Question 1(a) [3 marks]

Write key features of 4G and 5G system.

Answer:

Table: Key Features Comparison

Feature	4G System	5G System
Data Speed	Up to 100 Mbps	Up to 10 Gbps
Latency	30-50 ms	1-10 ms
Technology	LTE, OFDM	MIMO, Beamforming
Applications	Video streaming	IoT, AR/VR

Key Points:

4G: Uses LTE technology with OFDM modulation for high-speed data
5G: Ultra-low latency enables real-time applications like autonomous vehicles
Network Slicing: 5G allows virtual networks for specific applications

Mnemonic: "4G Fast, 5G Super-Fast"

Question 1(b) [4 marks]

Explain concept of frequency reuse in cellular mobile system.

Answer:

Diagram:

    F1      F2      F3
   +---+   +---+   +---+
   | A |   | B |   | C |
   +---+   +---+   +---+
    F4      F5      F6
   +---+   +---+   +---+
   | D |   | E |   | F |
   +---+   +---+   +---+
    F7      F1      F2
   +---+   +---+   +---+
   | G |   | A |   | B |
   +---+   +---+   +---+

Key Points:

Frequency Reuse: Same frequencies used in non-adjacent cells to increase capacity
Co-channel Distance: Minimum distance between cells using same frequency
Cluster Size: Group of cells using different frequencies (typically 3, 4, 7, 12)
Capacity Improvement: More users served with limited spectrum

Mnemonic: "Same Frequency, Different Places"

Question 1(c) [7 marks]

If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex communication. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels for cluster size of 7.

Answer:

Given Data:

Total bandwidth = 33 MHz
Channel bandwidth = 25 kHz (simplex)
Control spectrum = 1 MHz
Cluster size = 7

Calculations:

Step 1: Available spectrum for traffic Traffic spectrum = 33 - 1 = 32 MHz

Step 2: Total duplex channels Each duplex channel needs 2 × 25 kHz = 50 kHz Total channels = 32 MHz ÷ 50 kHz = 640 channels

Step 3: Control channels Control channels = 1 MHz ÷ 25 kHz = 40 channels

Step 4: Distribution per cell

Voice channels per cell = 640 ÷ 7 ≈ 91 channels
Control channels per cell = 40 ÷ 7 ≈ 6 channels

Final Distribution Table:

Parameter	Total	Per Cell
Voice Channels	640	91
Control Channels	40	6
Total Channels	680	97

Mnemonic: "Divide Total by Cluster"

Question 1(c OR) [7 marks]

List out types of cells and explain each.

Answer:

Table: Types of Cells

Cell Type	Coverage	Power	Applications
Macro Cell	1-30 km	High	Rural areas
Micro Cell	100m-1km	Medium	Urban areas
Pico Cell	10-100m	Low	Buildings
Femto Cell	10-50m	Very Low	Homes

Detailed Explanation:

Macro Cells:

Coverage: Large geographical areas (1-30 km radius)
Power: High transmission power (up to 40W)
Usage: Rural and suburban areas with low user density

Micro Cells:

Coverage: Medium areas (100m to 1km radius)
Power: Medium transmission power (1-10W)
Usage: Urban areas, highway coverage

Pico Cells:

Coverage: Small indoor/outdoor areas (10-100m)
Power: Low transmission power (100mW-1W)
Usage: Shopping malls, airports, offices

Umbrella Cells:

Special Type: Covers multiple smaller cells
Purpose: Handles high-speed mobile users
Advantage: Reduces handoffs for fast-moving users

Mnemonic: "Macro-Micro-Pico-Femto = Big to Small"

Question 2(a) [3 marks]

Define cell and cluster.

Answer:

Definitions:

Cell:

Definition: Geographical area covered by one base station
Shape: Typically hexagonal for planning purposes
Function: Serves mobile users within its coverage area

Cluster:

Definition: Group of cells using different frequency sets
Purpose: Enables frequency reuse pattern
Common Sizes: 3, 4, 7, 12 cells per cluster

Table: Cell vs Cluster

Parameter	Cell	Cluster
Unit	Single coverage area	Group of cells
Frequency	One frequency set	Multiple frequency sets
Reuse	Cannot reuse nearby	Enables frequency reuse

Mnemonic: "Cell = One Area, Cluster = Group Areas"

Question 2(b) [4 marks]

Explain effect of cluster size on capacity and interference.

