# Digital Communication Dec 2011

Note : Attempt one question from each Unit. All questions carry equal marks.

Unit —1

1- (a) Define and explain the following :

Cumulative distribution function

Probability density function

Variance and standard deviation of random variable

(b) An urn contains 4 white and 3 black balls. Two balls are drawn successively with X denoting the number of black balls :

Find the probability function of X.

Draw the bar chart and histogram.

Or

(a) Define and explain the following :

Correlation and autocorrelation

Cexural-limit theorem

Power spectral density of digital data

(b) A fair die is tossed 5 times. A toss is called a success if face 1 or 6 appears, find :

() the probability of 2 successes.

() the mean and standard deviation for the number of successes.

(a) Explain natural and flat tap sampling. Compare the two.

(b) Explain how PPM and PWM signals are generated from PAM signals and (ii) directly.

Or

(a) State and prove Sampling theorem. Also explain. Aliasing effect in detail.

(b) Draw and explain the PAM modulator and demodulator circuit.

Unit-III

(a) Explain quantization. What is quantization error ? How does it depend upon the step size and how it can be reduced.

(b) Describe della modulation systems. What arc its limitations ? How can they be overcome ?

Or

(a) Discuss generation ami detection, spectrum und

bandwidth of amplitude shift keying.

(b) What is matched filters ? Explain. Find its transfer function.

Or

(a) Explain frequency shift keying. Describe coherent

detection of FSK signals. What should be the relationship between bit rate and frequency shift for a better performance.

(b) Compare digital modulation techniques on the basis of probability of error and matched filter.

Unit—V

Explain spread spectrum modulation. Discuss generation and characteristics of p-n sequences.

Or

Explain any two of the following :