Question 1(a) [3 marks]

Draw block diagram of digital communication system.

Answer:

graph LR
    A[Information Source] --> B[Source Encoder]
    B --> C[Channel Encoder]
    C --> D[Digital Modulator]
    D --> E[Channel]
    E --> F[Digital Demodulator]
    F --> G[Channel Decoder]
    G --> H[Source Decoder]
    H --> I[Information Sink]
    
    J[Noise Source] --> E

Key Components:

Information Source: Generates message signal
Source Encoder: Converts analog to digital
Channel Encoder: Adds error correction codes
Digital Modulator: Converts digital bits to analog signal

Mnemonic: "Source Channel Modulator travels through Channel to Demodulator Channel Sink"

Question 1(b) [4 marks]

Write the function of transmitter and receiver of digital communication system.

Answer:

Component	Function
Transmitter	Converts information signal into suitable form for transmission
Source Encoder	Analog to digital conversion, sampling, quantization
Channel Encoder	Error detection and correction coding
Digital Modulator	Converts digital bits to analog waveform

Component	Function
Receiver	Recovers original information from received signal
Digital Demodulator	Converts received analog signal to digital bits
Channel Decoder	Error detection and correction
Source Decoder	Digital to analog conversion

Key Functions:

Signal Processing: Encoding, modulation, filtering
Error Control: Detection and correction of transmission errors
Signal Recovery: Demodulation and decoding at receiver

Mnemonic: "Transmitter Encodes Modulates, Receiver Demodulates Decodes"

Question 1(c) [7 marks]

Define and explain with example: Continues time and discrete time signals, Real and complex signals and even and odd signals.

Answer:

Signal Type	Definition	Example
Continuous Time	Signal defined for all time values	x(t) = sin(2πt)
Discrete Time	Signal defined only at specific time instants	x[n] = sin(2πn/8)
Real Signal	Signal with real values only	x(t) = 5cos(t)
Complex Signal	Signal with real and imaginary parts	x(t) = 3 + j4sin(t)

Even and Odd Signals:

graph LR
    A["Signal x[n]"] --> B{"Check x[-n]"}
    B --> C["x[n] = x[-n]<br/>EVEN SIGNAL"]
    B --> D["x[n] = -x[-n]<br/>ODD SIGNAL"]
    
    C --> E["Example: cos[n]"]
    D --> F["Example: sin[n]"]

Properties:

Even Signal: Symmetric about y-axis, x(t) = x(-t)
Odd Signal: Anti-symmetric about origin, x(t) = -x(-t)
Complex Signal: z(t) = x(t) + jy(t)
Discrete Signal: Sampled version of continuous signal

Mnemonic: "Continuous Everywhere, Discrete Specific, Real Simple, Complex Combined"

Question 1(c OR) [7 marks]

Define and explain with example: Unit step function, Unit impulse function, Unit ramp function

Answer:

Function	Definition	Mathematical Form
Unit Step	u(t) = 1 for t≥0, 0 for t<0	u(t) = 1 for t≥0
Unit Impulse	δ(t) = ∞ for t=0, 0 elsewhere	∫δ(t)dt = 1
Unit Ramp	r(t) = t for t≥0, 0 for t<0	r(t) = t·u(t)

Unit Step Function:        Unit Impulse Function:      Unit Ramp Function:
                          
    1 |----                   ∞ |                           |  /
      |                         | |                         | /
    0 |____                   0 |_|____                   0 |/____
      0    t                     0    t                     0    t

Applications:

Unit Step: Switch operations, system response analysis
Unit Impulse: System impulse response, convolution
Unit Ramp: System ramp response, integration

Properties:

Step: Derivative of ramp, integral of impulse
Impulse: Derivative of step function
Ramp: Integral of step function

Mnemonic: "Step Sudden, Impulse Instant, Ramp Rising"

Question 2(a) [3 marks]

Define: bit rate, baud rate and bandwidth.

Answer:

Parameter	Definition	Unit
Bit Rate	Number of bits transmitted per second	bps (bits per second)
Baud Rate	Number of signal changes per second	Baud (symbols per second)
Bandwidth	Range of frequencies in signal	Hz (Hertz)

Relationship:

Bit Rate = Baud Rate × log₂(M)
M = number of signal levels
Bandwidth ∝ Baud Rate

Key Points:

Higher bit rate: More data transmission
Baud rate: Symbol transmission rate
Bandwidth: Frequency spectrum occupied

Mnemonic: "Bits Baud Bandwidth - Data Symbol Frequency"

Question 2(b) [4 marks]

Explain Energy and power signal.

