Cognitive Radio Questions and Answers – Next Generation Wireless Network – Spectrum Awareness – 2

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This set of Advanced Cognitive Radio Questions and Answers focuses on “Next Generation Wireless Network – Spectrum Awareness – 2”.

1. Which among the following is an advantage of using Fast Fourier Transform (FFT) for spectrum sensing?
a) Analysis of computation complexity in the FFT window
b) Analysis of phase in the FFT window
c) Analysis of period enclosed in the FFT window
d) Analysis of frequencies enclosed in the FFT window
View Answer

Answer: d
Explanation: Fast Fourier transform is used for performing spectrum sensing by using a wideband analog to digital converter and extensive digital signal processor. This method proves advantageous as it enables the analysis of frequencies enclosed in the FFT window.
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2. Which among the following is not a limitation of employing FFT?
a) Wideband ADC is required
b) Restriction on bandwidth
c) Increased energy demand
d) High delay
View Answer

Answer: d
Explanation: FFT operation demands wideband ADC and extensive digital signal processing. The ADC imposes restrictions on the instantaneous bandwidth. The energy usage is increased due to the high amount of digital signal processing.

3. Which among the following parameters do not exhibit better performance in the spectrum analyzer when compared to the FFT technique?
a) Complexity
b) Bandwidth
c) Short signal detection
d) Power
View Answer

Answer: c
Explanation: Spectrum analyzers are employed for spectrum sensing by tuning to a band with the scan frequency adjusted to dwell at a frequency bin for a desired period. The spectrum analyzer exhibits reduced complexity and power requirements when receiver components are shared. It also scans over large ranges of signals.
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4. Which among the following is accomplished by sampling sub-Nyquist in wideband FFT spectrum sensing?
a) Reduction of complexity
b) Improved signal detection
c) Reduced power consumption
d) Shorter time delay
View Answer

Answer: d
Explanation: Time delay and interleaving time can be controlled by sampling sub-Nyquist and by using appropriate filters in wideband FFT spectrum sensing. The short delay simplifies compatibility with MAC layer timing.

5. Which among the following layers have a similar impact due to interference caused by primary user transmission?
a) Physical layer and MAC layer
b) MAC layer and network layer
c) Network layer and Transport layer
d) Transport layer and Physical layer
View Answer

Answer: c
Explanation: The effect of primary user transmission on the MAC layer and network layer is complete and irreversible blockage of a packet. It can be mitigated by retransmitting acknowledgments in the MAC layer. It can be mitigated in the network layer by selecting protocol with effective packet restoration mechanisms.
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6. Which among the following is the impact of primary user interference in the physical layer?
a) High bit error rate
b) High delay
c) High collision
d) High energy demand
View Answer

Answer: a
Explanation: The effect of primary user interference in the physical layer is a high bit error rate. It can be mitigated by adjusting the coding dynamically. It also can be resolved by allowing higher layers to tackle interference for a period of time.

7. Which among the layers fail to route a packet due to primary user interference?
a) Physical layer
b) Data link layer
c) Network layer
d) Transport layer
View Answer

Answer: d
Explanation: The effect of primary user interference in the transport layer is the failure to route a packet. It can be mitigated by identifying missing packets using the sequence number and requesting of the retransmission of the packets.
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8. Which among the following is the impact of having poor spur free dynamic range?
a) Difficulty in short signal detection
b) Overloading
c) Additive noise
d) High complexity
View Answer

Answer: c
Explanation: Spur free dynamic range is the maximum range over which self-generated signals are absent. Poor SFDR results in spur which resembles tonal signals and give the appearance of an occupied signal. This results in the addition of noise to the received signals.

9. Which among the following results in low sensitivity?
a) Poor collision mitigation mechanism
b) Poor signal processing
c) Poor noise figure
d) Poor linear noise amplifier performance
View Answer

Answer: c
Explanation: Noise figure quantifies the amount of self-generated noise at the receiver. It lower sensitivity and makes it difficult to find other users of the spectrum. Decreased sensitivity causes a reduction in the amount of power allowed for transmission.
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10. Under good propagation conditions, increasing the transmission energy is a poor technique to increase spectrum efficiency.
a) True
b) False
View Answer

Answer: a
Explanation: Effective spectrum usage considers both the amount of spectrum used and the area of propagation. Under good propagation conditions, increasing the transmission energy is a poor technique to increase spectrum efficiency. By increasing the efficiency of a single radio, the spectrum cannot provide proper communication conditions for other users of the spectrum.

11. Which among the following statements holds true about the give graph?

Find the energy required per bit from the given diagram

a) The energy required per bit increases exponentially with exponential increase in bits per Hertz
b) The energy required per bit increases linearly with exponential increase in bits per Hertz
c) The energy required per bit increases linearly with linear increase in bits per Hertz
d) The energy required per bit increases exponentially with linear increase in bits per Hertz
View Answer

Answer: d
Explanation: The graph shows that the energy required per bit increases linearly with exponential increase in bits per Hertz. The cost of energy varying from one to two bits per Hertz is equal to the variation from six to sever bits per Hertz. Bits per Hertz can be a valid measure for spectrum efficiency provided perfectly stable channel and power are available.

12. Which among the following may be implemented to reduce the amount of assumption about interference?
a) Installing multiple sensors to observe the same transmission
b) Developing multiple channel models
c) Developing multiple signal propagation models
d) Installing multiple resource managers
View Answer

Answer: a
Explanation: The removal of uncertainty can reduce the required amount of power at the cognitive transmitter with the certainty of non-interference to the victim receiver. Although multipath scenarios cannot be eliminated, installing multiple sensors to observe the same transmission can reduce the amount of assumption about interference.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter