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Posted By: sunil Member Level: Diamond Posted Date: 04 Sep 2008
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2006 Jawaharlal Nehru Technological University Information Technology III B.Tech I Semester Regular Examinations, November 2006 COMMUNICATION THEORY Question paper
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Code No: RR311202 Set No. 4
III B.Tech I Semester Regular Examinations, November 2006
COMMUNICATION THEORY
(Information Technology)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. (a) State and prove the symmetry theorem.
(b) Prove that F[f(at)]= 1/ |a f(?/a) ,i.e. The time scaling property where
F[f(t)]= f(?)
(c) Prove the attenuation property of Fourier Transform
{f(x)eax} = f (k-ai)
where f (k) = {f(x)} [5+5+6]
2. (a) In general the autocorrelation function R(t ) can be expressed as
R(t )=
t/2
R
-t/2
s(t) s(t + t )dt show that R(o) R(t ) [4]
(b) Obtain the time-average correlation function for a square wave. Does it make
any difference whether the square wave has even or odd symmetry. [12]
3. (a) What is the need for modulation? Derive the relation between the ouput
power of an AM transmitter and depth of modulation. Give output power in
case of critical modulation.
(b) The ouput RMS current of 60% modulated AM generator is 1.5 A. To what
values will this current rise if the generator is modulated additionally, by an-
other audio wave whose modulation index is 0.7? What will be the percentage
power saving if the carrier and one of the side bands are now suppressed?[8+8]
4. (a) Explain the principle of pre-emphasis and deemphasis with a circuit.
(b) Compare and contrast various FM modulation methods. [8+8]
5. (a) Explain the concept of quantization of sampled signal and hence discuss the
merits and demerits of it.
(b) Give the block diagram for PCM generation and reception and discuss the
features of each block. [6+10]
6. (a) Define various types of pulse modulations and distinguish between them. (06)
(b) Explain clearly the differences of sampled signal and PAM and hence discuss
the bandwidth criterion for PAM.
(c) Explain how signal can be recovered from PAM modulated signal. Give nec-
essary conditions.
7. (a) The voice frequency-modulating signal of a PCM system is quantized in 16
levels with the following probabilities,
P1 = P2 = P3 = P4 = 0.1
P5 = P6 = P7 = P8 = 0.05
P9 = P10 = P11 = P12 = 0.075
P13 = P14 = P15 = P16 = 0.025
Find the information rate taking the band limiting frequency of the modulating
signal at 3 KHz.
(b) Derive the capacity for binary symmetric channel. [8+8]
8. (a) A random variable has the density function
Find the corresponding entropy.
(b) Evaluate the capacity of the channel whose matrix is given as [8+8]
1 - ß ß 0
ß 1 - ß 0
0 0 1
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