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Posted Date: 06 May 2008 Posted By: neyha Member Level: Diamond
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2005 Anna University Electronics & Communications Engineering B.E./B.TECH.DEGREE EXAMINATION:DIGITAL COMMUNICATION Question paper
B.E./B.TECH.DEGREE EXAMINATION, NOV / DEC 2005
VI -SEMESTER
DIGITAL COMMUNICATION
TIME : 3 HRS
MAX MARKS : 100
PART - A (10 X 2 = 20 MARKS)
1. What do you understand by the term aliasing?
2. A bandpass signal has the spectral range that extends from 20 KHz to 82 KHz. Find the acceptable range of sampling frequency .
3. What is the purpose of using eye pattern?
4. Why do you need adaptive equalization in a switched telephone network ?
5. Draw an illustrative figure to show the operation of a correlation receiver.
6. Highlight the major difference between a QPSK signal and a MSK signal.
7. Explain the fundamental difference between block codes and convolational codes.
8. How will you define coding gain with reference to error control codes?
9. Write down the properies of PN sequences.
10. How do you define processing gain for a DS-CDMA system?
PART - B (5 X16 =80 MARKS)
11. (i) Derive expression for the quantization noise and signal-to -noise in a PCM system using a uniform quantizer
(ii) A sinusoidal signal is transmitted using PCM. An output SNR of 55.8 dB is required .Find the number of representation levels required to achieve this performance.
12 (a) What do you understand by the term inter symbol onterference (ISI) ? Discuss in detail the Nyquist criterian for minimizing ISI. Explain the difficulties in implementing it in a practical system.
(OR)
(b) (i) Discuss the merits and demerits of Duobimanary signalling.
(ii) The binary data (011100101) are applied to the input of Duobinary encoder.Construct the Duobinary output and the corresponding receiver output,without a precoder.Suppose due to error in transmission,the level produced by the third digit is reduced to zero, construct the new receiver output.
13 (a) Explain he BPSK signal transmission and coherent BPSK reception with suitable diagrams.Derive an expression for the probability of symbol error for the scheme.
(OR)
(b) Explain the basis of operation in a matched fiter receiver with suitable diagrams. Derive the condition for which the output signal -to -noise ratio is maximized
14 (a) A (15 , 5 ) linear cyclic code has a generator polynomial, g (D) = 1 + D + D ^2 + D^4 + D^5 + D^8 +D ^10.
Draw the block diagram of an encoder and syndrome calculatorfor this code. Find the code plynomial in systematic form , for the message polynomial m(D) = 1 + D^4 + D ^6 + D^8+ D^14, a code polynomial? If not , find the syndrome of y(D).
OR
(b) Consider a rate -1/2, a non-sstematic convolutional code with g ^ (1) ={1,0,1} and g ^ (2) = {1,1,1,}.Determine the encoder output corresponding to the data sequence {1 0 1 1 01 }.If the first and the fourth bits of the encoded sequence are affected during transmission, demonstrate the error correcting capability of viterbi algorithm.
15 (a) Derive the probablity error for a direct sequence spread BPSK system, affected only by an interfering signal , using the signal space approach. Also obtain the jamming margin. What does it imply?
OR
(b) Discuss a suitable method for generating PN sequences. Explain with suitale diagrams how the PN sequences is used in a frequency hoppng spread spectrum transmitter and receiver
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