正交分頻多工系統之通道估計與資料檢測技術研究

Abstract

正交分頻多工是可在無線通道下達高速資料傳輸的有效技術，應用多輸入多輸出技術於正交分頻多工系統被視為下一代無線通訊提升系統效能的熱門方式。於本論文中，吾人研究正交分頻多工系統中通道估計與資料檢測技術，研究主題包括多輸入多輸出通道之領航訊號設計、慢速時變通道之通道估計與追蹤以及快速時變通道之資料檢測。 本論文可分為四部分，第一部分提出以互補碼領航訊號為基礎之空時區塊碼-正交分頻多工系統。於此系統，一組預先定義順序之互補碼與資料訊號同時傳送，做為兩根天線傳送分集系統之領航訊號，使用於接收機端估計通道以達最佳資料檢測。吾人設計完整接收機架構，分析理論系統效能，同時利用電腦模擬來驗證系統於行動無線電衰退通道下之效能。 於論文第二部分中，吾人由牛頓法推導於空時區塊碼-正交分頻多工系統中以決策迴饋離散傅立葉轉換為基礎之通道估計方法，藉由導證過程，證明牛頓法與以決策迴饋離散傅立葉轉換為基礎方法之間的等效。吾人亦使用電腦模擬驗證位元錯誤率及正規化方差效能來展現兩方法之間的等效，此結論於傳統正交分頻多工系統亦成立。 於論文第三部分中，吾人研究於行動無線通道下正交分頻多工系統採用空時區塊碼之通道估計，根據典型以離散傅立葉轉換為基礎之通道估計方法，提出一涵蓋兩階段處理方法。於初始階段，吾人使用多重路徑干擾消除技術估得多重路徑延遲與複數增益，於追縱階段，吾人發展一種改善的以決策迴饋離散傅立葉轉換為基礎之通道估計方法，此方法應用少量相嵌於正交分頻多工資料符元之領航載波，在第一次迭代時，形成最佳梯度向量來減緩錯誤蔓延效應，並且利用近似的權重矩陣以降低反矩陣計算複雜度。吾人經由電腦模擬兩發射天線一接收天線之空時區塊碼-正交分頻多工系統以驗證所提出的方法，結果顯示所提出方法不僅優於典型以離散傅立葉轉換為基礎之方法，亦優於以空時區塊碼為基礎之最小均方差方法及卡爾曼濾波方法。模擬結果亦證明所提出方法可達成顯著的訊號雜訊比效能改善，尤其在使用高階調變方式（例如：十六點正交振幅調變）於高車速環境下。 於本論文最後部分中，吾人憑藉最大期望值演算法，來處理時變多路徑通道對於正交分頻多工系統以及位元交錯調變碼-正交分頻多工系統所造成的載波間干擾問題。吾人首先在頻域上分析載波間干擾以便使用減少的參數集，根據此分析，導證最大期望值演算法用於最大似然資料檢測。吾人又針對正交分頻多工系統提出最大似然-最大期望值接收機及位元交錯調變碼-正交分頻多工系統提出渦輪-最大期望值接收機，其主要概念在於將所提出之最大期望值演算法與群式載波間干擾消除方法結合，用以減少計算複雜度及獲得時間分集益處。不同於最大似然-最大期望值接收機，渦輪-最大期望值接收機藉由渦輪原理，進一步利用軟輸出維特比演算法與最大後驗之最大期望值檢測器交換訊息。電腦模擬證實所提出之二個接收機顯然勝於傳統一階等化器，且渦輪-最大期望值接收機的效能在正規化最大都卜勒頻率為0.1時，能逼近匹配濾波器界限。

Orthogonal frequency division multiplexing (OFDM) is an effective technique for high data rate transmission over wireless channels. Employing multiple-input multiple-output (MIMO) techniques in OFDM systems is viewed as a popular way to improve system performance for the next generation wireless communications. In this dissertation, we investigate channel estimation and data detection techniques for OFDM systems, covering the research topics of pilot signal designs for MIMO channels, channel estimation and tracking for slowly time-varying channels, and data detection for fast time-varying channels. This dissertation is divided into four parts. The first part presents a complementary codes (CC) pilot-based space-time block code (STBC)-OFDM system. In this system, a pair of complementary codes transmitted in a pre-defined order with the OFDM data signals is used as the pilot signals in a two-antenna transmit diversity system, and used to estimate the channels for optimal data detection at the receiver side. A complete receiver architecture has been designed, the theoretical system performance has been analyzed, and computer simulations have been used to verify the performance of the system in mobile radio fading channels. In the second part, we derive the decision-feedback (DF) discrete Fourier transform (DFT)-based channel estimation method from Newton's method for STBC-OFDM systems. Through our derivation, the equivalence between Newton's method and the DF DFT-based method is established. Computer simulations are also used to demonstrate the equivalence of the two methods in terms of bit error rate (BER) and normalized square error (NSE) performance. Finally, the results presented in this part also hold for conventional OFDM systems. In the third part, we investigate channel estimation for OFDM systems with STBC in mobile wireless channels. Our proposed method consists of two-stage processing and is developed on the basis of the classical DFT-based channel estimation method. In the initialization stage, we employ a multipath interference cancellation (MPIC) technique to estimate multipath delays and multipath complex gains. In the tracking stage, we develop a refined DF DFT-based channel estimation method in which a few pilot tones inserted in OFDM data symbols are applied to form an optimal gradient vector at the first iteration such that the error propagation effect is mitigated. In order to reduce computational complexity, an approximate weighting matrix is adopted to avoid matrix inversion. We demonstrate the proposed method through computer simulation of an STBC-OFDM system with two transmit antennas and a single receive antenna. The results show that our method outperforms the classical DFT-based method, the STBC-based minimum mean square error (MMSE) method, and the Kalman filtering method as well, and that significant signal-to-noise ratio (SNR) performance improvement can be achieved, especially when a high-level modulation scheme, e.g. 16-quadrature amplitude modulation (QAM), is adopted in low-mobility environments. In the final part, we resort to the expectation- maximization (EM) algorithm to tackle the inter-carrier interference (ICI) problem, caused by time-variant multipath channels, for both the OFDM systems and the bit-interleaved coded modulation (BICM)-OFDM systems. We first analyze the ICI in frequency domain with a reduced set of parameters, and following this analysis, we derive an EM algorithm for maximum likelihood (ML) data detection. An ML-EM receiver for OFDM systems and a TURBO-EM receiver for BICM-OFDM systems are then developed to reduce computational complexity and to exploit temporal diversity, the main idea of which is to integrate the proposed EM algorithm with a groupwise ICI cancellation method. Compared with the ML-EM receiver, the TURBO-EM receiver further employs a soft-output Viterbi algorithm (SOVA) decoder to exchange information with a maximum a posteriori (MAP) EM detector through the turbo principle. Computer simulation demonstrates that the two proposed receivers clearly outperform the conventional one-tap equalizer, and the performance of the TURBO-EM receiver is close to the matched-filter bound even at a normalized maximum Doppler frequency up to 0.1.