隨著多種先進無線通訊技術的出現，像是感測器網路和智慧型手機，數據傳輸速率越來越快。另一方面，有限的無線網路頻譜造成頻寬短缺的問題。靜態頻譜配置導致頻譜沒有效率的使用。為了增加短缺頻譜的使用率，感知無線電網路(CRN)被提出。此概念為沒有頻譜執照的使用者可以機會式使用有執照的頻譜，以減少頻譜的浪費，而且有執照的使用者並不受其影響。我們考慮了多重速率的感知無線電網路，其每一個主要用戶的頻寬需求為一個通道，而每一個次要用戶的頻寬需求是依據連結允入控制(CAC)來決定通道大小。我們研究三種連結允入控制的機制，分別為CAC1、CAC2和CAC3。在CAC1中，每一個次要使用者使用一個通道，在CAC2中，每一個次要使用者使用大於一個的固定數目通道，在CAC3中，每一個次要使用者使用的通道數在二個上下限之間。在這三種機制中，在任何時間點，主要使用者對於次要使用者擁有進接通道的佔先優先權。此外，為了減少被佔先的次要使用者的中斷機率，我們提出使用緩衝器去容納被佔先的次要使用者，直到超過他們的最大等候時間。當系統有足夠可用的通道時，在緩衝器排頭的次要使用者可以重新使用通道，並重新開始服務。針對所有考慮的連結允入控制機制，在具有或沒有緩衝器的情況下，我們開發了相對應的解析模型。我們提出一個疊代演算法去找出穩態機率分布且計算出我們感興趣的效能指標。這些指標分別為主要使用者和次要使用者的阻塞機率、成功送達率、系統平均人數和系統平均延遲，以及次要使用者的中斷機率。為了比較起見，我們也呈現所有通道允入控制在具有或沒有緩衝器的情況下的效能指標。最後但並非最不重要的，我們使用Dev C++來撰寫電腦模擬以驗證解析結果的準確性。

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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