UNIT 1: Cellular Mobile Communication - Exam-Focused Study Notes

1.0 Fundamental Concepts & System Architecture

1.1 Basic Principle of Operation of Cellular Systems

• Core Idea: Replace a single, high-power transmitter (large cell) with many low-power transmitters (small cells) to enable frequency reuse, dramatically increasing system capacity.

• Cellular Layout: Idealized as a hexagonal grid (closest packing, uniform distance to neighbors). Each hexagon is a cell served by a Base Station (BS) at its center.

• Key Components:

  • Mobile Station (MS): User device (phone) with transceiver and antenna.

  • Base Station (BS/Node B): Fixed transceiver in a cell, connects MS to the network.

  • Mobile Switching Center (MSC): Central switch; manages call setup, handoffs, and connects to PSTN/PDN.

  • Public Switched Telephone Network (PSTN) / Public Data Network (PDN): External networks.

• Rationale for Cellular Structure:

  1. Capacity Expansion: Frequency reuse allows same frequencies in non-adjacent cells.

  2. Coverage: Low-power BS provides reliable signal within its cell.

  3. Mobility: MSC tracks MS location and manages handoffs between cells.

1.2 Elements Establishing a Cellular Radio System

• HLR (Home Location Register): Permanent subscriber database.

• VLR (Visitor Location Register): Temporary subscriber data for MS in current MSC area.

• AuC (Authentication Center): Generates authentication parameters.

• EIR (Equipment Identity Register): Tracks stolen/faulty MS by IMEI.

1.3 Introduction to Cell Coverage: Signal & Traffic Conditions

• Signal Coverage: Determined by link budget (transmit power, gains, losses, receiver sensitivity). Cell radius R is the distance where received power equals receiver sensitivity.

• Traffic Coverage: Measured in Erlangs (offered load = call arrival rate × average call duration).

  • Grade of Service (GoS): Probability a call is blocked (Pb) due to all channels busy. Typical target: Pb ≤ 2%.

  • Erlang B Formula: Pb = (A^N / N!) / (Σ_{k=0}^{N} (A^k / k!)) where A = traffic intensity (Erlangs), N = number of channels.

• Trade-off: Larger R → fewer cells → lower infrastructure cost but lower capacity. Smaller R → more cells → higher capacity but higher cost. Optimal design balances both.

2.0 Mobile Radio Propagation & Path Loss Models

2.1 Free Space Propagation & Two-Ray Ground Reflection Model

• Friis Transmission Equation (Free Space):

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

where $$\displaystyle P_r $$ = received power, $$\displaystyle P_t $$ = transmitted power, $$\displaystyle G_t $$, $$\displaystyle G_r $$ = antenna gains, $\lambda$ = wavelength, $d$ = distance.

*   **Path Loss (dB):** $$\displaystyle L_{fs} = 32.45 + 20\log_{10}(f_{MHz}) + 20\log_{10}(d_{km}) $$

• Two-Ray Ground Reflection Model: Considers direct path + ground-reflected path.

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

*   **Critical Distance / Breakpoint ($$\displaystyle d_c $$):** Distance where two-ray model transitions from $$\displaystyle d^{-2} $$ to $$\displaystyle d^{-4} $$ path loss.

$$d_c = \frac{4\pi h_t h_r}{\lambda}$$

    where $$\displaystyle h_t $$, $$\displaystyle h_r $$ are antenna heights. For $$\displaystyle d << d_c $$, free-space dominates; for $$\displaystyle d >> d_c $$, two-ray dominates.

*   **Limitation:** Assumes specular reflection and flat earth; inaccurate for dense urban areas.

2.2 Propagation in Specific Environments

• Propagation over Water:

  • Mechanism: Smooth water surface causes specular reflection and ducting (atmospheric layer trapping signals), leading to reduced attenuation and long-distance propagation.

  • Challenges: Signals from distant cells cause co-channel interference; signal strength fluctuates due to changing water surface and atmospheric conditions.

• Mobile-to-Mobile Propagation:

  • Model: Two-ray model where both antennas are at mobile heights ($$\displaystyle H_m $$). Path loss exponent often steeper than BS-to-MS due to lower antennas and more obstructions.

