Digital Communications Questions and Answers – Signals and Noises

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Signals and Noises”.

1. The term heterodyning refers to
a) Frequency conversion
b) Frequency mixing
c) Frequency conversion & mixing
d) None of the mentioned
View Answer

Answer: c
Explanation: The method heterodyning means frequency conversion and mixing and this results in a spectral shift.

2. The causes for error performance degradation in communication systems are
a) Interference
b) Electrical noise
c) Effect of filtering
d) All of the mentioned
View Answer

Answer: d
Explanation: The main causes of error performance degradation are interference electrical noise effect of filtering and also due to the surroundings.

3. Thermal noise in the communication system due to thermal electrons
a) Can be eliminated
b) Cannot be eliminated
c) Can be avoided upto some extent
d) None of the mentioned
View Answer

Answer: b
Explanation: Thermal noise which cannot be eliminated is caused by the motion of thermal electrons causes degradation in system.
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4. White noise has _______ power spectral density.
a) Constant
b) Variable
c) Constant & Variable
d) None of the mentioned
View Answer

Answer: a
Explanation: The AWGN has constant power spectral density.

5. Which are called as hard decisions?
a) Estimates of message symbol with error correcting codes
b) Estimates of message symbol without error correcting codes
c) All of the mentioned
d) None of the mentioned
View Answer

Answer: b
Explanation: If error correcting codes are not present, the detector output consists of estimates of the message symbol which is also called as hard decisions.
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6. The filter which is used to recover the pulse with less ISI is called as
a) Matched filter
b) Correlator
c) Matched filter & Correlator
d) None of the mentioned
View Answer

Answer: c
Explanation: The optimum filter used to recover the pulse with best possible signal to noise ratio and less or no ISI is called as correlator or matched filter.

7. The composite equalizing filter is the combination of
a) Receiving and equalizing filter
b) Transmitting and equalizing filter
c) Amplifier and equalizing filter
d) None of the mentioned
View Answer

Answer: a
Explanation: The functions of both receiving and equalizing filter can be performed by only the equalizing filter alone. Thus equalizing filter is the combination of equalizing and receiving filter.
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8. The sample from the demodulation process consists of sample which is _______ to energy of the received symbol and _____ to noise.
a) Directly and inversely proportional
b) Inversely and directly proportional
c) Both directly proportional
d) Both inversely proportional
View Answer

Answer: a
Explanation: The output symbol of the sampler consists of sample which is directly proportional to the energy of the received signal and inversely proportional to the noise.

9. The average noise power of white noise is
a) 0
b) Infinity
c) 1
d) None of the mentioned
View Answer

Answer: b
Explanation: White noise is a idealized process with two sided spectral density equal to constant N0/2 and frequencies varying from minus infinity to plus infinity. Thus the average noise power is infinity.
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10. The channel may be affected by
a) Thermal noise
b) Interference from other signals
c) Thermal noise & Interference from other signals
d) None of the mentioned
View Answer

Answer: c
Explanation: A channel can be modelled as a linear filter with additional noise. The noise comes from thermal noise source and also from interference from other signals.

11. Channels display multi-path due to
a) Scattering
b) Time delayed reflections
c) Diffraction
d) All of the mentioned
View Answer

Answer: d
Explanation: Wireless wide-band channels display multi-path due to time delayed reflections, diffraction and also scattering.

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.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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