## December 17, 2011

### DIGITAL COMMUNICATIONS JNTU previous years question papers

Time: 3 hours Max Marks: 80
All Questions carry equal marks
1. (a) State and prove the sampling theorem for band pass signals.
(b) A signal m(t) = cos(200pt) + 2 cos(320pt) is ideally sampled at fS = 300Hz. If the sampled signal is passed through a low pass filter with a cutoff frequency of 250Hz. What frequency components will appear in the output? [6+10]
2. (a) Explain with a neat block diagram the operation of a continuously variable slope delta modulator (CVSD).
(b) Compare Delta modulation with Pulse code modulation technique. [8+8]
3. (a) Assume that 4800bits/sec. random data are sent over a band pass channel by BFSK signaling scheme. Find the transmission bandwidth BT such that the spectral envelope is down at least 35dB outside this band.
(b) Write the comparisons among ASK, PSK, FSK and DPSK. [8+8]
4. (a) What is meant by ISI? Explain how it differs from cross talk in the PAM.
(b) What is the ideal solution to obtain zero ISI and what is the disadvantage of this solution. [6+10]
5. A code is composed of dots and dashes. Assume that the dash is 3 times as long as the dots, has one-third the probability of occurrence.
Calculate
(a) the Information in a dot and that in a hash.
(b) average Information in the dot-hash code.
(c) Assume that a dot lasts for 10 ms and that this same time interval is allowed between symbols. Calculate average rate of Information. 
6. Explain Shannon-Fano algorithm with an example. 
7. Explain about block codes in which each block of k message bits encoded into block of n>k bits with an example. 
8. Sketch the Tree diagram of convolutional encoder shown in figure 8 with Rate= 1/2, constraint length L = 2. 

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