Answer:

Signal Type	Definition	Mathematical Form
Energy Signal	Finite energy, zero average power	E = ∫
Power Signal	Finite average power, infinite energy	P = lim(T→∞) 1/T ∫

Classification:

graph LR
    A[Signal] --> B{Energy Finite?}
    B -->|Yes| C[Energy Signal<br/>P = 0]
    B -->|No| D{Power Finite?}
    D -->|Yes| E[Power Signal<br/>E = ∞]
    D -->|No| F[Neither Energy<br/>nor Power]

Examples:

Energy Signal: Exponentially decaying signal e^(-t)u(t)
Power Signal: Sinusoidal signal sin(ωt)
Neither: Ramp signal t·u(t)

Properties:

Energy and power signals are mutually exclusive
Periodic signals are generally power signals
Non-periodic finite duration signals are energy signals

Mnemonic: "Energy Ends, Power Persists"

Question 2(c) [7 marks]

Give the comparison between ASK, FSK and PSK modulation techniques and draw their waveforms.

Answer:

Parameter	ASK	FSK	PSK
Full Form	Amplitude Shift Keying	Frequency Shift Keying	Phase Shift Keying
Varied Parameter	Amplitude	Frequency	Phase
Bandwidth	Narrow	Wide	Narrow
Noise Immunity	Poor	Good	Excellent
Power Efficiency	Poor	Good	Excellent
Implementation	Simple	Moderate	Complex

ASK Waveform:
Data:    1    0    1    1    0
        ___       ___  ___      
       |   |     |   ||   |     
    ___|   |_____|   ||   |_____
    
FSK Waveform:
       ~~~       ~~~ ~~~      
      ~   ~     ~   ~   ~     
    ~~     ~~   ~     ~   ~~   
    
PSK Waveform:
       ___       ___ ___      
      |   |     |   |   |     
    __|   |_____|   |   |_____
      phase shift at data change

Applications:

ASK: Optical communication, simple radio systems
FSK: Telephone modems, radio systems
PSK: Satellite communication, wireless systems

Advantages:

ASK: Simple implementation, low cost
FSK: Good noise performance, constant envelope
PSK: Best noise performance, bandwidth efficient

Mnemonic: "ASK Amplitude, FSK Frequency, PSK Phase"

Question 2(a OR) [3 marks]

A bit rate of signal generator from 8-bit generator is 1600 bps. Calculate the baud rate of signal.

Answer:

Given:

Bit rate = 1600 bps
Number of bits per symbol = 8 bits

Formula: Baud Rate = Bit Rate / Number of bits per symbol

Calculation: Baud Rate = 1600 bps / 8 bits Baud Rate = 200 Baud

Result: The baud rate of the signal is 200 Baud.

Explanation:

Each symbol carries 8 bits of information
1600 bits per second ÷ 8 bits per symbol = 200 symbols per second
Therefore, baud rate = 200 Baud

Mnemonic: "Bit Rate divided by Bits per Symbol gives Baud"

Question 2(b OR) [4 marks]

Find whether the signals are even or odd: 1. x(t) = e^(-5t) 2. x(t) = sin 2t 3. x(t) = cos 5t

Answer:

Signal	Test x(-t)	Result	Type
x(t) = e^(-5t)	x(-t) = e^(5t) ≠ x(t) ≠ -x(t)	Neither	Neither Even nor Odd
x(t) = sin 2t	x(-t) = sin(-2t) = -sin 2t = -x(t)	-x(t)	Odd Signal
x(t) = cos 5t	x(-t) = cos(-5t) = cos 5t = x(t)	x(t)	Even Signal

Test Procedure:

Even Signal Test: Check if x(t) = x(-t)
Odd Signal Test: Check if x(t) = -x(-t)

Properties Used:

Exponential: e^(-at) is neither even nor odd (a > 0)
Sine Function: sin(-x) = -sin(x) → Odd function
Cosine Function: cos(-x) = cos(x) → Even function

Results:

Signal 1: Neither even nor odd
Signal 2: Odd signal
Signal 3: Even signal

Mnemonic: "Cosine Even, Sine Odd, Exponential Neither"

Question 2(c OR) [7 marks]

Explain the Principle of QPSK signal. Draw its modulator and demodulator diagram. Also draw constellation diagram and waveforms of its.

Answer:

QPSK Principle: QPSK (Quadrature Phase Shift Keying) uses four different phase states to represent 2 bits per symbol.

Bits	Phase	I	Q
00	45°	+1	+1
01	135°	-1	+1
10	-45°	+1	-1
11	-135°	-1	-1

QPSK Modulator:

graph LR
    A[Data Stream] --> B[Serial to Parallel]
    B --> C[I Channel]
    B --> D[Q Channel]
    C --> E[Mixer 1]
    D --> F[Mixer 2]
    G["Carrier cos(ωt)"] --> E
    H["Carrier sin(ωt)"] --> F
    E --> I[Adder]
    F --> I
    I --> J[QPSK Output]

Constellation Diagram:

        Q
        |
   01   |   00
  (-1,1)| (1,1)
        |
  ------+------ I
        |
  (-1,-1)|(1,-1)
   11   |   10
        |

