Mobile & Wireless Communication (4351104) - Winter 2024 Solution

Question 1(a) [3 marks]

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Explain umbrella cell.

Answer: Umbrella cell is a large coverage area cell that overlays smaller cells to provide continuous coverage and handle overflow traffic.

Table: Umbrella Cell Characteristics

• Large coverage: Covers wide geographical area with high-power base station
• Traffic management: Handles calls when smaller cells are congested
• Mobility support: Serves fast-moving users crossing multiple cell boundaries

Mnemonic: “Umbrella Covers Large Areas”

Question 1(b) [4 marks]

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Define cell and cluster.

Answer: Cell and cluster are fundamental concepts in cellular communication systems.

Table: Cell vs Cluster Comparison

• Cell: Geographic area served by single base station with specific frequency allocation
• Cluster: Group of adjacent cells that collectively use entire frequency spectrum
• Frequency reuse: Same frequencies can be reused in different clusters
• Pattern repetition: Cluster pattern repeats throughout coverage area

Mnemonic: “Cells Cluster for Complete Coverage”

Question 1(c) [7 marks]

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Describe fundamental concept behind cellular communication systems.

Answer: Cellular communication divides service area into small cells to maximize spectrum efficiency and capacity.

Diagram:

Table: Cellular System Benefits

• Small cell concept: Service area divided into hexagonal cells for efficient coverage
• Frequency reuse: Limited spectrum used multiple times with adequate separation
• Base station control: Each cell served by low-power base station
• Capacity improvement: More users supported compared to single large coverage area
• Interference management: Co-channel interference controlled through proper cell planning

Mnemonic: “Small Cells Support Spectrum Sharing Successfully”

Question 1(c OR) [7 marks]

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Explain co-channel interference in cellular communication.

Answer: Co-channel interference occurs when cells using same frequencies are too close, causing signal degradation.

graph TD
    A[Cell A - f1] --> B[Interference Zone]
    C[Cell C - f1] --> B
    B --> D[Degraded Signal Quality]
    E[Distance D] --> F[Reduced Interference]

Table: Co-channel Interference Parameters

• Signal overlap: Same frequency signals from different cells interfere
• Quality degradation: Causes call drops and poor voice quality
• Distance factor: Interference reduces with square of distance
• Mitigation methods: Proper cell planning, power control, antenna design

Mnemonic: “Co-channel Causes Call Quality Concerns”

Question 2(a) [3 marks]

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Explain cell splitting.

Answer: Cell splitting divides congested cells into smaller cells to increase system capacity.

Diagram:

• Capacity increase: Each new cell handles fewer users with better service quality
• Power reduction: New base stations use lower power to cover smaller areas
• Frequency management: Original frequencies distributed among new smaller cells

Mnemonic: “Split Cells Serve Subscribers Successfully”

Question 2(b) [4 marks]

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Explain channel assignment strategies.

Answer: Channel assignment strategies determine how frequencies are allocated to cells for optimal performance.

Table: Channel Assignment Strategies

• Fixed assignment: Each cell has predetermined set of channels
• Dynamic assignment: Channels allocated in real-time based on traffic demand
• Load balancing: Distributes traffic evenly across available channels
• Interference avoidance: Considers co-channel interference in assignment decisions

Mnemonic: “Dynamic Distribution Delivers Optimal Performance”

Question 2(c) [7 marks]

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Calculate voice and control channels per cell for 33MHz bandwidth, 25KHz simplex channels, 7-cell reuse, 1MHz for control.

Answer: Calculation for channel allocation in cellular system.

Given Data:

• Total bandwidth = 33 MHz
• Channel bandwidth = 25 KHz (simplex)
• Full duplex requires = 2 × 25 KHz = 50 KHz
• Control spectrum = 1 MHz
• Cluster size = 7 cells

Calculations:

Step 1: Total available channels Total channels = 33 MHz ÷ 25 KHz = 1320 channels

Step 2: Control channels Control channels = 1 MHz ÷ 25 KHz = 40 channels

Step 3: Voice channels Voice channels = 1320 - 40 = 1280 channels

Step 4: Duplex voice channels Duplex voice channels = 1280 ÷ 2 = 640 channels

Step 5: Channels per cell Voice channels per cell = 640 ÷ 7 ≈ 91 channels Control channels per cell = 40 ÷ 7 ≈ 6 channels

Final Answer:

• Voice channels per cell: 91
• Control channels per cell: 6

Mnemonic: “Calculate Carefully for Channel Count”

Question 2(a OR) [3 marks]

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Write functions of FCCH and SCH in GSM.

Answer: FCCH and SCH are essential control channels in GSM system for synchronization.

