感知無線電(cognitive radio)技術具備有效解決頻譜不足使用之能力。為了提升頻譜的使用效率，次要使用者(secondary user)可以在主要使用者(primary user)不存在的情況下使用其頻帶來傳送資料。基於保護主要使用者不受到次要使用者的干擾，感知無線電系統需要可靠的頻譜偵測(spectrum sensing)技術。在此篇論文中，為了有比較低的運算複雜度，我們使用能量偵測器(energy detector)來探知主要使用者的動態。
為了保護主要使用者，次要使用者必須在傳送資料之前透過頻譜偵測技術得知主要使用者的動態。然而，探知主要使用者的動態所需要的時間可能會增加次要使用者的封包延遲;對於一個即時(real-time)的應用程式而言，封包延遲過大對其效能會有所影響。在週期性頻譜偵測架構(periodic sensing frame structure)中，偵測時間越長意謂著次要使用者需要等待越長的時間才能開始傳送資料。因此，封包延遲可能會隨著偵測時間的增加而增加。然而，偵測時間越短意謂著誤警機率(probability of false alarm)越高，所以封包延遲也可能會隨著偵測時間的減少而增加。
在此篇論文中，我們發現頻譜偵測效能與次要使用者的封包延遲之間存在一種權衡關係。之後我們建立一個封包的傳遞模型來探討封包延遲與頻譜偵測效能之間的關係，並且提出一個全新的目標函數。經由牛頓法(Newton’s method)，我們可以找到一個最佳的偵測時間使得封包的延遲最小。但是在求解最佳偵測時間的過程中，必須要滿足對主要使用者的保護以及次要使用者的服務質量(quality of service)。根據電腦模擬結果，我們可以發現理論結果跟模擬結果幾乎相符。除此之外，我們解出來的最佳偵測時間一定能滿足感知無線電系統的使用需求。

Cognitive radio (CR) which is one of the candidate communication techniques for solving the spectrum scarcity problem. In order to enhance the spectrum-usage efficiency, secondary users (SUs) are allowed to operate over the licensed bands while the bands are not being used by primary users (PUs) at a particular time or specific geographic area. CR systems require reliable spectrum sensing to protect communications of PUs from interference of SUs. In this thesis, an energy detector is equipped for spectrum sensing because of the low computational complexity.
In consideration of protecting PUs, the CR users must perform spectrum sensing before transmitting data. However, the spectrum sensing periods arranged in the frame makes the packet delay of CR transmissions increasing; a large packet delay is harmful to real-time applications. In the periodic sensing frame structure, the longer the sensing time, the later the SUs begin to transmit data. Hence, the packet delay may increase with the increasing of the sensing time. However, the shorter the sensing time, the higher the false alarm happened; the packet delay may increase with the decreasing of the sensing time.
In this thesis, we address a new sensing issue in which a tradeoff exists between sensing capability and the packet delay of SUs. Furthermore, we construct a novel scheme and formulate the issue as a convex optimization problem. Then, by using the Newton’s method, we can find an optimal sensing time with a minimal packet delay; the constraints include the requirement of the protection with PUs and the QoS of SUs. Computer simulation results indicate the theoretical result matches with the simulated result very well. In addition, the optimal sensing time in the proposed scheme satisfy the requirements of the CR systems in the IEEE 802.22.

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