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B.Tech. V-Semester Regular Examinations, Oct./Nov. 2017 Digital Communications

This document is an exam for a Digital Communications course, consisting of 10 questions across 5 units. The questions cover topics like: 1) Pulse code modulation and line encoding formats 2) Time division multiplexing of voice signals 3) Differential phase shift keying modulation and advantages over phase shift keying 4) Probability of error calculations for integrate and dump receivers and BPSK/FSK systems with matched filters 5) Entropy, Huffman coding, and properties of efficient variable length codes 6) Error correction capabilities of block codes and generator sequences of convolutional encoders

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Sarath Chandra Varma
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51 views2 pages

B.Tech. V-Semester Regular Examinations, Oct./Nov. 2017 Digital Communications

This document is an exam for a Digital Communications course, consisting of 10 questions across 5 units. The questions cover topics like: 1) Pulse code modulation and line encoding formats 2) Time division multiplexing of voice signals 3) Differential phase shift keying modulation and advantages over phase shift keying 4) Probability of error calculations for integrate and dump receivers and BPSK/FSK systems with matched filters 5) Entropy, Huffman coding, and properties of efficient variable length codes 6) Error correction capabilities of block codes and generator sequences of convolutional encoders

Uploaded by

Sarath Chandra Varma
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Course Code : 15EC1114/2015 R-2015 Reg.

No :

GAYATRI VIDYA PARISHAD COLLEGE OF ENGINEERING (AUTONOMOUS)


Madhurawada, Visakhapatnam
Affiliated to JNT University – K, Kakinada
B.Tech. V-Semester Regular Examinations, Oct./Nov. 2017
Digital Communications
(Electronics and Communication Engineering)

Date : 30-10-2017 Time : 3 Hours Max. Marks : 70


1. Answer ONE Question from each UNIT
2. All parts of a Question must be answered in one place to get valued.
3. All questions carry equal marks.

UNIT-I

1. a) Explain pulse code modulation with a neat sketch. 6 Marks


b) Derive and plot the PSD of Unipolar signaling line encoding format. 6 Marks
c) Five signals, each of Bandwidth 1KHz, are to be transmitted simultaneously by binary PCM. 2 Marks
The maximum tolerable error in sampling amplitude is 0.2% of peak signal amplitude. The
signals are sampled at a rate of 20% above the Nyquist’s rate. Determine the minimum band
width required to transmit this signal

2. a) 24 Voice signals are sampled uniformly and then time division multiplexed. The sampling 6 Marks
operation uses flat top samples with 1 microsec duration. The highest frequency component of
each voice signal is 3.4KHz. Assuming a sampling rate of 8KHz, find the spacing between
the successive pulses of the multiplexed signal.
b) Explain ߤ − ‫ ݓܽܮ‬Companding in PCM. 8 Marks
UNIT-II
3. a) Explain the Modulation and Demodulation process in a DPSK system, if the data sequence 8 Marks
to be transmitted is 1000101. Discuss about the advantage of DPSK system
b) Explain why DPSK system is called Non-Coherent PSK? Justify. Discuss about the 6 Marks
disadvantage of DPSK, relative to PSK.

4. a) For the data sequence b(t)= 1 0 0 1 0 1, give the transmission and recovery scheme using 7 Marks
DPSK modulation.
b) Derive and plot the power spectrum of BPSK signal 7 Marks
UNIT-III
5. a) Derive the expression for the bit error probability of Integrate and Dump Receiver. 8 Marks
b) Derive the expression for the probability of error in a BPSK system, employing Matched 6 Marks
filter receiver.

6. Derive the expression for the probability of bit error in the case of
(i) FSK assume a Matched Filter Receiver. 8 Marks
(ii) ASK assume a Matched Filter Receiver. 6 Marks

UNIT-IV

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Course Code : 15EC1114/2015 R-2015 Reg. No :

7. a) A communication system transmits two messages “0” and “1”. The message “1” is 3 times 7 Marks
more certain than a “0”. Due to the noise in the channel, on average, 1 message for every 100
transmitted will be received in error. Find the amount of information, per sec, received by the
user, if the source transmits 1000 messages per sec.
b) If X and Y are two signals , derive the relation between the marginal entropy , joint entropy 7 Marks
and the conditional entropy.

8. a) Derive and Explain about the Non uniqueness of Huffman Coding for a source transmitting 6 Marks
4 messages A,B, C and D with respective probabilities : 0.5, 0.25, 0.125, 0.125.
b) Discuss about the properties of efficient variable length source code words. Justify each with 8 Marks
an example.

UNIT-V
9. Find all the code words and the Error Correcting Capability of a (6,3) Block Code, with a Generator 14 Marks
Matrix equal to
0 1 1 1 0 1
1 1 1 0 1 0
1 1 0 0 0 1
10. Find the generator sequence of each adder in the following convolutional encoder 14 Marks

S1 S2 S3

+ +

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