QPSK Demodulator:

graph LR
    A[QPSK Input] --> B[Mixer 1]
    A --> C[Mixer 2]
    D["cos(ωt)"] --> B
    E["sin(ωt)"] --> C
    B --> F[LPF]
    C --> G[LPF]
    F --> H[Decision Device]
    G --> I[Decision Device]
    H --> J[Parallel to Serial]
    I --> J
    J --> K[Data Output]

Advantages:

Bandwidth Efficient: 2 bits per symbol
Good Noise Performance: Constant envelope
Widely Used: Standard in digital communication

Applications:

Satellite communication
Digital TV broadcasting
Wireless communication systems

Mnemonic: "QPSK - Quadrature Phase, 2 bits, 4 phases"

Question 3(a) [3 marks]

Draw the block diagram of FSK modulator

Answer:

graph LR
    A[Digital Data] --> B[Switch]
    C[Oscillator 1<br/>f1] --> B
    D[Oscillator 2<br/>f2] --> B
    B --> E[FSK Output]
    
    F[Data = 1] -.-> C
    G[Data = 0] -.-> D

Components:

Digital Data Input: Binary data stream (0s and 1s)
Two Oscillators: f₁ for bit '1', f₂ for bit '0'
Electronic Switch: Selects frequency based on input bit
FSK Output: Frequency modulated signal

Operation:

Bit '1': Switch connects oscillator 1 (higher frequency)
Bit '0': Switch connects oscillator 2 (lower frequency)
Output: Continuous frequency shifting based on data

Mnemonic: "FSK - Frequency Switch based on data Keys"

Question 3(b) [4 marks]

Draw and explain block diagram of PSK modulator.

Answer:

graph LR
    A[Digital Data] --> B[Balanced Modulator]
    C["Carrier Oscillator<br/>cos(ωt)"] --> B
    B --> D[PSK Output]
    
    E[Data = 1] -.-> F[0° phase]
    G[Data = 0] -.-> H[180° phase]

Components and Function:

Component	Function
Digital Data	Binary input stream (0s and 1s)
Carrier Oscillator	Generates reference carrier signal
Balanced Modulator	Multiplies data with carrier
PSK Output	Phase modulated signal

Operation:

Data '1': Output = +cos(ωt) (0° phase)
Data '0': Output = -cos(ωt) (180° phase)
Phase Shift: 180° difference between '1' and '0'

Mathematical Expression:

PSK Signal: s(t) = A·d(t)·cos(ωt)
Where d(t) = +1 for '1', -1 for '0'

Advantages:

Constant Envelope: Better noise immunity
Bandwidth Efficient: Occupies same bandwidth as ASK
Simple Detection: Coherent detection required

Mnemonic: "PSK - Phase Shift using balanced modulator Key"

Question 3(c) [7 marks]

Explain the block diagram of ASK modulator and de-modulator with waveform.

Answer:

ASK Modulator:

graph LR
    A[Digital Data] --> B[Multiplier]
    C["Carrier cos(ωt)"] --> B
    B --> D[ASK Output]

ASK Demodulator:

graph LR
    A[ASK Input] --> B[Multiplier] 
    C[Local Carrier] --> B
    B --> D[Low Pass Filter]
    D --> E[Decision Device]
    E --> F[Digital Output]
    G[Threshold] --> E

Waveforms:

Digital Data:
    1    0    1    1    0
   ___       ___  ___      
  |   |     |   ||   |     
__|   |_____|   ||   |_____

Carrier Signal:
  ∩   ∩   ∩   ∩   ∩   ∩
 ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩ ∩
∩   ∩   ∩   ∩   ∩   ∩   ∩

ASK Output:
  ∩∩∩     ∩∩∩ ∩∩∩    
 ∩   ∩   ∩   ∩   ∩   
∩     ∩ ∩     ∩   ∩

Modulation Process:

Data Bit	Carrier	ASK Output
'1'	A·cos(ωt)	A·cos(ωt)
'0'	A·cos(ωt)	0

Demodulation Process:

Multiplication: ASK signal × Local carrier
Low Pass Filtering: Remove high frequency components
Decision: Compare with threshold to recover data

Applications:

Optical Communication: LED/Laser on-off keying
Simple Radio Systems: AM radio modification
Short Range Communication: IR remote controls

Advantages/Disadvantages:

Advantages	Disadvantages
Simple implementation	Poor noise performance
Low cost	Bandwidth inefficient
Easy detection	Susceptible to fading

Mnemonic: "ASK - Amplitude Switch, multiply and filter Key"

Question 3(a OR) [3 marks]

Write Principle and draw the constellation diagram of MSK.

Answer:

MSK Principle: MSK (Minimum Shift Keying) is a form of continuous-phase FSK where the frequency deviation is exactly half the bit rate.

