UNIT 2: CELLULAR MOBILE COMMUNICATION - EXAM-FOCUSED 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), each covering a limited area. This allows frequency reuse—using the same frequency channels in geographically separated cells—dramatically increasing system capacity.

• Cell: The basic geographic service area, typically modeled as a hexagon for tessellation (no gaps/overlaps).

  DiagramSEARCH: hexagonal cellular layout

• Frequency Reuse: The fundamental capacity-enhancing principle. A cluster is a group of N cells, where each cell uses a unique set of frequencies. The same cluster pattern repeats over the service area.

  • Reuse Factor: 1/N. A smaller N means more frequent reuse, higher capacity, but requires better Signal-to-Interference Ratio (SIR).

  • Cluster Size N: Must satisfy N = i² + ij + j², where i, j are non-negative integers (e.g., N=1,3,4,7,9,...). N=7 is common in 1G/2G.

• System Components:

  • Mobile Station (MS): Subscriber's device (phone).

  • Base Station (BS/Cell Site): Fixed transceiver serving one cell. Contains antennas, RF equipment.

  • Mobile Switching Center (MSC): The central controller. Connects calls, manages handoffs, interfaces with PSTN/PLMN.

  • Public Switched Telephone Network (PSTN) / Public Land Mobile Network (PLMN): The external wired/wireless networks.

[!TIP] Exam Focus: Be prepared to define frequency reuse and derive/explain the cluster size formula N = i² + ij + j². Understand why N=7 is a common choice (balance between capacity and interference).

1.2 Cellular System Elements & Architecture

• Detailed Network Elements (GSM-centric view):

  • HLR (Home Location Register): Permanent database storing subscriber details (phone no., services, current VLR address).

  • VLR (Visitor Location Register): Temporary database for subscribers currently in the MSC area. Gets data from HLR.

  • EIR (Equipment Identity Register): Database to track stolen/defective mobile equipment (IMEI).

  • AUC (Authentication Center): Provides authentication and encryption parameters to ensure security.

• Channels:

  • Traffic Channels (TCH): Carry voice or data.

  • Control Channels:

    • Broadcast Control Channel (BCCH): Downlink only; broadcasts system info (cell ID, neighbor cells).

    • Common Control Channel (CCCH): Used for initial call setup (paging, access grant).

    • Dedicated Control Channel (DCCH): e.g., SDCCH (Stand-alone Dedicated Control Channel) for signaling, SMS; SACCH (Slow Associated Control Channel) for link maintenance; FACCH (Fast Associated Control Channel) for urgent signaling (steals voice frame).

• Evolution Context: 1G (Analog, FDMA), 2G (Digital, TDMA/FDMA - GSM), 3G (CDMA, WCDMA), 4G (OFDMA, LTE), 5G (mmWave, massive MIMO, network slicing).

1.3 Frequency Spectrum Utilization & Capacity

• Spectrum Allocation: Regulatory bodies (e.g., TRAI in India, FCC in US) allocate specific frequency bands (e.g., 850 MHz, 900 MHz, 1800 MHz, 2100 MHz) for cellular services.

• Capacity Relation: Total available channels S_total are divided among N cells in a cluster.

  • Channels per cell: S = S_total / N

  • System Capacity (total simultaneous calls) ≈ Number of cells * S.

  • Key Trade-off: Decreasing N (smaller cluster) increases S per cell (higher capacity) but reduces the reuse distance D (distance between co-channel cells), increasing Co-Channel Interference (CCI).

• Trunking Theory (Conceptual): Cellular systems use trunking to share a limited number of channels among many users.

  • Offered Traffic (A): Measured in Erlangs. A = λ * H, where λ = call arrival rate (calls/sec), H = average holding time (sec).

  • Grade of Service (GoS): The probability that a call is blocked (all channels busy). GoS = P_block.

  • Erlang B Formula: P_block = B(E, C) = (E^C / C!) / Σ_{k=0}^{C} (E^k / k!)

    • E = Offered traffic in Erlangs

    • C = Number of channels (circuits)

    • Assumes: Poisson call arrivals, Exponential holding times, No queue.

  • Erlang C Formula: Used when queuing is allowed (e.g., in switches).

[!TIP] Exam Focus: You may be asked to calculate the number of channels per cell given total spectrum, channel bandwidth, and cluster size N. Know the relationship: Capacity ∝ 1/N.

