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

(b) Compare DM and PCM systems. [10+6]
3. discuss the design and analysis of M-ary signaling schemes. List the waveforms in
quaternary schemes. [16]
4. (a) Derive an expression for fault probability of coherent PSK scheme.
(b) In a binary PSK scheme for using a correlator receiver, the local carrier wave-
form is Acos (wct + ?) instead of Acos(wct) due to poor carrier synchroniza-
tion. Derive an expression for the fault probability and calculate the increase
in fault probability when ?=150 and [A2Tb/?] = 10. [8+8]
5. Consider the transmitting Q1, Q2, Q3, and Q4 by symbols 0, 10, 110, 111
(a) Is the code uniquely decipherable? That is for every possible sequence is there
only 1 way of interpreting message.
(b) compute the avg. number of code bits per message. How does it compare
with H = 1.8 bits per messages. [16]
6. Show that H(X, Y) = H (X) + H (Y) and H(X/Y) = H (X). [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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1 of 1
Code No: R05310404 Set No. 3
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. The probability density function of the sampled values of an analog signal is shown
in figure 1.
Figure 1
(a) Design a four - level uniform quantizer.
(b) compute the signal power to quantization noise power ratio.
(c) Design a four - level minimum mean squared fault non - uniform quantizer.
[6+4+6]
2. A DM system is tested with a 10kHz sinusoidal signal, 1V peak to peak at the
input. The signal is sampled at 10times the Nyquist rate.
(a) What is the step size needed to prevent slope overload and to minimize the
granular noise.
(b) What is the power spectral density of the granular noise?
(c) If the receiver input is band limited to 200kHz, what is the avg. (S/NQ).
[6+5+5]
3. (a) Write down the modulation waveform for transmitting binary info over
base band channels, for the subsequent modulation schemes: ASK, PSK, FSK

Earning:   Approval pending.