用於多重輸入多重輸出正交分頻多工系統之遞迴偵測及解碼技術

Abstract

在未來的寬頻無線通訊系統中，最主要的兩個挑戰為通道多路徑衰減效應以及頻譜效率的提升。正交分頻多工（Orthogonal Frequency Division Multiplexing, OFDM）能夠在頻域上將頻率選擇性衰落通道轉變成平坦式衰落通道，為一種有效克服多路徑衰減效應之技術。另一方面，多重輸出多重輸入（Multi-Input Multi-Output, MIMO）技術可用來在空間中創造獨立的平行通道傳送不同資料串流，以提升系統的整體傳輸速率。MIMO所提供的此種空間多工（Spatial Multiplexing）技術可在不需佔用額外頻譜資源的前題下，大幅地提升頻譜效率。在本論文中，吾人將探討結合OFDM及MIMO技術的通訊系統，亦即MIMO OFDM系統。此種系統架構可分為兩種功能類型：一為提高資料傳輸速率，另一為藉由分集提高接收資料的可靠度。吾人將基於此兩種架構，藉由適當的訊號處理演算法實現高速傳輸以提高整體系統容量。吾人亦將引進遞迴處理的技巧，進一步改善系統效能。遞迴處理之概念來自渦輪碼（Turbo code），它可在接收機模組中遞迴地交換外在資訊（Extrinsic information），藉以降低系統錯誤率。吾人將提出一種具有遞迴處理的MIMO OFDM架構，並推導其詳細的演算法。藉由電腦模擬分析，吾人可證實：具有遞迴處理的接收機能在很低的訊號對雜訊比之下提供顯著的效能改善，其結果明顯優於傳統之非遞迴接收機。

Two major challenges in future broadband wireless communication systems are the multipath fading channels and need for enhanced spectral efficiency. OFDM can convert frequency-selective multipath fading channels into flat fading ones in the frequency domain, which makes it an effective technique for combating channel fading. On the other hand, the MIMO technique can create independent parallel channels in the spatial domain for sending multiple data streams, thereby increasing the overall transmission rate of the system. The technique of spatial multiplexing offered by MIMO can effectively enhance the spectral efficiency without the need of extra frequency resources. In this thesis, we will consider a new wireless communications system combining MIMO and OFDM techniques, called the MIMO OFDM system. The proposed system can be categorized into two functional types: one is to provide higher data rate, and the other is to provide higher reliability through diversity. We will realize a high data rate, high capacity system by incorporating appropriate signal processing algorithms based on the above two types. Furthermore, we will also introduce the iterative processing technique to improve the system performance. The iterative technique comes from the Turbo code, which involves iterative exchange of the extrinsic information between the decoding modules in the receiver in order to lower the bit error rate. We will develop an MIMO OFDM system with iterative processing, and derive the corresponding algorithms. From the simulation results, it can be shown that the presented iterative receiver outperforms conventional non-iterative ones in low SNR environments.