2.0 RADIO WAVE PROPAGATION & PATH LOSS MODELS

2.1 Large-Scale Path Loss

• Free Space Propagation Model: Ideal, line-of-sight (LOS) condition.

  • Friis Transmission Equation:

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

    Where `P_r` = Received power, `P_t` = Transmit power, `G_t, G_r` = Antenna gains, `λ` = wavelength, `d` = distance.

*   **Path Loss (dB)**: `PL(dB) = 10 log_{10} \left( \frac{P_t}{P_r} \right) = 10 log_{10} \left( \frac{(4\pi d)^2}{\lambda^2 G_t G_r} \right)`

*   **Path Loss Exponent `n`**: For free space, `n=2`. In real environments, `n` varies (2-6).

• Log-Distance Path Loss Model:

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

Where `d_0` = reference distance (1m or 1km), `n` = path loss exponent, `X_σ` = log-normal shadowing (zero-mean Gaussian with std dev `σ` in dB).

2.2 Specific Propagation Environments

• Propagation over Water/Flat Open Area:

  • Mechanism: Dominated by specular reflection from the water surface and diffraction. Can cause severe fading due to constructive/destructive interference between direct and reflected waves (similar to two-ray model).

  • Challenges: Very low path loss exponent (n≈2), signals travel very far. Causes co-channel interference over large distances. Requires careful frequency planning.

• Mobile-to-Mobile Propagation (Two-Ray Ground Reflection Model):

  • Scenario: Both Tx and Rx antennas are close to the ground (height h_t, h_r).

  • Model: Considers direct path and a single ground-reflected path.

  • Received Power:

$$P_r(d) \approx P_t G_t G_r \left( \frac{h_t h_r}{d^2} \right)^2 \quad \text{for large } d \gg h_t, h_r$$

*   **Path Loss Exponent**: `n=4` (instead of 2 in free space). This is a critical result.

*   **Cross-over Distance**: `d_c = \frac{4\pi h_t h_r}{\lambda}`. For `d < d_c`, free-space model dominates; for `d > d_c`, two-ray model dominates. 
DiagramCANVAS: two-ray model geometry showing direct and reflected rays, phase difference

• Urban/Suburban/Rural:

  • Urban: High buildings → severe multipath, shadowing, high n (3-5). Dense multipath → small-scale fading.

  • Suburban: Moderate clutter, n≈3-4.

  • Rural: Low clutter, n≈2-3, longer distances possible.

2.3 Small-Scale Fading & Multipath

• Causes: Multipath propagation (signals arrive via different paths with different delays) and Doppler spread (relative motion between MS and BS causes frequency shift).

• Key Parameters:

  • Delay Spread (τ_rm or σ_τ): Time difference between first and last significant multipath component. Causes intersymbol interference (ISI).

  • Coherence Bandwidth (B_c): Frequency range over which channel impulse response is highly correlated. Approx: B_c ≈ 1/(5σ_τ) for flat fading if signal BW < B_c.

  • Doppler Spread (f_d): Range of frequency shifts due to motion. f_d = v/λ (v=velocity).

  • Coherence Time (T_c): Time duration over which channel impulse response is invariant. Approx: T_c ≈ 1/(2f_d).

• Fading Types:

  • Flat Fading: Signal bandwidth W_s << B_c. All frequency components fade equally.

  • Frequency-Selective Fading: W_s > B_c. Different frequency components fade differently → ISI.

  • Slow Fading: Channel changes slowly relative to symbol rate (T_s << T_c).

  • Fast Fading: Channel changes rapidly (T_s > T_c).

[!TIP] Exam Focus: Distinguish clearly between flat/frequency-selective and slow/fast fading using the comparisons W_s vs B_c and T_s vs T_c. Be ready to define all four parameters (delay spread, coherence bandwidth, Doppler spread, coherence time) and state their significance.

3.0 INTERFERENCE MANAGEMENT

3.1 Co-Channel Interference (CCI)

• Definition: Interference caused by using the same frequency channel in two different cells that are too close (co-channel cells). It is the primary limiting factor for capacity in traditional cellular systems (FDMA/TDMA).

• Impact: Reduces SIR (Signal-to-Interference Ratio), leading to poor voice quality, dropped calls, and limits the reuse factor 1/N.

• Desired C/I Ratio: Minimum SIR required for acceptable performance (e.g., 18 dB for analog FM, 10-14 dB for digital).