  • Implications: Critical for Vehicle-to-Vehicle (V2V) and ad-hoc networks; shorter communication range, higher path loss.

• Propagation over Flat Open Area:

  • Characteristics: Minimal obstructions; dominant path is often the two-ray model with a higher path loss exponent ($n \approx 3-4$) due to diffraction over terrain.

  • Challenges: Long-distance co-channel interference possible due to good propagation; fading due to slight terrain variations.

2.3 Mobile Point-to-Point Model

• Conceptual model for direct communication between two mobile stations (MS-MS).

• Both antennas are low ($$\displaystyle H_m \sim 1.5m $$), resulting in a very small breakpoint distance ($$\displaystyle d_c $$).

• Practical Result: Two-ray $$\displaystyle d^{-4} $$ path loss regime applies almost immediately, leading to rapid signal decay with distance. This limits direct MS-MS range.

2.4 Log-Distance Path Loss Model & Shadowing

• Log-Distance Model (General):

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

where $$\displaystyle d_0 $$ = reference distance (e.g., 1m or 1km), $n$ = **path loss exponent** (environment-dependent: 2=free space, 3-6 urban, 4-6 indoor).

• Shadowing (Log-Normal Fading):

  • Random variation due to large obstacles (buildings, hills).

  • Modeled as a zero-mean Gaussian random variable in dB:

$$P_L(d) = \overline{P_L}(d) + X_\sigma$$

    where $$\displaystyle \overline{P_L}(d) $$ is the mean path loss from log-distance model, $$\displaystyle X_\sigma \sim \mathcal{N}(0, \sigma^2) $$.

*   $\sigma$ (standard deviation) is the **shadowing deviation** (typically 6-12 dB).

3.0 Antenna Theory for Cellular Systems

3.1 Cell Site Antenna Heights & Signal Coverage

• Base Station Antenna Height ($$\displaystyle H_b $$):

  • ↑ $$\displaystyle H_b $$ → ↑ cell radius (R) (horizon distance increases).

  • ↑ $$\displaystyle H_b $$ → ↑ interference to distant co-channel cells (larger footprint).

• Mobile Antenna Height ($$\displaystyle H_m $$):

  • ↑ $$\displaystyle H_m $$ → ↑ signal strength at cell edge, ↑ R.

  • Typically fixed (~1.5m).

• Horizon Distance (Approx.): $$\displaystyle d_{horizon} \approx 4.12 \left( \sqrt{H_b} + \sqrt{H_m} \right) $$ (with heights in meters, distance in km).

  • Sets an upper limit on cell radius due to line-of-sight.

3.2 Antenna Parameters: Gain, Pattern, Beamwidth, Directivity

3.3 Gain and Pattern Relationship

• For a rectangular (sectoral) pattern with half-power beamwidths $$\displaystyle \theta_1 $$ (horizontal) and $$\displaystyle \theta_2 $$ (vertical):

$$G \approx \frac{41250}{\theta_1 \cdot \theta_2}$$

where $$\displaystyle \theta_1, \theta_2 $$ are in degrees.

• Example: A 65° horizontal × 10° vertical sector antenna: $G \approx 41250/(65*10) \approx 63.5$ (18 dBi).

• Classification:

  • Omni: ~360° H-plane, wide V-plane.

  • Sectoral: 60°, 90°, 120° horizontal beamwidths (common: 65° for 120° sector).

  • Highly Directional: Parabolic dishes, very narrow beamwidth, high gain.

3.4 Umbrella Pattern Effect

• Concept: Use a high-gain, narrow-beam "umbrella" antenna at the BS to cover the cell's primary service area (near cells) while minimizing radiation toward co-channel cells in the same cluster (which are typically at larger distances and higher angles).

• Mechanism: The antenna pattern has a downtilt and a sharp roll-off beyond the main lobe.

• Benefit: Reduces the average co-channel interference (C) received from other clusters, improving the C/I ratio without reducing reuse distance D. Allows use of smaller cluster size N → higher capacity.

