Jaypee Institute of Information Technology (JIIT) 2008 B.E Electronics & Tele-Communication Engineering Test 3 : Digital Communication - Question Paper
JAYPF.F. 1NIVERSITY OF INFORMATION TECHNOLOGY WAKNAGHAT TEST-3 5 SEMESTER 2008
COURSE CODE: (PB5IKCI09 MAX. TIME: 1 HR 30 MIN.
COURSE NAME: Digital Communication
COURSE CREDITS: 4 MAX. MARKS: 30
Note: Attempt all five questions. Answers should be brief and to the point. Alt parts of a Question must be answered in one piece if you feel the question is incorrect or lacking data, make appropriate assumptions; state the assumptions in the answer script and answer the question. This paper has two pages.
Q.l Describe digital signals. What are the advantages and disadvantages of digital
communication in comparison with analogue communication? (11 Derive a formula for the quantization noise, under no overload condition, in a uniform
quantizer State the assumptions needed for this. (1.5)
A triangular signal is *(/) 4(1-1) with x(t-2 T) x(t) and T-025 msec This is to
be quantized by a 6 bit uniform quantizer.
Compute (i) the SNR of the quantized signal. (3)
(ii) the transmission rate (O.S)
Q.2a Explain companding for PCM coders. What are its advantages'' (1)
A signal x (t) has an adjacent sample correlation p,= 0.8. Determine the improvement in
SNR for DPCM over PCM for this signal. Derive any formula used (2)
b Determine the spectrum of an M-ary ASK signal. (2)
Draw and describe the phase trellis of MSK. (I)
Q.3 A signal x(t) S a* px (t*kT) with uncorrelated zero mean and pulse duration T, is passed through the transmitter filter having a transfer function Ht(Q, a channel having Hc<f) and a receiving filter H(f). There is additive Gaussian noise with power spectral density G(0 at the receiving input. To avoid intersymbol interference we want the pulse shape p(t) at ihe input of the decision device. Derive an expression for the Hr (f) and the Hr (f) to minimize P. Assume G.(0 and Hc(f) fixed and specified. (6)
Q.4 Explain how a binary' modulated signal can be demodulated using coherent detector. (I) Derive an expression for probability of error for the coherent detection of PSK. (2) In a PSK. system the received bit energy is 1012 J. The noise spectral density N0= 2.5 X 10,lJ W Hz- Determine the probability of error.
Assume Jexp( - %)dA * exp(" ) (2)
Sketch the Pe vs E*/ No curves for PSK and coherent dctcction of FSK For any given P, mark the difference on the Nt axis for the two curves. (I)
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