Mobile & Wireless Communication (4351104) - Winter 2023 Solution

Question 1(a) [3 marks]

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Draw & Explain umbrella cell.

Answer:

graph TD
    A[Large Coverage Area] --> B[Umbrella Cell Tower]
    B --> C[Micro Cell 1]
    B --> D[Micro Cell 2]
    B --> E[Micro Cell 3]
    C --> F[Users in Dense Area]
    D --> G[Users in Dense Area]
    E --> H[Users in Dense Area]

• Umbrella Cell: Large coverage cell overlaying smaller cells
• Purpose: Handles overflow traffic from micro/pico cells
• Coverage: Provides backup coverage for high-traffic areas

Mnemonic: “Under My Big Umbrella”

Question 1(b) [4 marks]

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Define full forms: (i) CCH (ii) TCH (iii) SCH (iv) BCCH

Answer:

Mnemonic: “Control Traffic Sync Broadcast”

Question 1(c) [7 marks]

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What is cell? Explain different types of cells.

Answer: Cell is the basic coverage area served by one base station in cellular communication.

graph LR
    A[Macro Cell] --> B[Large Area Coverage]
    C[Micro Cell] --> D[City Coverage]
    E[Pico Cell] --> F[Building Coverage]
    G[Femto Cell] --> H[Room Coverage]

• Function: Each cell provides wireless service to mobile users
• Frequency Reuse: Same frequencies used in non-adjacent cells
• Handoff: Users move between cells seamlessly

Mnemonic: “Many Mobile People Find coverage”

Question 1(c OR) [7 marks]

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What is handoff? Explain soft and hard handoffs.

Answer: Handoff is the process of transferring an ongoing call from one cell to another as mobile moves.

sequenceDiagram
    participant M as Mobile
    participant BS1 as Base Station 1
    participant BS2 as Base Station 2

    Note over M,BS2: Hard Handoff
    M->>BS1: Connected
    BS1-->>M: Signal weakens
    BS1->>BS2: Handoff request
    M->>BS2: New connection
    
    Note over M,BS2: Soft Handoff
    M->>BS1: Connected
    M->>BS2: Dual connection
    M-->>BS1: Drop weak signal

• Initiation: Based on signal strength measurements
• MAHO: Mobile Assisted Handoff improves decision accuracy

Mnemonic: “Hard Hurts, Soft Smooth”

Question 2(a) [3 marks]

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Define full forms: (i) SIM (ii) LTE (iii) WCDMA

Answer:

Mnemonic: “Subscriber’s Long Wideband connection”

Question 2(b) [4 marks]

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Draw mobile handset block diagram.

Answer:

graph TD
    A[Antenna] --> B[RF Section]
    B --> C[Baseband Processor]
    C --> D[Audio Section]
    C --> E[Display/Keypad]
    C --> F[Memory]
    G[Battery] --> H[Power Management]
    H --> B
    H --> C
    H --> D

• RF Section: Transmits/receives radio signals
• Baseband: Processes digital signals and protocols
• Audio: Handles voice input/output
• Power Management: Controls battery usage efficiently

Mnemonic: “Radio Baseband Audio Power”

Question 2(c) [7 marks]

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Explain GSM architecture with diagram.

Answer:

graph LR
    A[MS] --> B[BTS]
    B --> C[BSC]
    C --> D[MSC]
    D --> E[HLR]
    D --> F[VLR]
    D --> G[AuC]
    D --> H[PSTN]

    subgraph BSS
    B
    C
    end
    
    subgraph NSS
    D
    E
    F
    G
    end

• BSS: Base Station Subsystem handles radio interface
• NSS: Network Switching Subsystem manages calls
• Authentication: AuC verifies subscriber identity

Mnemonic: “Mobile Base Network calls Home”

Question 2(a OR) [3 marks]

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Define full forms: (i) RSSI (ii) MAHO (iii) NCHO

Answer:

Mnemonic: “Received Mobile Network signals”

Question 2(b OR) [4 marks]

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Draw baseband section block diagram.

Answer:

graph TD
    A[ADC/DAC] --> B[DSP]
    B --> C[Channel Codec]
    C --> D[Speech Codec]
    D --> E[Audio Interface]
    B --> F[Protocol Stack]
    F --> G[Control Interface]

• ADC/DAC: Analog to Digital conversion
• DSP: Digital Signal Processor
• Channel Codec: Error correction coding
• Speech Codec: Voice compression/decompression

Mnemonic: “Analog Digital Speech Protocol”

Question 2(c OR) [7 marks]

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Explain GSM signal processing with diagram.

