Communication Engineering (1333201) - Winter 2024 Solution

Question 1(a) [3 marks]

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What is modulation? What is the need of it?

Answer: Modulation is the process of varying one or more properties of a high-frequency carrier signal with a modulating signal containing information.

Table: Need for Modulation

• Practical transmission: Makes low-frequency information signals suitable for wireless transmission
• Signal separation: Enables different signals to be transmitted simultaneously

Mnemonic: “RARE Messages” (Range, Antenna, Reduce interference, Enable multiplexing)

Question 1(b) [4 marks]

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Compare AM and FM.

Answer:

Table: Comparison between AM and FM

Mnemonic: “BANC-QA” (Bandwidth, Amplitude/frequency, Noise, Complexity, Quality, Applications)

Question 1(c) [7 marks]

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Explain Amplitude modulation with waveform and derive voltage equation for modulated signal also Sketch the frequency spectrum of the DSBFC AM.

Answer:

Amplitude Modulation (AM) is a technique where the amplitude of a carrier wave is varied in proportion to the instantaneous amplitude of the modulating signal.

Voltage Equation:

• Carrier signal: v₁(t) = A₁ sin(ωct)
• Modulating signal: v₂(t) = A₂ sin(ωmt)
• Modulated signal: v(t) = A₁[1 + m sin(ωmt)] sin(ωct)
• Where m = A₂/A₁ (modulation index)

Diagram: AM Waveform

graph TD
    subgraph "AM Waveform"
    A[Carrier Wave] --> D[Modulated Wave]
    B[Modulating Signal] --> D
    end
    style D fill:#f9f,stroke:#333,stroke-width:2px

Frequency Spectrum of DSBFC AM

• Bandwidth: The bandwidth of AM signal is 2 × fm
• Sidebands: Upper sideband (USB) at fc+fm and Lower sideband (LSB) at fc-fm
• Power distribution: In carrier and two sidebands

Mnemonic: “CAM-SIP” (Carrier Amplitude Modified, Sidebands In Pair)

Question 1(c) OR [7 marks]

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Derive the equation for total power in AM, calculate percentage of power savings in DSB and SSB.

Answer:

Derivation of Total Power in AM:

• AM signal: v(t) = A₁[1 + m sin(ωmt)] sin(ωct)
• Total power: P = P₍carrier₎ + P₍sidebands₎
• P₍carrier₎ = A₁²/2
• P₍sidebands₎ = A₁²m²/4

Table: Power Distribution in AM

Power Savings:

• DSB-SC: 100% carrier power saved (66.67% of total power)

  • Only sidebands are transmitted
  • Percentage savings = (P₍c₎/P₍t₎) × 100 = 66.67%

• SSB: 50% of sideband power + 100% carrier power saved

  • One sideband + carrier removed
  • Percentage savings = (P₍c₎ + P₍s₎/2)/P₍t₎ × 100 = 83.33%

Diagram: Power Distribution

Mnemonic: “CAST-83” (Carrier And Sideband Transmission, 83% saved in SSB)

Question 2(a) [3 marks]

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Define (1) Modulation index for AM (2) Modulation index For FM.

Answer:

Table: Modulation Index Definitions

• AM Modulation Index: Controls the amplitude variation and power distribution
• FM Modulation Index: Determines bandwidth and signal quality

Mnemonic: “ARM-FDM” (Amplitude Ratio for Modulation, Frequency Deviation for Modulation)

Question 2(b) [4 marks]

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Draw and explain block diagram for envelope detector.

Answer:

Diagram: Envelope Detector

Table: Components and Their Functions

• Working principle: The diode conducts only during positive half-cycles
• Time constant: RC must be large enough to prevent ripple but small enough to follow modulation
• Condition: RC » 1/fc but RC « 1/fm

Mnemonic: “DEER” (Diode Extracts Envelope Representation)

Question 2(c) [7 marks]

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Draw block diagram of FM radio receiver and explain working of each block.

