感知無線電(cognitive radio)是基於提升頻譜使用效率所發展出的一項新興技術，其具備了有效解決頻譜使用不足之能力。為了提升頻譜的使用效率，次要使用者(secondary user)被授權得在主要使用者(primary user)不存在的情況下借用其頻帶來傳送資料。為了保障主要使用者的權利，使其不受到次要使用者的干擾，次要使用者必須在傳送資料之前透過頻譜感測(spectrum sensing)探知主要使用者的動態並決定其資料傳送與否。因此，可靠的頻譜感測技術對於感知無線電系統是相當重要的。
在此篇論文中，考量到較低的運算複雜度，我們選擇使用能量偵測器(energy detector)來偵測頻譜。傳統的能量偵測器使用固定的判定門檻值(decision threshold)來判斷主要使用者存在與否。當判定門檻值較高時，會得到較低的偵測機率(detection probability)以及錯誤警報機率(false-alarm probability)，進而可得相對應較高的次要使用者使用頻譜之機率；反之，當判定門檻值較低時，會得到較高的偵測和錯誤警報機率，進而使得次要使用者使用頻譜的機率降低。在此篇論文中，我們依據次要使用者傳送端的佇列長度(queue length)來調整判定門檻值，針對次要使用者使用頻譜的機率，將頻譜感測和排程技術(scheduling)結合在一起。另一方面，為了滿足對主要使用者之保護，我們會動態調整次要使用者的傳輸功率，使得主要使用者的中斷機率(Outage probability)維持在容許範圍內。
我們建立了一個封包的傳遞模型來探討封包延遲，並提出了佇列長度和判定門檻值之間的關係函數，用來控制判定門檻值的調整，藉以最佳化封包延遲的現象。根據模擬結果可以發現，我們所提出的機制相較於傳統的能量偵測機制，在封包延遲和封包遺失率表現上都有明顯的改善。

Cognitive radio (CR) is an emerging communication technique which can enhance the spectrum-usage efficiency by allowing some secondary users (SUs) to operate over the licensed spectrum as long as a primary user (PU) is inactive. To sufficiently protect PU communications from interference imposed by SUs, a CR system requires reliable spectrum sensing to determine whether an SU is allowed to access the licensed spectrum or not. However, such an opportunistic spectrum access mechanism would lead to serious latency and buffer overflow problems.
In this thesis, we propose a low-latency joint spectrum sensing and scheduling scheme based on an energy detector for opportunistic SU transmission. Unlike the conventional energy detector using a fixed threshold to detect the PU’s occurrence, the proposed scheme dynamically regulates the detection threshold according to the queue length at the SU transmitter, where the SU transmission power is adjusted properly to confine the PU’s outage probability at an acceptable level and two linear policy functions are adopted for the queue delay minimization. Numerical results indicate that the proposed approach has significant improvements over the conventional energy detection scheme with a fixed threshold in terms of the queue delay of SU transmission and the packet loss due to buffer overflow.

Abstract i
Contents iii
List of Figures v
Chapter 1 Introduction 1
1.1 Background 1
1.2 Cognitive Radio 1
1.3 Quality of Service 3
1.4 Outline of the Thesis 4
Chapter 2 Related Works 6
2.1 Basic Concepts of Spectrum Sensing 6
2.2 Spectrum Sensing with an Energy Detector 7
2.3 Adaptive Threshold Control for the Energy Detector 9
2.3.1 SU Transmission Rate Maximization 9
2.3.2 SU Buffer Overflow Constraint 11
2.4 Scheduling Method for Delay-Optimal Multiple Access 13
Chapter 3 Adaptive Threshold Control for Queue Delay Optimization 17
3.1 Motivation 17
3.2 System Model 18
3.3 The Proposed Scheme 21
3.3.1 SU Queuing Analysis 21
3.3.2 PU Outage Probability Restriction and SU Power Control 22
3.3.3 SU Queuing Delay Analysis 28
3.3.4 Linear Policy Functions 31
Chapter 4 Numerical Results 35
Chapter 5 Conclusions 43
Bibliography 45

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