在傳統的轉送解碼的合作式網路下，中繼點選擇方法主要遵循最大-最小策略，也就是說被選來幫忙傳送資料封包的中繼點與其最大-最小之通道增益相關。然而，最大-最小策略有可能對主要系統 (primary system) 造成多餘的干擾因此導致所謂的轉換比 (transfer ratio) 變得較差。值得注意的是我們把轉換比定義為次要系統 (secondary system) 的容量增益與主要系統的容量損失之比值，而主要系統之損失乃次要系統的通訊活動所造成的。在本篇論文，我們針對重疊式 (underlay)的感知無線電網路提出一種新的低干擾中繼點選擇方法。我們的選擇方法主要的概念在於額外考慮了次要系統對於主要系統干擾這一項因素。但是這也意味著最佳的中繼點，也就是有最大-最小通道增益的中繼點有可能不會被選到。為了闡明種現象，仍然最佳之機率 (PSO) 定義為最佳中繼點選擇方法與低干擾選擇方法選到同一個中繼點的機率。除了有仍然最佳之機率的數學分析式外，我們也分析了低干擾選擇方法對次要系統的容量與中斷機率之影響。最後，經由模擬結果我們發現我們所提出的低干擾中繼點選擇方法在轉換比上優於傳統的最大-最小法則，這也意涵著我們提出的方法可以達到較高的頻譜效率。

In the conventional decode-and-forward (DaF) cooperative networks, the relay selection method mainly follows the rule of max-min policy, which means the relay associated with the max-min channel gain can be selected to forward the data packets. However, the max-min policy may cause some extra amount of interference to the primary system (PS) such that the so-called transfer ratio (TR) can be lower. Note that the transfer ratio is defined as the ratio of the secondary system (SS)'s capacity gain to the PS's capacity loss due to the activities of the SS. In this thesis, we proposed a novel low-interference relay selection for the underlay cognitive radio (CR) networks. The key idea here is to select a relay by taking an additional term reflecting the interference from the SS to PS. This also means that the optimal relay, i.e. the one with max-min channel gain, may not be picked. To clarify this phenomenon, the still optimal probability (Pso) is defined as the probability of optimal relay selection by the proposed low-interference strategy. In addition to the analytical expression of Pso, the impacts of the low-interference selection on the SS's capacity and outage probability are also analyzed. At last, via simulation results, we find that the proposed low-interference relay selection can own a higher TR compared with the conventional max-min rule, which indicates that the higher spectrum efficiency can be achieved by the proposed method.

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