• SIR Calculation (Omni-directional Antennas):

  Let D = reuse distance (center-to-center distance between co-channel cells). R = cell radius.

  Assume i=1, j=1 for N=3 (worst-case, closest co-channel cells). There are n_0 = 6 co-channel interferers at distance D.

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

Where `n` = path loss exponent.

Rearranging for `D/R`:

$$\left( \frac{D}{R} \right) = \left( 6 \times \frac{1}{(SIR)_{req}} \right)^{1/n}$$

*   **Example**: For `n=4`, `(SIR)_{req}=18 dB` (63.1 linear), `D/R ≈ 4.56`. For `N=7` cluster, `D/R = √(3N) ≈ 4.58`. So `N=7` is just sufficient for `n=4`.

• Co-Channel Interference Reduction Factor Q:

  • Q = D/R is the reuse ratio.

  • For a hexagonal grid with cluster size N: Q = D/R = √(3N).

  • Higher Q means more separation between co-channel cells → lower CCI → better SIR, but lower capacity (since N = Q²/3).

• SIR Analysis with Sectoring: Using 120° or 60° directional antennas reduces the number of dominant interferers (n_0). For 3-sector (120°), n_0=2 (instead of 6). The SIR formula becomes:

$$SIR = \frac{1}{n_0} \left( \frac{D}{R} \right)^n$$

This allows a **smaller `Q` (smaller `N`)** for the same SIR requirement, **increasing capacity**.

[!TIP] Exam Focus: Derive the SIR formula for omni-directional cells (step-by-step). Be able to calculate the minimum D/R or required N given n and (SIR)_{req}. Understand how sectoring improves SIR by reducing n_0.

3.2 Adjacent Channel Interference (ACI)

• Causes:

  1. Imperfect Receiver Filters: Adjacent channel signals "leak" into the desired channel passband.

  2. Near-Far Problem: A nearby mobile on an adjacent channel, even with normal power, can cause interference to a distant mobile on the desired channel because its signal is much stronger at the receiver.

• Mitigation:

  • Increase frequency separation between adjacent channels.

  • Use high-quality base station filters.

  • Implement power control (especially in CDMA).

  • Assign adjacent channels to cells with sufficient physical separation.

3.3 System Design for Interference-Limited Areas

• Design Goal: Serve a predefined geographic area with a specified traffic density and GoS (blocking probability), while meeting SIR requirements.

• Methodology:

  1. Determine cell radius R from coverage requirements (path loss, shadowing, required P_r at cell edge).

  2. Determine cluster size N from interference requirements (SIR formula, path loss exponent n).

  3. Calculate number of cells needed to cover the area.

  4. Calculate total channels needed = (Traffic per cell in Erlangs) / (Erlangs per channel) using Erlang B.

  5. Total channels = N * Channels per cell. This must be ≤ available spectrum.

  6. If constraints conflict, use capacity enhancement techniques (splitting, sectoring) or accept lower GoS.

• Interference Reduction Tools:

  • Cell Splitting: Increases capacity by reducing R (more cells), but N (and thus Q) may need to increase to control CCI.

  • Sectoring: Reduces n_0, allowing smaller N (higher capacity) for same SIR.

  • Zone Concept (Microcell): Uses a single BS with multiple zone controllers/antennas. Reduces MS transmission power, mitigating near-far effect and ACI.

4.0 ANTENNA THEORY IN CELLULAR SYSTEMS

4.1 Cell Site Antenna Parameters

• Antenna Height:

  • BS Height (H_bs): Primary factor determining cell radius R (via radio horizon: d ≈ 4.12(\sqrt{H_bs} + \sqrt{H_ms}) km). Higher H_bs → larger R → fewer cells (lower cost) but increases interference to distant co-channel cells (larger D needed).

  • MS Height (H_ms): Typically ~1.5m. Affects mobile-to-mobile propagation and signal strength at cell edge.

• Radiation Pattern:

  • Omni-directional: 360° horizontal coverage. Simple, but high CCI (n_0=6).

  • Directional (Sector): Focuses power in a specific angle (e.g., 120°, 60°). Reduces CCI (n_0=2 for 120°), increases gain in the sector, but requires more BS sites.

• Gain and Pattern Relationship: Gain is the ratio of radiation intensity in a specific direction to the average intensity. A directional pattern has higher gain in the main lobe compared to an omni antenna. Gain (dBi) = 10 log_{10}(4π / Ω_A), where Ω_A is the beam solid angle.