Key Properties:

Continuous Phase: No phase discontinuities
Minimum Frequency Separation: Δf = Rb/2
Constant Envelope: Good for nonlinear amplifiers

Constellation Diagram:

        Q
        |
        ●  (I=0, Q=1)
        |
   ●----+----●  I
        |
        ●  (I=0, Q=-1)
        |
        
Points rotate continuously
between ±1 on I and Q axes

Mathematical Representation:

Bit '1': f₁ = fc + Rb/4
Bit '0': f₂ = fc - Rb/4
Frequency Deviation: Δf = Rb/2

Characteristics:

Spectral Efficiency: Better than conventional FSK
Continuous Phase: Reduces out-of-band radiation
Orthogonal Detection: Can be detected as OQPSK

Mnemonic: "MSK - Minimum Shift, Continuous phase Key"

Question 3(b OR) [4 marks]

Draw and explain the constellation diagram of 16-QAM

Answer:

16-QAM Constellation:

           Q
           |
     ●  ●  |  ●  ●
           |
     ●  ● -3-1 1 3 ● I
           |
     ●  ●  |  ●  ●
           |
     ●  ●  |  ●  ●
           |

16-QAM Mapping Table:

Bits	I	Q	Amplitude	Phase
0000	-3	-3	√18	225°
0001	-3	-1	√10	198.4°
0010	-3	+1	√10	161.6°
0011	-3	+3	√18	135°
0100	-1	-3	√10	251.6°
0101	-1	-1	√2	225°
...	...	...	...	...

Key Features:

16 Symbol Points: 4 bits per symbol
Gray Coding: Adjacent symbols differ by 1 bit
Variable Amplitude: Different power levels
High Data Rate: 4 times QPSK data rate

Signal Representation: s(t) = I(t)·cos(ωt) - Q(t)·sin(ωt)

Applications:

Digital Cable TV: High data rate transmission
Microwave Links: Point-to-point communication
WiFi Systems: 802.11 standards

Advantages:

High Spectral Efficiency: 4 bits per symbol
Good BER Performance: With proper coding
Flexible Implementation: Software defined radio

Trade-offs:

Higher Complexity: More complex than QPSK
Power Variation: Requires linear amplifiers
Noise Sensitivity: Higher than constant envelope schemes

Mnemonic: "16-QAM - 16 points, 4 bits, Quadrature Amplitude Modulation"

Question 3(c OR) [7 marks]

Compare Bits PER Symbol for digital modulation techniques-ASK, FSK, PSK, QPSK,8-PSK, MSK and 16-QAM

Answer:

Bits per Symbol Comparison:

Modulation	Bits per Symbol	Symbol Rate	Data Rate Relationship
ASK	1	Rs = Rb	Rb = Rs × 1
FSK	1	Rs = Rb	Rb = Rs × 1
PSK (BPSK)	1	Rs = Rb	Rb = Rs × 1
QPSK	2	Rs = Rb/2	Rb = Rs × 2
8-PSK	3	Rs = Rb/3	Rb = Rs × 3
MSK	1	Rs = Rb	Rb = Rs × 1
16-QAM	4	Rs = Rb/4	Rb = Rs × 4

Detailed Analysis:

graph LR
    A[Digital Modulation] --> B[M-ary Modulation]
    B --> C["Bits per Symbol = log₂(M)"]
    C --> D[Higher M = More bits per symbol]
    D --> E[Higher Data Rate]
    E --> F[But Higher Complexity]

Bandwidth Efficiency:

Modulation	M	Bits/Symbol	Bandwidth Efficiency
ASK, FSK, PSK	2	1	1 bit/s/Hz
QPSK	4	2	2 bits/s/Hz
8-PSK	8	3	3 bits/s/Hz
16-QAM	16	4	4 bits/s/Hz

Power Requirements:

Modulation	Relative Power	BER Performance
PSK	Reference	Best
ASK	+3dB penalty	Poor
FSK	Same as PSK	Good
QPSK	Same as PSK	Same as PSK
8-PSK	+2.5dB penalty	Moderate
16-QAM	+4dB penalty	Good with coding

Trade-offs:

Higher M: More bits per symbol but higher complexity
Bandwidth vs Power: Trade-off between spectral and power efficiency
Implementation: Higher order modulation needs better hardware

Applications:

Low Rate: ASK, FSK, PSK for simple systems
Medium Rate: QPSK for balanced performance
High Rate: 8-PSK, 16-QAM for high-speed systems

Formula: Bits per Symbol = log₂(M), where M = number of symbols

Mnemonic: "More symbols, More bits, More complexity"

Question 4(a) [3 marks]

Define probability and write it Significance of in communication

Answer:

Definition of Probability: Probability is the measure of likelihood that an event will occur, expressed as a number between 0 and 1.

P(Event) = Number of favorable outcomes / Total number of possible outcomes

Significance in Communication:

Application	Significance
Error Analysis	Calculate bit error rate (BER)
Channel Modeling	Noise and fading statistics
Coding Theory	Error correction probability
Signal Detection	Detection and false alarm rates

Key Applications:

BER Calculation: P(error) = Q(√(2Eb/N0))
Channel Capacity: Shannon's theorem uses probability
Information Theory: Entropy based on probability
System Design: Performance prediction

Mathematical Tools:

Gaussian Distribution: For noise analysis
Rayleigh Distribution: For fading channels
Poisson Distribution: For arrival processes

Mnemonic: "Probability Predicts Performance in communication systems"

Question 4(b) [4 marks]

Explain Huffman code with suitable example

Answer:

Huffman Coding Principle: Variable length coding where frequently occurring symbols get shorter codes.

