Digital Communications Questions and Answers – Signal Time Spreading

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Signal time spreading”.

1. Multiple isolated peaks in multipath components is called as
a) Fingers
b) Returns
c) Fingers & Returns
d) None of the mentioned
View Answer

Answer: c
Explanation: The received signal usually consists of several discrete multipath components causing multipath intensity profile to exhibit multiple isolated peaks also called as fingers or returns.

2. If delay time is greater than symbol time, _______ fading occurs.
a) Frequency selective
b) Time selective
c) Frequency non selective
d) None of the mentioned
View Answer

Answer: a
Explanation: When delay time is greater than symbol time, frequency selective fading occurs.

3. If delay time is lesser than symbol time, ______ fading occurs.
a) Frequency non selective
b) Flat
c) Frequency non selective & Flat
d) None of the mentioned
View Answer

Answer: c
Explanation: If delay time is lesser than the symbol time, frequency non selective or flat fading occurs.
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4. When channel coherence bandwidth is greater than the transmitted signal bandwidth ______ occurs.
a) Time selective
b) Frequency selective
c) Frequency non selective
d) None of the mentioned
View Answer

Answer: c
Explanation: Frequency non selective or flat fading occurs if channel coherence bandwidth is greater than the transmitted signal bandwidth.

5. Channel’s time spreading properties are
a) Coherence bandwidth
b) Signal dispersion
c) Coherence bandwidth & Signal dispersion
d) None of the mentioned
View Answer

Answer: c
Explanation: Signal dispersion and coherence bandwidth characterizes the channel’s time spreading properties in a local area.
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6. When channel coherence time is less than the time duration of a transmission symbol, then ______ fading occurs.
a) Fast
b) Slow
c) Time selective
d) None of the mentioned
View Answer

Answer: a
Explanation: Fast fading occurs when the channel coherence time is less than the time duration of a transmission symbol.

7. Minimum time required for magnitude change or phase change is called as
a) Shift time
b) Coherence time
c) Delay time
d) None of the mentioned
View Answer

Answer: b
Explanation: Coherence time is the minimum time required for magnitude or phase change of the channel to become uncorrelated from the previous value.
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8. When a user’s moving, the user’s velocity causes shift in frequency. This is called as
a) Doppler shift
b) Frequency shift
c) Velocity shift
d) None of the mentioned
View Answer

Answer: a
Explanation: When a user’s moving, the user’s velocity causes shift in frequency of the transmitted signal along each signal path. This is called as Doppler shift.

9. Coherence time is _______ to Doppler spread.
a) Directly proportional
b) Inversely proportional
c) Not dependent
d) None of the mentioned
View Answer

Answer: b
Explanation: Coherence time is inversely proportional to Doppler spread.
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10. Block fading occurs in
a) Frequency domain
b) Time domain
c) Frequency & Time domain
d) None of the mentioned
View Answer

Answer: c
Explanation: Block fading can occur in both frequency domain and time domain. This is called as doubly block fading.

11. If coherence bandwidth is smaller than the bandwidth of the signal, _____ fading occurs.
a) Flat
b) Frequency selective
c) Fast fading
d) Time selective
View Answer

Answer: b
Explanation: Frequency selective fading occurs when coherence bandwidth is smaller than the bandwidth of the transmitted signal.

12. Flat fading can be avoided or stopped by
a) Error coding
b) Equalization
c) Adaptive bit loading
d) All of the mentioned
View Answer

Answer: d
Explanation: Flat fading can be combated by means of error coding, equalization and adaptive bit loading.

13. The effect of Doppler shift can be counterattacked by
a) OFDM
b) By using two receivers with separate antennas
c) By using diversity receiver
d) All of the mentioned
View Answer

Answer: d
Explanation: The effect of Doppler shift can be removed by using diversity scheme such as orthogonal frequency division multiplexing, by using two receivers with separate antennas or by using specially designed diversity receiver.

14. When transmitter and receiver moves towards each other, the magnitude of frequency shift is
a) Positive
b) Negative
c) Zero
d) Infinity
View Answer

Answer: a
Explanation: When transmitter and receiver moves towards each other, the magnitude of frequency shift is positive, where as if it moves away from each other then it is negative.

15. If signalling rate is less than fading rate, _____ fading occurs.
a) Slow
b) Fast
c) Time selective
d) None of the mentioned
View Answer

Answer: b
Explanation: A channel is said to be fast fading if symbol rate or signalling rate is less than fading rate.

16. Frequency tracking loop can minimize irreducible error rate by using
a) GMSK
b) DMSK
c) MSK
d) QAM
View Answer

Answer: b
Explanation: Frequency tracking loop can minimize the irreducible error rate in a mobile system by using differential minimum shift keying modulation.

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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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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