P_r (\text{dB}) = P_r(d_2) - P_r(d_1) = -10n \log_{10}(2) \approx -3n \ \text{dB} $$

Example: For n=4, doubling distance reduces power by 12 dB.

2.3 Specific Propagation Environments

2.3.1 Propagation Over Water & Flat Open Areas

• Principle: Dominated by surface wave propagation and reflection from the water surface (which acts as a smooth, highly conductive reflector).

• Characteristics:

  • Very low path loss (often n < 2 due to constructive interference).

  • Signals can travel very long distances (hundreds of km) due to low absorption and strong groundwave.

  • Causes severe co-channel interference over large areas.

• Challenges: Requires careful frequency planning to manage interference; signals can be unstable due to atmospheric ducts.

2.3.2 Mobile-to-Mobile Propagation

• Principle: Both transmitter and receiver are in motion, typically at low heights (e.g., vehicles).

• Model Differences from Fixed-to-Mobile:

  • Antenna heights are similar and low (h_tx ≈ h_rx).

  • Dominated by diffraction around obstacles and scattering from surrounding objects.

  • Path loss exponent is typically higher (n ≈ 4-5) than in fixed-to-mobile with elevated BS.

  • No dominant line-of-sight (LoS) component on average.

[!TIP] Common Pitfall: Do not confuse "propagation over water" (low loss, long range) with "mobile-to-mobile" (high loss, short range). They are opposite extremes.

3.0 CELL COVERAGE & ANTENNA DESIGN

3.1 Cell Site Antenna Heights & Signal Coverage

• Higher BS Antenna:

  • Increases cell radius R (reduces number of cells needed for area coverage).

  • Increases coverage area (Area = 2.6 R^2 for hexagonal cells).

  • BUT increases interference to distant cells (larger D/R ratio needed for same C/I).

• Lower BS Antenna:

  • Reduces cell radius, requiring more cells (higher infrastructure cost).

  • Reduces interference, allowing smaller cluster size N (increases capacity).

• Trade-off: System design balances coverage (large R) and capacity/interference (small R).

3.2 Antenna Parameters for Cellular Systems

3.3 Gain and Pattern Relationship

• Omni-directional Pattern: 360° horizontal coverage. Low gain. Used in rural areas with large cells.

• Directional (Sector) Pattern: 60°-120° coverage. Higher gain. Used in urban areas to reduce CCI. Typically 3 sectors (120°) per cell site.

• Umbrella Pattern: A pattern with a main lobe (for serving cell) and a high-angle secondary lobe (to fill coverage gaps caused by terrain or building penetration). Used in hilly areas or where the main lobe is obstructed.

3.3.1 Umbrella Pattern Effect & Application

• Effect: The secondary lobe provides coverage to areas that would otherwise be in a shadow zone behind a hill or large building.

• Application: Cell sites located in valleys or near tall structures where the primary line-of-sight is blocked for some user locations. The "umbrella" lobe radiates over the obstacle.

3.4 Antenna Pattern Classification & Use in Cellular Layout

[!TIP] Key Formula Link: Antenna gain G is directly related to beamwidth. Narrower beamwidth → higher gain → better C/I but smaller coverage area.

4.0 INTERFERENCE & CARRIER-TO-INTERFERENCE RATIO (C/I)

4.1 Co-Channel Interference (CCI)

• Definition: Interference caused by another cell using the same frequency channel.

• Source: Reuse of channels in non-adjacent cells (distance D apart).

• Impact: Primary limiting factor on system capacity. Increases with smaller cluster size N (since D/R decreases).

4.2 Desired C/I Ratio in Omni-Directional Antenna Systems

• Worst-Case Scenario: Mobile at edge of its serving cell, equidistant to i=6 interfering co-channel cells (hexagonal layout).

• Signal Power (from serving BS): S ∝ 1/R^n

• Interference Power (from each interfering BS): I_i ∝ 1/D_i^n

• Total C/I:

$$ \left(\frac{C}{I}\right) = \frac{S}{\sum_{i=1}^{6} I_i} = \frac{R^{-n}}{\sum_{i=1}^{6} D_i^{-n}} $$

For a regular hexagonal grid, `D/R = √(3N)`. Assuming all `D_i ≈ D`:

$$ \left(\frac{C}{I}\right) = \frac{1}{6} \left(\frac{D}{R}\right)^{-n} = \frac{1}{6} \left(\sqrt{3N}\right)^{n} = \frac{(3N)^{n/2}}{6} $$

**Required C/I:** For acceptable voice quality (AMPS), `C/I ≥ 18 dB` (analog) or `C/I ≥ 14 dB` (digital).

4.3 Co-Channel Interference Reduction Factor (Q)

• Definition: Q = D/R = √(3N). It is the reuse distance ratio.