Algorithm:

List symbols with probabilities
Combine two lowest probability symbols
Repeat until single symbol remains
Assign codes: left = 0, right = 1

Example:

Symbol	Probability	Huffman Code
A	0.4	0
B	0.3	10
C	0.2	110
D	0.1	111

Huffman Tree Construction:

graph LR
    A1[1.0] --> B1[A: 0.4]
    A1 --> C1[0.6]
    C1 --> D1[B: 0.3] 
    C1 --> E1[0.3]
    E1 --> F1[C: 0.2]
    E1 --> G1[D: 0.1]

Code Assignment:

A: 0 (1 bit)
B: 10 (2 bits)
C: 110 (3 bits)
D: 111 (3 bits)

Average Code Length: L = 0.4×1 + 0.3×2 + 0.2×3 + 0.1×3 = 1.9 bits/symbol

Advantages:

Optimal: Minimum average code length
Prefix Property: No code is prefix of another
Efficient: Reduces transmission bandwidth

Mnemonic: "Huffman - Frequent symbols get Shorter codes"

Question 4(c) [7 marks]

Explain concept and key features of Internet of Things (IoT).

Answer:

IoT Concept: Internet of Things is the network of physical devices embedded with sensors, software, and connectivity to collect and exchange data.

IoT Architecture:

graph LR
    A[Physical Devices] --> B[Connectivity Layer]
    B --> C[Data Processing]
    C --> D[Application Layer]
    D --> E[Business Layer]
    
    F[Sensors] --> A
    G[Actuators] --> A
    H[WiFi/Bluetooth] --> B
    I[Cellular/LoRa] --> B
    J[Cloud Computing] --> C
    K[Edge Computing] --> C

Key Features:

Feature	Description	Example
Connectivity	Devices connected to internet	WiFi, 4G, 5G
Intelligence	Smart decision making	AI algorithms
Sensing	Data collection from environment	Temperature, humidity
Actuation	Control physical processes	Motors, valves
Interoperability	Devices work together	Standard protocols

IoT Protocol Stack:

Layer	Protocols	Function
Application	HTTP, CoAP, MQTT	Data exchange
Transport	TCP, UDP	Reliable transmission
Network	IPv6, 6LoWPAN	Routing
Physical	WiFi, ZigBee, LoRa	Connectivity

Applications:

Smart Home: Automated lighting, security
Industrial IoT: Manufacturing automation
Healthcare: Remote patient monitoring
Smart Cities: Traffic management, utilities

Challenges:

Security: Device vulnerabilities, data privacy
Scalability: Billions of devices
Interoperability: Different standards
Power Consumption: Battery-operated devices

Benefits:

Automation: Reduced human intervention
Efficiency: Optimized resource usage
Real-time Monitoring: Instant data access
Cost Reduction: Predictive maintenance

Technologies:

Communication: WiFi, Bluetooth, Cellular, LoRa
Processing: Edge computing, cloud computing
Analytics: Big data, machine learning
Security: Encryption, authentication

Mnemonic: "IoT - Internet of Things, Smart Connected Devices everywhere"

Question 4(a OR) [3 marks]

Define error correction code and list common error correcting code.

Answer:

Error Correction Code Definition: Error correction codes are techniques that add redundant bits to data to detect and correct transmission errors automatically.

Common Error Correcting Codes:

Code Type	Description	Capability
Hamming Code	Single error correction	Correct 1-bit error
Reed-Solomon	Block code for burst errors	Correct multiple errors
BCH Code	Binary cyclic code	Correct t errors
Convolutional Code	Continuous encoding	Good for noisy channels
Turbo Code	Iterative decoding	Near Shannon limit
LDPC Code	Low density parity check	Excellent performance

Applications:

Memory Systems: ECC RAM
Storage Devices: Hard drives, CDs
Communication: Satellite, cellular
Broadcasting: Digital TV, radio

Mnemonic: "Error Correction Codes - Hamming Reed BCH Convolutional Turbo LDPC"

Question 4(b OR) [4 marks]

Explain Shanon Fano code with suitable example

Answer:

Shannon-Fano Coding Algorithm: Top-down approach that divides symbols into two groups with approximately equal probabilities.

