Mumbai University Previous year question papers

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.