在最近幾年中，有鑑於無線通訊系統通道的開放特性(broadcast, open nature)，如何防止訊息被竊聽者(eavesdropper)接收的保密傳輸(secure communication)技術受到人們越來越多的探討。此篇論文將研究如何使用傳送波束成型(transmit beamforming)達成保密傳輸的需求；也就是傳送者(transmitter)如何加強合法使用者(intended receiver)的服務質量(Quality of Service, QoS)與同時限制竊聽者的接收品質(reception QoS)。本論文採用訊號干擾加雜訊比(signal-to-interference-and-noise-ratio, SINR)做為服務質量的基準，進而探討保密傳送波束成型技術以達成 i)竊聽者的訊號干擾加雜訊比不超過事先所設定的臨界值(threshold) ii)最大化或保證合法使用者的服務質量不低於所設定的訊號干擾加雜訊比。為達此些目的，我們考慮傳送者同時傳送訊息資料及人造雜訊(artificial noise)以干擾竊聽者之接收能力，不同於現存文獻中並未最佳化(optimize)人造雜訊之使用，我們將探討傳送者如何經由合法使用者與竊聽者的通道狀態信息(channel state information, CSI)共同最佳化傳送波束成型與人造雜訊之使用。
由於本論文中所探討的問題並非屬於凸面最佳化(convex optimization)問題，而是屬於非定常多項式時間難解(NP-hard)問題，因此一般來說此問題並無普遍有效率的算法。在此篇論文中，我們將使用半正定鬆弛演算法(semidefinite relaxation, SDR)來處理此保密通道波束問題；此外，我們證明在某些特定具代表性的通道條件下，半正定鬆弛演算法可被用以取得保密傳送波束成型問題的最佳解。在本論文中，我們亦探討如何延伸保密傳送波束演算法用以解決額外的單天線功率限制(per-antenna power constraint)或存在多個合法使用者的情形。經由電腦模擬，我們發現使用人造雜訊對於所探討之保密傳送波束成型問題無論在所消耗的傳輸功率或合法使用者的服務質量皆比不使用人造雜訊的方法有顯著的改進。

Secure signal transmission techniques have been receiving growing attention in recent years, as a powerful lternative to blocking eavesdropping attempts in an open wireless medium. This thesis proposes a secure transmit beamforming approach whose goal is to simultaneously enhance an ntended receiver's quality-of-service (QoS) and degrade eavesdroppers' QoSs. Specically, we establish design formulations that i) constrain the maximum allowable signal-to-interference-and-noise ratios (SINRs) of the eavesdroppers; and that ii) provide the intended receiver with a satisfactory SINR through either a guaranteed SINR constraint or SINR maximization. The proposed designs incorporate articial noise (AN), where a suitable amount of AN is added
in the transmitted signal to degrade the eavesdroppers' interception. Unlike existing AN-aided designs where the use of AN is usually not optimized, our designs advocate
joint optimization of the transmit beamforming weights and AN spatial distribution in accordance with the channel state information (CSI) of the intended receiver and
eavesdroppers. Our formulated design problems are nonconvex, and they are shown to be NP-hard in general. We handle the design problems by semidenite relaxation (SDR). We prove that SDR can exactly solve the design problems for a practically representative class of problem instances. Extensions to the per-antenna power constraint case and a challenging multiple intended receivers case are also examined. Simulation results illustrate that the proposed AN-aided designs can yield signicant power savings or SINR enhancement over their no-AN counterparts.

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