EC-502 Digital Communication - Jun 2025 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 Ideal sampling. Also, derive an expression for it.
   How to answer

   • Define ideal (impulse) sampling with sampling frequency and impulse train
   • Write time-domain expression $s(t)=m(t)\sum \delta(t-nT_s)$
   • Derive spectrum $S(f)=f_s\sum M(f-nf_s)$ showing spectral replication
   • State Nyquist condition $f_s \ge 2f_m$ for alias-free recovery
2. 7 Marks
   bDiscuss about the generation of PPM and PWM signal.
   How to answer

   • Define PPM and PWM in terms of PAM reference
   • Explain generation using comparator / sawtooth and monostable circuits
   • Draw block diagram and waveforms for both
   • Compare position vs width variation with message amplitude
3. 7 Marks
   aDescribe Quantization. Derive an expression for quantization noise. Why is quantization required?
   How to answer

   • Define quantization and quantizer steps
   • Derive quantization error power $N_q = \Delta^2/12$ and SQNR expression
   • Explain need: ADC, digital transmission and processing
   • Mention uniform vs non-uniform quantization
4. 7 Marks
   bDefine Bandwidth. Transfer 01101001 in to Manchester code.
   How to answer

   • Define bandwidth as range of frequencies occupied
   • State Manchester encoding rule: 0 as low-to-high, 1 as high-to-low transition
   • Convert bits 01101001 to Manchester waveform / sequence
   • Show clock recovery advantage
5. 7 Marks
   aDiscuss the concept of QPSK with the help of block diagram.
   How to answer

   • Define QPSK as 4-phase modulation carrying 2 bits per symbol
   • Explain block diagram: serial-to-parallel, I/Q BPSK modulators, carrier oscillator
   • Show constellation with phases 45, 135, 225, 315 degrees
   • Mention demodulation and coherent detection
6. 7 Marks
   bDescribe how QPSK modulates digital signals using phase shifts, and discuss the advantages of QPSK over simpler modulation schemes like BPSK (Binary Phase Shift Keying).
   How to answer

   • Explain dibit to phase mapping in QPSK
   • Describe phase shifts representing 00, 01, 11, 10
   • Compare spectral efficiency and bandwidth with BPSK
   • List advantages: double data rate, better bandwidth efficiency
7. 7 Marks
   aExplain the basic idea behind Gram-Schmidt orthogonalization and how it is used to transform a set of linearly independent vectors into a set of orthogonal vectors.
   How to answer

   • Define inner product, orthogonal and orthonormal vectors
   • Explain iterative projection and subtraction steps of Gram-Schmidt
   • Show normalization to get orthonormal basis
   • Mention use in signal space representation
8. 7 Marks
   bDiscuss the applications where each modulation technique is commonly used and the factors that influence the choice of modulation scheme in different digital communication systems.
   How to answer

   • List common modulation schemes: ASK, FSK, PSK, QAM
   • Give applications: wireless, satellite, optical, modem
   • Discuss factors: bandwidth, power, BER, complexity, channel
   • Relate choice to data rate and noise requirements
9. 7 Marks
   aDiscuss the applications of error correcting codes in digital communication systems, highlighting the importance of ECCs in ensuring reliable data transmission over noisy channels.
   How to answer

   • Define error correcting codes and redundancy
   • Give examples: Hamming, LDPC, Turbo, convolutional
   • Explain ARQ vs FEC and noisy channel reliability
   • List applications: mobile, satellite, storage, deep space
10. 7 Marks
    bDescribe the fundamental concepts behind the Shannon-Hartley theorem, including the relationship between channel capacity, bandwidth and signal-to-noise ratio (SNR). https://www.rgpvonline.com
    How to answer

    • State Shannon-Hartley law $C = B \log_2(1+SNR)$
    • Define channel capacity, bandwidth and SNR terms
    • Explain trade-off between bandwidth and SNR
    • Mention Shannon limit for error-free transmission
11. 7 Marks
    aGiven a digital signal with a bit rate of 1 Mbps and a bandwidth efficiency of 0.8, calculate the required bandwidth for transmission. Define Aliasing.
    How to answer

    • Define bandwidth efficiency $\eta = R_b/B$
    • Calculate $B = R_b/\eta = 1/0.8 = 1.25$ MHz
    • Define aliasing as spectral overlap due to undersampling
    • Mention prevention by Nyquist sampling and anti-alias filter
12. 7 Marks
    bConsider an analog signal with a voltage range of $-5V$ to $+5V$ that needs to be digitized using 8-bit quantization. Determine the step size (quantization interval) and the number of quantization levels. Give advantages of differential encoding.
    How to answer

    • Calculate levels $L=2^8=256$ and range 10 V
    • Calculate step size $\Delta = 10/256 \approx 0.039$ V
    • Explain differential encoding as encoding differences
    • List advantages: reduced bits, better efficiency for correlated signals
13. 7 Marks
    aDiscuss the concept of M-ary PSK and M-ary FSK.
    How to answer

    • Define M-ary PSK with M phase levels
    • Define M-ary FSK with M frequencies
    • Explain constellation and bandwidth comparison
    • Mention applications for higher data rates
14. 7 Marks
    bIf a Hamming code with a block length of 7 is used to correct errors, calculate the number of data bits than can be corrected by this code. Describe Convolution code.
    How to answer

    • Use Hamming relation $2^r \ge m+r+1$ with $n=7$
    • Solve $m=4$, $r=3$ for single-error correction
    • Define convolutional code with shift registers
    • Mention constraint length, coding gain and Viterbi decoding
15. 14 Marks
    Write short notes on (any two):

    1. a)Natural Sampling
    2. b)RZ Signalling
    3. c)Matched Filter
    4. d)Linear Block Codes
    How to answer

    • Select any two topics from the list
    • Give definition, principle and diagram for each
    • Mention mathematical expression and key features
    • Give applications in digital communication