• Significance: Larger Q means more separation between co-channel cells → less interference → better C/I.

• Relation to Cluster Size: N = (Q^2)/3. To achieve a target C/I, solve for minimum N:

$$ N_{min} = \left( \frac{6 \cdot 10^{(C/I)_{dB}/10}}{3^{n/2}} \right)^{2/n} $$

> **Example:** For `n=4`, `(C/I) = 18 dB` → `N_min ≈ 6.5` → **N=7** (next integer).

4.4 Adjacent Channel Interference (ACI)

• Interference from channels adjacent in frequency to the desired channel.

• Causes: Imperfect receiver filters (roll-off), near-far problem (a nearby mobile on adjacent channel is much stronger).

• Mitigation: Sufficient frequency separation between channels assigned to same cell, high-quality filters, power control.

4.5 Effects of Power Decrease (Power Control)

• On Coverage: Reduces cell radius R (since P_r ∝ P_t). May create coverage holes if not managed.

• On Interference: Dramatically reduces CCI because interfering signal power drops as 1/d^n. A 3 dB reduction in P_t reduces interference by 3 dB but also reduces desired signal by 3 dB → net C/I improvement because interference comes from many cells.

• Purpose: In CDMA, power control is essential to combat the near-far problem. In FDMA/TDMA, it extends battery life and reduces interference.

5.0 SYSTEM DESIGN, CAPACITY ENHANCEMENT & CHANNEL ASSIGNMENT

5.1 System Design for Areas Experiencing Co-Channel Interference

Design Steps:

1. Define Requirements: Coverage area A, traffic density (Erlangs/km²), grade of service (blocking probability P_b).

2. Choose Cell Radius R:

   • From coverage (link budget): R = 10^{(PL(d0) - P_{rx(min)} + G_{BS}+G_{MS} - L_{other})/(10n)} * d_0.

   • From interference: Choose N from Q = √(3N) and C/I requirement. R must be ≤ D/(√(3N)).

3. Determine Cluster Size N: Minimum N from C/I formula (Section 4.3). Often rounded up (e.g., 4, 7, 12).

4. Calculate Number of Cells: Number of cells = A / (2.6 R^2).

5. Total Channels Needed: Total = (Traffic per cell * Number of cells) / (1 - P_b).

6. Channels per Cell: S = Total / Number of cells. Must be ≤ Total available channels / N.

5.2 Capacity Expansion Techniques

5.2.1 Cell Splitting

• Principle: Divide a congested cell into smaller cells (microcells). New cells have half the radius (R_new = R/2).

• Implementation: Place new BSs at the center of old cells. Requires additional spectrum or frequency reuse (new cluster pattern).

• Effect: Increases capacity by factor of 4 (area reduces by 4, so 4 times more cells in same area). N may need to be reduced to maintain D/R.

5.2.2 Sectoring

• Principle: Replace an omni-directional antenna at a BS with multiple directional antennas (e.g., 3 × 120°).

• Effect on C/I: A co-channel interferer in a different sector of the same cell is now behind a high-gain, high-F/B antenna → its signal is attenuated. Effective N is reduced by factor N_sector (e.g., 3).

• Capacity: Allows use of smaller cluster size (e.g., from N=7 to N=4 or 3), increasing capacity by 7/3 ≈ 2.3 times.

5.2.3 Microcell Zone Concept

• Principle: A single cell site (with one frequency set) is divided into multiple zones (microcells) using low-power, directional antennas all connected to the same BS.

• Advantage: Reduces handoff traffic (since all zones are under one BS) and allows frequency reuse within the macrocell.

• Use: High-capacity hotspots (airports, malls).

5.3 Channel Assignment Algorithms

5.4 Traffic Engineering & Call Blocking

• Call Blocking (Subscriber View): A call attempt fails because no channel is available in the serving cell (FCA) or system (DCA).

• Erlang B Formula (for FCA):

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

Where `A` = offered traffic (Erlangs), `C` = number of channels per cell.

• Practical Solutions to Reduce Blocking:

  1. Cell Splitting: Increases number of cells → more channels available per unit area.

  2. Sectoring: Increases effective channels per cell site.

  3. Adding More Spectrum: Increases total C in the system.

  4. Using DCA: More efficient channel utilization.

6.0 HANDOFF & CALL MANAGEMENT

6.1 Classification of Handoff

• Based on Nature:

  • Hard Handoff: Break-before-make. Connection with old BS is terminated before connection with new BS is made. (Used in GSM, FDMA, TDMA).

  • Soft Handoff: Make-before-break. Mobile maintains connections with multiple BSs simultaneously during transition. (Used in CDMA).

• Based on Control:

  • Network-Controlled Handoff (NCHO): MSC (network) measures signal strength (via BSs) and decides. (e.g., GSM).