• Beamwidth: Width of main lobe (typically between half-power points). Narrower beamwidth → higher gain → better interference rejection but requires more precise pointing.

• Front-to-Back Ratio (F/B): Ratio of power in the forward direction to that in the backward direction. High F/B (>20 dB) reduces interference from cells behind.

• Polarization: Linear (vertical/horizontal) or circular. Vertical polarization is standard for cellular to reduce ground reflection losses with vertically polarized mobile antennas.

4.2 Antenna Patterns & Coverage

• Umbrella Pattern Effect:

  • Concept: A specially designed antenna pattern with a high-elevation angle lobe (tilted up) in addition to the standard main lobe.

  • Purpose: To provide extended coverage to distant cells in the first tier (co-channel cells) without increasing interference to second-tier cells. It "shapes" the coverage to be flatter at longer distances.

    DiagramSEARCH: umbrella pattern antenna radiation pattern

  • Application: Used when D/R ratio is small (tight frequency reuse), to prevent the signal from the serving cell from reaching too far and causing CCI.

• Pattern Classification:

  • 3-sector: 120° antennas per cell site. Most common in GSM.

  • 6-sector: 60° antennas. Higher capacity, more BS equipment, more handoffs.

• Antenna Tilting:

  • Mechanical Tilt: Physically tilting the antenna mast.

  • Electrical Tilt: Phase shifters inside the antenna to tilt the pattern electronically (more flexible, remote control).

  • Purpose: Reduce interference to neighboring cells by lowering the antenna's main lobe, and control cell size (downtilt shrinks cell, uptilt expands it).

4.3 Diversity Techniques

• Need: To combat fading (deep fades occur at specific frequencies/locations/times). Diversity provides multiple independent signal copies.

• Types:

  • Space Diversity: Multiple antennas separated by > λ/2 at receiver or transmitter (e.g., dual receive antennas at BS).

  • Frequency Diversity: Transmit same signal on multiple frequencies (e.g., spread spectrum in CDMA).

  • Time Diversity: Transmit same signal at different times (e.g., interleaving in GSM).

  • Polarization Diversity: Use two orthogonal polarizations (vertical/horizontal).

[!TIP] Exam Focus: Explain Umbrella Pattern Effect clearly—it's a frequent short note. Know the impact of antenna height and tilting on coverage and interference. Link sectoring directly to reducing n_0 in CCI calculation.

5.0 CAPACITY ENHANCEMENT TECHNIQUES

5.1 Cell Splitting

• Concept: Subdivide a congested cell into smaller cells (microcells/picocells). This increases the number of cells in a given area, allowing the same cluster pattern to be reused more times → capacity increases.

• Implementation (Splitting Factor k):

  • Original cell radius R. New smaller cell radius R' = R/√k (if area is split into k smaller cells).

  • To maintain the same SIR, the reuse ratio Q = D/R must remain constant.

  • Therefore, new reuse distance D' = Q * R' = D/√k.

  • New cluster size N' must satisfy √(3N') = D'/R' = Q (same as old √(3N)). So, N remains the same.

  • Result: Number of cells increases by factor k. Total channels = (k * old number of cells) * (S_total / N) → Capacity increases by k.

• Impact: Requires more BS sites, more handoffs, more control channels. Can be done gradually (splitting only hot spots).

5.2 Sectoring

• Definition: Replacing an omni-directional antenna at a BS site with several directional antennas (e.g., three 120° antennas), each serving a sector of the cell.

• Use in Reducing CCI:

  • Reduces the number of dominant co-channel interferers (n_0). For 3-sector (120°), n_0=2 instead of 6.

  • From SIR formula: SIR ∝ 1/n_0. So SIR improves by factor 6/2=3 (or 4.8 dB).

  • This allows using a smaller cluster size N (e.g., from N=7 to N=4 or 3) for the same SIR requirement.

  • Capacity Gain: Channels per cell remain same, but N decreases → channels per cell increase. Capacity gain ≈ 7/3 ≈ 2.33x.

• Trade-offs:

  • Increased number of BS transceivers (more cost).

  • Increased number of handoffs (more signaling load on MSC).

  • Requires careful frequency planning within the sectorized cell.

5.3 Microcells & Picocells

• Definition:

  • Macrocell: Traditional cell, radius 1-30 km, BS on tall tower/mast.

