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Digital Communication Question Paper - Jun 19 - Electronics Engineering (Semester 5) - Mumbai University (MU)
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Digital Communication - Jun 19

Electronics Engineering (Semester 5)

Total marks: 80
Total time: 3 Hours
INSTRUCTIONS
(1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Draw neat diagrams wherever necessary.

1. Attempt any four

1.a. Explain why digital communication is preferred over analog communication?
(5 marks) 00

1.b. A rate 1$/ 3$ convolutional coder with constraint length of 3 " uses the generating vectors as given $g_{1}=100, g_{2}=101, g_{3}=111$
(5 marks) 00

1.c. Represent the following bit sequence, $1011101011,$ using

i)Unipolar RZ

ii) Unipolar NRZ

iii) Bipolar NRZ

iv) AMI RZ

v) Manchester

(5 marks) 00

1.d. In the presence of White Gaussian noise, with a constant signal power the channel capacity reaches its upper limit with the increase in the bandwidth B. Prove that this upper limit of $\mathrm{C}$ is given by $\mathrm{C}_{\alpha}=1.44\left(\mathrm{S} / \mathrm{N}_{0}\right)$
(5 marks) 00

1.e. Write a note on optimum receiver.
(5 marks) 00

2.a. Why MSK is called 'shaped QPSK " explain. For the bit sequence, 10110101100 , draw the MSK waveform (let $m=5 )$

(10 marks) 00

2.b. A discrete memory less source generates symbols every one millisecond as given below:

S S1 S2 S3 S4 S5 S6 S7 S8
P 1/4 1/8 1/16 1/16 1/16 1/4 1/16 1/8

Construct Shannon-Fano code. Also find the source entropy, information rate and code efficiency.

(10 marks) 00

3.a. What is ISI? Derive an expression for ISI and explain methods to overcome ISI. State the Nyquist's condition for zero ISI.
(10 marks) 00

3.b. Show that duobinary signaling suffers from error propagation while pre-coded duobinary signaling does not. Explain with encoder and decoder block diagrams and decoding logic.
(10 marks) 00

4.a. Explain with the help of a neat block diagram, the transmitter and receiver of M-ary FSK. Also sketch the PSD of M-ary FSK. What is the bandwidth requirement of M-ary FSK?
(10 marks) 00

4.b. Find the generator matrix G for a systematic $(7,4)$ cyclic code using generator polynomial $g(x)=1$ $+x^{2}+x^{3} .$ Design an encoder for the code and verify its operation to determine the codeword for the message vector $(1100)$ .
(10 marks) 00

5.a. The parity check matrix $\mathrm{H}$ of a linear $(7,4)$ block code is given as :

enter image description here

Determine the code words for the messages:

(i) 0011

(ii) 0100 and

(iii) 0110

Also show how error is detected when $2^{\text { nd }}$ bit is detected erroneously for data word 0011$?$ \lt/div\gt \ltspan class='paper-ques-marks'\gt(10 marks)\lt/span\gt \ltspan class='paper-page-id'\gt00\lt/span\gt \lt/div\gt \ltDIV class='paper-question'\gt \ltDIV class='paper-ques-desc'\gt \ltb\gt5.b.\lt/b\gt Compare BASK, BPSK and BFSK, based on following parameters: Bandwidth, detection method, noise immunity, transmission rate and signal space representation. \lt/div\gt \ltspan class='paper-ques-marks'\gt(5 marks)\lt/span\gt \ltspan class='paper-page-id'\gt00\lt/span\gt \lt/div\gt \ltDIV class='paper-question'\gt \ltDIV class='paper-ques-desc'\gt \ltb\gt5.c.\lt/b\gt Compare OQPSK with MSK. \lt/div\gt \ltspan class='paper-ques-marks'\gt(5 marks)\lt/span\gt \ltspan class='paper-page-id'\gt00\lt/span\gt \lt/div\gt \ltDIV class='paper-question'\gt \ltDIV class='paper-ques-desc'\gt \ltb\gt6.a.\lt/b\gt Write short notes on : i) optical communication system ii) Satellite Communication System \lt/div\gt \ltspan class='paper-ques-marks'\gt(10 marks)\lt/span\gt \ltspan class='paper-page-id'\gt00\lt/span\gt \lt/div\gt \ltDIV class='paper-question'\gt \ltDIV class='paper-ques-desc'\gt \ltb\gt6.b.\lt/b\gt Sketch the signal constellation diagram $(\mathrm{d}=2 \mathrm{a})$ for $16-\mathrm{QAM}$ system and Derive the expression for its Symbol energy, Es. Prove that the noise immunity of $16-$ QAM is better than that of 16 -PSK system.

(10 marks) 00

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