Answer:

Diagram: FM Radio Receiver

flowchart LR
    A[Antenna] --> B[RF Amplifier]
    B --> C[Mixer]
    D[Local Oscillator] --> C
    C --> E[IF Amplifier]
    E --> F[Limiter]
    F --> G[FM Discriminator]
    G --> H[Audio Amplifier]
    H --> I[Speaker]

Table: Functions of Each Block

• Superheterodyne principle: Uses frequency conversion to process signals at fixed IF
• Distinctive FM feature: Limiter removes noise in amplitude before demodulation

Mnemonic: “RAMLIDASS” (RF, Amplifier, Mixer, Local oscillator, IF, Discriminator, Audio, Speaker System)

Question 2(a) OR [3 marks]

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Draw only Waveform For frequency modulation and Phase modulation.

Answer:

Diagram: FM and PM Waveforms

Key Characteristics:

• FM: Frequency increases when modulating signal is positive
• PM: Phase shifts immediately with amplitude changes

Mnemonic: “FIP-PAF” (Frequency Increases with Positive signal, Phase Advances with Faster changes)

Question 2(b) OR [4 marks]

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Define any FOUR characteristics of radio receiver.

Answer:

Table: Characteristics of Radio Receiver

Additional characteristics:

• Signal-to-Noise Ratio: Ratio of signal power to noise power
• Bandwidth: Range of frequencies that can be received
• Stability: Ability to maintain tuned frequency

Mnemonic: “SFIS-BSS” (Sensitivity, Fidelity, Image rejection, Selectivity - Better Signal Stability)

Question 2(c) OR [7 marks]

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Draw block diagram of AM radio receiver and explain working of each block.

Answer:

Diagram: AM Radio Receiver

flowchart LR
    A[Antenna] --> B[RF Tuner & Amplifier]
    B --> C[Mixer]
    D[Local Oscillator] --> C
    C --> E[IF Amplifier]
    E --> F[Detector]
    F --> G[AGC]
    G --> E
    F --> H[Audio Amplifier]
    H --> I[Speaker]

Table: Functions of Each Block

• Superheterodyne principle: Uses frequency conversion for better selectivity
• AGC feedback loop: Maintains constant output despite signal strength variations

Mnemonic: “ARMLESS” (Antenna, RF, Mixer, Local oscillator, Envelope detector, Sound System)

Question 3(a) [3 marks]

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Define quantization. Explain non uniform quantization in brief.

Answer:

Quantization is the process of converting continuous amplitude values into discrete levels for digital representation.

Table: Non-uniform Quantization

• Working principle: Smaller step sizes for lower amplitudes, larger steps for higher amplitudes
• Effect: Improves SNR for weak signals at the expense of strong signals

Mnemonic: “QUEST-CS” (QUantization with Enhanced Steps - Compressing Small signals)

Question 3(b) [4 marks]

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Explain Sample and hold Circuit with Waveform.

Answer:

Diagram: Sample and Hold Circuit

Diagram: Sample and Hold Waveform

Sample and Hold Operation:

• Sampling mode: Switch closes, capacitor charges to input voltage
• Hold mode: Switch opens, capacitor maintains voltage
• Parameters: Acquisition time, aperture time, hold time, droop rate

Mnemonic: “CHASED” (Capacitor Holds Amplitude Samples for Extended Duration)

Question 3(c) [7 marks]

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What is sampling? Explain types of sampling in brief.

Answer:

Sampling is the process of converting a continuous-time signal into a discrete-time signal by taking measurements at regular intervals.

Table: Types of Sampling

Diagram: Sampling Types

• Nyquist criterion: Sampling frequency must be at least twice the highest frequency in the signal

Mnemonic: “INFUN” (Ideal, Natural, Flat-top, Uniform, Non-uniform)

Question 3(a) OR [3 marks]

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Explain quantization process and its necessity.

Answer:

Quantization Process maps continuous amplitude values to finite discrete levels for digital representation.