Table: FCCH and SCH Functions

• FCCH function: Enables mobile to synchronize with base station frequency
• SCH function: Carries BSIC (Base Station Identity Code) and frame number
• Timing correction: Both channels help mobile achieve proper timing synchronization

Mnemonic: “FCCH Fixes Frequency, SCH Synchronizes System”

Question 2(b OR) [4 marks]

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Write GSM 900 specifications.

Answer: GSM 900 operates in 900 MHz frequency band with specific technical parameters.

Table: GSM 900 Specifications

• Frequency bands: Separate uplink and downlink frequencies for full duplex operation
• TDMA structure: 8 time slots per carrier frequency

Mnemonic: “GSM 900 Gives Great Global Coverage”

Question 2(c OR) [7 marks]

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Draw and explain GSM architecture.

Answer: GSM architecture consists of three main subsystems working together for mobile communication.

graph TB
    MS[Mobile Station] --> BSS[Base Station Subsystem]
    BSS --> NSS[Network Switching Subsystem]
    BSS --> BTS[Base Transceiver Station]
    BSS --> BSC[Base Station Controller]
    NSS --> MSC[Mobile Switching Center]
    NSS --> HLR[Home Location Register]
    NSS --> VLR[Visitor Location Register]
    NSS --> AuC[Authentication Center]
    MSC --> PSTN[Public Switched Telephone Network]

Table: GSM Architecture Components

• Mobile Station: Consists of mobile equipment and SIM card for user identification
• Base Station Subsystem: Handles radio communication and resource management
• Network Switching Subsystem: Manages call switching, routing, and subscriber databases
• Interfaces: A-bis (BTS-BSC), A (BSC-MSC) interfaces connect subsystems

Mnemonic: “Mobile Base Network - Complete Communication Chain”

Question 3(a) [3 marks]

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Draw block diagram of signal processing in GSM.

Answer: Signal processing in GSM involves multiple stages for voice and data transmission.

Diagram:

• Speech coding: Converts analog speech to 13 kbps digital data using RPE-LTP
• Channel coding: Adds error correction bits increasing rate to 22.8 kbps
• Interleaving: Reorders data to combat burst errors from fading

Mnemonic: “Speech Signals Systematically Processed Successfully”

Question 3(b) [4 marks]

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Write functions of Common Control Channels in GSM.

Answer: Common Control Channels manage system information and access procedures in GSM.

Table: Common Control Channels Functions

• Broadcast function: BCCH continuously transmits system information
• Access management: RACH allows mobiles to request service
• Channel assignment: AGCH allocates resources for active calls
• Paging service: PCH notifies mobiles of incoming calls

Mnemonic: “Common Channels Control Communication Completely”

Question 3(c) [7 marks]

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Explain GSM identifiers.

Answer: GSM identifiers uniquely identify subscribers, equipment, and network elements.

Table: GSM Identifiers

• IMSI structure: MCC (3) + MNC (2-3) + MSIN (9-10 digits)
• Security purpose: TMSI protects subscriber identity over radio interface
• Location management: LAI helps in efficient paging and location updates
• Network planning: BSIC prevents confusion between adjacent cells

Mnemonic: “Important Mobile System Identifiers Ensure Security”

Question 3(a OR) [3 marks]

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Compare Fast and Slow frequency hopping.

Answer: Frequency hopping techniques differ in hopping rate relative to symbol rate.

Table: Fast vs Slow Frequency Hopping

• Fast hopping: Multiple hops per symbol, better security but more complex
• Slow hopping: Multiple symbols per hop, simpler implementation

Mnemonic: “Fast Frequently Flips, Slow Stays Stable”

Question 3(b OR) [4 marks]

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Calculate number of users in GSM 900 band without frequency reuse.

Answer: Calculation for maximum users in GSM 900 without frequency reuse.

Given GSM 900 Parameters:

• Uplink: 890-915 MHz (25 MHz)
• Downlink: 935-960 MHz (25 MHz)
• Channel spacing: 200 KHz
• Time slots per channel: 8

Calculations:

Step 1: Available channels Total channels = 25 MHz ÷ 200 KHz = 125 channels

Step 2: Usable channels Guard channels removed ≈ 124 channels

Step 3: Simultaneous users Users per channel = 8 time slots Total users = 124 × 8 = 992 users

Answer: 992 users can talk simultaneously

Mnemonic: “Calculate Channels Times Time-slots”

Question 3(c OR) [7 marks]

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Draw and explain general block diagram of mobile handset.