Answer:

Effects Table:

Cluster Size	Capacity	Interference	Co-channel Distance
Small (3,4)	High	High	Short
Large (7,12)	Low	Low	Long

Key Effects:

On Capacity:

Smaller Cluster: More channels per cell, higher capacity
Larger Cluster: Fewer channels per cell, lower capacity
Formula: Channels per cell = Total channels ÷ Cluster size

On Interference:

Smaller Cluster: Higher co-channel interference
Larger Cluster: Lower co-channel interference
Trade-off: Capacity vs. Quality

Co-channel Distance:

Relationship: D = R√(3N) where N = cluster size
Effect: Larger N means larger distance between co-channel cells

Mnemonic: "Small Cluster = More Capacity, More Interference"

Question 2(c) [7 marks]

Write key features of IS-95, CDMA2000 and WCDMA.

Answer:

Comparison Table:

Feature	IS-95	CDMA2000	WCDMA
Generation	2G	3G	3G
Data Rate	14.4 kbps	2 Mbps	2 Mbps
Chip Rate	1.2288 Mcps	3.6864 Mcps	3.84 Mcps
Bandwidth	1.25 MHz	1.25 MHz	5 MHz

IS-95 Features:

Technology: First commercial CDMA system
Voice Quality: Better than GSM in some conditions
Soft Handoff: Maintains multiple connections during handoff
Power Control: Precise power control reduces interference

CDMA2000 Features:

Backward Compatibility: Works with IS-95 networks
High Data Rate: Up to 2 Mbps for 1xEV-DO
Multimedia: Supports voice, data, and video
Efficiency: Better spectrum efficiency than IS-95

WCDMA Features:

Global Standard: Used worldwide for 3G
High Capacity: Supports more simultaneous users
QoS Support: Different service classes for applications
International Roaming: Global compatibility

Mnemonic: "IS-95 First, CDMA2000 Faster, WCDMA Global"

Question 2(a OR) [3 marks]

Explain cell splitting.

Answer:

Definition: Cell splitting is a technique to increase system capacity by subdividing congested cells into smaller cells.

graph TD
    A[Original Large Cell] --> B[Split into 4 Smaller Cells]
    B --> C[Cell 1]
    B --> D[Cell 2]
    B --> E[Cell 3]
    B --> F[Cell 4]

Process:

Step 1: Identify congested cell with high traffic
Step 2: Install new base stations with lower power
Step 3: Reduce original base station power
Step 4: Create multiple smaller coverage areas

Benefits:

Capacity Increase: More channels available in same area
Better Signal Quality: Shorter distances improve signal strength

Mnemonic: "Split Big Cell into Small Cells"

Question 2(b OR) [4 marks]

Write functions of HLR and VLR in GSM.

Answer:

Functions Table:

Database	Full Form	Primary Functions
HLR	Home Location Register	Permanent subscriber data
VLR	Visitor Location Register	Temporary visitor data

HLR Functions:

Subscriber Profile: Stores permanent subscriber information (IMSI, services)
Location Tracking: Maintains current location area of subscriber
Authentication: Provides authentication keys for security
Service Management: Controls subscribed services and restrictions

VLR Functions:

Temporary Storage: Stores visiting subscriber data temporarily
Local Services: Enables services for roaming subscribers
Call Routing: Assists in routing calls to visiting subscribers
Authentication Copy: Maintains copy of authentication data from HLR

Interaction:

HLR updates VLR when subscriber roams to new area
VLR requests subscriber data from HLR during registration

Mnemonic: "HLR = Home Data, VLR = Visitor Data"

Question 2(c OR) [7 marks]

Describe RFID technology.

Answer:

RFID Overview: Radio Frequency Identification uses electromagnetic fields to identify and track tags attached to objects.

System Components:

graph LR
    A[RFID Reader] --> B[Radio Waves]
    B --> C[RFID Tag]
    C --> D[Stored Data]
    C --> B
    B --> A

Types Table:

Type	Power Source	Range	Applications
Passive	Reader's energy	0.1-10m	Access cards
Active	Internal battery	10-100m	Vehicle tracking
Semi-passive	Battery + Reader	1-30m	Temperature sensors

Key Features:

No Line of Sight: Works without direct visual contact
Multiple Reading: Can read multiple tags simultaneously
Data Storage: Can store and update information
Durability: Resistant to environmental conditions

Applications:

Inventory Management: Warehouse and retail tracking
Access Control: Building and vehicle access
Payment Systems: Contactless payment cards
Supply Chain: Product tracking from manufacturing to sale

Advantages:

Fast Reading: Instant identification without scanning
Automation: Reduces manual data entry errors
Real-time Tracking: Continuous monitoring of assets

Mnemonic: "Radio Frequency Identifies Everything"

Question 3(a) [3 marks]

Draw GSM architecture.