  • Microcell: Radius 0.1-1 km, BS on lamp-post/building side, below rooftop level.

  • Picocell: Radius < 100 m, indoor/office coverage, very low power.

• Application: High-capacity dense areas (city centers, malls, airports, stadiums).

• Propagation Differences:

  • Lower BS height → less dominant LOS, more street-level propagation, higher path loss exponent (n can be 3-5 in streets).

  • Shadowing from buildings is more pronounced.

  • Near-far problem is severe because MS powers are similar, but distances vary greatly.

  • Requires tight frequency reuse (N=1,3), sophisticated power control, and often sectoring is not used (omni antennas common on lamp-posts).

6.0 CHANNEL ASSIGNMENT & TRAFFIC MANAGEMENT

6.1 Channel Assignment Strategies

• Fixed Channel Assignment (FCA):

  • Algorithm: A pool of S channels is permanently assigned to each cell. A call in cell i can only use one of its pre-assigned channels.

  • Blocking: If all S channels in cell i are busy, the call is blocked (no queue).

  • Advantage: Simple, no signaling for channel assignment.

  • Disadvantage: Inefficient; channels in a lightly loaded cell cannot be used by a heavily loaded neighbor.

  • Blocking Probability: Calculated using Erlang B formula per cell, assuming traffic is uniformly distributed.

• Dynamic Channel Assignment (DCA):

  • Algorithm: All channels are in a central pool. When a call arrives in cell i, the MSC dynamically assigns any available channel that meets SIR constraints (i.e., not used by co-channel or adjacent channel cells within interference range).

  • Advantage: More efficient, adapts to traffic variations, lower blocking for same total channels.

  • Disadvantage: Complex MSC processing, requires real-time interference monitoring, more signaling (setup delay).

• Borrowing Strategies (Hybrid):

  • Borrowing with Channel Locking: A cell can borrow a channel from a neighbor if all its own are busy, but the borrowed channel is locked for the duration of the call and cannot be used by its owner.

  • Borrowing with Channel Yield: A cell can lend a channel to a neighbor in need, but can reclaim it if a high-priority call arrives.

6.2 Traffic Engineering & Blocking

• Offered Traffic (A): A = λ * H (Erlangs). λ = call arrival rate per cell, H = average call duration.

• Grade of Service (GoS): Probability of call blocking P_b. Typical target: P_b = 0.01 to 0.02 (1-2%).

• Call Blocking Probability (Erlang B):

$$P_b = B(A, C) = \frac{\frac{A^C}{C!}}{\sum_{k=0}^{C} \frac{A^k}{k!}}$$

Where `C` = number of channels in the cell.

*   **Application**: Given `A` per cell and desired `P_b`, find required `C`. Or, given `C` and `A`, find `P_b`.

• Dropped Call Rate (HANDOFF FAILURE RATE):

  • Definition: Probability that a call in progress fails during a handoff attempt.

  • Derivation:

    Let λ_h = handoff arrival rate (per cell), μ_h = handoff service rate (successful handoffs/sec). Assume handoff attempts have higher priority than new call attempts (guard channels).

    Dropped Call Rate P_d ≈ Probability that no channel is available for handoff.

    If C_g guard channels are reserved for handoffs:

$$P_d = B(A_h, C_g)$$

    Where `A_h = λ_h / μ_h` is the offered handoff traffic in Erlangs.

*   **Factors Affecting `P_d`**: Cell size `R` (affects `λ_h`), velocity of users (affects handoff rate), number of guard channels `C_g`.

6.3 Handoff (Handover)

• Classification based on Nature:

  • Hard Handoff (Break-before-Make): Used in GSM (TDMA/FDMA). MS releases connection to old BS before connecting to new BS. momentary break in service.

  • Soft Handoff (Make-before-Break): Used in CDMA. MS maintains connection with old BS while establishing with new BS. Macro-diversity (simultaneous connection to multiple BSs) reduces fade probability.

  • Softer Handoff: A special case of soft handoff where the multiple BSs are under the same MSC/BSC (same cell site with multiple sectors).

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

  • Inter-cell Handoff: Changing cell (due to mobility).

  • Mobile-Assisted Handoff (MAHO): MS measures signal strength of neighbor BSs and reports to network. Used in GSM (NCMA).

  • Network-Controlled Handoff (NCHO): Network (BS/MSC) makes measurements and decision. Used in older analog systems.