Table: Quantization Process and Necessity

• Step size calculation: Step size = (Vmax - Vmin)/2ⁿ
• Quantization error: Maximum error is ±Q/2 where Q is step size
• Applications: Digital communication, audio/video processing, data storage

Mnemonic: “SEND” (Step-size Establishes Noise in Digitization)

Question 3(b) OR [4 marks]

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State and explain Nyquist Criteria for sampling of signal.

Answer:

Nyquist Sampling Theorem states that to perfectly reconstruct a bandlimited signal, the sampling frequency must be at least twice the highest frequency component in the signal.

Table: Nyquist Criteria

Diagram: Sampling Effects

• Consequences of undersampling: Aliasing (frequency folding)
• Practical application: Anti-aliasing filters used before sampling

Mnemonic: “TRAP-A” (Twice Rate Avoids Problematic Aliasing)

Question 3(c) OR [7 marks]

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Explain PAM, PWM and PPM with waveform.

Answer:

Table: Pulse Modulation Techniques

Diagram: Pulse Modulation Waveforms

• PAM: Simplest form, most susceptible to noise
• PWM: Better noise immunity, easy generation
• PPM: Best noise immunity, requires precise timing

Mnemonic: “AWP-PAW” (Amplitude, Width, Position - Pulse Alteration Ways)

Question 4(a) [3 marks]

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What is slop overload noise and granular noise in DM?

Answer:

Table: Noise Types in Delta Modulation

Diagram: DM Noise Types

Mnemonic: “FAST-SLOW” (Fast signals cause Slope overload, SLOW signals cause Granular noise)

Question 4(b) [4 marks]

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Draw and explain TDM frame.

Answer:

Diagram: TDM Frame Structure

Table: TDM Frame Components

• Working principle: Allocates different time slots to different channels
• Synchronization: Essential for proper demultiplexing
• Types: Synchronous TDM (fixed slots) and Statistical TDM (dynamic allocation)

Mnemonic: “FAST-Ch” (Frame And Slots for Transmitting Channels)

Question 4(c) [7 marks]

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Describe the function of each block of PCM transmitter and Receiver. Give application, advantage and disadvantage of PCM system.

Answer:

Diagram: PCM System

flowchart LR
    subgraph "PCM Transmitter"
    A[Sampler] --> B[Quantizer]
    B --> C[Encoder]
    C --> D[Line Coder]
    end
    subgraph "PCM Receiver"
    E[Line Decoder] --> F[Decoder]
    F --> G[Reconstruction Filter]
    end
    D --> E

Table: PCM Block Functions

Applications, Advantages and Disadvantages:

Table: PCM System Characteristics

Mnemonic: “SEQUEL-DR” (Sample, Quantize, Encode - Line code, Decode, Reconstruct)

Question 4(a) OR [3 marks]

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Give difference between DM and ADM modulation.

Answer:

Table: Comparison between DM and ADM

Diagram: DM vs ADM Tracking

Mnemonic: “FAST-VAR” (Fixed And Simple Tracking vs Variable Adaptive Response)

Question 4(b) OR [4 marks]

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Explain Block diagram of basic PCM-TDM system.

Answer:

Diagram: PCM-TDM System

flowchart LR
    A[Input 1] --> B[Low-pass Filter]
    C[Input 2] --> D[Low-pass Filter]
    E[Input n] --> F[Low-pass Filter]
    B & D & F --> G[Multiplexer]
    G --> H[PCM Encoder]
    H --> I[Transmission Channel]
    I --> J[PCM Decoder]
    J --> K[Demultiplexer]
    K --> L[Output 1] & M[Output 2] & N[Output n]

Table: PCM-TDM System Components

• Working principle: Combines time division multiplexing with pulse code modulation
• Applications: Digital telephony, digital audio broadcasting, communication networks

Mnemonic: “FLIMPED” (Filter, Limit, Multiplex, PCM Encode, Decode)

Question 4(c) OR [7 marks]

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Explain DPCM modulator with equation and waveform.

Answer:

Differential Pulse Code Modulation (DPCM) encodes the difference between the current sample and a predicted value based on previous samples.

