Wireless & Mobile Communications Multiple Choice Questions on “Diversity Techniques”.
1. Diversity requires a training sequence.
a) True
b) False
Answer: b
Clarification: Unlike equalization, diversity requires no training overhead since a training sequence is not required by the transmitter. Diversity is a powerful communication receiver technique that provides wireless link improvement at a relatively low cost.
2. Diversity decisions are made by ____________
a) Receiver
b) Transmitter
c) Channel
d) Adaptive algorithms
Answer: a
Clarification: In virtually all applications, diversity decisions are made by the receiver and are unknown to the transmitter. Diversity exploits the random nature of radio propagation by finding independent signal paths for communication.
3. Small scale fades are characterized by ____________ amplitude fluctuations.
a) Large
b) Small
c) Rapid
d) Slow
Answer: c
Clarification: Small scale fades are characterized by deep and rapid fluctuations. They occur as the mobile system moves over distances of just a few wavelengths. These fades are caused by multiple reflections from the surrounding in the vicinity of the mobile.
4. ____________ is used to prevent deep fade for rapidly varying channel.
a) Modulation
b) Demodulation
c) Macroscopic diversity technique
d) Microscopic diversity technique
Answer: d
Clarification: In order to prevent deep fades from occurring, microscopic diversity techniques can exploit the rapidly changing signal. By selecting the best signal at all times, a receiver can mitigate small scale fading effects.
5. Large scale fading can be mitigated with the help of _________
a) Modulation
b) Demodulation
c) Macroscopic diversity technique
d) Microscopic diversity technique
Answer: c
Clarification: Large scale fading is mitigated with macroscopic diversity techniques. It is done by selecting a base station which is not shadowed when others are, the mobile can improve substantially the average signal to noise ratio.
6. Space diversity s also known as ________
a) Antenna diversity
b) Time diversity
c) Frequency diversity
d) Polarization diversity
Answer: a
Clarification: Space diversity is also known as antenna diversity. It is one of the popular forms of diversity used in wireless communications. Signals received from the spatially separated antenna on the mobile would have essentially uncorrelated envelopes for antenna separation.
7. Which of the following is not a category of space diversity technique?
a) Selection diversity
b) Time diversity
c) Feedback diversity
d) Equal gain diversity
Answer: b
Clarification: Space diversity reception methods can be classified into four categories. They are selection diversity, feedback diversity, maximal ratio combining and equal gain diversity.
8. In selection diversity, the gain of each diversity branch provides different SNR.
a) True
b) False
Answer: b
Clarification: Selection diversity uses m demodulators to provide m diversity branches. Their gain is adjusted to provide the same average SNR for each branch.
9. Polarization diversity uses the ________ as the diversity element.
a) Modulation index
b) Carrier frequency
c) Reflection coefficient
d) Coherence time
Answer: c
Clarification: Decorrelation of the signal in each polarization is caused by multiple reflections in the channel between mobile and base station antenna. Reflection coefficient for each polarization is different, which results in different amplitudes and phases for each reflection.
10. Which of the factor does not determine the correlation coefficient?
a) Polarization angle
b) Cross polarization discrimination
c) Offset angle from the main beam direction
d) Coherence time
Answer: d
Clarification: The correlation coefficient is determined by three factors, polarization angle, offset angle from the main beam direction of the diversity antenna, and the cross polarization discrimination. The correlation coefficient generally becomes higher as offset angle becomes large.
11. Frequency diversity is implemented by transmitting information on more than one ___________
a) Carrier frequency
b) Amplitude
c) Phase
d) Modulation scheme
Answer: a
Clarification: Frequency diversity is implemented by transmitting information on more than one carrier frequency. Frequency diversity is often employed in microwave line of sight links which carry several channels in frequency division multiplex mode.
12. Frequency diversity uses ________ as a diversity element.
a) Correlation coefficient
b) Coherence time
c) Coherence bandwidth
d) SNR
Answer: c
Clarification: The rationale behind the frequency diversity is that frequencies separated by more than the coherence bandwidth of the channel will be uncorrelated. Thus, they will not experience the same fade.
13. Frequency diversity is good for low traffic conditions.
a) True
b) False
Answer: b
Clarification: Frequency diversity is not good for low traffic conditions. This technique has a disadvantage that it not only requires spare bandwidth but also requires that there be as many receivers as there are channels used for frequency diversity. However, for critical traffic, the expense may be justified.
14. Time diversity repeatedly transmits information at time spacings that exceed ___________
a) Coherence bandwidth
b) Dwell time
c) Run time
d) Coherence time
Answer: d
Clarification: Time diversity repeatedly transmits information at time spacings that exceed coherence time of the channel. Thus, multiple repetitions of the signal will be received with independent fading conditions, thereby providing for diversity.
15. In maximal ratio combining, the output SNR is equal to __________
a) Mean of all individual SNRs
b) Maximum of all SNRs
c) Sum of individual SNR
d) Minimum of all SNRs
Answer: c
Clarification: Maximal ratio combining produces an output SNR equal to the sum of the individual SNRs. Thus, it has the advantage of producing an output with an acceptable SNR even when none of the individual signals are themselves acceptable.
.