EC-502 Digital Communication - Nov 2023 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
   aDefine and explain sampling theorem for low pass signal. Also explain cross talk and aliasing effect.
   How to answer

   • Statement of sampling theorem for low-pass signal with Nyquist rate fs >= 2fm
   • Explanation / proof using spectrum replication and reconstruction with LPF
   • Definition of aliasing effect due to under-sampling and cross-talk
   • Methods to avoid aliasing and cross-talk
2. 7 Marks
   bExplain the generation and detection of PWM signal.
   How to answer

   • Principle of PWM - pulse width variation with message amplitude
   • Block diagram of PWM generator using comparator / sawtooth
   • Detection using low-pass filter / demodulator steps
   • Waveforms for generation and detection
3. 7 Marks
   aWhat is quantization and derive the expression of quantization noise?
   How to answer

   • Definition of quantization and quantizer types
   • Quantization error model and step size
   • Derivation of quantization noise power / SNR expression
   • Effect of number of bits on noise
4. 7 Marks
   bWhat is differential coding? What are its advantages and disadvantages?
   How to answer

   • Definition / principle of differential coding
   • Encoding-decoding rule with example
   • Advantages such as phase ambiguity resolution
   • Disadvantages such as error propagation
5. 7 Marks
   aHow to generate and detect ADM signal?
   How to answer

   • Principle of adaptive delta modulation (ADM)
   • Block diagram of ADM transmitter with adaptive step
   • Block diagram of ADM receiver / detection
   • Advantage over delta modulation - slope overload and granular noise
6. 7 Marks
   bWhat is inter symbol interference? How can it be minimized?
   How to answer

   • Definition of inter symbol interference and its cause
   • Effect on eye diagram and BER
   • Nyquist criterion for zero ISI
   • Techniques to minimize - equalization, raised cosine filtering
7. 14 Marks
   With the help of block diagram explain the generation and detection of QPSK signal.
   How to answer

   • QPSK signal representation and constellation
   • Block diagram of QPSK modulator / transmitter
   • Block diagram of QPSK demodulator / coherent detection
   • Waveforms / phasor explanation
8. 14 Marks
   With the help of block diagram explain the generation and detection of coherent and non coherent BFSK signal.
   How to answer

   • Principle of BFSK with two frequencies for binary 1/0
   • Block diagram of coherent BFSK transmitter and receiver
   • Block diagram of non-coherent BFSK transmitter and envelope detector
   • Comparison of coherent vs non-coherent detection
9. 7 Marks
   aExplain the working of matched filter and correlator detector.
   How to answer

   • Impulse response of matched filter h(t)=s(T-t)
   • Maximization of output SNR at sampling instant
   • Structure of correlator / integrate and dump detector
   • Equivalence of matched filter and correlator
10. 7 Marks
    bDetermine the error probability of QPSK signal.
    How to answer

    • QPSK signal model as two orthogonal BPSK
    • Decision statistics and Euclidean distance
    • Integration of Gaussian Q-function for BER
    • Final error probability expression in terms of Eb/N0
11. 7 Marks
    aDerive the expression of channel capacity in terms of mutual information. Also calculate the channel capacity for noise free channel.
    How to answer

    • Definitions of entropy, conditional entropy and mutual information
    • Channel capacity as max mutual information
    • Derivation C = max I(X;Y)
    • Noise-free channel case where H(Y|X)=0
12. 7 Marks
    bFor a BSC shown in fig. 1 find the channel capacity for

    1. i)P = 0.9
    2. ii)P = 0.6

    

    How to answer

    • BSC capacity formula C = 1 - H(P) where H is binary entropy
    • Calculation for P=0.9 using H(0.9)
    • Calculation for P=0.6 using H(0.6)
    • Interpretation of capacity values
13. 14 Marks
    Write short notes on any two of the following:

    1. a)Gram Schmidt Orthogonalisation
    2. b)Error probability of QAM
    3. c)Cyclic codes
    How to answer

    • Brief principle, steps / expression for each chosen topic
    • Relevant block diagram / formula / example
    • Applications / significance
    • Conclusion for each short note