Answer:

graph TD
    A[Mobile Station] --> B[BTS - Base Transceiver Station]
    B --> C[BSC - Base Station Controller]
    C --> D[MSC - Mobile Switching Center]
    D --> E[HLR - Home Location Register]
    D --> F[VLR - Visitor Location Register]
    D --> G[PSTN/ISDN]

    H[Authentication Center] --> D
    I[Equipment Identity Register] --> D

Components:

MS: Mobile Station (handset + SIM)
BTS: Radio interface with mobile
BSC: Controls multiple BTS, handles handoffs
MSC: Switching and call control
HLR/VLR: Database for subscriber information

Mnemonic: "Mobile Talks Through BTS-BSC-MSC"

Question 3(b) [4 marks]

Write GSM 900 specifications.

Answer:

GSM 900 Specifications Table:

Parameter	Specification
Frequency Band	890-915 MHz (Uplink), 935-960 MHz (Downlink)
Channel Spacing	200 kHz
Total Channels	124 channels
Modulation	GMSK (Gaussian MSK)
Access Method	TDMA/FDMA
Frame Duration	4.615 ms
Time Slots	8 per frame
Speech Coding	13 kbps RPE-LTP

Key Features:

Digital Transmission: Superior voice quality compared to analog
International Roaming: Global compatibility standard
Security: Encryption and authentication built-in
SMS Support: Short message service capability

Coverage:

Cell Radius: Up to 35 km (rural areas)
Power Classes: 5 classes from 0.8W to 20W

Mnemonic: "900 MHz, 200 kHz spacing, 8 time slots"

Question 3(c) [7 marks]

Explain mobile to landline and landline to mobile call procedure in GSM.

Answer:

Mobile to Landline Call Procedure:

sequenceDiagram
    participant MS as Mobile Station
    participant BTS as BTS/BSC
    participant MSC as MSC
    participant PSTN as PSTN/Landline

    MS->>BTS: Call Request
    BTS->>MSC: Forward Request
    MSC->>MSC: Authenticate User
    MSC->>PSTN: Route Call
    PSTN->>MSC: Ring Response
    MSC->>BTS: Ring Indication
    BTS->>MS: Ring Back Tone
    PSTN->>MSC: Call Answered
    MSC->>MS: Connect Call

Steps:

Call Initiation: Mobile dials landline number
Channel Assignment: BSC assigns traffic channel
Authentication: MSC verifies subscriber
Routing: MSC routes call to PSTN gateway
Connection: End-to-end connection established

Landline to Mobile Call Procedure:

sequenceDiagram
    participant PSTN as PSTN/Landline
    participant MSC as Gateway MSC
    participant HLR as HLR
    participant VMSC as Visited MSC
    participant MS as Mobile Station

    PSTN->>MSC: Call to Mobile
    MSC->>HLR: Location Request
    HLR->>VMSC: Get Routing Number
    VMSC->>MSC: Return Routing Number
    MSC->>VMSC: Route Call
    VMSC->>MS: Page Mobile
    MS->>VMSC: Page Response
    VMSC->>MS: Ring Mobile

Steps:

Call Reception: PSTN receives call to mobile number
HLR Query: Gateway MSC queries HLR for location
Location Update: HLR provides current MSC information
Paging: Visited MSC pages mobile in location area
Connection: Mobile responds and call is connected

Key Differences:

Mobile Originating: Direct routing through serving MSC
Mobile Terminating: Requires location lookup through HLR

Mnemonic: "Mobile Out = Direct, Mobile In = Find First"

Question 3(a OR) [3 marks]

Explain fast and slow frequency hopping.

Answer:

Frequency Hopping Types:

Table: Fast vs Slow Hopping

Parameter	Fast Hopping	Slow Hopping
Hop Rate	> Symbol Rate	< Symbol Rate
Symbols per Hop	< 1	> 1
Complexity	High	Low
GSM Usage	Not used	Used (217 hops/sec)

Fast Frequency Hopping:

Definition: Frequency changes multiple times per symbol
Characteristics: Very high hop rate, complex implementation
Advantage: Excellent interference resistance

Slow Frequency Hopping:

Definition: Multiple symbols transmitted per frequency
GSM Implementation: 217 hops per second
Advantage: Simple to implement, effective interference averaging

Mnemonic: "Fast = Many hops per symbol, Slow = Many symbols per hop"

Question 3(b OR) [4 marks]

Explain authentication process in GSM.

