# Mumbai University Previous year question papers

## VI Sem Electronics Examination Dec 2007

## Digital Communication

N.B.(1) Question NO.1 is ,compulsory.

(2) Attempt any four questions out of remai,ningsix questic;>ns.

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

1. Attempt any four from the following :-

(a) Defirie Probability. Explain Conditional and Joint Probabilities.

(b) What is Intersymbollnterference? Justify the correlative coding introduces controlled amount of ISI ?

(c) Explain tbe advantage of gray coding of the Input to 8-ary PSK system? Also draw the phaser diagram of 8-ary PSK.

(d) Distinguish between Matched filter and CoreJator.Holt\f.they are relate~ to each other?

(e) What is the necessity of spread spectrum modulation?Differentiate between slow frequency hopping and fast frequency hopping.

2. (a). Cpnsider a binary input-output channels shown below: 5

If the source symbols are equiprobable, find the receiver probability distribution pry] and the Joint probability distribution P[XY].

(b) A discrete memoryless source has an alphabet of five symbol with there probabilities as shown:

(i) Construct a Shannon-Fano code for the source and calculate code efficiency Y and redundancy of the code.

(ii) Repeat the same for Huffman Code

(iii) Compare the Huffman and Shannon-Fano Code.

3. (a) For a systematic linear block code the three parity check digits C4. Cs’ C6 are given by 10

C4 = d1 EBd2 EB d3

Cs = d1 EB d2

C6 = d1 EB d3

(i) Construct generator matrix

(ii) Construct code generated by this matrix

iii) Determine error correcting capability

(iv) Prepare suitable decoding table

v) Decode the received code word 1011 * ocr *and 000110

(b) Explain the Viterbi algorithm of convolutional codes.

(c) Compare linear Block codes, Cyclic codes and Convolutional codes.

symbol S1 S2 S3 S4 Ss

Probability 0-40 0.19 0.16 0.15 0.15

4. (a) State andexplairffheconditionfor orthogonalityof the BFSKsignal. Determineits spectr.um and hence the bandwidth requirement for transmission of signal.

(b) Explain 4-ary PSK alongwith the lollowing lil’le&:-

(i) Offset and Non-offset QPSK

(ii) Modulationand demodulation block diagram of offset QPSK.

(iii) Plot of power spectral density .

iv) Signal space-representation and hence Eucledian distance.

5. (a) Show that for an input signal<which is an secjuence of rectangular positive and negative pulses, the integrator<is the matched filter.

(b) Explain the meaning of equalizer. How is equalization achieved? with the help of neat block diagram. Explain fapped-delay line equalizer.

6. (a) What is duo-binary ~ncoding? Explain with neat block diagram. How the duo-binary encoding reduces the Bandwid.th requirement?

b) Draw the block diagram of DS-‘SSS transmitter and receiver. Obtain the expression for the signal at the output of each block and show that the original sequence can be recovered at the receiver output. .

7. Write short notes on anyfour ‘:-

(a) Linear preditive vecoders .

(b)Line codes

(c) P-N sequence generator

(d) Lempel-Ziv coding

e) Optimum filter.