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

and DPSK.
(b) elaborate the advantages and disadvantages of digital modulation schemes?
(c) explain base band transmission of M-ary data. [4+6+6]
4. (a) Draw the block diagram of band pass binary data transmission system and
discuss every block.
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Code No: R05310404 Set No. 3
(b) A band pass data transmitter used a PSK signaling scheme with
s1(t) =?A coswct; 0  t  Tb
s2(t) = +A coswct; 0  t  Tb
Where Tb = 0.2msec; wc = 10p /Tb.
The carrier amplitude at the receiver input is 1mV and the power spectral
density of the additive white gaussian noise at the input is 10-11w/Hz. presume
that an ideal correlation receiver is used. compute the avg. bit fault rate
of the receiver. [8+8]
5. A Discrete Memory less Source (DMS) has an alphabet of 5 letters, xi, i =1,
2,3,4,5, every occurring with probabilities 0.15, 0.30, 0.25, 0.15, 0.10, 0.08, 0.05,
0.05.
(a) Determine the Entropy of the source and compare with it N.
(b) Determine the avg. number N of binary digits per source code. [16]
6. (a) compute the bandwidth limits of Shannon-Hartley theorem.
(b) What is an Ideal system? What type of method is proposed by Shannon for
an Ideal system? [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. Sketch the Tree diagram of convolutional encoder shown in figure eight with Rate=
1/2, constraint length L = 2. [16]
Figure 8
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Code No: R05310404 Set No. 4
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
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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) discuss 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) presume that 4800bits/sec. random data are sent over a band pass channel by
BFSK signaling scheme. obtain the transmission bandwidth BT such that the
spectral envelope is down at lowest 35dB outside this band.
(b) Write the comparisons among ASK, PSK, FSK and DPSK. [8+8]
4. (a) What is meant by ISI? discuss how it differs from cross talk in the PAM.
(b) What is the ideal solution to find zero ISI and what is the disadvantage of
this solution. [6+10]
5. A code is composed of dots and dashes. presume that the dash is three times as long
as the dots, has one-third the probability of occurrence.
Calculate
(a) the info in a dot and that in a hash.
(b) avg. info in the dot-hash code.
(c) presume that a dot lasts for 10 ms and that this identical time interval is allowed
ranging from symbols. compute avg. rate of info. [16]
6. discuss Shannon-Fano algorithm with an example. [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. Sketch the Tree diagram of convolutional encoder shown in figure eight with Rate=
1/2, constraint length L = 2. [16]
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Code No: R05310404 Set No. 4
Figure 8
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Earning:   Approval pending.