West Bengal Institute of Technology (WBIT) 2005 B.Tech Electronics and Tele-Communication Engineering digital communication - Question Paper

(A) structured codes. (B) syndromes.

(C) systematic codes. (D) fault controlled codes.





PART I
ans any 3 ques.. every ques. carries 14 marks.



Q.2 a. discuss -law and A-law of compression Show that for uniform quantization process, quantization fault is proportional to the square of the quantization step size. (8)



b. The info in an analog waveform whose maximum frequency is to be transmitted using 16 level PAM system. The quantization distortion must not exceed of the peak to peak analog signal.

(i) What is the minimum number of bits/sample or bits/PCM word that should be used in this PAM transmission system?

(ii) What is the minimum needed sampling rate and what is the resulting bit rate?

(iii) What is the 16-ary PAM symbol transmission rate? (6)


Q.3 a. The term matched filter is often used synonymously with Correlator. How is that possible in spite of their mathematical operations being different? (6)


b. Show that the impulse response of a matched filter h(t) matched to a signal s(t) is where T is the duration of the signal and K is the constant of proportionality. (8)



Q.4 a. discuss how Inter Symbol Interference (ISI) can be decreased by pulse shaping. Compare the performance degradation of communication system due to noise interference and ISI. (8)



b. An analog signal is PCM formatted and transmitted using binary waveforms over a channel that is bandlimited to 100 KHz. presume that 32 quantization levels are used and that overall equivalent transfer function is of the raised cosine kind with roll off factor, r = 0.6.



(i) obtain the maximum bit rate that can be used by this system without introducing ISI.

(ii) obtain the maximum bandwidth of the original analog signal that can be accommodated with these parameters. (6)


Q.5 a. Derive an expression for the probability of bit fault for coherently detected BPSK signal in AWGN channel with noise power spectral density watts/Hz. (8)



b. obtain the bit fault for a BPSK system with a bit rate of one Mbps. The received waveform and are coherently detected with a matched filter. The value of A is 10 mv. presume that the single sided noise power spectral density is W/Hz and that signal power and energy per bit are normalized relative to load. (6)



Q.6 a. discuss DPSK modulation and demodulation. (8)



b. compute the minimum needed bandwidth for a non-coherently detected orthogonal binary FSK system. The higher frequency signalling tone is one MHz and the symbol duration is one ms. What is the minimum needed bandwidth for a non coherent MFSK system having the identical symbol duration? (6)



PART II
ans any 3 ques.. every ques. carries 14 marks.



Q.7 Consider a (7, 4) code whose generator matrix is
(i) obtain all the code words of the code and the parity check matrix (H) of the code.

(ii) calculate the syndrome for the received vector 1101101. Is this a valid vector? What is the fault correcting capability of the code? What is the fault detecting capability of the code? (14)


Q.8 A (15, 5) cyclic code has generator polynomial as follows :


(i) Draw a diagram of an encoder for this code.

(ii) obtain the code polynomial (in systematic form) for the message . Is a code polynomial in this system? Justify your ans. (14)



Q.9 a. discuss why a syndrome can be computed by evaluating the received polynomial at every of the roots of the codes generator polynomial. (8)


b. discuss in brief Turbo codes. (6)


Q.10 a. elaborate the advantages of correlative coding? discuss duobinary coding and decoding. discuss precoded duobinary signalling. (9)


b. Consider a binary sequence where the 1st digit of the sequence is the begin up digit and not a part of data. Illustrate duobinary coding and decoding rules when differential precoding is used. (5)


Q.11 a. How does processing gain parameter differ for direct sequence systems as compared to frequency-hopping systems? (6)



b. A CDMA system uses direct sequence modulation with a data bandwidth of 10 KHz and a spread bandwidth of 10 MHz. With only 1 signal being transmitted, the receive is 16 dB. If the needed is 10 dB, how many equal power users can share the band? Do not neglect receiver noise. If every user power is decreased by three dB, how many equal power users can share the band? Also obtain out the number of users that can share the band if . (8)

Earning:   Approval pending.