  • Mobile-Assisted Handoff (MAHO): Mobile measures BS signals and reports to network. Network decides. (Common in 2G/3G).

  • Mobile-Controlled Handoff (MCHO): Mobile autonomously decides and executes handoff. (Used in some cordless systems).

6.2 Types of Handoff in Practice

6.3 Handoff Procedure & Parameters

• Initiation: Based on signal strength measurements.

  • Threshold (T): Minimum signal level from serving BS to consider handoff.

  • Hysteresis (H): Prevents "ping-pong" effect. Handoff only if Signal_New > Signal_Old + H.

  • Time Delay (t): New BS signal must be stronger by H for a duration t to trigger handoff.

• Dropped Call Rate (DCR) Formula:

  The probability that a call is dropped during handoff due to failure.

$$ P_{drop} = P(\text{handoff required}) \times P(\text{handoff failure}) $$

*   `P(handoff required)` depends on cell radius, user speed, and call duration.

*   `P(handoff failure)` depends on channel availability in target cell and handoff algorithm delay.

6.4 Call Processing in Cellular Systems (General)

1. Call Initiation: MS sends access request on control channel.

2. Authentication & Verification: AUC/VLR checks subscriber.

3. Channel Assignment: MSC assigns a traffic channel (TCH).

4. Call Establishment: Connection set up through BS to PSTN/other MS.

5. Handoff: During call, if signal weakens, procedure from 6.3 is triggered.

6. Call Termination: Either party hangs up; resources released.

7.0 GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS)

7.1 GSM Architecture & Functional Entities

[MS] <--Um--> [BTS] <--Abis--> [BSC] <--A--> [MSC] <--E--> [HLR/VLR/AUC/EIR]

                                 |

                                 +-----> [PSTN/ISDN]

• MS (Mobile Station): ME + SIM.

• BSS (Base Station Subsystem): BTS (transceiver) + BSC (controls multiple BTSs, manages radio resources).

• NSS (Network Switching Subsystem): MSC (call switching), HLR (home DB), VLR (visitor DB), AUC (auth), EIR (equipment ID).

• PSTN/ISDN: External networks.

7.2 Key Features of GSM Architecture

• Separation of Switching (MSC) and Radio (BSS).

• Centralized Databases (HLR/VLR) for mobility management.

• Modular Design: Allows independent evolution of radio access (BSS) and core network (NSS).

• Security: Authentication (AUC) and encryption (A5 algorithm).

7.3 GSM Channel Structure

• Physical Channel: A time slot in a TDMA frame on a specific frequency.

• Logical Channel: Information carried on a physical channel. Classified as:

  • Traffic Channels (TCH): Carry user data (voice/data).

    • TCH/F (Full rate, 13 kbps), TCH/H (Half rate, 6.5 kbps).

  • Control Channels: Carry signaling.

    • Broadcast Control Channel (BCCH): BS → MSs. System info (cell ID, freq list).

    • Frequency Correction Channel (FCCH): BS → MS. For frequency synchronization.

    • Synchronization Channel (SCH): BS → MS. Frame timing & BS identity.

    • Common Control Channel (CCCH): Bidirectional for initial access.

      • RACH (Random Access Channel): MS → BS. Call request.

      • AGCH (Access Grant Channel): BS → MS. Assigns SDCCH.

      • PCH (Paging Channel): BS → MS. Pages incoming call.

    • Dedicated Control Channel (DCCH): Point-to-point.

      • SDCCH (Stand-alone DCCH): For call setup, SMS, location update.

      • SACCH (Slow Associated DCCH): Associated with TCH/SDCCH. Carries measurement reports, SMS.

      • FACCH (Fast Associated DCCH): "Steals" TCH frames for urgent signaling (e.g., handoff command).

Frame Structure: 8 time slots/frame (TDMA). One physical channel = one TS per frame. A multiframe (26 or 51 frames) defines logical channel mapping.

7.4 Call Processing in GSM (Mobility Management & Call Setup)

1. Location Update: MS registers with new VLR when entering new Location Area (LA).

2. Call Origination (Mobile Initiated):

   • MS listens to BCCH for system info.

   • MS sends request on RACH.

   • BS responds on AGCH with SDCCH assignment.

   • Authentication & ciphering on SDCCH.

   • MSC assigns TCH via SDCCH/FACCH.

   • Call connected on TCH.

3. Call Termination (Mobile Terminated):

   • MSC queries HLR for MS location (VLR address).

   • MSC sends paging message via PCH in that LA.

   • MS responds on RACH.

   • Rest same as origination (SDCCH → TCH).

4. Handoff: MSC measures signal strength from neighboring BSs (via SACCH reports). MSC decides, assigns new TCH, and sends handoff command on FACCH.

