WBUT Question Papers BE EC 5th Semester
Digital Communication Systems Dec 2007
 Choose the correct alternatives for any ten of the following : 4 The spectral density of white noise is
a) exponential b) uniformc) Poisson d) Gaussian.


. HI! < l««l _av.T— 
GROUP B ( Short Answer Type Questions )
 What are the advantages of digital communication system over analog communication ?
 What is companding ? Why is it needed ?
 Given the data stream 1110010100,
sketch the transmitted sequence of rectangular pulses for each of the following line codes :a) Unipolar NRZb) Unipolar RZc) Polar RZd) Polar NRZe) Manchester.
 A discrete memoryless source X has five symbols x_{f} , ( i = 1. … 5 ) with P(Xj ) = 04. P(x_{2} ) = 019, P(x_{3} ) = 013, P(x_{4} ) = 016 and P(x_{5}) = 012.
Construct the Huffman code for X and calculate the efficiency of the code.
 For a BSC shown in the following figure find the channel capacity for p = 06.
GROUP – C ( Long Answer Type Questions)Answer any three of the following questions. 3 x 15 = 45
 a) Draw the block diagram for generation and detection of the BFSK signal and
explain clearly its operation.b) Draw the signal space representation and find the distance between symbols.c) What Is the difference between MSK and QPSK ? 6 + (3 + 2) + 4
 a) What is Nyqulst criterion for zero Intersymbol interference ?
b) What are the limitations of ideal solution and how can it be solved with the help of raised cosine function ?c) A communication channel of bandwidth 75 kHz is required to transmit binary data at a rate of 01 Mbps using raised cosine pulses. Determine the roll of factor a. 4 + 7 + 4
 a) What is a matched filter ?
b) Draw the Impulse response of an optimum filter matched to the following input/(t). \
c) If a rectangular pulse of amplitude A and with T Is applied to a matched filter, find the spectrum of the matched filter output and also the output signal in the time domain. 3 + 4 + 8
10. a) A discrete memoryless source has an alphabet of 7symbols with probabilities for its output:

Apply ShanonFano algorithm to this & calculate the efficiency & variance of this coding.
b) What is the disadvantage of Huffman coding ?
c) Write down the algorithm for Huffman coding for moving a “combined” symbol as high as possible. Explain with suitable example. 8 + 2 + 5
 a) One parity check code has parity check matrix as :
H =  101  100 
110  010  
011  001 
1) Determine generator matrix
ii) Find the code word that begins [ 101 )
111) If received word is [ 110110 ], then decode this word,
b) Write the propereties of Hamming distance d _{min} •