Digital Communication Interview Questions & Answers

Question 1. Explain How Qpsk Differs From Psk In Term Of Transmission Bandwidth And Bit Information It Carries?

Answer :

For a given bit rate 1/Tb, a QPSK wave requires half the transmission bandwidth of the corresponding binary PSK wave. Equivalently for a given transmission bandwidth, a QPSK wave carries twice as many bits of information as the corresponding binary PSK wave

Question 2. Give The Equation For Average Probability Of Symbol Error For Coherent Binary Psk?

Answer :

Average probability of signal error, 

Pe = 1 / 2 erfc? Eb / No

Question 3. Define Qpsk?

Answer :

QPSK is Quadriphase –shift keying. In QPSK the phase of the carrier takes on one of the four equally spaced values Such as ?/4 , 3?/4, 5?/4 and 7?/4.

Question 4. Define Dibit?

Answer :

dibits 10, 00, 01 & 11

Question 5. Give The Two Basic Operation Of Dpsk Transmitter?

Answer :

1. differential encoding of the input binary wave 
2. Phase –shift keying hence, the name differential phase shift keying

Question 6. Define Information Capacity?

Answer :

Information capacity represents the number of independent symbols that can be carried in the system for a given time.

Question 7. What Is The Unit Of Information Capacity?

Answer :

Unit is bits per second

Question 8. Why Digital Amplitude Modulation Is Commonly Called On –off Keying?

Answer :

Carrier is either in ON or Off state so it is called as On Off Keying.

Question 9. Why Digital Amplitude Modulation Is Called As Continuous Wave Modulation?

Answer :

When the carrier is being transmitted it has constant amplitude, constant frequency and constant phase.

Question 10. Define Bit Rate?

Answer :

The rate of change at the input to the modulation is called bit ra and is expressed in bits per second.

Question 11. Define Baud?

Answer :

Baud is the rate of change and is equal to the reciprocal of one signaling element.

Question 12. Define Phase Reversal Keying?

Answer :

Since Phase of the output carrier shifts between two phases PSK is also called as Phase reversal keying.

Question 13. What Is Offset Qpsk?

Answer :

IT is a modified form of QPSK where the bit waveforms on the I and Q channels are offset or phase shifted in phase from each other one by one half of a bit time.

Question 14. What Is Eight Phase Psk?

Answer :

Eight Phase PSK (8-PSK) is an M-ary encoding technique where M=8.

Question 15. What Is The Need Of Maximum Distance Code?

Answer :

It is used to reduce the number of transmission errors.

Question 16. What Is Quad Bits?

Answer :

A 16 bit PSK modulator acts on the incoming data in groups of four bit is called Quad bits.

Question 17. Define Dpsk?

Answer :

DPSK is the difference between two successive signalling elements rather than the absolute phase.

Question 18. What Is The Need For Ber?

Answer :

Bit error Rate is the empirical record of Systems actual bit error performance.