由於無線通訊的訊號基本上是以廣播的型態傳送出去，所以送出的訊息很容易就能被惡意的竊聽者所竊聽。因此，近年來無線通訊的實體層安全性得到越來越多的關注。許多用來提升實體層安全性的方法也紛紛被提出，如犧牲部份傳送功率來製造人為噪聲以用來干擾竊聽者就是其中之一。在我們的研究中，我們考慮一個擁有多輸入傳送端、單輸出接收端、以及單輸出竊聽者的正交分頻多工系統，並提出一個新的改善實體層安全性的方法。這個方法的基本構想是利用周期性延遲會同時改變接收者及竊聽者所看到的等效通道的特性，藉由適當設計一個乘於數據符號上的回饋因子，我們能夠單方面的復原接收端的等效通道，但保留對竊聽者等效通道的擾亂。模擬分析顯示被提出的方法確實能維持對竊聽者通道的干擾，相較於產生人為噪聲的方法，更能在二位元相位偏移調變及十六位元正交振幅調變使竊聽者有更差的誤碼率。此外，在竊聽者方通道容量上的模擬也能與誤碼率的結果相符合。

Because the signal transmission in wireless communication is essentially broadcasting, the transmitted information is easily wire-tapped by malicious eavesdroppers. As a result, physical layer security of wireless communication has gained more and more attention in recent years. Methods such as artificial noise have been proposed to guarantee physical layer security, where part of the transmit power is sacrificed to generate artificial noise for interfering eavesdroppers. In this work, we consider a MISOSE OFDM system and propose a new approach to improve physical layer security. The idea of the proposed method is to exploit the property that cyclic delays change the effective channels of the intended receiver and eavesdroppers. By properly designing a compensation factor and multiply it to the transmitted data symbol, we can recover the channel of the intended receiver while keeping the distortion on the channel of eavesdroppers. Simulation results show that the proposed method can distort the channel at eavesdroppers and degrade their BPSK and 16-QAM BER more than the method of using artificial noise. Moreover, simulation results on the channel capacity of eavesdropper also match with the BER results.

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