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Cognitive Radio Questions and Answers for Entrance exams on “Next Generation Wireless Network – Spectrum Sensing – 2”.
1. Which among the following is achieved by coupling a modulated signal with a sine wave carrier?
a) Causality
b) Periodicity
c) Linearity
d) Stability
Answer: b
Clarification: Modulated signals are coupled with sine wave carriers, hopping sequences, or cyclic prefixes to attain periodicity. The signals are regarded as cyclostationary due to periodic mean and autocorrelation.
2. Which among the following is not an advantage of using cyclostationary feature detection?
a) Better performance than energy detection
b) Does not require prior knowledge of the signal transmitted by the primary user
c) Robust noise performance
d) Rapid attainment of high processing gain
Answer: d
Clarification: The primary advantage of using cyclostationary feature detection is its ability to differentiate noise signals from modulated signals. This is due to incorporated redundancy in modulated signals. However cyclostationary feature detection is computationally intensive and demands long observation time.
3. What is the purpose of out-of-band sensing?
a) Fast switching
b) Fast routing
c) Fast discovery
d) Fast handoff
Answer: c
Clarification: Out-of-band sensing computes the time duration required by a cognitive radio user to detect a completely available spectrum band for transmission. Sensing order and stopping rule are factors that overlook out-of-band sensing.
4. Which among the following is a factor of concern for in-band sensing?
a) Identification of stop period for the search process
b) Identification of the start period for the search process
c) Selection of spectrum for evaluation
d) Selection of duration for transmission
Answer: d
Clarification: In-band sensing is mainly concerned with interference avoidance. Sensing time and transmission time are the major factors computed by in-band sensing. The duration and the frequency at which sensing should be carried out to achieve satisfying sensing accuracy is determined by in-band sensing.
5. Which among the following are problems of transmitter detection?
a) Secondary user uncertainty and xG base station uncertainty
b) xG base station uncertainty and primary base station uncertainty
c) Receiver uncertainty and shadowing uncertainty
d) Transmitter uncertainty and receiver uncertainty
Answer: d
Clarification: When the xG user communication range does not include the transmitter, interference is unavoidable as receiver information is unavailable. This is known as receiver uncertainty. In certain situations, the xG user has a good line of sight with the receiver but is unaware, this is known as shadowing uncertainty.
6. What is the terminology used when xG base-station is involved in gathering sensing information?
a) Cumulative
b) Centralized
c) Distributed
d) Aggregated
Answer: b
Clarification: Cooperation detection can be carried out with a centralized structure or with a distributed structure. Centralized cooperation detection involves the collection of sensing information by the xG base station to determine spectrum holes. Distributed cooperation detection detects spectrum holes by using information exchanged by xG users.
7. Which among the following is an interference of interest in xG architecture?
a) Expected interference at primary user
b) Expected interference at xG user
c) Expected interference at channel
d) Expected interference at spectrum broker
Answer: a
Clarification: The expected amount of interference on the primary user due to an xG user is the interference of interest in xG architecture. Interference at the primary transmitter can be controlled through the radiated power, the out-of-band emissions, and the location at the transmitters.
8. Which among the following is not a disadvantage of cooperation detection?
a) Overhead traffic
b) Receiver uncertainty
c) Shadowing uncertainty
d) Additional storage
Answer: c
Clarification: The main drawback of cooperation detection is the increased strain on networks with limited resources as the number of operations and overhead traffic is increased. When the xG user communication range does not include the transmitter, interference is unavoidable as receiver information is unavailable. Receiver uncertainty is not solved by cooperation detection.
9. What is an interference temperature model?
a) Interference temperature model is constructed by using the cumulative RF energy from multiple transmissions
b) Interference temperature model is constructed by using the RF energy from a single transmission
c) Interference temperature model is constructed by using the sampling RF energy from periodic transmissions
d) Interference temperature model is constructed by using the sampling IF energy from periodic transmissions
Answer: a
Clarification: An interference temperature model is constructed by using the cumulative RF energy from multiple transmissions. A threshold limit is set on the aggregate level. The xG users are permitted to use the spectrum band provided their transmission levels are within the threshold limit.
10. Which among the following is not a disadvantage of the interference detection method?
a) Requirement of modulation of unlicensed signal
b) Requirement of the activity level of licensed and unlicensed users
c) Management of interference at the receiver
d) Disregard for the effect of multiple xG users
Answer: c
Clarification: The interference detection method requires information such as type of unlicensed signal modulation, power control, and the activity level of licensed and unlicensed users. It does not consider the effect of multiple xG users. It requires the location of nearby primary users.
11. Spectrum decisions are affected by the co-location of data transmission and sensing function.
a) True
b) False
Answer: a
Clarification: Spectrum decisions are affected by the co-location of data transmission and sensing function. This problem is overcome by developing a separate sensor network for cooperative spectrum sensing while an operational network is constructed for data transmission.
12. Which among the following condition is represented in the following diagram?
a) Transmitter uncertainty
b) Shadowing uncertainty
c) Receiver uncertainty
d) Access point uncertainty
Answer: b
Clarification: Shadowing uncertainty is the situation when the xG user cannot sense the receiver when there is a clear line of sight between the receiver and xG user. It also cannot sense the transmitter since the line of sight between the transmitter and the xG user is hindered by the receiver.