| Summary: | 碩士 === 國立臺灣科技大學 === 電機工程系 === 104 === With the emergence of various advanced wireless communication technologies, such as sensor network and smart phones, the data transmission rate is getting faster and faster. On the other hand, limited wireless spectrum causes the problem of spectrum scarcity. The static spectrum allocation has resulted in an inefficient utilization of spectrum. To enhance the utilization of scarce spectrum, the cognitive radio network (CRN) is proposed. The idea is that the users without spectrum license can utilize the licensed spectrum opportunistically to reduce the spectrum wastage, while the spectrum access of the users with license is not affected. We consider the multi-rate CRN, where the bandwidth requirement of each PU is one channel, whereas that of each SU depends on the call admission control (CAC) used. We study three CAC schemes: CAC1, CAC2 and CAC3. With CAC1, the bandwidth requirement of an SU is one channel, that with CAC2 is a constant greater than one, and that with CAC3 is between a lower bound and an upper bound. In three CACs considered, PUs have the preemptive priority over SUs at any time. Furthermore, to reduce the dropping probability of preempted SUs, we propose to use a buffer to accommodate the preempted SUs until their maximum waiting time expires. If there are enough available channels, the SU at the head of buffer will reoccupy the channels and resume the service. We derive the analytical models for all CAC schemes with or without buffer. An iterative algorithm is developed to find the steady state probability distribution and the performance measures of interest are computed. The performance measures of interest are the blocking probability, throughput, the average number in the system, and the average system delay for PUs and SUs, respectively, and the dropping probability for SUs. For comparison, we also present the performance of all CACs with and without buffer. Last but not least, computer simulation is written in dev C++ to verify the accuracy of the analytical results.
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