Equation:

• Error signal: e(n) = x(n) - x̂(n)
• Where x(n) is current sample, x̂(n) is predicted sample
• Prediction: x̂(n) = Σ(aᵢ × x(n-i))
• Transmitted signal: DPCM output = Q[e(n)]

Diagram: DPCM Modulator

flowchart LR
    A[Input x(n)] --> B((+))
    B --> C[Quantizer]
    C --> D[Encoder]
    D --> E[Output]
    C --> F[Predictor]
    F -->|x̂(n)| G((−))
    G --> B

Diagram: DPCM Waveform

Table: DPCM Characteristics

Mnemonic: “PQED” (Predict, Quantize Error, Encode Difference)

Question 5(a) [3 marks]

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Define Antenna and radiation pattern and polarization.

Answer:

Table: Antenna Definitions

Types of Polarization:

• Linear: Electric field oscillates in one direction (vertical, horizontal)
• Circular: Electric field rotates with constant amplitude (RHCP, LHCP)
• Elliptical: Electric field rotates with varying amplitude

Mnemonic: “WAVE-PRO” (Wireless Antenna Validates Electromagnetic Propagation, Radiation, Orientation)

Question 5(b) [4 marks]

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Explain Microstrip Antenna with sketch.

Answer:

Diagram: Microstrip Patch Antenna

Table: Microstrip Antenna Components

• Working principle: Fringing fields at edges cause radiation
• Advantages: Low profile, lightweight, easy fabrication, compatible with PCB
• Applications: Mobile devices, satellites, aircraft, RFID tags

Mnemonic: “SPGF” (Substrate, Patch, Ground, Feed)

Question 5(c) [7 marks]

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Explain delta modulation with necessary sketch and waveform.

Answer:

Delta Modulation (DM) is the simplest form of differential pulse code modulation where the difference between successive samples is encoded into a single bit.

Diagram: Delta Modulator

flowchart LR
    A[Input Signal] --> B((+))
    B --> C[1-bit Quantizer]
    C --> D[Output]
    C --> E[Delay]
    E --> F[Integrator]
    F -->|Approximated Signal| G((−))
    G --> B

Diagram: Delta Modulation Waveform

Table: Delta Modulation Characteristics

Mnemonic: “SIGN-UP” (SInGle bit, Next step Up or down, Predict)

Question 5(a) OR [3 marks]

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What is smart antenna? list application of it.

Answer:

A Smart Antenna is an adaptive array system that uses digital signal processing algorithms to dynamically adjust its radiation pattern to enhance communication performance.

Table: Smart Antenna Applications

• Working principle: Uses beamforming to focus signal energy toward desired users
• Types: Switched beam systems and adaptive array systems

Mnemonic: “SWIM-CM” (Smart Wireless In Mobile-Cellular-Military)

Question 5(b) OR [4 marks]

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Explain parabolic reflector antenna With Sketch.

Answer:

Diagram: Parabolic Reflector Antenna

Table: Parabolic Reflector Components

• Working principle: Incoming parallel rays are reflected to focus at focal point
• Characteristics: High gain, directivity, narrow beamwidth
• Applications: Satellite communication, radio astronomy, radar, microwave links

Mnemonic: “PFGH” (Parabolic Focus Gives High-gain)

Question 5(c) OR [7 marks]

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Explain Adaptive Delta modulation with necessary sketch and waveform.

Answer:

Adaptive Delta Modulation (ADM) improves on standard DM by dynamically adjusting the step size according to the input signal characteristics.

Diagram: Adaptive Delta Modulator

flowchart LR
    A[Input Signal] --> B((+))
    B --> C[1-bit Quantizer]
    C --> D[Output]
    C --> E[Step Size Control]
    E --> F[Integrator]
    F -->|Approximated Signal| G((−))
    G --> B

Diagram: ADM Waveform

Table: ADM Characteristics

• Step size adaptation: μ(n) = μ(n-1) × K if consecutive bits are same
• Step size adaptation: μ(n) = μ(n-1) / K if consecutive bits change

Mnemonic: “ADVISED” (ADaptive Variable Increment Step for Enhanced Delta modulation)