近年來，頻寬資源的縮減已經成為通訊系統裡的重要問題。感知無線通訊(Cognitive Radio)被視為解決這個問題最有效的技術。它最主要的精神便是在不干擾主要使用者(Primary User)的情況下，將未被使用的頻寬資源供應給次要使用者(Cognitive User)，以期達到最佳的頻寬使用效率。截至今日，學界已累積了許關於如何減少干擾並提高解碼效能的研究。在這篇論文裡，我們提出利用次要使用者的其中一根天線，傳送一組足夠抵消次要使用者對主要使用者所產生之干擾的信號。我們的系統架構是由一對次要使用者的傳送端與接受端和主要使用者的收發端所組成，並且假設主要使用者的頻寬小於次要使用者。由於頻寬大小不等，我們可以確信這兩者的取樣頻率是相異的。
在這篇論文中，我們考慮兩個干擾的來源：(1)自次要使用者的天線所傳來的干擾。(2)次要使用者與主要使用者重疊的頻寬內，子載波間的干擾。我們利用主要使用者的其中一根天線來傳送抵銷干擾的信號給主要使用者。模擬中，這個方法成功得抵銷干擾。在次要使用者的接收端，我們也採用最大概似方法搭配QR分解以得到最佳的解碼效能。在這樣的搭配之下，這篇論文所提出的創新方法不但可以抵銷干擾並兼顧雙方使用者的解碼效能。

In recent years, the lack of spectrum resources has becomed a widely concerned issue in the communication system. Cognitive radio is considered as an efficient solution to this spectrum utilization problem. As we know, the fundamental idea of cognitive radio is not to interfere with the authorized user. There are a lot literature have investigated on how to null the interference cause by the cognitive users but increase the decoding performance at the
cognitive user’s receiver. In this thesis, we present a novel scheme that aligns the interference from the antennas of the cognitive user such that the interference to the primary user is nullified or minimized. The two-user multiple-input multiple-output(MIMO) interference channel is considered. We assume that bandwidth of primary user and the cognitive user is unequal and the transmission bandwidth of the primary user is called victim band.Because of the unequal transmission bandwidth, the sampling rates of the two users are not necessarily equal. Two sources of interferences are considered in the alignment. One source is the spatial interference from different antennas of the cognitive user. The other source is the inter-carrier interference (ICI) from the in-victim band of cognitive user to the victim band of primary user. An active antenna of the cognitive user is used to align with the spatial
interference from the antennas of the cognitive user and ICI such that the interference to the primary user is nullified. The simulation shows that the scheme created a > 100dB notch on the transmission spectrum of the victim band. This scheme allows the cognitive user to utilize the victim band simultaneously with the primary user and leads to underlay cognitive radio transmission. We also present a soft successive interference cancellation with maximum likelihood(ML) detection method at the receiver of the cognitive user to detect the signal within the victim band. The error performance at the cognitive receiver of this underlay cognitive radio system is presented as well.

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