# Mumbai University Previous year question papers

## VI Sem Electronics Examination June 2008

## Digital Communication

N. S.: (1) Question’ NO.1 is compulsory.

(2) Attempt any four questions out of remaining six questions.

(3) Assume any suitable data wherever required but justify the same.

1. Attempt any four from the following :-

(a) Explain linearity and cyclic property of cyclic code.

(b) Determine the bandwidth required for M-ary PSK system. Draw the geometrical repr.esentation of M-ary PSK an9 find out distance between signal point.

(c) Using duo-binary encoding, how is the bandwidth requirement reduced by half.

(9) What is matched filter? How it differs from optimum filter?

e) Draw the model of spread spectrum digital communication system and explain correlation and Run property of maximum length sequence.

2. (a) A (7, 4) cyclic code is generated using the polynomial X3+ x + 1. 10

(i) What would be the generated codeward for the data sequence 1000 and 1100 ?

(ii) Draw the circuit diagram to generate this code and show how parity bits are generated”for the data sequence 1000.

(iii) Draw the circuit for syndrome calculator and obtain the syndrome for the received code word 1000100.

(iv) Draw the block diagram of cyclic code decoder.

b) A convolutional encoder has single shift register with three modulo-2 adder and an output multiplexer. The following generator sequences are combined by the multiplexerto produce the encoder output.

g1=100; g2 = 111 and g3 =.101

(i) Draw block diagram of the encoder-

(ii) If input messagesequenceis 10110determine the output sequenceof the encoder.

(iii) Drawthe codetree, state and trellier diagram for the same.

3. (a) The binary sequence 1011011110 is applied to the DPSK transmitter:

(i) Draw the block diagram of DPSK transmitter and r~ceiver and sketch the resultin g waveform at theoutput.

(ii) Show that, in the’ absence of noise the original binary

sequence is ‘reconstructed ~t the receiver output. Calculate the Bandwidth of DPSK signal.

(b) pifferentiate between:-

(i) QASK and QPSK

(ii) BPSK and BFSK

4. (a) Show that duobinary signalling suffers from error propogation while precoded duobinary saignalling doesnot. Explain with encoder and decoder block diagrams and decoding logic.

(b) Derive the expression for error Probability of a matched filter and justify that Pe doesnot depend on the shape of the Input waveform.

5. (a) What is Intersymbol Interference? Show how transversal equalizer reduces ISI.

(b) Derive the expression for a si£jnal to noise ratio of an integrater and dump circuit and hence explain how th~ signal is emphasized relative to noise. What is the ideal sam”pling time?

6. (a) What are Pseudo-noise sequences in spread spectrum technology? Why they are used in spread spectrum modulation? Explain the method to generate a Pseudo-noise sequence.

(b) Explainthe basic principleof frequency Hop spread spectrum. Explainwith waveforms,slow frequency hopping and fast frequency hopping.

7. Write short notes on any **four :-**

(i) Huffman coding

(ii) QASK (Quadarature Amplitude Shift Keying)

(iii) Viterbi decoding

(iv) Direct sequence spread spectrum

(v) ISI and Eye Pattern.