Interview Questions and Answers on Performance Analysis of Digital Communication System

Q.1. What is meant by noise ?

Ans. When the signal voltage arrives at the demodulator of a receiver, it will be accompanied by a voltage waveform which varies with time in an entirely ” unpredictable manner ” . This unpredictable voltage waveform is a random process called “Noise”. The signal accompanied by noise is said to be”contaminated” or corrupted by noise.

Noise is an unwanted electrical disturbance which gives rise to audible or visual disturbances in communication system in the digital communication.

Q.2. What are the sources of noise.

Ans. Noise can arise from different types of sources. The noise sources are :

  1. Natural sources
  2. Man made sources
  3. Fundamental or internal sources

Q.3. What is meant by shot noise?

Ans. The shot noise is produced due to shot effect.

The shot noise is produced in all the amplifying devices rather in all the active devices.

The shot noise is produced due to the random variations in the arrival of electrons (or holes) at the output of an amplifying device.

Therefore, it appears as a randomly varying noise current superimposed on the output. The shot noise sounds like a shower of lead shots falling on metal sheet.

Q.4. What is meant by detection and estimation ?


Detection: The detection theory helps in designing a decision making device which looks at the input signals and makes a guesses about whether  a 0 or 1 was transmitted.

Estimation : The estimation theory has been developed in order to design a processor which uses the information in the received signal to extract the estimates of some physical parameters or waveforms. There is always a possibility of error getting introduced into the detection as well as estimation.

Q.5. Write the expression for minimum error probability of matched filter.

Ans. Minimum error probability of matched filter:

Q.6. What is meant by quadrature modulation?

Ans. In digital modulations, instead of transmitting one bit at a time, we transmit two or more bits simultaneously. This is known as M-ary transmission. This type of transmission results in reduced channel bandwidth. However, sometimes, we use quadrature carriers for modulation. This process is known as quadrature modulation.

Q.7. What is signal space ?

Ans. The signal space is an N-dimensional Euclidean space which has N mutually perpendicular axes labelled


Q.8. Define probability of error (Pe) ?

Ans. The receiver or detector is supposed to make a correct decision about which symbol is transmitted by a transmitter. But it is not always possible for a practical receiver to make a correct decision. The wrong decision of receiver is called as an error and the probability of such error is called as error probability.

The error probability is denoted by Pe. All the errors should be made to reduce the error probability to its minimum possible value. The error probability Pis also called as the average probability of symbol error.

A system with smaller value of Pis better than the system having larger value of P.