Answer:

Authentication Process:

sequenceDiagram
    participant MS as Mobile Station
    participant MSC as MSC/VLR
    participant HLR as HLR/AuC

    MS->>MSC: Location Update Request
    MSC->>HLR: Send IMSI
    HLR->>HLR: Generate RAND, SRES, Kc
    HLR->>MSC: Return Triplet (RAND, SRES, Kc)
    MSC->>MS: Authentication Request (RAND)
    MS->>MS: Calculate SRES using A3 algorithm
    MS->>MSC: Authentication Response (SRES)
    MSC->>MSC: Compare SRES values
    MSC->>MS: Accept/Reject

Key Components:

RAND: Random number (128 bits)
SRES: Signed response (32 bits)
Kc: Cipher key (64 bits)
Ki: Individual subscriber authentication key

Process Steps:

Challenge: Network sends random number (RAND)
Response: Mobile calculates SRES using Ki and RAND
Verification: Network compares received and expected SRES
Result: Authentication success or failure

Security Features:

Mutual Authentication: Prevents fake base stations
Unique Keys: Each subscriber has individual Ki
Challenge-Response: Prevents replay attacks

Mnemonic: "Random Challenge, Signed Response, Compare and Accept"

Question 3(c OR) [7 marks]

Draw and explain block diagram of Signal processing in GSM.

Answer:

GSM Signal Processing Block Diagram:

graph LR
    A[Speech Input] --> B[Speech Coder]
    B --> C[Channel Coder]
    C --> D[Interleaver]
    D --> E[Burst Formatter]
    E --> F[Modulator]
    F --> G[RF Section]
    G --> H[Antenna]

    I[Antenna] --> J[RF Section]
    J --> K[Demodulator]
    K --> L[Burst Detector]
    L --> M[De-interleaver]
    M --> N[Channel Decoder]
    N --> O[Speech Decoder]
    O --> P[Speech Output]

Transmitter Processing:

Speech Coding:

Function: Converts analog speech to 13 kbps digital
Algorithm: RPE-LTP (Regular Pulse Excitation - Long Term Prediction)
Frame Size: 20 ms speech frames

Channel Coding:

Purpose: Adds redundancy for error correction
Types: Convolutional coding, block coding
Output: Protected 22.8 kbps data stream

Interleaving:

Function: Spreads coded bits across multiple time slots
Benefit: Combats burst errors from fading
Types: Block interleaving over 8 time slots

Burst Formatting:

Process: Organizes data into GSM burst structure
Components: Training sequence, guard bits, data bits
Types: Normal burst, access burst, sync burst

Modulation:

Technique: GMSK (Gaussian Minimum Shift Keying)
Bandwidth: 200 kHz channel spacing
Symbol Rate: 270.833 kbps

Receiver Processing:

Demodulation: Recovers digital bits from RF signal
Equalization: Compensates for multipath distortion
Error Correction: Uses channel coding redundancy
Speech Decoding: Reconstructs original speech

Key Features:

Digital Processing: All operations in digital domain
Error Protection: Multiple levels of error correction
Adaptive: Parameters adjust to channel conditions

Mnemonic: "Speech-Code-Interleave-Burst-Modulate-Transmit"

Question 4(a) [3 marks]

Draw block diagram of baseband section.

Answer:

Baseband Section Block Diagram:

graph TD
    A[DSP Processor] --> B[Audio Codec]
    B --> C[Speaker]
    D[Microphone] --> B
    A --> E[Memory Interface]
    E --> F[Flash Memory]
    E --> G[RAM]
    A --> H[Control Interface]
    H --> I[Keypad]
    H --> J[Display]
    A --> K[RF Interface]
    A --> L[SIM Interface]

Components:

DSP: Digital signal processing for speech and data
Audio Codec: Analog-to-digital conversion
Memory: Program storage (Flash) and working memory (RAM)
Control: User interface management
Interfaces: RF section, SIM card connections

Functions:

Signal Processing: Speech coding, echo cancellation
Protocol Stack: GSM layer 1, 2, 3 protocols
User Interface: Display, keypad, audio management

Mnemonic: "DSP Controls Audio, Memory, Display, RF"

Question 4(b) [4 marks]

Explain EDGE.

Answer:

EDGE Overview: Enhanced Data rates for GSM Evolution - improves data transmission in GSM networks.

