# Digital Communications Questions and Answers – Random Signals

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Random signals”.

1. Random variables give relationship between _____
a) Two random events
b) Probability of occurence of two random events
c) Random event and a real number
d) Random event and its probability of occurrence

Explanation: A random variable gives a functional relationship between a random event and a real number.

2. The distribution function of random variable is
a) P(X less than or equal to x)
b) P(X greater than or equal to x)
c) P(X less than x)
d) P(X greater than x)

Explanation: The distribution function of a random variable is the probability that the value taken by the random variable is less than or equal to the real number x.

3. The distribution function of -(infinity) and (infinity) is _____
a) 0 and 1
b) 1 and 0
c) Both 0
d) Both 1

Explanation: F(minus infinity) is 0 and F(infinity) is 1.

4. The value of the probability density function of random variable is
a) Positive function
b) Negative function
c) Zero
d) One

Explanation: The probability density function is always greater than 0. It is a non negative function with the area of 1.

5. Which gives the measure of randomness of the random variable?
a) Mean
b) Variance
c) Standard variance
d) Pdf

Explanation: Variance gives the randomness of the random variable. It is the difference between the mean square value and square of the mean.

6. Random process is a function of ______
a) Random event and time
b) Random event and frequency
c) Random event and real number
d) None of the mentioned

Explanation: Random process is a function of two variables: a random event and its time of occurrence.

7. A random process is called as stationary in strict sense if
a) Its statistics vary with shift in time origin
b) Its statistics does not vary with shift in time origin
c) Its autocorrelation vary with shift in time
d) Its autocorrelation does not vary with shift in time

Explanation: A random process is defined to be stationary in a strict sense if its statistics varies with a shift in time origin.

8. For a stationary process, autocorrelation function depends on
a) Time
b) Time difference
c) Does not depend on time
d) None of the mentioned

Explanation: Autocorrelation function depends on the time difference between t1 and t2.

9. The autocorrelation function is maximum at
a) Origin
b) Infinity
c) Origin & Infinity
d) None of the mentioned

Explanation: On substituting different values in the formula for autocorrelation function it wil be maximum at the origin.

10. Standard deviation is ______
a) Rms value of dc
b) Rms value or ac
c) Either ac or dc
d) Neither dc nor ac

Explanation: The standard deviation of a random variable gives the rms value of an ac component.

11. The average power of white noise is
a) Zero
b) Unity
c) Infinity
d) Between zero and one

Explanation: The average power of white noise is infinity because its bandwidth is infinite.

12. White noise has _____ mean and ______ variance.
a) Zero and zero
b) Finite and zero
c) Zero and finite
d) One and zero

Explanation: White noise is a zero mean function with infinite average power, finite variance and infinite bandwidth.

Sanfoundry Global Education & Learning Series – Digital Communications.

To practice all areas of Digital Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.

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