對無線通訊系統而言，由於其向四面八方廣播的特性，如何對其傳送的資訊加以保密，是個很值得研究的問題，除了利用應用層的加密技術之外，在文獻中亦有實體層保密(physical layer secrecy)技術之相關探討，其重點在於如何利用實體層通道的傳輸特性，進一步讓竊聽者無法竊取重要資訊，讓合法的傳輸更加可靠。本文考慮一個放大後前送(amplify-and-forward, AF)的合作式傳輸網路，網路中資料源與目的端皆配備多根天線，且有多個具備單天線的中繼端(relay)，中繼端扮演著資料源與目的端之間溝通的橋梁角色，然而在本文中，我們假設中繼端是不可靠的，可能會在未經許可的狀況下，竊取資料源的訊息。有鑑於此，在尋求不可靠的中繼端幫助傳送的同時，也要避免中繼端成功竊聽，因此在資料傳送訊號的同時，由目的端產生一個人工雜訊(artificial noise, AN)，以期破壞中繼端對來源訊號的解碼，因為人工雜訊由目的端產生，所以其接收到中繼端傳送的訊號時，可自行消除人工雜訊的部分，不至於影響正常通訊。我們在資料源與目的端各佈置一個預編碼器(precoder)，用來設計傳送端訊號與目的端人工雜訊的統計特性，在中繼端乘上預編碼係數後，傳送給目的端，為了要系統的具備保密的效能，欲藉由確保服務品質(quality of service, QoS)為系統考慮依據，將使用最佳化的方式來聯合設計傳送訊號與人工雜訊的統計特性，以及中繼端放大後前送的預編碼係數，達到需求品質下的最佳保密容量(secrecy capacity)，透過模擬方式可以得知，使用人工雜訊干擾竊聽者，以及最佳化傳輸訊號與人工雜訊的統計特性，確實可以提升合作式系統中，對於不可靠中繼端的保密容量。

Due to the broadcast nature of wireless medium, how to ensure the security of wireless transmission is worth investigating. In addition to the encryption techniques commonly adopted in the application-layer, physical-layer secrecy communications drawn significant research concern recently. Physical-layer secrecy communication is achieved by exploiting spatial property of the physical-layer channel, such that the eavesdroppers can’t decode source important information successfully, and the legitimate transmission becomes more reliable. This thesis considers an amplify-and-forward cooperative network, where the source and destination are equipped with multiple antennas, while multiple relays have single antenna. The relays act a bridge between the source and destination, but we assume that these relays are not reliable and may wiretap the information without authorization. As a result, while being assisted by those untrusted relays, we must avoid relays eavesdropping source information. In order to prevent relay from wiretapping information, the destination generates artificial noise(AN) simultaneously to interfere signal reception at relays, when the source transmits information. Since AN is generated by the destination, the destination can eliminate AN by itself after receiving signal forwarded from the relays to fully eliminate additional interference at the destination. We employ linear precoder at the source and destination to optimize the statistical properties of the transmission signal and artificial noise. On the other hand, the relays scale the received signal by a precoding coefficient before retransmission. Instead of maximizing the secrecy rate, we adopt quality of service(QoS)-based criterion to ensure secrecy transmission implicitly since the secrecy rate is non-convex. More specifically, the precoders at the source, relays and destination are jointly optimazed in terms of maximum SNR of the destination subject to the SINRs at all relays are constrained. Through computer simulation, it shows that secrecy rate is improved obviously through introducing artificial noise and joint optimization of precoders compare with the scheme without AN and the scheme using arbitrary precoders.

論文審定書i
致謝ii
摘要iii
Abstract iv
目錄vi
圖次vii
第一章 簡介1
第二章 文獻探討5
2.1 使用人工雜訊干擾的保密5
2.2 合作式系統中的保密8
2.3 以確保QoS為系統考慮依據的保密11
第三章 系統模型16
第四章 預編碼器之設計22
4.1 多中繼系統中的保密22
4.2 設計最佳化問題23
4.3 中繼端預編碼向量的最佳化25
4.4 Rx與Q的最佳化28
第五章 模擬結果31
第六章 結論43
參考文獻44

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