Answer:

graph LR
    A[Speech] --> B[Speech Codec]
    B --> C[Channel Codec]
    C --> D[Interleaving]
    D --> E[Burst Formatter]
    E --> F[GMSK Modulator]
    F --> G[RF Transmitter]

• Processing Rate: 270.833 kbps gross bit rate
• Frame Structure: 8 time slots per TDMA frame
• Frequency Hopping: 217 hops per second

Mnemonic: “Speech Channel Interleaves Modulated Radio”

Question 3(a) [3 marks]

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Explain cell splitting.

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

• Process: Replace high-power cell with multiple low-power cells
• Benefit: Increases system capacity by frequency reuse
• Implementation: Reduce antenna height and transmit power

Mnemonic: “Split Small Cells”

Question 3(b) [4 marks]

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Explain Li-Ion type batteries used in mobile handset with its advantages and disadvantages.

Answer:

• Chemistry: Lithium ions move between electrodes
• Voltage: 3.7V nominal per cell
• Capacity: Measured in mAh (milliampere-hours)

Mnemonic: “Light Ion Energy Safety”

Question 3(c) [7 marks]

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Explain GPRS.

Answer: GPRS (General Packet Radio Service) provides packet-switched data service over GSM.

graph TD
    A[Mobile] --> B[BSS]
    B --> C[PCU]
    C --> D[SGSN]
    D --> E[GGSN]
    E --> F[Internet]

• PCU: Packet Control Unit manages packet data
• SGSN: Serving GPRS Support Node
• GGSN: Gateway GPRS Support Node
• Classes: Class 1-12 with different speed/slot combinations

Mnemonic: “General Packet Radio Service”

Question 3(a OR) [3 marks]

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Explain cell sectoring.

Answer: Cell sectoring divides omnidirectional cell into sectors using directional antennas.

• Common: 3-sector (120°) or 6-sector (60°) configurations
• Benefit: Reduces co-channel interference
• Implementation: Directional antennas at same site

Mnemonic: “Sector Reduces Interference”

Question 3(b OR) [4 marks]

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Explain Li-Po type batteries used in mobile handset with its advantages and disadvantages.

Answer:

• Technology: Lithium Polymer electrolyte
• Form Factor: Can be molded into various shapes
• Voltage: 3.7V nominal per cell

Mnemonic: “Polymer Flexible Thin Light”

Question 3(c OR) [7 marks]

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Explain EDGE.

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

graph LR
    A[Data] --> B[Adaptive Coding]
    B --> C[8-PSK Modulation]
    C --> D[Link Adaptation]
    D --> E[Enhanced Reception]

• 8-PSK: 8-Phase Shift Keying provides 3 bits per symbol
• Link Adaptation: Adjusts coding scheme based on channel quality
• Incremental Redundancy: Improves error correction efficiency

Mnemonic: “Enhanced Data GSM Evolution”

Question 4(a) [3 marks]

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Draw DSSS transmitter and receiver block diagram.

Answer:

• Spreader: Multiplies data with PN sequence
• Despreader: Correlates received signal with same PN code
• Processing Gain: Ratio of spread to original bandwidth

Mnemonic: “Direct Sequence Spread Spectrum”

Question 4(b) [4 marks]

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Compare CDMA and GSM.

Answer:

Mnemonic: “Code Division vs Time Division”

Question 4(c) [7 marks]

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Explain concept of spread spectrum with applications.

Answer: Spread Spectrum spreads signal bandwidth much wider than required for data transmission.

graph TD
    A[Narrowband Signal] --> B[Spreading Code]
    B --> C[Wideband Signal]
    C --> D[Transmission]
    D --> E[Despreading]
    E --> F[Original Signal]

Benefits:

• Anti-jamming: Resistant to interference
• Low Power Density: Difficult to detect
• Multiple Access: Many users share spectrum
• Multipath Resistance: Resolves delayed signals

Applications: GPS, WiFi, Bluetooth, Military communications

Mnemonic: “Spread Signal Spectrum Security”

Question 4(a OR) [3 marks]

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Draw FHSS transmitter block diagram.