• Classification based on Type:

  • Cell Site Handoff: Between cells controlled by the same MSC.

  • Intersystem Handoff: Between cells controlled by different MSCs or even different systems (e.g., GSM to CDMA, 4G to 5G).

• Handoff Procedure & Parameters (GSM NCMA Example):

  1. MS continuously measures BCCH signal levels of serving and neighbor cells (using MAHO).

  2. MS sends Measurement Reports to serving BS on SACCH.

  3. BS evaluates reports. Handoff decision based on:

     • Threshold (TH) : Minimum signal level for serving cell to consider handoff.

     • Hysteresis (HYS) : Prevents "ping-pong" effect. New cell's signal must be HYS dB stronger than serving cell.

     • Time Delay (T) : Condition must be met for T consecutive measurement periods (e.g., 5-10 reports).

  4. If criteria met, BS sends Handoff Request to MSC.

  5. MSC assigns a new traffic channel in target cell, sends Handoff Command to MS via serving BS.

  6. MS switches to new channel, sends Handoff Complete.

• Handoff Rate & Dropped Call Probability:

  • Handoff Rate per Cell (λ_h): λ_h ≈ (v / (πR)) * A (approximation for uniform traffic). v = average velocity, R = cell radius, A = offered traffic.

  • Dropped Call Probability P_d: As derived in 6.2, depends on λ_h and available guard channels.

[!TIP] Exam Focus: Distinguish Hard vs. Soft handoff (GSM vs. CDMA) with reasons. Explain GSM's NCMA procedure step-by-step with parameters (Threshold, Hysteresis, Time Delay). Derive/explain Dropped Call Rate formula linking λ_h, R, v.

7.0 MULTIPLE ACCESS & SYSTEM ARCHITECTURES

7.1 GSM Architecture & Procedures

• Functional Architecture:

  • BSS (Base Station Subsystem): BTS (Base Transceiver Station) + BSC (Base Station Controller). BSC manages radio resources, handoffs, frequency hopping.

  • NSS (Network Switching Subsystem): MSC + HLR + VLR + AUC + EIR. Core network for switching, mobility management, authentication.

  • OSS (Operation Support Subsystem): For network management.

• GSM Channels (Key Ones):

  • TCH (Traffic Channel): Full-rate (TCH/F), Half-rate (TCH/H).

  • BCCH (Downlink): Broadcast system info.

  • CCCH (Bidirectional): Paging, access grant (PCH, AGCH, RACH).

  • SDCCH (Bidirectional): Call setup, SMS, location update.

  • SACCH (Bidirectional): Slow control (power control, time alignment), associated with TCH or SDCCH.

  • FACCH (Bidirectional): Fast control (handoff commands), steals voice frames.

• Call Processing Flow (Mobile Originated):

  1. Access: MS sends Channel Request on RACH.

  2. Immediate Assignment: BSC assigns SDCCH via AGCH.

  3. Signaling on SDCCH: Authentication, ciphering start, TMSI assignment.

  4. Assignment: BSC assigns a TCH (and SACCH) via Assignment Command on SDCCH.

  5. Conversation: MS switches to TCH.

• Handoff in GSM (NCMA):

  • As detailed in 6.3. It is a hard handoff. MSC coordinates. BSC can make "internal" handoff decisions (between BTSs under same BSC).

7.2 CDMA System Principles

• Direct Sequence Spread Spectrum (DS-CDMA):

  • Each user's data is multiplied by a unique high-rate pseudo-random (PN) sequence (chip sequence). Spreading factor SF = chip rate / data rate.

  • All users transmit simultaneously in the same frequency band.

  • Receiver uses correlation with the correct PN code to despread desired signal; other users appear as wideband noise.

• Call Processing & Handoff in CDMA:

  • Soft Handoff: Fundamental feature. MS can be in active set (connected to multiple BSs simultaneously). MSC combines signals (selection diversity).

  • Procedure:

    1. MS continuously measures Pilot PN offsets of neighbor BSs (from Neighbor List in BCCH).

    2. MS sends Pilot Strength Measurement message to serving BS when a neighbor's pilot exceeds Add Threshold.

    3. MSC adds that BS to MS's active set, assigns a new forward traffic channel from that BS.

    4. MS now receives from multiple BSs (macrodiversity).

    5. When a BS's pilot drops below Drop Threshold for T_drop time, it's removed from active set.

  • Softer Handoff: If multiple sectors of the same cell site are in active set, it's softer handoff (combining at BSC).

