This set of Cognitive Radio Questions and Answers for Campus interviews focuses on “Next Generation Wireless Network – Spectrum Management – 2”.
1. Which among the following is the expression for channel capacity as per Shannon-Hartley theorem?
B is bandwidth, N is the average noise power, and S is the average received signal power.
a) C=B+log2(1+ \(\frac{N}{S})\)
b) C=B+log2(1+ \(\frac{S}{N})\)
c) C=B log2(1+ \(\frac{N}{S})\)
d) C=B log2(1+ \(\frac{S}{N})\)
View Answer
Explanation: Shannon Hartley theorem states the maximum rate of transmission over a channel of specified bandwidth containing noise. The calculated channel capacity creates an upper bound for maximum error-free information transmission.
C=B log2(1+ \(\frac{S}{N})\)
Here B is bandwidth, N is the average noise power, and S is the average received signal power.
2. Which among the following an expression for spectrum capacity in OFDM based xG networks?
α refers to a set of unused spectrum units, G(f) is the channel power gain at frequency f, S0 is the signal power per unit frequency and N_0is the noise power per unit frequency.
a) C=∫α\(\frac{1}{2}\)log2(1+\(\frac{G(f)S_0}{N_0})\)
b) C=∫α\(\frac{1}{2}\)log2(1+\(\frac{S_0}{N_0})\)
c) C=∫αB log2(1+\(\frac{G(f)S_0}{N_0})\)
d) C=∫αB log2(1+\(\frac{G(f)N_0}{S_0})\)
View Answer
Explanation: Orthogonal Frequency Division Multiplexing is a digital modulation scheme. The data stream is divided into several closely spaced subcarrier signals at different frequencies. The spectrum capacity of OFDM xG networks is given by the expression,
C=∫α\(\frac{1}{2}\)log2(1+\(\frac{G(f)S_0}{N_0})\)
Here α refers to a set of unused spectrum units, G(f) is the channel power gain at frequency f, S0 is the signal power per unit frequency, and N0 is the noise power per unit frequency.
3. Which among the following is not selected in accordance with the user requirement?
a) Interference
b) Data rate
c) Tolerable error rate
d) Transmission mode
View Answer
Explanation: Following the characterization of the spectrum, the spectrum band for transmission is selected by weighing the spectrum characteristics and the quality of service requirements. Spectrum manager fixes the data rate, tolerable error rate, transmission mode, the bandwidth of transmission, and the delay bound according to the user requirements.
4. Which among the following is not a challenge for spectrum management?
a) Interference avoidance
b) Quality of service awareness
c) Seamless communication
d) Interference temperature measurement
View Answer
Explanation: xG users should not cause interference to primary user communication. The handoff to a different spectrum band upon arrival of the primary user should be seamless. The dynamic spectrum environment and quality of service requirements should be analyzed and managed for effective communication.
5. Which among the following is a challenge for spectrum decision models?
a) Combining multiple characterization parameters
b) Supporting OFDM networks
c) Supporting multiple spectrum bands
d) Maintaining a decision model for each characterization parameter
View Answer
Explanation: Spectrum decision models can be built for various characterization parameters such as signal to noise, date rate, error rate, and other parameters that affect the quality of service. However a decision model combining several spectrum parameters is not available. Spectrum decision models support OFDM networks that have multiple spectrum bands operating simultaneously for transmission.
6. Which among the following is not a requirement for multiple spectrum band transmission by xG user?
a) Spectrum holes
b) Compatibility with internal policies
c) Contiguous spectrum band
d) Compatibility with quality of service
View Answer
Explanation: xG users can transmit packets on multiple spectrum bands provided the available spectrum meets the quality of service requirements and the spectrum policies required for communication. However it does not require the multiple spectrum bands to be contiguous. This exhibits a vast improvement in communication quality during the spectrum handoff.
7. Which among the following is not an advantage of avoiding contiguous spectrum bands for transmission?
a) Low delay
b) Low power consumption
c) Low interference
d) Mitigation of quality of service degradation
View Answer
Explanation: The primary advantage of using a contiguous spectrum band is the mitigation of the quality of service degradation. This is because when a user has to vacate a spectrum band on the arrival of the primary user, the other bands continue to transmit maintaining communication. Low power is consumed in each band and less interference with the primary user is achieved.
8. Which among the following replaces the static sine pulses of OFDM to improve flexibility?
a) Wavelet bases
b) Triangular pulses
c) Impulse bases
d) Ramp pulses
View Answer
Explanation: In multi-carrier wavelet packet modulation, wavelet bases replace the static sine/cosine pulses of OFDM. This technique provides higher sideband suppression. It also reduces inter-channel interference and inter-symbol interference.
9. Which among the following is not a disadvantage of using OFDM?
a) Complex computation is necessary
b) Sensitive to frequency offset
c) High peak to average power ratio
d) Low data rate
View Answer
Explanation: OFDM requires only the computation of fast Fourier transform and inverse fast Fourier transform of the signal to be transmitted. However it requires the subcarriers to remain orthogonal and exhibits a large peak to average power ratio due to the presence of complex sinusoids in time domain OFDM signals.
10. What is the full form of CE-OFDM?
a) Closed Envelope Orthogonal Frequency Division Multiplexing
b) Constant Envelope Orthogonal Frequency Division Multiplexing
c) Constant Evolute Orthogonal Frequency Division Multiplexing
d) Closed Evolute Orthogonal Frequency Division Multiplexing
View Answer
Explanation: In Constant Envelope Orthogonal Frequency Division Multiplexing, the complex modulated signals are positioned in a complex conjugated arrangement to obtain a real value inverse fast Fourier transform output. Also, phase modulation is applied in the real value time domain. This technique reduces the high peak to average power ratio of OFDM.
11. Spectrum decision over heterogeneous spectrum band with different characteristics is a challenge in spectrum management.
a) True
b) False
View Answer
Explanation: xG users operate in licensed and unlicensed spectrum bands. In licensed spectrum bands, the operations of the primary users are collected during spectrum analysis and the decision taken should not affect the primary user transmission. Likewise, intelligent spectrum sharing algorithms should be selected for xG users operating on the licensed bands which have equal rights for spectrum access.
12. Which among the following terms should replace the labels ‘A’, ‘B’, and ‘C’ in the diagram?
a) A – Sensing information, B – Spectrum sensing, C – Reconfiguration
b) A – Sensing information, B – Reconfiguration, C – Spectrum sensing
c) A – Spectrum sensing, B – Reconfiguration, C – Sensing information
d) A – Reconfiguration, B – Spectrum sensing, C – Sensing information
View Answer
Explanation: Spectrum sensing examines the available spectrum bands, gathers information about the spectrum band, and detects spectrum holes. It is implemented in the physical layer. In-band sensing is carried out to sense the arrival of primary users and transfer the information to the spectrum mobility function unit. Reconfiguration is necessary to alter the transmission parameters in accordance with the dynamic radio environment.
Sanfoundry Global Education & Learning Series – Cognitive Radio.
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