Key Features Table:

Parameter	GSM/GPRS	EDGE
Modulation	GMSK	8-PSK
Data Rate	9.6-171 kbps	Up to 473 kbps
Generation	2.5G	2.75G
Symbol Rate	270.833 ksps	270.833 ksps

Technical Improvements:

Advanced Modulation: 8-PSK carries 3 bits per symbol vs 1 bit in GMSK
Link Adaptation: Automatically switches between GMSK and 8-PSK
Enhanced Coding: Better error correction schemes
Incremental Redundancy: Improved retransmission strategy

Benefits:

Higher Data Rates: 3x faster than GPRS
Backward Compatibility: Works with existing GSM infrastructure
Cost Effective: Software upgrade to existing networks
Multimedia Support: Enables better mobile internet experience

Applications:

Mobile Internet: Faster web browsing
Email: Quick email with attachments
Multimedia Messaging: MMS support
Video Calls: Basic video communication

Mnemonic: "EDGE = Enhanced Data rates for GSM Evolution"

Question 4(c) [7 marks]

Draw and explain block diagram of mobile handset.

Answer:

Mobile Handset Block Diagram:

graph TD
    A[Antenna] --> B[Antenna Switch]
    B --> C[RF Transceiver]
    C --> D[Baseband Processor]
    D --> E[Audio Section]
    E --> F[Speaker/Microphone]

    D --> G[Display Controller]
    G --> H[LCD Display]
    
    D --> I[Keypad Interface]
    I --> J[Keypad]
    
    D --> K[Memory Controller]
    K --> L[Flash Memory]
    K --> M[RAM]
    
    D --> N[SIM Interface]
    N --> O[SIM Card]
    
    P[Battery] --> Q[Power Management]
    Q --> C
    Q --> D
    Q --> R[Charging Circuit]

Major Sections:

RF Section:

Antenna: Transmits and receives radio signals
Duplexer: Separates TX and RX signals
RF Transceiver: Up/down conversion, amplification
Frequency Synthesizer: Generates carrier frequencies

Baseband Section:

DSP: Digital signal processing for speech and data
Protocol Stack: Implements GSM protocols
Control Unit: Manages all mobile functions
Memory Interface: Controls program and data storage

Audio Section:

Audio Codec: A/D and D/A conversion
Audio Amplifier: Drives speaker
Microphone Amplifier: Amplifies voice input
Hands-free Support: External audio accessories

User Interface:

Display: Shows information to user (LCD/OLED)
Keypad: User input interface
LED Indicators: Status indication
Vibrator: Alert mechanism

Power Management:

Battery: Energy storage (Li-ion typically)
Charging Circuit: Battery charging control
Power Regulation: Voltage regulation for all sections
Power Saving: Sleep modes and power optimization

Memory System:

Flash Memory: Program storage and user data
RAM: Working memory for program execution
SIM Interface: Secure element for subscriber identity

Interconnections:

Control Bus: Command and control signals
Data Bus: Information transfer
Power Bus: Power distribution
Audio Bus: Voice and audio signals

Operation:

Receive: Antenna → RF → Baseband → Audio → Speaker
Transmit: Microphone → Audio → Baseband → RF → Antenna
Control: User input → Baseband → Display output
Processing: All operations controlled by baseband processor

Mnemonic: "Antenna-RF-Baseband-Audio-Display-Power"

Question 4(a OR) [3 marks]

Explain radiation hazards due to mobile.

Answer:

Radiation Hazards:

SAR (Specific Absorption Rate):

Definition: Rate of energy absorption by human body
Unit: Watts per kilogram (W/kg)
Limit: 2.0 W/kg (Europe), 1.6 W/kg (USA)

Health Concerns Table:

Effect	Risk Level	Symptoms
Thermal	Confirmed	Tissue heating
Non-thermal	Under study	Headaches, fatigue
Long-term	Uncertain	Cancer concerns

Prevention Measures:

Distance: Keep phone away from body during calls
Duration: Limit call duration
Hands-free: Use headsets or speakerphone
Low SAR: Choose phones with lower SAR values

Safety Guidelines:

Avoid sleeping with phone near head
Use airplane mode when not needed
Keep calls short and use text when possible

Mnemonic: "SAR measures absorption rate"

Question 4(b OR) [4 marks]

Describe working of charging section in mobile handset.

Answer:

Charging Section Block Diagram:

graph LR
    A[AC Adapter] --> B[Rectifier]
    B --> C[Voltage Regulator]
    C --> D[Charging Controller]
    D --> E[Battery]
    D --> F[Current Monitor]
    F --> G[Protection Circuit]
    G --> H[Temperature Sensor]

Components & Functions:

Charging Controller:

Function: Controls charging current and voltage
Types: Linear and switching mode controllers
Protection: Prevents overcharging and overheating

Charging Process:

Constant Current: Initial high current charging (fast charge)
Constant Voltage: Voltage maintained, current decreases
Trickle Charge: Low current maintenance charging
Cut-off: Charging stops when battery full

Protection Features:

Over-voltage Protection: Prevents damage from high voltage
Over-current Protection: Limits maximum charging current
Temperature Monitoring: Stops charging if battery gets too hot
Reverse Polarity: Prevents damage from wrong connection

Battery Management:

Fuel Gauge: Monitors battery capacity
Cell Balancing: Ensures equal charging of battery cells
Health Monitoring: Tracks battery condition over time

Mnemonic: "Control Current, Voltage, Temperature, and Time"

Question 4(c OR) [7 marks]

Draw and explain block diagram of DSSS transmitter and receiver.

Answer:

DSSS Transmitter Block Diagram:

graph LR
    A[Data Input] --> B[Data Modulator]
    B --> C[Spreader/Mixer]
    D[PN Code Generator] --> C
    C --> E[RF Modulator]
    E --> F[Power Amplifier]
    F --> G[Antenna]

DSSS Receiver Block Diagram:

graph LR
    H[Antenna] --> I[RF Amplifier]
    I --> J[RF Demodulator]
    J --> K[Correlator/Despreader]
    L[PN Code Generator] --> K
    K --> M[Data Demodulator]
    M --> N[Data Output]
    K --> O[Synchronization]
    O --> L

Transmitter Operation:

Data Modulation:

Input: Original data stream (low rate)
Modulation: BPSK or QPSK modulation
Output: Modulated narrowband signal

Spreading Process:

PN Code: Pseudo-random binary sequence (high rate)
Spreading: XOR operation between data and PN code
Result: Wideband spread spectrum signal

RF Modulation:

Carrier: High frequency carrier signal
Modulation: Spread signal modulates RF carrier
Transmission: Signal transmitted through antenna

Receiver Operation:

RF Processing:

Reception: Antenna receives spread spectrum signal
Amplification: Low noise amplifier boosts weak signal
Demodulation: Recovers baseband spread signal

Despreading Process:

Correlation: Received signal correlated with same PN code
Synchronization: PN code timing synchronized with received signal
Output: Original narrowband data signal recovered

Key Parameters:

Processing Gain: Ratio of spread bandwidth to data bandwidth
Chip Rate: Rate of PN code (higher than data rate)
Spreading Factor: Processing gain value

Advantages:

Interference Rejection: Resistant to narrowband interference
Low Probability of Intercept: Difficult to detect and jam
Multiple Access: Many users can share same frequency
Multipath Resistance: Reduces fading effects

Applications:

CDMA Cellular: IS-95, CDMA2000, WCDMA
GPS: Global positioning system
WiFi: 802.11b spread spectrum mode
Military: Secure communications

Mnemonic: "Data Spreads with PN, Correlates to Recover"

Question 5(a) [3 marks]

Explain the concept of spread spectrum.

Answer:

Spread Spectrum Concept: A communication technique where the transmitted signal bandwidth is much wider than the minimum required bandwidth.

Basic Principle:

Parameter	Before Spreading	After Spreading
Bandwidth	Narrow (data rate)	Wide (chip rate)
Power Density	High	Low
Interference	Vulnerable	Resistant

Key Characteristics:

Bandwidth Expansion: Signal spread over wide frequency range
Processing Gain: Improvement in signal-to-noise ratio
Pseudo-random Sequence: Spreading code known only to intended receiver
Security: Difficult for unauthorized users to intercept

Benefits:

Jam Resistance: Immune to intentional interference
Low Power Density: Coexists with narrowband systems
Multiple Access: Many users share same spectrum
Privacy: Encrypted-like transmission

Mnemonic: "Spread Wide, Gain Processing Power"

Question 5(b) [4 marks]

Write criteria of spread spectrum and its applications.

Answer:

Spread Spectrum Criteria:

Technical Criteria:

Bandwidth: Transmitted bandwidth >> Information bandwidth
Processing Gain: Gp = Spread BW / Data BW ≥ 10 dB
Pseudo-random: Spreading sequence appears random
Synchronization: Receiver must sync with transmitter code

Performance Criteria Table:

Criteria	Requirement	Benefit
Processing Gain	> 10 dB	Interference rejection
Code Length	Long period	Security and randomness
Cross-correlation	Low	Multiple user separation
Auto-correlation	Sharp peak	Synchronization

Applications:

Military Communications:

Anti-jam: Resistant to enemy jamming
LPI/LPD: Low probability of intercept/detection
Secure: Encrypted transmission

Cellular Systems:

CDMA: IS-95, CDMA2000, WCDMA
Capacity: Multiple users per frequency
Quality: Reduced interference

Satellite Communications:

GPS: Global positioning system
Weather: Satellite data transmission
Broadcasting: Satellite radio/TV

Wireless Networks:

WiFi: 802.11b DSSS mode
Bluetooth: Frequency hopping
Cordless Phones: 2.4 GHz band

Mnemonic: "Military, Cellular, Satellite, Wireless use Spread Spectrum"

Question 5(c) [7 marks]

Explain call processing in CDMA.

