dc

) What are the two basic types of spread-spectrums systems? Explain the basic principle of
each of them. [15]
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 Nyquist rate and Nyquist interval?
b) What is aliasing and how it is reduced?
c) A band limited signal x(t) is sampled by a train of rectangular pulses of width τ and period
T.
i) Find an expression for the sampled signal.
ii) Determine the spectrum of the sampled signal and sketch it. [15]
2. a) What is Companding? Explain how Companding improves the SNR of a PCM system?
b) The input of a PCM and a Data Modulational(DM) is a sine wave of 4KHz. PCM and
DM are both designed to yield an output SNR of 30dB. Assuming PCM sampling at 5
times the Nyquist rate. Compare the bandwidth required for each system. [15]
3. a) Draw the block diagram of QPSK system and explain its working.
b) Derive an expression for the probability of error for PSK. [15]
4. a) What is an optimum receiver? Explain it with suitable derivation.
b) Describe briefly baseband M-ary PAM system. [15]
5. a) Explain the importance of source coding.
b) Apply Haffman’s encoding procedure to the following message ensemble and determine
the average length of the encoded message
{}{}12345678910,,,,,,,,,Xxxxxxxxxxx=.
{}{}0.18,0.17,0.16,0.15,0.10,0.08,0.05,0.05,0.04,0.02XP=.
The encoding alphabet is {D} = {0,1,2,3}. [15]
6. a) What is a systematic block code?
b) What is a syndrome vector? How is it useful?
c) A linear (n, k) block code has a generated matrix
10110110G⎡⎤=⎢⎥⎣⎦
i) Find all its code words
ii) Find its H matrix
iii) What is the minimum distance of the code and its error correcting capacity?
[15]
7. a) What is a convolutional code?
b) What is meant by the constraint length of a convolutional encoder?
c) A convolutional encoder has a single shift register with two stages i.e., constraint length
k=3, three mod-2 adders and an output multiplexer. The generator sequence of the
encoder are as follows.
()()()()()()1231,0,1,1,1,0,1,1,1ggg===.
Draw the block diagram of the encoder. [15]
8. a) What are the advantages of spread – spectrum communication.
b) What are PN sequences? Discuss their characteristics.
c) Explain the principle of direct sequence spread spectrum. [15]