4.0 Interference & Carrier-to-Interference Ratio (C/I)

4.1 Types of Interference

4.2 Desired C/I in Omni-directional Antenna System

• Assumptions: Hexagonal grid, first-tier interferers only (6 cells), omni-directional antennas, path loss exponent $n$.

• Signal Power (C): From serving BS at distance R.

$$C \propto \frac{1}{R^n}$$

• Interference Power (I): Sum from 6 first-tier co-channel BSs, each at distance D.

$$I \propto 6 \cdot \frac{1}{D^n}$$

• Average C/I Ratio:

$$\left( \frac{C}{I} \right)_{avg} = \frac{D^n}{6 R^n} = \frac{1}{6} \left( \frac{D}{R} \right)^n$$

• Required Threshold: For acceptable analog FM voice quality, 18 dB is typical.

• Design Implication: To meet C/I ≥ 18 dB with $$\displaystyle n=4 $$, need $$\displaystyle (D/R)^4 \geq 6 \times 10^{1.8} \approx 1209 $$ → $D/R \geq 5.6$.

4.3 Co-channel Interference Reduction Factor (q)

• Definition: $$\displaystyle q = \frac{D}{R} $$ (reuse distance to cell radius ratio).

• Role in Cluster Size (N): For a square grid of cells, $$\displaystyle N = q^2 $$.

  For a hexagonal grid, $$\displaystyle N = q^2 $$ (approx.) or exact: $$\displaystyle N = i^2 + ij + j^2 $$, where $$\displaystyle q = \sqrt{3N} $$ for $$\displaystyle i=j $$.

• Example: For $$\displaystyle q=5.6 $$, $N \approx 31$ (square) or $$\displaystyle N=19 $$ (hexagonal). Larger $q$ → better C/I but lower capacity (smaller 1/N).

4.4 Effects of Power Control & Cell Site Power Decrease

• Power Decrease (↓ $$\displaystyle P_t $$):

  • Coverage: Reduces cell radius R (from link budget: $$\displaystyle R \propto \sqrt[4]{P_t} $$ for two-ray).

  • Interference: ↓ Transmit power → ↓ interference to all neighboring cells (improves their C/I).

  • Trade-off: Smaller cells require more BS sites for same area → higher cost, but increases total system capacity via more cells.

5.0 Frequency Reuse, System Design & Capacity Expansion

5.1 Frequency Reuse Concept & Cluster Size (N)

• Reuse Factor: $1/N$. Each cell in a cluster of N cells uses a unique set of m channels. Total channels: $$\displaystyle S = N \times m $$.

• Cluster Layout: Hexagonal pattern. Common sizes: N=4, 7, 12, 19, 21, 28...

• Reuse Distance (D): $$\displaystyle D = R \sqrt{3N} $$ (for hexagonal grid, distance between co-channel cell centers).

5.2 Designing a System for a Predefined Area with CCI

Step-by-Step Procedure:

1. Traffic Estimate: Calculate total Erlangs (A_total) from user density and call parameters.

2. Determine Channels per Cell (m): From Erlang B table, find channels/cell needed for target Pb (e.g., 2%).

3. Select Cluster Size (N): Based on required C/I (from 4.2). Calculate minimum $$\displaystyle q = (6 \cdot 10^{C/I_{dB}/10})^{1/n} $$, then $$\displaystyle N \approx q^2 $$.

4. Calculate Cell Radius (R): From link budget (path loss model, $$\displaystyle P_t $$, $G$, $$\displaystyle L_{sys} $$, $$\displaystyle S_{min} $$).

5. Layout Frequency Plan: Assign channel groups (1 to N) to cells in a repeating cluster pattern.

6. Check Interference Margins: Ensure C/I meets requirement at cell edge considering shadowing (add interference margin, e.g., 3-5 dB).

5.3 Capacity Expansion Techniques

6.0 Traffic Management: Channel Assignment & Handoff

6.1 Channel Assignment Algorithms

6.2 Call Blocking & Dropped Call Rate

• Call Blocking (Pb): Probability a call is blocked in the originating cell due to no free channels. Modeled by Erlang B formula (loss system, no queue).

