# WBUT  Previous Years Question Papers EC

## Digital Communication Systems B Tech Fifth Sem 2008

Time : 3 Hours ]

{ Full Marks ; 70

GROUP-A

(Multiple Choice Type Questions)

Choose the correct alternatives for any ten of the following :  10 x 1 = 10

i) The frequency spectrum of a square wave or rectangle wave in time domain is a) Impulse function b) Sine function

c) Sine, function         d) Gaussian function.

11) In quantization has 8 bit code, then SMR will be a) 32 dB          b) 16 dB

c) 48 dB                  d) 8 dB.

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ill) In Hofmann coding, the probabilities are arranged in

a)            ascending order

b)            descending order

c)            no specification.

iv)            How many bits would be required to represent a 256 level quantization in PCM ? a) 6 b) 8

c) 5                      d) 7.

v)              In PCM, the amplitude levels are transmitted in a 7-unit channel code. The sampling is done at the rate of 10 Hz. The bandwidth preferred minimum should be

a)                                                                                                                              5 kHz    b) 35 kHz

c) 70 kHz                 d) 85 kHz.                1 ■

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vi)             If the quantization has 4 levels which occurs with probability Pj = P4 = — and

8

p2 s P3 = —, then the information rate is 8

a)               1-8 bits/message

c)                1 –v6 bits / message

3-8 bits/message 2-6 bits/message.

vli) The Nyquist rate of the composite signal x (/) * 5 cos 1000 nt cos 3000 nt is

a)                                                                                                                                                 3000 Hz  b) 1000 Hz

c)                                                                                                                             4000 Hz d) 2000 Hz.

viii)         The channel capacity of a band-limited Gaussian channel is

 a) C – B log2 (l+|) c> c4’Mf)
 b) d)
 C-B

ix)                                                                                                                                              Which of the following system is digital ?

a)                                                                                                                                                 pulse-position modulation b)

c)                                                                                                                                                 pulse-width modulation d)

PCM generation requires LPF (low-pass filter ) at the beginning to

a)               eliminate aliasing effect

b)               eliminate quantization noise

c)               eliminate decoding noise d} none of these.

xi)            Spread spectrum scheme increases

a)               processing gain

c)               transmission gain

xii)          Eye pattern is used to study

a)               ISI

c)               Error rate

b)                  spectral efficiency

d)               none Of these.

b)                Quantization noise

d)                None of these.

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GROUP-B (Short Answer Type guestions )

Answer any three of the following.

 2. a) What are the proportion of line coding. b) Draw the Manchester coding and PNRZ coding for m = 1001011 c) Write down the PSD function of PNRZ signal. 3. a) Explain the advantages of digital communication system. b) Draw block diagram and explain such a system briefly. 4. a) What is direct sequence spread spectrum ? b) Explain the generation of DSSS. 5. What is eye pattern ? b) How is it generated in CRO. c) What information we get from it ? 6. Explain the principle of operation of QPSK modulator. 7. a) Explain the implication ‘aliasing1. b) How is the ‘aliasing’ effect removed. GROUP-C (Long Answer Type Questions )Answer any three of the following questions. 8. a) What are the disadvantages of DPSK. b) What are the disadvantages of BPSK and how is it improved ? c) What is BFSK ? d) Describe orthogonal BFSK. 9. a) State sampling theorem and explain its important. b) What is Nyquist rate of sampling.

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c)            Explain how an analogue signal isconverted into a digital signal noising PCM system.  6

d)           A television signal has a bandwidth of 4*5 MHz. The signal is sampled and converted into PCM signal. Determine the sampling rate if the signal is to be sampled at a rate 20% above Nyqulst rate.     3

1. a) What is quantization ? Classify them.                                                                                       3

b)            How is non-uniform quantization utilised.                                                                  2

c)            Deduce the relation of signal to quantization noise.                                                   4

d)            Calculate the number of quantization levels for the signal x{t) = 5 sin (500 n + 0)        4

e)            What are therelated laws for non-uniform quantization.                                             2

1. a)     What do you mean by matched filter for digital reception ?                                                 5

b)            Derive an expression for error probability of a matched filter ?                                   3

c)            State and explain Nyqulst criterion for zero ISI.                                                          3

d)            What is the roll of an equalizer ?                                                                                 4

1. a)     Define information.                                                                                                                2

b)            Define’Discrete Memoryless Source’.                                                                            2

c)            What is ‘Entropy1 ?                                                                                                      2

d)            Calculate amount of information if binary digits occur with equal llkelyhood in a binary PCM system. 3

e)            A discrete source emits one of the five symbols once every millisecond with

probabilities —, —, —, — and —. Determine source entropy and information 2 4 8 16 32

rate.                                                                                                                                       6

1. Write sh6rt notes for the following :

a)            Compounding                                                                                                              5

b)            Differential Encoding                                                                                                   5

c)            Linear Prediction Coder.                                                                                              5

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