Answer:

• Frequency Synthesizer: Changes carrier frequency rapidly
• Hopping Sequence: Pseudo-random frequency pattern
• Dwell Time: Time spent on each frequency

Mnemonic: “Frequency Hopping Spread Spectrum”

Question 4(b OR) [4 marks]

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Explain call processing in CDMA.

Answer:

• Rake Receiver: Combines multipath signals
• Power Control: 800 times per second
• Soft Capacity: Degrades gracefully with load

Mnemonic: “Code Division Multiple Access”

Question 4(c OR) [7 marks]

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Explain HSDPA.

Answer: HSDPA (High Speed Downlink Packet Access) enhances WCDMA downlink data rates.

graph LR
    A[NodeB] --> B[HS-DSCH]
    B --> C[16-QAM]
    C --> D[HARQ]
    D --> E[Mobile]

• HS-DSCH: High Speed Downlink Shared Channel
• AMC: Adaptive Modulation and Coding
• Fast Cell Selection: Improves cell edge performance
• MIMO: Multiple antenna configurations possible

Mnemonic: “High Speed Downlink Packet Access”

Question 5(a) [3 marks]

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List LTE specifications.

Answer:

Mnemonic: “Long Term Evolution specifications”

Question 5(b) [4 marks]

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Draw OFDM receiver and explain its working.

Answer:

graph LR
    A[RF Input] --> B[ADC]
    B --> C[Remove CP]
    C --> D[FFT]
    D --> E[Demodulator]
    E --> F[Data Output]

• FFT: Fast Fourier Transform converts time to frequency domain
• Cyclic Prefix: Guards against inter-symbol interference
• Subcarriers: Parallel transmission on multiple frequencies
• Demodulation: QPSK/16QAM/64QAM per subcarrier

Mnemonic: “Orthogonal Frequency Division Multiplexing”

Question 5(c) [7 marks]

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Explain Bluetooth Technology & list its applications.

Answer: Bluetooth is short-range wireless communication technology for personal area networks.

graph TD
    A[Master Device] --> B[Slave 1]
    A --> C[Slave 2]
    A --> D[Slave 3]
    E[Scatternet] --> A
    E --> F[Another Piconet]

Protocol Stack:

• RF Layer: Physical radio interface
• Baseband: Medium access control
• L2CAP: Logical Link Control
• Applications: Various profiles (A2DP, HID, etc.)

Applications:

• Audio devices (headphones, speakers)
• File transfer between devices
• Input devices (keyboard, mouse)
• Health monitoring devices
• Smart home automation

Mnemonic: “Blue Tooth Personal Area Network”

Question 5(a OR) [3 marks]

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List advantages of 5G Technology.

Answer:

Mnemonic: “Fifth Generation advantages”

Question 5(b OR) [4 marks]

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Draw OFDM transmitter and explain its working.

Answer:

graph LR
    A[Data Input] --> B[Modulator]
    B --> C[IFFT]
    C --> D[Add CP]
    D --> E[DAC]
    E --> F[RF Output]

• Modulation: Maps bits to symbols (QPSK/QAM)
• IFFT: Inverse FFT converts frequency to time domain
• Cyclic Prefix: Copies end samples to beginning
• DAC: Digital to Analog Converter for transmission

Mnemonic: “Orthogonal Frequency Division Multiplexing Transmitter”

Question 5(c OR) [7 marks]

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Explain Zigbee Technology & list its applications.

Answer: Zigbee is low-power wireless mesh networking standard based on IEEE 802.15.4.

graph TD
    A[Coordinator] --> B[Router 1]
    A --> C[Router 2]
    B --> D[End Device 1]
    B --> E[End Device 2]
    C --> F[End Device 3]
    C --> G[Router 3]
    G --> H[End Device 4]

Network Roles:

• Coordinator: Network manager
• Router: Forwards messages
• End Device: Simple sensors/actuators

Applications:

• Home automation (lights, thermostats)
• Industrial monitoring
• Smart grid systems
• Healthcare monitoring
• Agricultural sensors
• Building management systems

Features:

• Self-healing: Automatic route discovery
• Low cost: Simple implementation
• Secure: AES encryption
• Reliable: Mesh redundancy

Mnemonic: “Zigbee Mesh Network Applications”