This set of Digital Communication test focuses on “Equalization and Non Coherent Detection”.
1. The range of amplitude difference gives the value of
a) Width
b) Distortion
c) Timing jitter
d) Noise margin
View Answer
Explanation: In the eye pattern, the amplitude difference gives the value of distortion caused by ISI.
2. As the eye opens, ISI _______
a) Increases
b) Decreases
c) Remains the same
d) None of the mentioned
View Answer
Explanation: As the eye closes, ISI increases and as the eye opens ISI decreases.
3. Pseudo noise signal has _______ and _______ SNR for the same peak transmitted power.
a) Larger, smaller
b) Smaller, larger
c) Larger, larger
d) Smaller, smaller
View Answer
Explanation: A training pulse is applied to the equalizer and corresponding impulse response is observed. Pseudo noise is preferred as the training pulse as it has larger SNR value and larger average power value.
4. The index value n, in transversal filter can be used as.
a) Time offset
b) Filter coefficient identifier
c) Time offset & Filter coefficient identifier
d) None of the mentioned
View Answer
Explanation: The index n can be used as both time offset and the filter coefficient identifier, which is the address in the filter.
5. The over-determined set of equations can be solved using
a) Zero forcing
b) Minimum mean square error
c) Zero forcing & Minimum mean square error
d) None of the mentioned
View Answer
Explanation: The matrix x in transversal equalizer if non square with dimensions 4N+1 and 2N+1. Such equations are called as over-determined set. This can be solved by two methods called as zero forcing method and minimum mean square error method.
6. If the filter’s tap weight remains fixed during transmission of data, then the equalization is called as
a) Preset equalization
b) Adaptive equalization
c) Fixed equalization
d) None of the mentioned
View Answer
Explanation: If the weight remains fixed during transmission of data then the equalization is called as preset equalization. It is a simple method which consists of setting the tap weight according to some average knowledge of the channel.
7. Equalization method which is done by tracking a slowly time varying channel response is
a) Preset equalization
b) Adaptive equalization
c) Variable equalization
d) None of the mentioned
View Answer
Explanation: This method is implemented to perform tap weight adjustment periodically or continually. Equalization is done by tracking a slowly varying channel response.
8. Preamble is used for
a) Detect start of transmission
b) To set automatic gain control
c) To align internal clocks
d) All of the mentioned
View Answer
Explanation: The receiver uses preamble for detecting the start of transmission, to set automatic gain control, and to align internal clocks and local oscillator with the received signal.
9. The disadvantage of preset equalizer is that
a) It doesnot requires initial training pulse
b) Time varying channel degrades the performance of the system
c) All of the mentioned
d) None of the mentioned
View Answer
Explanation: The disadvantage of preset equalization is that it requires an initial training period that must be invoked at the start of any new transmission. Also time varying channel can degrade system performance due to ISI, since the tap weights are fixed.
10. For AWGN, the noise variance is
a) N0
b) N0/2
c) 2N0
d) N0/4
View Answer
Explanation: The noise variance out of the correlator for AWGN is N0/2.
11. Performance of BFSK signal is ________ than BPSK.
a) 3db worse
b) 3db better
c) 6db worse
d) 6db better
View Answer
Explanation: The performance of BFSK is 3db worse than BPSK signalling, since for a given signal power, the distance squared between orthogonal vectors is a factor of two less than the distance squared between orthopodal signals.
12. A Gaussian distribution into the non linear envelope detector yields
a) Rayleigh distribution
b) Normal distribution
c) Poisson distribution
d) Binary distribution
View Answer
Explanation: The two output signals of Gaussian distribution yields Rayleigh and Rician distribution.
13. The non coherent FSK needs ________ Eb/N0 than coherent FSK.
a) 1db more
b) 1db less
c) 3db more
d) 3db less
View Answer
Explanation: The non coherent receiver is easier to implement. The non coherent FSK needs 1db more Eb/N0 than coherent FSK.
14. The DPSK needs ________ Eb/N0 than BPSK.
a) 1db more
b) 1db less
c) 3db more
d) 3db less
View Answer
Explanation: The DPSK system is easier to implement than PSK and it needs 1db more Eb/N0 than BPSK.
15. Coherent PSK and non coherent orthogonal FSK have a difference of ______ in PB.
a) 1db
b) 3db
c) 4db
d) 6db
View Answer
Explanation: The difference of PB is approximately 4db for the best ( coherent PSK ) and the worst (non coherent orthogonal FSK).
16. Which is easier to implement and is preferred?
a) Coherent system
b) Non coherent system
c) Coherent & Non coherent system
d) None of the mentioned
View Answer
Explanation: A non coherent system is desirable because there may be difficulty is establishing and maintaining a coherent reference.
17. Which is the main system consideration?
a) Probability of error
b) System complexity
c) Random fading channel
d) All of the mentioned
View Answer
Explanation: The major system considerations are error probability, complexity and random fading channel. Considering all this non coherent system is more desirable than coherent.
Sanfoundry Global Education & Learning Series – Digital Communications.
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