Algorithm Steps:

Arrange symbols in decreasing probability order
Divide into two groups with nearly equal total probability
Assign '0' to first group, '1' to second group
Repeat for each subgroup

Example:

Symbol	Probability	Shannon-Fano Code
A	0.4	00
B	0.3	01
C	0.2	10
D	0.1	11

Construction Tree:

graph TD
    A1[A,B,C,D: 1.0] --> B1[A,B: 0.7]
    A1 --> C1[C,D: 0.3]
    B1 --> D1[A: 0.4]
    B1 --> E1[B: 0.3]
    C1 --> F1[C: 0.2]
    C1 --> G1[D: 0.1]

Code Assignment:

Group 1 (A,B): Code starts with '0'
Group 2 (C,D): Code starts with '1'
A: 00, B: 01, C: 10, D: 11

Comparison with Huffman:

Shannon-Fano: Top-down approach
Huffman: Bottom-up approach
Huffman: Always optimal
Shannon-Fano: May not be optimal

Average Code Length: L = 0.4×2 + 0.3×2 + 0.2×2 + 0.1×2 = 2.0 bits/symbol

Mnemonic: "Shannon-Fano - Split groups, assign codes Top-down"

Question 4(c OR) [7 marks]

Explain different standard formats of audio signal.

Answer:

Audio Signal Formats:

Format	Full Form	Compression	Quality	File Size
WAV	Waveform Audio File	Uncompressed	Highest	Largest
MP3	MPEG Layer 3	Lossy	Good	Small
AAC	Advanced Audio Coding	Lossy	Better than MP3	Small
FLAC	Free Lossless Audio Codec	Lossless	Original	Medium
OGG	Ogg Vorbis	Lossy	Good	Small

Audio Parameters:

graph TD
    A[Audio Signal] --> B[Sampling Rate]
    A --> C[Bit Depth]
    A --> D[Channels]
    A --> E[Compression]
    
    B --> F[44.1 kHz CD Quality]
    C --> G[16-bit Standard]
    D --> H[Mono/Stereo]
    E --> I[Lossy/Lossless]

Sampling Standards:

Standard	Sampling Rate	Bit Depth	Application
CD Quality	44.1 kHz	16-bit	Consumer audio
Studio Quality	48 kHz	24-bit	Professional recording
High Resolution	96 kHz	24-bit	Audiophile
Telephone	8 kHz	8-bit	Voice communication

Compression Types:

Lossless: Original quality preserved (FLAC, ALAC)
Lossy: Some quality lost for smaller size (MP3, AAC)
Uncompressed: No compression (WAV, AIFF)

Applications:

Broadcasting: AAC for digital radio
Streaming: MP3, AAC for internet
Professional: WAV, FLAC for studios
Mobile: AAC for smartphones

File Size Comparison (3-minute song):

WAV: 30 MB
FLAC: 20 MB
MP3: 3 MB
AAC: 2.5 MB

Quality vs Size Trade-off:

Highest Quality: WAV (uncompressed)
Best Balance: AAC (lossy compressed)
Smallest Size: Low bitrate MP3
Lossless Compressed: FLAC

Mnemonic: "WAV MP3 AAC FLAC - Wave, Layer3, Advanced, Free Lossless"

Question 5(a) [3 marks]

Explain E1 carrier multiplexing hierarchy.

Answer:

E1 Carrier System: European digital transmission standard for multiplexing voice channels.

E1 Hierarchy:

Level	Name	Bit Rate	Voice Channels	Multiplexing
E0	Basic Channel	64 kbps	1	-
E1	Primary Rate	2.048 Mbps	30	30 × E0 + 2
E2	Secondary Rate	8.448 Mbps	120	4 × E1
E3	Tertiary Rate	34.368 Mbps	480	4 × E2
E4	Quaternary Rate	139.264 Mbps	1920	4 × E3

E1 Frame Structure:

Frame (125 μs, 256 bits)
|TS0|TS1|TS2|...|TS15|TS16|TS17|...|TS31|
 32 time slots × 8 bits = 256 bits

TS0: Synchronization + Alarm
TS16: Signaling (voice channels)
TS1-15, 17-31: 30 voice channels

Multiplexing Process:

Level 1: 30 voice channels + 2 control → E1
Level 2: 4 E1 streams → E2
Level 3: 4 E2 streams → E3
Level 4: 4 E3 streams → E4

Applications:

ISDN: Primary rate interface
Cellular: Base station connectivity
Enterprise: Private branch exchange (PBX)
Internet: Digital subscriber line (DSL)

Mnemonic: "E1 - 30 voices, 2.048 Mbps, European standard"

Question 5(b) [4 marks]

Compare FDMA with TDMA.

