EC-502 Digital Communication - Dec 2024 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 about the generation and detection of PWM signal.
   How to answer

   • Definition of PWM signal and principle of pulse width variation with message amplitude
   • Generation method using comparator / sawtooth generator with circuit/block diagram
   • Detection/demodulation using low-pass filtering / integration and waveform steps
   • Relevant waveforms and applications
2. 7 Marks
   bState and explain sampling theorem of low pass signal.
   How to answer

   • Statement of low-pass sampling theorem: fs >= 2fm
   • Explanation of Nyquist rate and Nyquist interval
   • Proof outline using spectrum / aliasing concept
   • Diagram of sampled spectrum and reconstruction condition
3. 7 Marks
   aDraw block diagram of PCM system and explain it is detail.
   How to answer

   • Block diagram of PCM transmitter: sampler, quantizer, encoder
   • Block diagram of PCM receiver: decoder, filter
   • Explanation of sampling, quantization and encoding steps
   • Neat labelled block diagram
4. 7 Marks
   bWith the help of block diagram explain the working of Delta modulator.
   How to answer

   • Block diagram of Delta modulator: comparator, delay/integrator, pulse generator
   • Working as 1-bit DPCM tracking input by staircase approximation
   • Role of step size, slope overload and granular noise
   • Waveforms showing input vs staircase
5. 7 Marks
   aDiscuss about different line encoding techniques.
   How to answer

   • Need for line coding and its properties
   • Classification: unipolar, polar, bipolar, Manchester, AMI etc.
   • Waveform examples for sample bit sequence
   • Comparison of bandwidth, DC content and synchronization
6. 7 Marks
   bExplain the polar signaling and RZ signaling.
   How to answer

   • Definition and waveforms of polar signaling (polar NRZ, polar RZ)
   • Definition and waveforms of RZ signaling
   • Power spectral and synchronization features
   • Advantages and disadvantages
7. 7 Marks
   aWith the help of block diagram explain the working of BPSK system.
   How to answer

   • Block diagram of BPSK transmitter: NRZ encoder, balanced modulator/carrier
   • Block diagram of BPSK receiver: coherent demodulator, bit synchronizer
   • Working with phase shifts 0 and 180 degrees and constellation
   • Waveforms and coherent detection steps
8. 7 Marks
   bDiscuss about the spectral properties of QPSK and MSK.
   How to answer

   • Signal representation and PSD expression for QPSK
   • Signal representation and PSD for MSK as continuous-phase FSK
   • Comparison of main-lobe width, sidelobe roll-off and bandwidth
   • Spectral plots showing QPSK wider sidelobes vs MSK faster decay
9. 7 Marks
   aExplain the DPSK in detail.
   How to answer

   • Principle of differential encoding to avoid carrier phase ambiguity
   • DPSK transmitter block diagram with differential encoder and BPSK modulator
   • Receiver with delay and compare / non-coherent detection
   • Waveforms and advantages/limitations
10. 7 Marks
    bExplain the minimum shift keying in detail.
    How to answer

    • Definition of MSK as CPFSK with modulation index 0.5
    • Phase continuity, frequency separation and orthogonality condition
    • Transmitter/receiver block diagram and signal expressions
    • Properties: constant envelope, spectral efficiency
11. 7 Marks
    aExplain about Gram Schmidt Orthogonalization procedure.
    How to answer

    • Purpose: constructing orthonormal basis from signal set
    • Steps: normalization and subtraction of projections
    • Mathematical formulas for Gram-Schmidt procedure
    • Example with two signals and signal-space diagram
12. 7 Marks
    bCalculate the error probability for QPSK system.
    How to answer

    • QPSK signal-space and decision regions
    • Derivation using two orthogonal BPSK channels
    • Expression for bit error and symbol error probability with Q-function
    • Dependence on Eb/N0
13. 7 Marks
    aExplain the information capacity theorem of channel coding.
    How to answer

    • Statement of Shannon channel capacity theorem: C = B log2(1+S/N)
    • Meaning of information capacity and channel coding limit
    • Implications for bandwidth-SNR tradeoff
    • Example calculation
14. 7 Marks
    bExplain about block codes and cyclic codes.
    How to answer

    • Definition and parameters (n,k) of linear block codes
    • Definition and generator polynomial of cyclic codes
    • Encoding/decoding and error detection/correction principle
    • Examples of each
15. 14 Marks
    Write short notes on any two of the following.

    1. a)Matched filter
    2. b)Shannon's theorem
    3. c)Error correcting codes
    How to answer

    • Brief principle, block diagram/formula for each selected topic
    • Key features and advantages
    • Applications in digital communication
    • Neat diagram where applicable