Answer: Mobile handset consists of several functional blocks working together.

graph TB
    A[Antenna] --> B[RF Section]
    B --> C[IF Section]
    C --> D[Baseband Processor]
    D --> E[Audio Section]
    D --> F[Display Unit]
    D --> G[Keypad]
    H[Power Management] --> D
    I[Battery] --> H
    J[SIM Interface] --> D

Table: Mobile Handset Blocks

• RF processing: Handles radio frequency transmission and reception
• Digital processing: Baseband performs channel coding, speech processing
• User interface: Provides interaction through display, keypad, audio
• Power control: Manages battery usage and charging functions

Mnemonic: “Mobile Manages Multiple Modules Simultaneously”

Question 4(a) [3 marks]

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Write radiation hazards due to mobile.

Answer: Radiation hazards from mobile phones are a health concern due to RF energy exposure.

Table: Mobile Radiation Hazards

• SAR (Specific Absorption Rate): Measures RF energy absorbed by body tissue
• Thermal effects: RF energy can cause localized heating of tissues
• Non-thermal effects: Possible impacts on cellular functions and DNA

Mnemonic: “Safety Awareness Reduces Radiation Risk”

Question 4(b) [4 marks]

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Explain working of baseband section in mobile handset.

Answer: Baseband section performs digital signal processing functions in mobile handset.

Table: Baseband Section Functions

• Digital signal processor: Executes speech coding algorithms (GSM: RPE-LTP)
• Error correction: Implements convolutional coding for reliable transmission
• Control functions: Manages call setup, handoff, and power control
• Interface: Connects RF section with user interface components

Mnemonic: “Baseband Brings Better Communication Control”

Question 4(c) [7 marks]

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Explain working of DSSS transmitter and receiver.

Answer: DSSS (Direct Sequence Spread Spectrum) spreads signal bandwidth using pseudorandom codes.

Transmitter Diagram:

graph LR
    A[Data Input] --> B[PN Code Generator]
    A --> C[XOR Gate]
    B --> C
    C --> D[Modulator]
    D --> E[RF Output]

Receiver Diagram:

graph LR
    F[RF Input] --> G[Demodulator]
    G --> H[XOR Gate]
    I[PN Code Generator] --> H
    H --> J[Data Output]

Table: DSSS Process

• Spreading process: Original data XORed with high-rate pseudorandom sequence
• Bandwidth expansion: Signal bandwidth increased by processing gain factor
• Despreading: Receiver uses same PN code to recover original data
• Interference rejection: Spread spectrum provides resistance to jamming

Mnemonic: “Direct Sequence Spreads Signals Successfully”

Question 4(a OR) [3 marks]

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Calculate processing gain for DSSS system with 10 Mcps chip rate and 1 Mbps data rate.

Answer: Processing gain determines spread spectrum system’s performance improvement.

Given:

• Chip rate (Rc) = 10 million chips per second = 10 × 10⁶ cps
• Data rate (Rd) = 1 Mbps = 1 × 10⁶ bps

Calculation: Processing Gain (Gp) = Chip rate ÷ Data rate Gp = Rc ÷ Rd = (10 × 10⁶) ÷ (1 × 10⁶) = 10

In dB: Gp (dB) = 10 log₁₀(10) = 10 × 1 = 10 dB

Answer: Processing Gain = 10 or 10 dB

Mnemonic: “Processing Power Provides Protection”

Question 4(b OR) [4 marks]

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Explain how data rate is improved in EDGE.

Answer: EDGE (Enhanced Data rates for GSM Evolution) improves data rates through advanced modulation.

Table: EDGE Improvements

• 8-PSK modulation: Transmits 3 bits per symbol instead of 1 bit in GMSK
• Link adaptation: Dynamically selects coding scheme based on channel quality
• Backward compatibility: Works with existing GSM infrastructure
• Enhanced applications: Supports multimedia and higher data rate services

Mnemonic: “EDGE Enhances Exchange Efficiently”

Question 4(c OR) [7 marks]

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Explain call processing in CDMA.

Answer: CDMA call processing involves unique procedures for code-based multiple access.

graph TD
    A[Mobile Power On] --> B[Pilot Channel Search]
    B --> C[Sync Channel Read]
    C --> D[Paging Channel Monitor]
    D --> E[Access Channel Request]
    E --> F[Traffic Channel Assignment]
    F --> G[Active Call State]
    G --> H[Soft Handoff]

Table: CDMA Call Processing Stages

• Pilot channel: Provides timing reference and system identification
• Rake receiver: Combines multipath signals for improved performance
• Power control: Maintains optimal signal levels for all users
• Soft handoff: Mobile communicates with multiple base stations simultaneously
• Code assignment: Each user assigned unique spreading code

Mnemonic: “CDMA Calls Connect Carefully and Clearly”

Question 5(a) [3 marks]

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Compare CDMA and GSM.

