EC-502 Digital Communication - Nov 2022 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain different types of Sampling. What is Aperture effect and aliasing effect?
   How to answer

   • Define sampling and sampling theorem
   • Explain natural, flat-top and ideal/instantaneous sampling
   • Define aperture effect with formula
   • Define aliasing effect and how to avoid
2. 7 Marks
   bWith the help of suitable diagram explain the generation of PWM wave.
   How to answer

   • Block diagram of PWM generator using comparator
   • Comparison of message with sawtooth carrier
   • Waveforms showing pulse width variation
   • Detection by low-pass filtering
3. 7 Marks
   aExplain about need of quantization, what and how is quantization done? What is quantization noise and how can it be reduced?
   How to answer

   • Need of quantization in ADC/PCM
   • Steps: sampling, quantizing, encoding - uniform/non-uniform
   • Define quantization noise and error $q = x - x_q$
   • Methods to reduce: increase levels, companding, non-uniform quantization
4. 7 Marks
   bExplain the generation and detection of delta modulated signal.
   How to answer

   • Block diagram of delta modulator with comparator and integrator
   • Generation: 1-bit difference encoding with step size
   • Detection using integrator and low-pass filter
   • Slope overload and granular noise
5. 7 Marks
   aExplain the concept of AMI encoding technique.
   How to answer

   • Definition of AMI as bipolar alternate mark inversion
   • Encoding rule with example bit sequence
   • Waveform and DC-free property
   • Clock recovery advantage
6. 7 Marks
   bExplain the concept of differential encoding and its advantages and disadvantages.
   How to answer

   • Principle of differential encoding with XOR
   • Advantages: phase ambiguity resolution, etc.
   • Disadvantages: error propagation, complexity
   • Example waveform
7. 7 Marks
   aExplain the generation and detection of following: BPSK
   How to answer

   • BPSK transmitter block diagram with balanced modulator
   • BPSK signal expression $s(t)=A_c\cos(2\pi f_c t+\pi d)$
   • Coherent receiver with carrier recovery
   • Constellation and waveforms
8. 7 Marks
   bExplain the generation and detection of following: QAM
   How to answer

   • QAM transmitter using I and Q ASK modulators
   • Signal constellation for 16-QAM
   • Coherent QAM receiver structure
   • Waveforms and bandwidth efficiency
9. 7 Marks
   aDiscuss the concept of M-ary PSK.
   How to answer

   • M-ary PSK signal expression and constellation
   • Transmitter and receiver block diagram
   • Bandwidth and power efficiency vs M
   • Example: BPSK, QPSK, 8-PSK
10. 7 Marks
    bDiscuss the spectral properties of QPSK and MSK.
    How to answer

    • PSD expression for QPSK and MSK
    • Main lobe width and sidelobe decay comparison
    • Constant envelope property of MSK
    • Spectral efficiency diagram
11. 7 Marks
    aExplain the concept of matched filter and correlator.
    How to answer

    • Impulse response $h(t)=s(T-t)$ for matched filter
    • Block diagrams of matched filter and correlator
    • Output SNR maximization
    • Equivalence of two structures
12. 7 Marks
    bCalculate the probability of error for BPSK system.
    How to answer

    • BPSK signal model in AWGN
    • Decision rule and error function $Q(\sqrt{2E_b/N_0})$
    • Derivation steps via matched filter output statistics
    • Final $P_e = Q\left(\sqrt{2E_b/N_0}\right)$ curve
13. 7 Marks
    aWhat is channel capacity, discuss about Shannon's theorem?
    How to answer

    • Definition $C = B\log_2(1+S/N)$
    • Statement of Shannon's channel coding theorem
    • Implications for error-free transmission
    • Channel capacity for AWGN
14. 7 Marks
    bDiscuss about linear block codes and cyclic codes.
    How to answer

    • Generator and parity-check matrix for linear block codes
    • Properties: linearity, syndrome decoding
    • Cyclic codes with generator polynomial
    • Example (n,k) code
15. 7 Marks
    aWrite short notes on: DPSK
    How to answer

    • Definition of DPSK and differential encoding rule
    • Transmitter and receiver block diagram
    • Advantage of non-coherent detection
    • Waveform example
16. 7 Marks
    bWrite short notes on: Gram Schmidt orthogonalization procedure.
    How to answer

    • Need for orthonormal basis functions
    • Steps of Gram-Schmidt procedure with formulas
    • Normalization $\phi_i = u_i/||u_i||$
    • Application to signal constellation