# Communication Systems Important Questions – Past Papers

## Paper 1: Communication Systems – University Past Papers.

Time Allowed: 3 hours

Total Marks:    70, Passing Marks (35)

Q1: a) What is Cumulative Distribution Function (CDF). Write its properties as well.
b) Define:
i. Sample Space ii. Random process iii. Complement of an event
Q2: a) A binary-symmetric channel has error probability = . The probability of transmitting 1 is Q while
that of transmitting 1 is 1-Q.
b) Determine the probabilities of receiving 1 and 0 at the receiver.

Q3: a) Define correlation and discuss its different types.
b) Define moments. Write the equations for nth moment, nth central moment and 2nd central moment.
c) Define continuous random variable and discrete random variable.
Q4: Show that the random process where θ is a random variable uniformly
distributed in the range (0, 2π) is a wide-sense stationary process.

Q5: With the help of a figure, discuss the behavior of DSB-SC system in the presence of noise by deriving the
equation for .

Q6: Describe the transmission of random processes through linear systems by determining the autocorrelation
function and PSD.

Q7: Draw the figures for the Coherent and Non-coherent Detection of digital modulated signals.

Q8: Write short notes on any two of the following:
a) Stationary Random Processes
b) Ergodic Processes