• Power Control (Critical in CDMA):

  • Why? Near-far problem: a close MS can drown out a distant MS on same frequency. All MSs must arrive at BS with similar power.

  • Open Loop: MS estimates path loss from received BS power and sets its initial transmit power.

  • Closed Loop (Fast Power Control): BS measures SIR of each MS. Sends power control bits (up/down) on the reverse channel every 1.25 ms (800 bps). MS adjusts power by 1 dB step. Maintains SIR at target.

7.3 Other Multiple Access Techniques

• FDMA (Frequency Division Multiple Access): Each user gets a dedicated frequency channel (e.g., 1G AMPS, satellite).

• TDMA (Time Division Multiple Access): Users share a frequency but transmit in different time slots (e.g., GSM, DECT).

• SDMA (Space Division Multiple Access): Uses spatial separation via directional/smart antennas to reuse frequencies in the same cell.

• TDD (Time Division Duplexing):

  • Concept: Uplink and downlink share the same frequency band but are separated in time (different time slots).

  • Application: Used in systems where uplink/downlink traffic is asymmetric (e.g., internet browsing: more downlink). Allows flexible spectrum allocation. Used in DECT, TD-SCDMA, Wi-Fi (802.11), 5G NR (in some bands).

  • Advantage: No need for paired spectrum, flexible DL/UL ratio.

  • Disadvantage: Requires fast switching, guard times, and can have hidden node problem.

[!TIP] Exam Focus: Contrast GSM (TDMA/FDMA) and CDMA (DS-CDMA). Explain CDMA's soft handoff and why power control is essential. Define TDD and state one advantage and one disadvantage.

8.0 SPECIAL TOPICS & SHORT NOTE SYLLABUS

8.1 Umbrella Pattern Effect

• Concept: An antenna radiation pattern designed with a secondary high-elevation angle lobe in addition to the main lobe.

• Purpose: To extend coverage to cells in the first tier (co-channel cells) without significantly increasing interference to second-tier cells. It flattens the coverage contour at longer distances.

• Use Case: When frequency reuse ratio D/R is small (tight reuse, high capacity), the signal from a serving cell can reach too far and interfere with co-channel cells. The umbrella pattern controls this long-distance radiation.

8.2 Mobile Point-to-Point Model (Two-Ray Ground Reflection)

• Model: Considers direct path and single ground-reflected path between Tx and Rx antennas at heights h_t, h_r.

• Path Difference: δ = √(d² + (h_t - h_r)²) - √(d² + (h_t + h_r)²) ≈ -2h_t h_r / d for d >> h_t, h_r.

• Phase Difference: Δφ = (2π/λ) * δ.

• Received Power:

$$P_r(d) = P_t G_t G_r \left( \frac{\lambda}{4\pi d} \right)^2 |1 + \Gamma e^{jΔφ}|^2$$

Where `Γ` = ground reflection coefficient (≈ -1 for vertical polarization, perfect conductor).

• For large d: |1 + \Gamma e^{jΔφ}|^2 ≈ (4π h_t h_r / λ d)^2.

$$P_r(d) \propto \frac{1}{d^4}$$

**Path loss exponent `n=4`**.

• Cross-over Distance d_c = 4π h_t h_r / λ. For d > d_c, two-ray model dominates over free-space (n=2).

8.3 UHF-TV Interference

• Sources:

  1. TV Broadcast Stations: Operate in UHF band (470-890 MHz), overlapping with early cellular bands (e.g., 800 MHz band).

  2. Intermodulation: Non-linearities in BS amplifiers can mix strong TV signals with cellular carriers, producing interference within cellular band.

  3. TV Receivers: Poorly shielded TV sets can radiate interference.

• Impact: Causes continuous wideband noise or intermittent spurious signals in cellular receivers, degrading sensitivity and capacity.

• Mitigation: Use high-quality filters at BS (pre-selectors), shielded equipment, proper site selection (away from TV towers), and use of higher frequency bands (e.g., 1800 MHz, 2100 MHz) away from TV spectrum.

8.4 Non-Cellular Systems

• Paging Systems:

  • One-way (to pager) or two-way.

  • Architecture: Paging Terminal → Transmitters (simulcast or selective). Simple, low-cost, long battery life.

