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B. Tech III Year II Semester Examinations, April/May - 2012
DIGITAL COMMUNICATIONS
(ELECTRONICS AND COMMUNICATION ENGINEERING)
Time: 3 hours Max. Marks: 75
Answer any five questions
All questions carry equal marks
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1. a) What is natural sampling? Explain it with sketches.
b) Specify the Nyquist rate and Nyquist intervals for each of the following signals
i) x(t) = Sinc200t ii) x(t) = Sinc 2 200t iii) x(t) = Sinc200t+ Sinc 2 200t.
[15]
2. a) Derive an expression for signal to quantization noise ratio of a PCM encoder using
uniform quantizer when the input signal is uniformly distributed.
b) A PCM system uses a uniform quantizer followed by a 7 bit binary encoder. The bit rate of
the system is equal to 50 x 10 6 bits/sec.
i) What is the maximum message bandwidth?
ii) Determine the signal to quantization noise ratio when f m
= 1 MHz is applied.
[15]
3. a) Draw the correlation receiver structure for QPSK signal and explain its working
principle.
b) Write the power spectral density of BPSK and QPSK and draw the power spectrum of
each. [15]
4. a) Draw the block diagram of baseband communication receiver and explain the
importance of each block.
b) What is matched filter?
c) Represent BFSK system using signal space diagram. What are the conclusions one can
make with that of BPSK system? [15]
5. a) Define and explain the following.
i) Information
ii) Efficiency of coding
iii) Redundancy of coding.
b) Prove that H(X,Y) = H(X) + H(Y/X) = H(Y) + H(X/Y). [15]
6. a) Explain the principle and operation of encoder for Hamming code.
b) An error control code has the following parity check matrix.
101100110010011001H⎡⎤⎢⎥=⎢⎥⎢⎥⎣⎦
i) Determine the generator matrix ‘G’