本論文針對多輸入多輸出系統(MIMO)提出可用以區分合法接收者和非法接收者間通道估計效能的雙向訓練訊號設計。本研究延伸自過去已提出的差異性通道估計技術(discriminatory channel estimation)
。差異性通道估計技術旨在最小化合法接收者的通道估計錯誤並同時將非法接收者的通道估計錯誤維持在某個標準以上。如果訓練訊號只能單從傳送者發送，合法接收者和非法接收者間的效能差異則將有所限制。這是因為傳送者所發出的訓練訊號會同時幫助兩方接收者執行其通道估計。本論文所提出的雙向訓練方法允許傳送者和合法接收者都發送訓練訊號。在此情況下，由合法接收者發送的訓練訊號將幫助傳送者得到關於傳送者至合法接收者間的通道資訊，但卻無助於非法接收者去估計傳送者至非法接收者間的通道。當傳送者估得它到合法接受者間的通道後，其可在下一次的訓練訊號上加上人工雜訊(artificial noise)以擾亂非法接收者的通道估計。此人工雜訊應散佈於傳送者到合法接收者間通道估計的零空間上以減少對合法接收者的干擾。基於此概念，我們分別就對等通道(reciprocal channel)和非對等通道(non-reciprocal channel)提出各自的雙向差異性通道估計技術。在總和與個別能量限制下，我們求得訓練訊號和人工雜訊間的最佳能量分配，並藉由數值模擬結果驗證所提出雙向差異性通道估計的效能。

Thisworkexaminestheuseoftwo-waytraininginmultiple-inputmultiple-output(MIMO)wirelesssystemstodiscriminatethechannelestimationperformancesbetweenalegitimatereceiver(LR)andanunauthorizedreceiver(UR).Thisthesisextendsuponthepreviouslyproposeddiscriminatorychannelestimation(DCE)schemethatallowsonlythetransmittertosendtrainingsignals.ThegoalofDCEistominimizethechannelestimationerroratLRwhilerequiringthechannelestimationerroratURtoremainbeyondacertainlevel.Ifthetrainingsignalissentonlybythetransmitter,theperformancediscriminationbetweenLRandURwillbelimitedsincethetrainingsignalshelpbothreceiversperformestimatesoftheirdownlinkchannels.Inthiswork,weconsiderinsteadthetwo-waytrainingmethod-ologythatallowsboththetransmitterandLRtosendtrainingsignals.Inthiscase,thetrainingsignalsentbyLRhelpsthetransmitterobtainknowledgeofthetransmitter-to-LRchannel,butdoesnothelpURestimateitsdownlinkchannel(i.e.,thetransmitter-to-URchannel).Withtransmitterknowledgeoftheestimatedtransmitter-to-LRchannel,artificialnoise(AN)canthenbeembeddedinthenullspaceofthetransmitter-to-LRchanneltodisruptUR’schannelestimationwithoutseverelydegradingthechannelestimationatLR.Basedontheseideas,two-wayDCEtrainingschemesaredevelopedforbothreciprocalandnon-reciprocalchannels.TheoptimalpowerallocationbetweentrainingandANsignalsisdevisedunderbothaverageandindividualpowerconstraints.Numericalresultsareprovidedtodemonstratetheefficacyoftheproposedtwo-wayDCEtrainingschemes.

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