感知無線電(cognitive radio)為一個有效提升頻譜使用效率的技術。在感知無線電系統中，頻譜偵測是最重要的部份。但由於隱藏節點(hidden nodes)、遮蔽(shadowing)與衰減通道(fading channel)的問題，造成單一次要使用者(secondary user)無法準確地偵測出主要使用者(primary user)的訊號是否存在；為了解決這些問題，合作式頻譜偵測(cooperative spectrum sensing)的方法被提出。在傳統的合作式頻譜偵測方法中，參與的次要使用者數目較多時，次要使用者回傳偵測結果所需要的控制通道(control channel)頻寬較顯著；另外，當次要使用者的偵測環境不同時，所有次要使用者對於最後決策結果的貢獻仍為相同，這些問題均顯示出傳統的合作式頻譜偵測並非一個好的偵測方式。
在此篇論文中，我們針對感知無線電系統，基於注水原理(water-filling)的概念提出一個權值計算方法，並將提出的方法應用至合作式雙決策值能量偵測(double threshold energy detection)。在我們所提出的合作式頻譜偵測方法中，次要使用者會依照其偵測通道的瞬時訊號與雜訊比(signal-to-noise ratio)而被分配一個權重值；當瞬時訊號與雜訊比太低，則次要使用者的權重值為零，即次要使用者在偵測過程為無效的，因此不需要回傳本身的決策結果給次要使用者的基地台。為了評估此篇論文所提出方法的偵測效能及決定能量組合所需的決策值，我們提供在雷利衰減通道(Rayleigh fading channel)下之誤警機率(false alarm probability)與正規化平均偵測位元數(normalized average number of sensing bits)的理論分析。經由模擬結果我們可以看出，在相等的誤警機率下，所提出的方法相較於平等權值方法(equal weighting scheme)，可以達到較佳的偵測效能與較小的正規化平均偵測位元數，且驗證模擬結果與理論分析為一致。

Cognitive radio is an effective technique to enhance the spectrum utilization. In cognitive radio systems, spectrum sensing is the most important part. However, a single secondary user cannot exactly detect the existence of the primary user due to the effects of hidden nodes, shadowing, and fading channels. In order to combat these effects, cooperative spectrum sensing has been proposed. For the conventional cooperative spectrum sensing method, the control channel bandwidth required for reporting the sensing results of secondary users would be significant as the number of secondary users is large. It is also not a good approach to use equal weighting for all secondary users’ decisions in cooperative spectrum sensing when they are under different sensing environments.
In this thesis, we propose a weighting scheme based on the water-filling principle and apply it to cooperative spectrum sensing with double threshold energy detection for cognitive radio systems. For the proposed cooperative spectrum sensing scheme, each secondary user is allocated a weighting factor based on the instantaneous signal-to-noise ratio (SNR) of the corresponding channel, where the weighting factor is set to be zero as the instantaneous SNR is low enough. Since the zero-weighting secondary user is ineffective in the sensing process, it does not need to report the local decision to the secondary-user base station. To assess the detection performance of the proposed scheme as well as to determine the threshold for energy combination, we also provide analyses of the false alarm probability and the normalized average number of sensing bits under Rayleigh fading channels. Simulation results show that, under the same false alarm probability, the proposed weighting scheme achieves better detection performance with a smaller normalized average number of sensing bits (i.e., a smaller control channel bandwidth) than the equal weighting scheme, and they agree well with the analysis results.

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