8.0 CDMA (CODE DIVISION MULTIPLE ACCESS) SYSTEMS

8.1 CDMA System Overview (contrast with FDMA/TDMA)

• FDMA/TDMA: Users separated by frequency (FDMA) or time (TDMA). One user per channel at a time.

• CDMA: All users transmit simultaneously in the same frequency band. Users separated by unique spreading codes (pseudo-random sequences).

• Key Concept: Direct Sequence Spread Spectrum (DS-SS). User data multiplied by a high-rate code (chip rate >> data rate).

• Advantages: Soft capacity (graceful degradation), soft handoff, interference limited, high security.

• Disadvantages: Complex power control (near-far problem), requires precise synchronization.

8.2 Call Processing in CDMA

1. Access: MS sends access probe on Access Channel (ACH) (a common code channel).

2. Response: BS replies on Paging Channel (PCH) with assignment to a Traffic Channel (TCH) (a dedicated code).

3. Traffic: Call proceeds on TCH. All signaling (e.g., handoff direction) is sent on Forward Traffic Channel (F-TCH) and Reverse Traffic Channel (R-TCH).

4. Power Control: Continuous closed-loop power control at 800-1600 bps to keep all received powers equal at BS (combats near-far problem).

8.3 Handoff Procedure in CDMA

• Type: Soft Handoff (make-before-break).

• Procedure:

  1. Pilot Strength Measurement: MS continuously measures pilot signals (synchronization codes) from all nearby BSs.

  2. Reporting: MS reports a Active Set (currently connected BSs) and Neighbor Set (candidate BSs) to serving BS via Reverse Traffic Channel.

  3. Decision: Serving BS/MSC adds a pilot to Active Set if Pilot_Strength > T_add (threshold). Removes if < T_drop for duration T_Tdrop.

  4. Execution: MS now has simultaneous connections to all BSs in Active Set. Data is combined (selection diversity) at MSC.

  5. Completion: When one BS's pilot becomes strongest and others drop below T_drop, connection to weaker BSs is terminated.

• Advantage: No dropped calls during handoff, higher reliability.

9.0 ADVANCED TOPICS & SYSTEM COMPARISONS

9.1 Time Division Duplex (TDD) Systems

• Principle: Uplink and downlink share the same frequency but use different time slots. (Contrast with FDD: separate freq bands for UL/DL).

• Application in Cellular: TD-SCDMA (3G standard), LTE-TDD (4G), used in unpaired spectrum.

• Advantages: Flexible UL/DL ratio (asymmetric traffic), no duplexer needed in MS, efficient for bursty data.

• Disadvantages: Requires strict time synchronization, guard periods increase overhead, less efficient for symmetric voice traffic.

9.2 Non-Cellular Systems: Overview and Comparison

• Examples: Cordless phones (DECT), PMR (TETRA), Wireless Local Loop (WLL), Satellite phones.

• Comparison with Cellular:

  • Coverall: Non-cellular typically has larger cells (satellite: global, cordless: 100m-1km).

  • Mobility: Cellular designed for high-speed mobility (cars). Non-cellular often for low-speed/stationary (indoor, pedestrian).

  • Capacity: Cellular uses frequency reuse for high capacity. Non-cellular often single cell or limited reuse.

  • Infrastructure: Cellular has complex MSC/VLR databases. Non-cellular simpler (direct connection to PSTN).

9.3 Layer Modelling in Cellular Systems

• OSI Model Application:

  • Physical Layer (L1): Modulation (GMSK, QPSK), channel coding, spreading (CDMA), RF transmission.

  • Data Link Layer (L2): MAC protocol (TDMA, CDMA codes), error control (ARQ), framing (GSM multiframe).

  • Network Layer (L3): Routing, mobility management (location update, handoff signaling), call control (GSM MAP, IS-41).

  • Transport Layer (L4): TCP/UDP over wireless links (with adaptations for high BER).

  • Application Layer (L7): Voice codec (AMR), SMS, data services (WAP, HTTP).

• Practical Application: Understanding where handoff signaling (L3), power control (L1/L2), or SMS delivery (L7) occurs.

9.4 Other Interference Sources: UHF-TV Interference

• Source: TV broadcast stations in UHF band (470-862 MHz) can interfere with cellular systems operating in adjacent bands (e.g., 850 MHz cellular band).

• Mechanism: Intermodulation or receiver front-end overload if TV signal is very strong.

• Mitigation: Use of high-quality filters at BS receiver input, sufficient guard bands between TV and cellular allocations, and careful site selection to avoid strong TV signals.

[!TIP] Exam Focus: For short notes (like Dec 2024 Q5), be ready to write concise 3-4 point explanations for TDD, cell splitting, UHF-TV interference, GSM channels, non-cellular systems. Use bullet points.