Answer:

FDMA vs TDMA Comparison:

Parameter	FDMA	TDMA
Full Form	Frequency Division Multiple Access	Time Division Multiple Access
Domain	Frequency	Time
Channel Allocation	Each user gets different frequency	Each user gets different time slot
Bandwidth per User	Narrow bandwidth continuously	Full bandwidth for short duration
Guard Bands	Required between frequencies	Not required
Synchronization	Not critical	Critical
Flexibility	Less flexible	More flexible
Handoff	Simple	Complex
Near-Far Effect	Less problematic	More problematic

FDMA System:

graph TD
    A[Total Bandwidth] --> B[User 1: f1]
    A --> C[User 2: f2]
    A --> D[User 3: f3]
    A --> E[User N: fn]
    
    F[Guard Band] --> B
    F --> C
    F --> D

TDMA System:

gantt
    title TDMA Time Slots
    dateFormat X
    axisFormat %s
    
    section Frame
    User 1 :done, u1, 0, 1
    User 2 :done, u2, 1, 2
    User 3 :done, u3, 2, 3
    User 4 :done, u4, 3, 4

Advantages/Disadvantages:

FDMA Advantages	FDMA Disadvantages
Simple implementation	Waste of bandwidth due to guard bands
No synchronization needed	Less flexible
Continuous transmission	Difficult to accommodate varying rates

TDMA Advantages	TDMA Disadvantages
Efficient bandwidth usage	Complex synchronization
Flexible data rates	Battery life issues (burst transmission)
Easy to add/remove users	Near-far problem

Applications:

FDMA: AMPS (1G), satellite communication
TDMA: GSM (2G), satellite communication

Mnemonic: "FDMA Frequency, TDMA Time - different domains for multiple access"

Question 5(c) [7 marks]

Explain CDMA technique in detail.

Answer:

CDMA Principle: Code Division Multiple Access allows multiple users to share the same frequency and time by using unique spreading codes.

CDMA System Architecture:

graph LR
    A[User Data] --> B[Spreading Code]
    B --> C[Modulator]
    C --> D[Channel]
    D --> E[Correlator]
    E --> F[Despreading]
    F --> G[Data Recovery]
    
    H[Pseudo-random Code] --> B
    I[Same PN Code] --> F

Spreading Process:

Parameter	Before Spreading	After Spreading
Data Rate	Rb	Rb
Chip Rate	-	Rc (= N × Rb)
Bandwidth	Rb	Rc
Processing Gain	1	N = Rc/Rb

CDMA Properties:

Original Data:    1  0  1
PN Code:         101 010 101
XOR Result:      101 010 101
(Spread Signal)

At Receiver:
Received:        101 010 101
Same PN Code:    101 010 101  
XOR Result:       1   0   1
(Original Data)

Key Features:

Feature	Description	Benefit
Spreading	Data XOR with PN code	Bandwidth expansion
Processing Gain	Rc/Rb ratio	Interference rejection
Soft Handoff	Simultaneous connections	Better quality
Power Control	Dynamic power adjustment	Near-far solution

CDMA Advantages:

Capacity: Higher user capacity than FDMA/TDMA
Security: Encrypted by spreading code
Soft Handoff: No call dropping during handoff
Anti-jamming: Spread spectrum immunity
No Frequency Planning: Same frequency reuse

CDMA Disadvantages:

Near-Far Problem: Requires power control
Complexity: More complex than FDMA/TDMA
Self Interference: Users interfere with each other
Breathing Effect: Coverage varies with loading

Mathematical Analysis:

Processing Gain: G = Rc/Rb = 10log₁₀(Rc/Rb) dB
Capacity: M ≈ 1 + G/(Eb/I₀)
BER: Depends on number of active users

Power Control:

Open Loop: Based on received signal strength
Closed Loop: Base station commands mobile
Requirement: ±1 dB accuracy needed

Applications:

IS-95 (cdmaOne): 2G CDMA standard
WCDMA: 3G UMTS system
GPS: Satellite navigation
WiFi: Spread spectrum option

PN Code Properties:

Autocorrelation: High for synchronized, low for unsynchronized
Cross-correlation: Low between different codes
Balance: Equal number of 1s and 0s
Run Length: Distribution of consecutive bits

Mnemonic: "CDMA - Code Division, same frequency/time, unique codes for Multiple Access"

Question 5(a OR) [3 marks]

Draw block diagram of Time Division Multiplexing technique (TDM).

Answer:

TDM Block Diagram:

graph LR
    A[Input 1] --> E[Multiplexer]
    B[Input 2] --> E
    C[Input 3] --> E
    D[Input N] --> E
    
    E --> F[TDM Signal]
    F --> G[Channel]
    G --> H[Demultiplexer]
    
    H --> I[Output 1]
    H --> J[Output 2] 
    H --> K[Output 3]
    H --> L[Output N]
    
    M[Clock/Sync] --> E
    N[Clock/Sync] --> H

TDM Frame Structure:

|<---- Frame Period T ---->|
|Ch1|Ch2|Ch3|...|ChN|Sync|
 TS1 TS2 TS3     TSN
 
Each time slot = T/N
Frame Rate = 1/T

Components:

Multiplexer: Samples inputs sequentially
Clock/Synchronization: Controls switching timing
Channel: Transmission medium
Demultiplexer: Separates multiplexed signal

Operation:

Each input channel gets dedicated time slot
Sampling rate must satisfy Nyquist criterion
Frame synchronization required at receiver

Mnemonic: "TDM - Time Division, sequential sampling, Multiplexing"

Question 5(b OR) [4 marks]

Write a short not on classification of multiplexing techniques.