Answer:

CDMA Call Processing Sequence:

sequenceDiagram
    participant MS as Mobile Station
    participant BTS as Base Station
    participant BSC as Base Station Controller
    participant MSC as Mobile Switching Center

    Note over MS,MSC: Call Origination
    MS->>BTS: Access Request (Random Access)
    BTS->>MS: Access Grant (Assign Code)
    MS->>BTS: Call Setup Request
    BTS->>BSC: Forward Call Request
    BSC->>MSC: Route Call Setup
    MSC->>BSC: Assign Traffic Channel
    BSC->>BTS: Allocate Walsh Code
    BTS->>MS: Traffic Channel Assignment
    MS->>BTS: Confirm Assignment
    Note over MS,MSC: Call in Progress

Call Origination Process:

Step 1: System Access

Random Access: Mobile sends access probe on access channel
Power Control: Gradually increases power until acknowledged
Code Assignment: Base station assigns unique spreading code

Step 2: Authentication

Challenge: Network sends authentication challenge
Response: Mobile responds with calculated authentication
Validation: Network validates mobile identity

Step 3: Channel Assignment

Walsh Code: Unique orthogonal code assigned for forward link
PN Offset: Base station identified by PN sequence offset
Power Level: Initial transmission power set

Step 4: Traffic Channel Setup

Service Options: Voice, data, or multimedia service negotiated
Rate Set: Transmission rate configured (Rate Set 1 or 2)
Handoff Parameters: Neighboring cell information provided

Call Processing Features:

Soft Handoff:

Multiple Connections: Mobile maintains links to multiple base stations
Diversity: Improves call quality and reliability
Make-before-Break: New connection established before old one dropped

Power Control:

Closed Loop: Rapid power adjustments (800 Hz rate)
Open Loop: Initial power estimation
Purpose: Minimize interference, maximize capacity

Variable Rate Vocoder:

Rate Adaptation: Transmission rate varies with speech activity
Silence Detection: Lower rates during speech pauses
Capacity: Increases system capacity

Call Termination Process:

sequenceDiagram
    participant PSTN as PSTN
    participant MSC as MSC
    participant HLR as HLR
    participant BSC as BSC/BTS
    participant MS as Mobile Station

    PSTN->>MSC: Incoming Call
    MSC->>HLR: Location Request
    HLR->>MSC: Routing Information
    MSC->>BSC: Page Mobile
    BSC->>MS: Paging Message
    MS->>BSC: Page Response
    BSC->>MSC: Page Response
    MSC->>BSC: Setup Traffic Channel
    BSC->>MS: Channel Assignment
    MS->>BSC: Assignment Complete
    Note over PSTN,MS: Call Connected

Key CDMA Features:

Rake Receiver:

Multipath Combining: Combines multiple signal paths
Diversity Gain: Improves signal quality
Finger Assignment: Each finger tracks different path

Capacity Advantages:

Frequency Reuse: Same frequency used in all cells
Interference Limited: Capacity limited by interference, not frequency
Voice Activity: Statistical multiplexing increases capacity

Quality Features:

Error Correction: Forward error correction coding
Interleaving: Protects against burst errors
Adaptive Rates: Data rate adapts to channel conditions

Call States:

Idle: Mobile monitoring paging channel
Access: Attempting to access system
Traffic: Active call in progress
Handoff: Transitioning between base stations

Mnemonic: "Access-Authenticate-Assign-Traffic-Handoff"

Question 5(a OR) [3 marks]

Write features of Zigbee and advantages.

Answer:

Zigbee Features:

Technical Specifications Table:

Parameter	Specification
Standard	IEEE 802.15.4
Frequency	2.4 GHz, 915 MHz, 868 MHz
Data Rate	250 kbps (2.4 GHz)
Range	10-100 meters
Power	Ultra-low power

Key Features:

Mesh Network: Self-organizing and self-healing network
Low Power: Battery life up to years
Low Cost: Inexpensive hardware implementation
Simple Protocol: Easy to implement and deploy

Advantages:

Long Battery Life: Optimized for battery-powered devices
Network Reliability: Multiple routing paths available
Scalability: Supports thousands of nodes
Interoperability: Standard ensures device compatibility

Applications:

Home automation, Industrial monitoring, Smart lighting

Mnemonic: "Low Power, Mesh Network, Many Applications"

Question 5(b OR) [4 marks]

Explain OFDM with block diagram.

