This set of Cognitive Radio Multiple Choice Questions & Answers (MCQs) focuses on “Next Generation Wireless Network – Spectrum Sensing – 1”.
1. Which among the following is the function of out-of-band sensing?
a) Detect primary base-station
b) Detect spectrum holes
c) Detect primary user
d) Detect xG users
View Answer
Explanation: A cognitive radio should be aware of its surrounding radio environment. Spectrum sensing allows a radio to find the unused spectrum in the radio environment. In out-of-band sensing, cognitive radio users focus on the identification of spectrum holes over a wide frequency range to transmit their information.
2. What is the purpose of in-band sensing?
a) Interference avoidance
b) Sharing spectrum information
c) Sharing spectrum resources
d) Spectrum handoff
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Explanation: Spectrum sensing identifies unused spectrum holes that are available for transmission for cognitive radio users. During transmission, the cognitive radio users continue to monitor the spectrum band. They also sense other users of the primary network in order to avoid interference. Spectrum sensing carried out during transmission is called in-band sensing.
3. Why is cooperation required in spectrum sensing?
a) Elimination of base-station
b) Elimination of xG users
c) Interference avoidance
d) Improvement of sensing accuracy
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Explanation: The cognitive radio user monitors and studies the local radio environment to identify the current spectrum availability. The information gathered by each user is shared between its neighbours. This practice is known as cooperation in sensing and it improves the sensing accuracy.
4. Which among the following is the most practical method to identify spectrum holes?
a) Detect primary user transmission
b) Detect primary user receiving information
c) Detect secondary user transmission
d) Detect secondary user receiving information
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Explanation: The detection of primary users receiving information within the communication range of xG users is considered as the most efficient means for determining spectrum holes. However this is not always feasible as it is laborious to measure the channel between a transmitter and a primary receiver. Hence sensing is predominantly carried out by detecting the transmission operation of primary users.
5. Which among the following is not a transmission detection technique?
a) Matched filter detection
b) Energy detection
c) Cyclostationary detection
d) Interference based detection
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Explanation: Transmission detection is based on the identification of weak signals sent by the primary transmitter obtained by the local observations of the xG users. Transmitter detection is also known as non-cooperative method. Interference based detection and cooperation detection are other spectrum sensing techniques.
6. What is the terminology used to represent the absence of a licensed user in the spectrum?
a) Grounded hypothesis
b) Fixed hypothesis
c) Null hypothesis
d) Zero hypothesis
View Answer
Explanation: The following equation represents the basic hypothesis model for transmitter detection,
s(t) = n(t) [U1]
s(t) = a*i(t) + n(t) [U2]
where s(t) is the signal available at the xG user, i(t) is the signal transmitted by the primary user, n(t) is the AWGN, and a is the amplitude gain of the channel.
U1 is the null hypothesis which indicates the absence of signals from the licensed users in the spectrum band. U2 is termed alternate hypothesis which indicates the presence of signals from the licensed users in the spectrum band.
7. Which among the following techniques requires prior knowledge of the primary signal?
a) Matched filter detection
b) Energy detection
c) Cooperation detection
d) Cyclostationary detection
View Answer
Explanation: The matched filter detection method requires prior knowledge of the signal to be transmitted by the primary user such as the pulse shape, the packet format, and the order and modulation type. If the information is available, then the matched filter is the most compatible detection method. Matched filter can maximize the signal to noise ratio.
8. What is the advantage of using matched filter detection?
a) Rapid increase in processing gain
b) Simultaneous detection
c) High accuracy
d) Low power consumption
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Explanation: Matched filter detection achieves high processing gain rapidly due to coherency. However when accurate information is not available about the signal transmitted by the primary user, then the matched filter detection method exhibits poor performance.
9. Which among the following is not an operational unit in energy detection?
a) Integrator
b) Squaring unit
c) Correlator
d) Threshold device
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Explanation: Energy detection consists of three operational units namely squaring device, integrator, and threshold device. The received signal energy is measured by squaring and integrating the received signal over an interval T. The output of the integrator is then evaluated by the threshold device to determine the existence of the primary user in the licensed spectrum.
10. Which among the following are factors of significance in the energy detection technique?
a) Probability of detection and probability of exception
b) Probability of detection and probability of false alarm
c) Probability of detection and probability of useful information
d) Probability of false alarm and probability of useful information
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Explanation: Probability of detection and probability of false alarm are factors of significance in energy detection technique. They depend on the signal to noise ratio, time-bandwidth product, complete and incomplete gamma functions, and generalized Marcum Q-function.
11. Sensing control is implemented in the physical layer of the OSI model.
a) True
b) False
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Explanation: Sensing control is essential to adapt sensing operation in accordance with the dynamic radio environment. It operates in the physical and data-link layer of the OSI model. It coordinates the sensing operations among xG users in a distributed manner. It helps to avoid false alarms in cooperative sensing.
12. What is the detection technique employed in the following diagram?
a) Matching filter detection
b) Cooperation detection
c) Energy detection
d) Cyclostationary detection
View Answer
Explanation: In cyclostationary detection, modulated signals are coupled with sine wave carriers or pulse trains to incorporate periodicity. By exploiting the periodicity, noise energy is differentiated from modulated signal energy. Hence it yields better noise performance than other transmitter detection techniques.
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