• Dropped Call Rate (DCR) / Handoff Failure Probability:

  • Definition: Probability a call in progress is dropped during a handoff attempt.

  • Derivation (Simplified): If $$\displaystyle P_h $$ = probability a call needs handoff, $$\displaystyle P_{bh} $$ = blocking probability during handoff (often higher than initial blocking due to priority schemes), then:

$$DCR \approx P_h \cdot P_{bh}$$

*   **Factors:** Handoff delay, hysteresis/threshold settings, channel availability in target cell.

*   **Design Goal:** DCR << Pb (e.g., DCR < 1%, Pb = 2%).

6.3 Handoff (Handover)

• Classification based on Nature:

  | Type | Process | Used In | Key Feature | | :--- | :--- | :--- | :--- | | Hard Handoff | Break-before-make. Connection to old BS terminated before connecting to new BS. | GSM, CDMA (typical) | Simple, but risk of momentary disconnect. | | Soft Handoff | Make-before-break. MS connects to multiple BSs simultaneously before breaking old link. | CDMA | Seamless, better reliability, uses active set (connected BSs). | | Softer Handoff | Soft handoff between sectors of the same BS (same BSC). | CDMA | More efficient than soft handoff (same BSC control). |

• Cell Site Handoff vs. Intersystem Handoff:

  • Cell Site (Intra-MSC): Between cells controlled by the same MSC. Simpler, MSC coordinates.

  • Intersystem (Inter-MSC/Inter-PLMN): Between cells under different MSCs/PLMNs. Requires inter-MSC signaling, more complex, may involve PSTN/PDN.

7.0 Specific System Architectures: GSM & CDMA

7.1 GSM System Architecture & Working

• Functional Entities:

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

  • BTS (Base Transceiver Station): Radio transceivers for a cell.

  • BSC (Base Station Controller): Controls multiple BTSs; manages radio resources, handoffs.

  • MSC (Mobile Switching Center): Core switch; call routing, mobility management.

  • HLR (Home Location Register), VLR (Visitor Location Register): Subscriber databases.

  • AuC (Authentication Center), EIR (Equipment Identity Register): Security.

• GSM Channels (TDMA/FDMA):

  | Channel Type | Logical Channels | Function | | :--- | :--- | :--- | | Traffic Channels (TCH) | TCH/F (full-rate), TCH/H (half-rate) | Speech/data user traffic. | | Control Channels (CCH) | BCCH (Broadcast), CCCH (Common Control: RACH, AGCH, PCH), DCCH (Dedicated Control: SDCCH, SACCH, FACCH) | Signaling, synchronization, paging, SMS, handoff. |

• Frame & Slot Structure: 200 kHz carrier, 8 time slots (TDMA), 26/51 multiframe for traffic/control.

7.2 CDMA System: Call Processing & Handoff

• Call Processing:

  1. Access: MS acquires pilot channel, syncs to system time.

  2. Paging: MSC pages MS on paging channel.

  3. PN Code Assignment: MSC assigns unique PN offset for forward traffic channel to MS.

  4. Power Control: Open-loop (MS estimates path loss from pilot) & Closed-loop (BS commands MS to adjust power in 1 dB steps, 800 bps).

• Handoff in CDMA (Soft/Softer):

  1. Measurement: MS measures pilot signal strengths of neighbor/active/candidate sets.

  2. Decision: MS or BSC (depending on mode) decides to add/drop pilots based on T_ADD and T_DROP thresholds.

  3. Execution: BSC instructs MSC to establish new traffic channel with new BS. MS combines signals from active set (max 6) using RAKE receiver.

  4. Active Set: Pilots currently assigned to the MS's traffic channel.

  5. Candidate Set: Pilots strong enough to consider adding (≥ T_ADD).

  6. Neighbor Set: Pilots on same frequency, neighboring cells.

8.0 Advanced Topics & Short Note Syllabus Integration

8.1 Frequency Spectrum Utilization

• Efficiency Metric: Spectral Efficiency = $$\displaystyle \frac{\text{Number of users} \times \text{bits per user}}{\text{Bandwidth (Hz)} \times \text{cell area}} $$ (bits/sec/Hz/cell).

