Wireless & Mobile Communications Multiple Choice Questions on “Pulse Shaping Techniques”.
1. Intersymbol interference (ISI) leads to ________ probability of the receiver for making an error in detecting the symbols.
a) Increased
b) Decreased
c) Zero
d) One
Answer: a
Clarification: ISI leads to increased probability of the receiver making an error in detecting a symbol. When rectangular pulses are passed through a bandlimited channel, the pulses will spread in time, and the pulse for each symbol will smear into the time intervals of succeeding symbols.
2. ISI is ________ by increasing channel bandwidth.
a) Maximized
b) Minimized
c) Zero
d) Infinite
Answer: b
Clarification: Increasing channel bandwidth is one of the method to minimize intersymbol interference. But mobile communication systems use minimal bandwidth, thus other methods to reduce ISI are desirable.
3. Why is pulse shaping technique used?
a) To increase ISI
b) To increase spectral width of modulated signal
c) To reduce ISI
d) To reduce power spectral density
Answer: c
Clarification: Pulse shaping techniques reduces the intersymbol interference. They are also used to reduce the spectral width of the modulated digital signal.
4. Who was the first to solve the problem of ISI?
a) Manchester
b) Faraday
c) Graham Bell
d) Nyquist
Answer: d
Clarification: Nyquist was the first to solve the problem of ISI. He overcome the problem of ISI while keeping the transmission bandwidth low. He observed that ISI can be completely nullified if at every instant, the response due to all symbols except the current symbol is equal to zero.
5. According to Nyquist, the impulse response of the overall communication system should have ______ decay with _______ magnitude for sample values not equal to zero.
a) Fast, small
b) Slow, small
c) Slow, large
d) Fast. Large
Answer: a
Clarification: According to Nyquist, the impulse response of the overall communication system should have fast decay with small magnitude for sample values not equal to zero. If the channel is ideal then it should be possible to realize approximate shaping filters at both transmitter and receiver.
6. Raised cosine filter does not satisfy Nyquist criteria.
a) True
b) False
Answer: b
Clarification: Raised cosine filter is the most popular pulse shaping filter used in mobile communication. It belongs to the class of filters that satisfy Nyquist criterion.
7. As the roll off factor in raised cosine rolloff filter __________ the occupied bandwidth ________
a) Increases, decreases
b) Decreases, constant
c) Increases, increases
d) Decreases, increases
Answer: c
Clarification: As the rolloff factor increases, the bandwidth of the filter also increases and the time sidelobe levels decrease in adjacent symbol slots. Thus, it implies that increasing rolloff factor decreases the sensitivity to timing jitter but increases the occupied bandwidth.
8. Gaussian pulse shaping filter follows Nyquist criterion.
a) True
b) False
Answer: b
Clarification: Gaussian pulse shaping filter uses non Nyquist technique. It is effective when used in conjunction with minimum shift keying (MSK) modulation, or other modulation which is well suited for power efficient nonlinear amplifiers.
9. Gaussian filter has zero crossings at adjacent symbol peaks.
a) True
b) False
Answer: b
Clarification: Nyquist filters have zero crossings at adjacent symbol peaks and a truncated transfer function. Gaussian filter does not follow Nyquist criterion and has a smooth transfer function with no zero crossings.
10. Which of the following is true for a Gaussian filter?
a) Large bandwidth
b) Minimum ISI
c) High overshoot
d) Sharp cut off
Answer: d
Clarification: The Gaussian filter has a narrow absolute bandwidth, and has a sharp cut off, low overshoot and pulse area preservation properties. This makes it attractive for use in mobile communication that uses nonlinear RF amplifiers.
11. Gaussian pulse shaping filter reduces the spectral occupancy and ISI.
a) True
b) False
Answer: b
Clarification: Gaussian pulse shaping does not satisfy Nyquist criterion for ISI cancellation. Thus, it reduces the spectral occupancy but there is degradation in the performance due to increased ISI.
12. Gaussian pulses are used when cost and power efficiency are major factors.
a) True
b) False
Answer: a
Clarification: Gaussian pulses are used when cost and power efficiency are major factors. But the bit error rates due to ISI are deemed to be lower than what is nominally required. Thus, there is a trade-off between desired RF bandwidth and irreducible error due to ISI.
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