應用於數位廣播系統之編碼正交分頻多工傳輸技術及基於渦輪等化之編碼正交分頻多工/分碼多重存取傳輸技術之比較

Abstract

由於編碼正交分頻多工傳輸技術無法利用多路徑分集增益來提升訊號抵抗雜訊的能力，導致系統效能受限於多路徑通道干擾。針對上述的問題，本論文提出一種使用具有完美正交性的華氏碼作為展頻碼，也就是將編碼正交分頻多工傳輸技術結合分碼多重存取傳輸技術，使訊號分散在各個次載波上傳送，並在系統接收端使用包含軟性多路徑干擾消除器之渦輪等化技術，以消除通道對傳送訊號造成的多路徑干擾，並且利用等化器及解碼器以遞迴方式傳遞軟性資訊，來提升等化器進行多路徑干擾消除及解碼器解碼的能力。此外我們也針對現有的編碼正交分頻多工傳輸技術之接收架構做改良，將帶有通道狀態資訊的軟性資訊提供給接收機之威特比演算法做分析。最後經由電腦模擬驗證上述所有技術在多路徑衰減通道中可有效的提高效能。

OFDM transmission technique cannot utilize the multipath diversity gains to improve signal-to-noise performance. To solve this problem, in this thesis we combine the OFDM and CDMA transmission technique with walsh codes which has perfect orthogonality for data spreading. And we also propose a turbo equalization based OFDM/CDMA transmission techniques for digital broadcasting application to improve the performance of the multipath interference cancellation and the channel decoder by using the iterative pass of soft information between equalizer and decoder. Besides, for COFDM system we propose a modified Viterbi algorithm to improve performance effectively by using the channel state information. Finally, the performances of these techniques mentioned above were evaluated by computer simulation in multipath fading channels and from the simulation results, the performances are improved effectively.