• Improvement Techniques:

  • Modulation: Higher order (e.g., 8PSK, 16QAM) → more bits/symbol.

  • Multiple Access: FDMA, TDMA, CDMA → share spectrum among users.

  • Trunking Efficiency: Depends on offered load and number of channels per cell (Erlang B). Smaller cells (more cells) → better trunking efficiency.

8.2 TDD Systems (Time Division Duplexing)

• Principle: Uplink and downlink use same frequency band but separate time slots.

• Advantages:

  • Flexible asymmetric traffic (more slots for DL if needed).

  • No duplexer needed in MS (cheaper).

  • Can use same RF hardware for both directions.

• Disadvantages:

  • Requires tight synchronization between BS and MS.

  • Large propagation delay limits cell radius.

  • Potential for uplink/downlink interference if guard time insufficient.

• Examples: DECT, TD-SCDMA, Wi-Fi, LTE-TDD.

8.3 UHF-TV Interference

• Source: TV broadcast channels in UHF band (470-890 MHz, channels 14-83). Early cellular (e.g., AMPS 800 MHz) was adjacent.

• Impact: Strong TV signals can desensitize cellular receivers (blocker problem), causing adjacent channel interference.

• Mitigation:

  • Guard Bands: Unused spectrum between TV and cellular bands.

  • High-Q Filters: In BS and MS receivers to reject TV channels.

  • Channel Planning: Avoid cellular channels immediately adjacent to strong TV stations.

8.4 GSM Channels (Short Note)

• Physical Channels: Defined by frequency and time slot number.

• Logical Channels (Mapped to Physical):

  • Broadcast Control Channel (BCCH): Downlink only. System info (cell ID, frequencies).

  • Common Control Channel (CCCH): Bidirectional. RACH (MS→BS, random access), AGCH (BS→MS, access grant), PCH (BS→MS, paging).

  • Dedicated Control Channel (DCCH): Point-to-point. SDCCH (stand-alone, for call setup/SMS), SACCH (slow associated control, e.g., measurement reports), FACCH (fast associated control, steals TCH frames for handoff/urgent messages).

  • Traffic Channel (TCH): User data (speech/data).

8.5 Non-Cellular Systems (Short Note)

8.6 Layer Modelling with Practical Applications

• OSI/TCP-IP Model in Cellular (e.g., GSM/GPRS/UMTS):

  • Physical Layer: Modulation (GMSK, QPSK), coding, TDMA/CDMA, RF transmission.

  • Data Link Layer: LAPDm (GSM), RLC/MAC (UMTS). Handles framing, ARQ, access control.

  • Network Layer: IP (for GPRS/UMTS), GSM MAP (mobility management). Routing, addressing.

  • Transport Layer: TCP (reliable data), UDP (voice/video).

  • Application Layer: SMS, WAP, HTTP, custom apps.

• Practical App: A web browse on a 3G phone: Application (HTTP) → Transport (TCP) → Network (IP over GPRS) → Data Link (RLC/MAC) → Physical (QPSK on UMTS).

[!TIP] Exam Strategy

• Definitions are Key: Always start answers with clear definitions (e.g., "Frequency reuse is...").

• Derive the C/I Formula: This is a guaranteed 7-mark question. Practice the derivation step-by-step.

• Compare & Contrast: For questions like "Design a system" or "Effects of power control," use bullet points or tables to show trade-offs.

• Draw Diagrams: For GSM architecture, handoff types, antenna patterns – a simple labeled sketch can earn 2-3 extra marks.

• Units: Always include units (dB, km, MHz, Erlangs).

• Common Pitfall: Confusing path loss exponent (n) with co-channel reuse ratio (q). Remember: $$\displaystyle C/I \propto (D/R)^n = q^n $$.

• Short Notes: For "UHF-TV Interference" or "TDD," structure as: 1) What is it? 2) How does it affect cellular? 3) Mitigation/Advantages-Disadvantages.