Answer:

OFDM Block Diagram:

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

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

OFDM Principle: Orthogonal Frequency Division Multiplexing divides high-speed data into multiple parallel low-speed streams transmitted simultaneously on different frequencies.

Key Components:

IFFT/FFT:

IFFT: Inverse Fast Fourier Transform creates orthogonal subcarriers
FFT: Fast Fourier Transform recovers data at receiver
Orthogonality: Subcarriers don't interfere with each other

Cyclic Prefix:

Function: Prevents inter-symbol interference
Implementation: Copy of signal end added to beginning
Length: Longer than channel delay spread

Advantages:

Spectral Efficiency: High data rate in limited bandwidth
Multipath Immunity: Resistant to fading channels
Flexible: Easy to implement with DSP

Applications:

4G LTE: Mobile communication standard
WiFi: 802.11a/g/n/ac standards
Digital TV: DVB-T, ISDB-T standards

Mnemonic: "Orthogonal Frequencies Divide Multiplexed data"

Question 5(c OR) [7 marks]

Describe MANET.

Answer:

MANET Overview: Mobile Ad-hoc Network is a self-configuring network of mobile devices connected wirelessly without fixed infrastructure.

Network Topology:

     A ---- B
     |    / |
     |   /  |
     C ---- D ---- E
       \   /
        \ /
         F

Key Characteristics:

Architecture Table:

Parameter	MANET	Cellular Network
Infrastructure	No fixed base stations	Base stations required
Topology	Dynamic, changes frequently	Fixed cell structure
Routing	Multi-hop peer-to-peer	Single hop to base station
Cost	Low deployment cost	High infrastructure cost

MANET Features:

Dynamic Topology:

Mobile Nodes: All nodes can move freely
Changing Links: Network connections change as nodes move
Self-Organization: Network automatically reconfigures

Multi-hop Communication:

Relay Function: Nodes act as routers for other nodes
Path Discovery: Dynamic route finding to destination
Distributed Control: No central coordination needed

Routing Protocols:

Proactive Protocols:

DSDV: Destination Sequenced Distance Vector
Characteristic: Maintain routing tables continuously
Advantage: Routes available immediately
Disadvantage: High overhead in mobile environment

Reactive Protocols:

AODV: Ad-hoc On-demand Distance Vector
DSR: Dynamic Source Routing
Characteristic: Find routes only when needed
Advantage: Lower overhead
Disadvantage: Route discovery delay

Hybrid Protocols:

ZRP: Zone Routing Protocol
Combination: Proactive within zone, reactive between zones
Balance: Overhead vs. delay optimization

Advantages:

No Infrastructure: Quick deployment without base stations
Flexibility: Network adapts to changing topology
Cost Effective: Lower setup and maintenance costs
Robustness: No single point of failure

Disadvantages:

Limited Bandwidth: Shared wireless medium
Power Consumption: Routing functions drain battery
Security Issues: Vulnerable to attacks
Scalability: Performance degrades with network size

Applications:

Military Operations:

Battlefield Communications: Soldier-to-soldier communication
Emergency Response: Disaster relief coordination
Surveillance: Sensor network deployment

Commercial Applications:

Vehicular Networks: Car-to-car communication
Sensor Networks: Environmental monitoring
Conference Networks: Temporary meeting networks
Personal Area Networks: Device interconnection

Challenges:

Technical Challenges:

Routing Overhead: Control message bandwidth consumption
Quality of Service: Difficulty in guaranteeing service levels
Power Management: Energy-efficient operation
Interference: Co-channel interference from multiple hops

Security Challenges:

Authentication: Verifying node identity
Data Integrity: Ensuring message authenticity
Privacy: Protecting user information
Denial of Service: Preventing network attacks

Performance Metrics:

Throughput: Data delivery rate
Delay: End-to-end packet delivery time
Packet Loss: Percentage of lost packets
Energy Consumption: Battery life optimization

Future Trends:

Integration: Combination with cellular and WiFi networks
IoT Applications: Internet of Things device networks
5G Integration: Part of 5G network architecture
AI-based Routing: Machine learning for optimal routing

Mnemonic: "Mobile Nodes, Ad-hoc Routing, No Infrastructure, Temporary Networks"