Jawaharlal Nehru Technological University Kakinada 2008 B.Tech Electronics and Communications Engineering DIGITAL COMMUNICATIONS - Question Paper

III B.Tech I Semester Supplimentary Examinations, February 2008
DIGITAL COMMUNICATIONS
( Common to Electronics & Communication Engineering and Electronics &
Telematics)
Time: three hours Max Marks: 80
ans any 5 ques.
All ques. carry equal marks
? ? ? ? ?
1. (a) State and prove the sampling theorem for band pass signals.
(b) A signal m(t) = cos(200pt) + two 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) Derive an expression for channel noise and quantization noise in DM system.
(b) Compare DM and PCM systems. [10+6]
3. (a) Draw the signal space representation of MSK.
(b) Show that in a MSK signaling scheme, the carrier frequency in integral mul-
tiple of ‘fb/4’ where ‘fb’ is the bit rate.
(c) Bring out the comparisons ranging from MSK and QPSK.
4. (a) Derive an expression for fault probability of non - coherent ASK scheme.
(b) Binary data is transmitted over an RF band pass channel with a usable band-
width of 10MHz at a rate of 4,8 × 106 bits/sec using an ASK singling method.
The carrier amplitude at the receiver antenna is 1mV and the noise power
spectral density at the receiver input is 10-15w/Hz.
i. obtain the fault probability of a coherent receiver.
ii. obtain the fault probability of a coherent receiver. [8+8]
5. Figure five illustrates a binary erasure channel with the transmission probabilities
probabilities P(0|0) = P(1|1) = one - p and P(e|0) = P(e|1) = p. The probabilities
for the input symbols are P(X=0) =a and P(X=1) =1- a.
Figure 5
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Code No: R05310404 Set No. 1
Determine the avg. mutual info I(X; Y) in bits. [16]
6. Show that H(X, Y) = H(X) + H(Y |X) = H(Y ) + H(X|Y ). [16]
7. discuss about block codes in which every block of k message bits encoded into block
of n>k bits with an example. [16]
8. discuss different methods for describing Conventional Codes. [16]
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2 of 2
Code No: R05310404 Set No. 2
III B.Tech I Semester Supplimentary Examinations, February 2008
DIGITAL COMMUNICATIONS
( Common to Electronics & Communication Engineering and Electronics &
Telematics)
Time: three hours Max Marks: 80
ans any 5 ques.
All ques. carry equal marks
? ? ? ? ?
1. (a) State sampling theorem for low pass signals and band pass signals.
(b) What is aliasing effect? How it can be eliminated? discuss with neat diagram.
[4+4+8]
2. (a) Derive an expression for channel noise and quantization noise in DM system.

Earning:   Approval pending.