Answer: CDMA and GSM represent different approaches to cellular communication.

Table: CDMA vs GSM Comparison

• Multiple access: CDMA uses unique codes, GSM uses time slots
• Call quality: CDMA provides soft handoff, GSM has hard handoff

Mnemonic: “Choose CDMA or GSM Carefully”

Question 5(b) [4 marks]

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Write advantages of CDMA.

Answer: CDMA advantages make it suitable for high-capacity cellular systems.

Table: CDMA Advantages

• Spectrum efficiency: Higher capacity compared to FDMA/TDMA systems
• Quality advantage: Soft handoff eliminates call drops during cell transitions
• Security benefit: Spread spectrum provides inherent privacy protection
• Simplified planning: No frequency reuse planning required

Mnemonic: “CDMA Creates Considerable Communication Capacity”

Question 5(c) [7 marks]

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Explain MANET in brief and write its applications.

Answer: MANET (Mobile Ad Hoc Network) is infrastructure-less network of mobile devices.

graph TB
    A[Mobile Node A] -.-> B[Mobile Node B]
    B -.-> C[Mobile Node C]
    C -.-> D[Mobile Node D]
    A -.-> C
    B -.-> D
    A -.-> D

    style A fill:#f9f
    style B fill:#9ff
    style C fill:#ff9
    style D fill:#9f9

Table: MANET Characteristics vs Applications

Applications:

• Military operations: Battlefield communications without infrastructure
• Emergency services: Disaster response and rescue operations
• Sensor networks: Environmental monitoring and data collection
• Vehicular networks: Car-to-car communication for traffic management
• Personal area networks: Device-to-device communication
• Academic research: Collaborative computing environments

Advantages:

• Rapid deployment: No infrastructure setup required
• Self-healing: Automatic route reconfiguration when nodes fail
• Cost effective: No base station installation costs

Disadvantages:

• Limited bandwidth: Shared wireless medium
• Security challenges: Vulnerable to attacks
• Power constraints: Battery-dependent operation

Mnemonic: “Mobile Ad Hoc Networks Enable Everywhere”

Question 5(a OR) [3 marks]

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Write key features of WCDMA.

Answer: WCDMA (Wideband CDMA) is the 3G standard offering enhanced capabilities.

Table: WCDMA Key Features

• Wideband operation: 5 MHz bandwidth provides high data rates
• Variable spreading: Adapts to different service requirements

Mnemonic: “WCDMA Widens Communication Data Magnificently”

Question 5(b OR) [4 marks]

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Enlist advantages of 5G.

Answer: 5G advantages represent significant improvements over previous generations.

Table: 5G Advantages

• Enhanced mobile broadband: Supports AR/VR and 4K/8K video streaming
• Ultra-reliable communications: Enables autonomous vehicles and remote surgery
• Massive machine communications: Supports smart cities and Industry 4.0
• Flexible network architecture: Software-defined networking capabilities

Mnemonic: “5G Generates Great Gigabit Growth”

Question 5(c OR) [7 marks]

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Explain working of OFDM with block diagram.

Answer: OFDM (Orthogonal Frequency Division Multiplexing) uses multiple subcarriers for high-speed data transmission.

OFDM Transmitter:

graph LR
    A[Serial Data] --> B[Serial to Parallel]
    B --> C[QAM Mapping]
    C --> D[IFFT]
    D --> E[Add Cyclic Prefix]
    E --> F[Parallel to Serial]
    F --> G[RF Transmission]

OFDM Receiver:

graph LR
    H[RF Reception] --> I[Serial to Parallel]
    I --> J[Remove Cyclic Prefix]
    J --> K[FFT]
    K --> L[QAM Demapping]
    L --> M[Parallel to Serial]
    M --> N[Serial Data]

Table: OFDM Process Steps

Key Features:

• Orthogonal subcarriers: Multiple parallel low-rate data streams prevent interference
• FFT/IFFT processing: Efficient digital implementation using fast transforms
• Cyclic prefix: Guard interval prevents inter-symbol interference from multipath
• Spectral efficiency: High data rates achieved in limited bandwidth
• Multipath resistance: Individual subcarriers experience flat fading

Applications:

• WiFi (802.11): Wireless LAN communications
• LTE/4G: Mobile broadband networks
• Digital TV: DVB-T terrestrial broadcasting
• WiMAX: Broadband wireless access

Advantages:

• High spectral efficiency: Optimal bandwidth utilization
• Robustness: Resistant to frequency selective fading
• Flexibility: Adaptive modulation per subcarrier
• Implementation: Digital signal processing simplifies hardware

Table: OFDM Parameters

Mnemonic: “OFDM Offers Outstanding Data Multiplexing”