Answer:

Classification of Multiplexing Techniques:

graph TD
    A[Multiplexing] --> B[Analog Multiplexing]
    A --> C[Digital Multiplexing]
    
    B --> D[FDM - Frequency Division]
    B --> E[WDM - Wavelength Division]
    
    C --> F[TDM - Time Division]
    C --> G[CDM - Code Division]
    C --> H[SDM - Space Division]
    
    F --> I[Synchronous TDM]
    F --> J[Asynchronous TDM]

Detailed Classification:

Type	Method	Domain	Application
FDM	Frequency separation	Frequency	Radio, TV broadcasting
TDM	Time slot allocation	Time	Digital telephony
CDM	Code separation	Code	CDMA cellular
WDM	Wavelength separation	Wavelength	Optical fiber
SDM	Space separation	Space	MIMO systems

Synchronous vs Asynchronous TDM:

Parameter	Synchronous TDM	Asynchronous TDM
Time Slots	Fixed allocation	Dynamic allocation
Efficiency	Lower	Higher
Complexity	Simple	Complex
Bandwidth Waste	May occur	Minimal

Selection Criteria:

Nature of Signal: Analog → FDM, Digital → TDM
Bandwidth: Limited → TDM, Abundant → FDM
Synchronization: Critical → Synchronous, Flexible → Asynchronous
Application: Voice → TDM, Data → Statistical TDM

Modern Techniques:

OFDM: Orthogonal Frequency Division Multiplexing
MIMO: Multiple Input Multiple Output
Carrier Aggregation: Multiple frequency bands

Mnemonic: "FDM TDM CDM WDM SDM - Frequency Time Code Wave Space Division Multiplexing"

Question 5(c OR) [7 marks]

Describe the procedure to troubleshoot the code division multiplexing circuit

Answer:

CDMA Troubleshooting Procedure:

1. System Overview Check:

graph TD
    A[CDMA System] --> B[Transmitter Section]
    A --> C[Channel Section]
    A --> D[Receiver Section]
    
    B --> E[Data Input OK?]
    B --> F[PN Code Generation OK?]
    B --> G[Spreading Function OK?]
    
    C --> H[Path Loss Measurement]
    C --> I[Interference Check]
    
    D --> J[Correlation OK?]
    D --> K[Despreading OK?]
    D --> L[Data Recovery OK?]

2. Step-by-Step Troubleshooting:

Step	Parameter	Test Method	Expected Result
1	Input Data	Verify data stream	Clean digital signal
2	PN Code	Check code generation	Proper sequence
3	Spreading	Monitor XOR output	Spread spectrum signal
4	Transmission**	Measure power level	Adequate signal strength
5	Reception	Check received signal	Above noise floor
6	Correlation	Verify correlator output	Peak at correct timing
7	Despreading	Check XOR with local PN	Despread signal
8	Data Recovery**	Verify output data	Original data recovered

3. Common Problems and Solutions:

Problem	Symptoms	Possible Causes	Solutions
No Signal	Zero output	Power supply failure	Check power connections
High BER	Many bit errors	Poor correlation	Adjust timing/power
Interference	Degraded performance	Other users/noise	Power control adjustment
Sync Loss	Intermittent signal	PN code mismatch	Verify code sequences

4. Test Equipment Required:

Equipment	Purpose	Measurement
Spectrum Analyzer	Signal analysis	Power spectral density
BER Tester	Error measurement	Bit error rate
Power Meter	Power measurement	Transmitted/received power
Oscilloscope	Waveform analysis	Time domain signals
Vector Analyzer	Modulation quality	EVM, constellation

5. Measurement Procedures:

Processing Gain Verification:

Gp = 10 log₁₀(Rc/Rb) dB
Where: Rc = chip rate, Rb = bit rate

BER vs Eb/N0 Measurement:

BER = Q(√(2Eb/N0))
Measure at various power levels

Near-Far Effect Check:

Measure power levels of different users
Verify power control operation
Check dynamic range requirements

6. Performance Optimization:

Parameter	Optimization Method	Target Value
Power Control	Adjust loop gain	±1 dB accuracy
Code Selection	Choose orthogonal codes	Low cross-correlation
Timing	Synchronize PN generators	<0.5 chip accuracy
Filtering	Bandlimit signals	Minimize ISI

7. Documentation:

Record all measurements
Document problem symptoms
Note solutions applied
Create troubleshooting log

Systematic Approach:

Isolate: Identify faulty section
Measure: Use appropriate test equipment
Analyze: Compare with specifications
Correct: Apply appropriate solution
Verify: Confirm problem resolution

Safety Considerations:

Power levels within safe limits
Proper grounding procedures
RF exposure guidelines
Equipment calibration status

Mnemonic: "CDMA Troubleshoot - Check Data, PN code, Spreading, Channel, Correlation, Recovery"