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16. Question papers:
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) Discuss the advantages and disadvantages of digital communication system.
b) State and prove sampling theorem in time domain. [15]
2. a) With a relevant diagram, describe the operation of DPCM system.
b) A TV signal with a bandwidth of 4.2 MHz is transmitted using binary PCM. The number
of representation level is 512. Calculate:
i) Code word length ii) Final bit rate iii) Transmission bandwidth. [15]
3. a) What are different digital modulation techniques available? Compare them with regard
to the probability error.
b) Draw the block diagram of DPSK modulator and explain how synchronization problem is
avoided for its detection. [15]
4. a) Draw the block diagram of a baseband signal receiver and explain.
b) What is an Eye pattern? Explain.
c) What is matched filter? Derive the expression for its output SNR. [15]
5. a) State and prove the condition for entropy to be maximum.
b) Prove that H(Y/X) ≤ H(Y) with equality if and only if X and Y are independent.
[15]
6. a) Explain the advantages and disadvantages of cyclic codes.
b) Construct the (7, 4) linear code word for the generator polynomial G(D) = 1+D 2 +D 3 for the
message bits 1001 and find the checksum for the same.
[15]
7. a) Briefly describe the Viterbi algorithm for maximum-likelihood decoding of
convolutional codes.
b) For the convolutional encoder shown in figure7, draw the state diagram and the trellis
diagram.
8. a) Explain how PN sequences are generated. What are maximal-length sequences? What
are their properties and why are they preferred?
b) With the help of a neat block diagram, explain the working of a DS spread spectrum
based CDMA system. [15]