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Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
PDDC SEMESTER-VI EXAMINATION WINTER 2018
Subject Code:X61101 Date: 16/11/2018
Subject Name: Digital Communication
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1

Define following terms:
1. Random variable
2. Marginal probability
3. Correlation coefficient
03

List advantages of digital communication over analog communication.
04

Explain sampling theorem. What is aliasing effect?
07
Q.2

An experiment consists of drawing two cards from a deck in succession (without replacing the first card drawn). Assign a value to the probability of obtaining two red aces in two draws.
03

Draw the following data format for the bit stream 1 1 1 0 0 1 1 0 1
Polar NRZ AMI Manchester RZ ON-OFF
04

Derive the formula of signal to quantization ratio for PCM system that uses uniform quantization.
07
OR

Draw and explain block diagram of ADPCM system. Compare PCM and ADPCM.
07
Q.3

Find the PSD of NRZ Polar signaling and discuss the advantages and disadvantages of Polar signaling.
07

Derive the formula of channel capacity of a discrete memoryless channel.
07
OR
Q.3

What is scrambling? Explain scrambling and unscrambling process with block diagram and example.
07

Find the channel capacity of the Binary Symmetric Channel (BSC).
07
Q.4

What is cumulative distribution function State and prove its properties.
07

Explain the decoding of cyclic code with suitable example.
07
OR
Q.4

If the random variable z is sum of two random variables x and y i.e. z x find mean and variance of random variable z. What is the variance of z if x and y are independent random variables?
07

For a systematic LBC the three parity check digits c4, c5 and
c6 are
c4=d1+d3
c5=d1+d2+d3
c6=d1+d2
1. Construct the code generated by this matrix
2. Prepare a suitable decoding table
3. Decode 100100 and 101011.
07
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Q.5

What is the difference between coherent and non-coherent detection techniques? Describe non-coherent detection of FSK signal.
07

Write short note on Optimum Binary Receiver
07
OR
Q.5

With necessary diagram and waveforms explain the principle of Binary Phase Shift Keying (BPSK).
07

Write short note on Convolution code.
07