  • Example: POCSAG, FLEX.

• Cordless Telephony (e.g., DECT):

  • Short-range (indoors/office), connects portable handset to fixed base station connected to PSTN.

  • DECT (Digital Enhanced Cordless Telecommunications): Uses TDMA/TDD. 10 carriers, each with 12 time slots (6 for uplink, 6 for downlink). Very high capacity in small area. No handoff between base stations (roaming limited).

8.5 Cell Site Antenna Heights

• Impact on Coverage:

  • BS Height (H_bs): Increases radio horizon d ≈ 4.12(√H_bs + √H_ms) km → larger cell radius R.

  • MS Height (H_ms): Minor effect on coverage from BS, but significant for mobile-to-mobile links.

• Impact on Interference:

  • Higher H_bs → signal propagates farther → co-channel cells at distance D receive stronger interference → worse C/I.

  • Requires larger reuse distance D (larger N) to maintain SIR → reduces capacity.

• System Planning Trade-off: High H_bs → fewer BS sites (lower CAPEX) but lower capacity. Lower H_bs → more sites (higher CAPEX) but higher capacity (smaller R, smaller N possible). Sectoring can offset the capacity loss from higher H_bs.

8.6 Frequency Reuse & Co-Channel Interference (Deep Dive)

• Planning Parameters:

  • N (Cluster Size): i² + ij + j². Determines how many cells share the total frequency set.

  • D/R (Reuse Ratio): √(3N). Key parameter linking geometry to interference.

  • Q = D/R: Interference reduction factor.

• Design Equation:

$$\left( \frac{D}{R} \right) = Q = \left( n_0 \times \frac{1}{(SIR)_{req}} \right)^{1/n}$$

Where `n_0` = number of co-channel interferers (6 for omni, 2 for 120° sector).

• Steps to Determine N:

  1. From coverage: find R (max cell radius from path loss, fade margin, P_r_min).

  2. From interference: find required Q from SIR formula (given n, (SIR)_{req}, n_0).

  3. Compute N = Q² / 3. Choose smallest integer N from the sequence 1,3,4,7,... that is ≥ Q²/3.

  4. Check total channels: S = S_total / N must meet traffic demand (Erlang B).

8.7 Power Control

• Role:

  1. Reduce Interference: By lowering transmit power of mobiles that are close to BS, interference to other cells/users is reduced.

  2. Increase Capacity: Less interference → better SIR → can use smaller N (higher frequency reuse) or more users per channel.

  3. Compensate for Fading: Rapid power adjustments (closed-loop) combat fast fading.

  4. Reduce Battery Drain: MS transmits only as much power as needed.

• Types:

  • Open Loop: MS estimates path loss from received BS power and sets P_tx = P_rx_bs * (d/d_0)^n. Fast but inaccurate due to fading/asymmetry.

  • Closed Loop: BS measures SIR/ received power, sends power control commands to MS. Slower (feedback delay) but accurate. Essential in CDMA.

  • Downlink Power Control: In CDMA, BS adjusts power to each MS to equalize received power at MS (reduces near-far on downlink).

8.8 Layer Modelling (OSI/Protocol Stack in Cellular)

• Physical Layer (L1): Modulation (GMSK in GSM, QPSK in CDMA), Coding (convolutional, turbo), Spreading (CDMA), Frequency bands, RF.

• Data Link Layer (L2):

  • MAC (Medium Access Control): Controls channel access (e.g., GSM's TDMA frame structure, slotted ALOHA on RACH).

  • LLC (Logical Link Control): Error correction (ARQ), framing (e.g., GSM's LAPDm on SDCCH).

• Network Layer (L3): Mobility management (location update, handoff), call control (call setup, release), routing (MSC).

• Transport Layer (L4): Not heavily used in traditional cellular voice (circuit-switched). Used in GPRS/UMTS for TCP/UDP.

• Application Layer (L7): SMS, supplementary services (call waiting, conferencing).

[!TIP] Final Exam Strategy: For short notes (7 marks), structure as: 1. Definition/Concept, 2. Key Principle/Mechanism, 3. Purpose/Advantage, 4. Example/Application. For long answers (14 marks), include diagrams (e.g., SIR geometry, handoff flowchart, protocol stack), derivations (SIR, two-ray, Erlang B statement), and numerical examples where applicable. Always link theory to system performance (capacity, coverage, interference).