in owen words explain the following a.Explain the ‘hidden station problem’ in the context of cognitive radio schemes based on sensing the radio environment. b.How is the hidden station problem avoided in database type cognitive radio schemes?
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In cognitive radio schemes that rely on sensing the radio environment, this can lead to interference. For example, if a secondary user (cognitive radio) is trying to access a frequency that is currently being used by a primary user, but it cannot sense the primary Show more…
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Wireless communications questions Q1. Explain the use of carrier sense multiple access (CSMA) in communication networks and provide its step-by-step procedure to prevent a collision in the network. Explain what the cause of a possible collision can be when CSMA is used. Also, by further research, clarify how CSMA/CD (collision detection) or CSMA/CA (collision avoidance) can monitor the traffic within the network. Q2. (i) Provide the advantages of CDMA (code division multiple access) over other multiple access channel sharing systems (e.g. TDMA, FDMA). (ii) With the help of an example, explain how CDMA (code division multiple access) is used to multiplex three binary signals, each with 2 bits only. In your example use different spreading (chipping) codes of 8 bits each. Q3. Orthogonal frequency division multiple access (OFDM) is popular as a high fidelity multiplexing system for wireless communication systems. Briefly describe the block diagram of the overall (transmitter – receiver) system, explain its advantages, and clarify why the OFDM system is an efficient modulation system. Q4. For a line-of-sight (LoS) wireless transmission, the carrier frequency is 0.5 GHz and the distance between the transmitter and receiver antennas is 2 Km. In the transmitter we use a parabolic antenna with the face area of 0.8m2 and in the receiver an infinitesimal dipole is used. Considering the path loss and antenna gains, calculate the overall gain of the (transmitter-receiver) system in decibel (dB). Q5. Provide the importance of evaluation of Link Budget is satellite communication systems and by doing some research explain various factors and parameters in the budget. Clearly explain what each block and its value/parameter refer to. Q6. Explain the importance, limits, advantages and disadvantages of cellular wireless communication systems compared to non-cellular systems particularly with regards to mobility within a wireless network. Q7. (i) In Figure 1. Use 64-QAM as your PSK modulator. binary source generator -> PSK modulator -> Channel -> PSK demodulator Figure 1 Use random binary source generator and AWGN (additive white Gaussian noise) as your channel effect. Use oscilloscopes and constellation maps to inspect the signals at right points of the system. Increase the noise level, draw the bit error rate (BER) versus signal to noise ratio (SNR), and specify at which noise level the demodulator is not able to recover the original signal accurately any more. Draw your schematic and provide the constellation maps together with the input and output binary signals. (ii) Explain the advantages and disadvantage of this system over BPSK and QPSK. (iii) Replace the AWGN channel with a Rayleigh fading channel for both QPSK and 64-QAM systems. For each system (QPSK and 64-QAM) increase the fading level and specify at which fading level the demodulator is not able to fully recover the original signal any more. Explain the differences
A wireless LAN card reports the state of the radio channel for each second. The channel has 3 states which are poor (0), fair (1), good (2) or excellent (3) as described by the following conditions: (a) In 'poor' state, the next state is equally likely to be poor or fair. (b) In states 1, 2 and 3, there is a probability of 0.9 that the next system state will remain unchanged from the previous state and a probability of 0.04 that the next system state will be poor. (c) In states 1 and 2, there is a probability of 0.06 that the next state is one state up in quality. (d) When the channel is excellent, the next state is either good with probability 0.04 or fair with probability 0.02. Please answer the following. (i) Sketch the state diagram for the radio channel described above and calculate the state transition matrix, P. (ii) State whether the Markov chain derived for the state of the radio channel is irreducible. Please justify your answer. (iii) Initially, the channel is at state good (2). Compute the state transition matrix after 3 (THREE) iterations and discuss the possible channel states after 3 iterations. (iv) After waiting for a long time after the LAN card has been activated, an operator observes the radio channel state at random. State the probability that the observed state is fair (1) and justify your answer.
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