用於多輸入多輸出無線通訊系統之空-時信號處理：空-時信號模式與干擾抑制

Abstract

近年來，許多研究組織均積極投入於多輸入多輸出(Multi-Input Multi-Output, MIMO)技術的研發，以提升無線行動網際網路服務及下世代細胞式無線行動通訊系統之性能。這些新技術必須能夠有效克服無線通訊環境中嚴苛的挑戰，而基於適應性天線陣列或智慧型天線形式之天線系統恰可提供一套有效且良好的解決方案，以達到『高可靠度』及『高資料率』的傳輸品質。在此一研究範疇裡，已有許多研發成果大量產出，而許多用於無線通訊系統的商業化產品也已相繼問世。

本論文即在探討於無線通訊系統中，同時在傳輸端及接收端使用多根天線情況下的相關研究。吾人試圖提供無線通訊天線系統一個完整的綜合評述及概要說明，並介紹一些環繞在此一系統周圍的重要議題。其關鍵點為所探討的系統必須是具有適應性且於收發端使用多根天線。吾人亦將討論用於多輸入多輸出無線通訊架構下，有關空-時(Space-Time, ST)信號模式及干擾抑制之空-時信號處理的相關議題。在本論文中，吾人首先將研究重點放在發展一種基於空-時接收機架構之干擾消除技術上；具體而言，為研究一個適用於頻率選擇式多路徑通道下，基於排序式漸次干擾消除(Ordered Successive Interference Cancellation, OSIC)技術的多輸入多輸出等化器(Equalizer)。此一多輸入多輸出等化器將基於傳統最小均方誤差(Minimum Mean Square Error, MMSE)決策迴授等化器(Decision Feedback Equalizer, DFE)的形式以降維(Reduced Rank, RR)技術方式實現之。其中，在排序式漸次干擾消除架構下每一個階段中之最小均方誤差的權值將以廣義旁波瓣干擾消除器(Generalized Sidelobe Canceller, GSC)技術求得，而降維處理則以共軛梯度(Conjugate Gradient, CG)法則實現，以降低計算量。隨後，吾人將探討在頻率平坦式通道下之多輸入多輸出收發機的研究設計。在此一部份，吾人將首先研究一種通用多用戶雙模信號(Dual-Signaling)傳輸系統。在此一雙模信號傳輸系統中，每個用戶終端的資料可基於其當時通道狀況，以正交空-時編碼(Orthogonal Space-Time Block Code, O-STBC)方式傳輸以獲取傳輸多樣(Transmit Diversity, TD)亦或是以空間多工(Spatial Multiplexing, SM)方式進行傳輸以獲取高頻譜效益(Spectral Efficiency, SE)。接下來，吾人將發展一種具有微量回傳資訊之高效率多輸入多輸出收發機架構（主要針對空-時編碼/空-時解碼設計）。此一收發機架構在群組式空-時區塊編碼(Grouped Space-Time Block Code, G-STBC)系統，於位元錯誤率(BER)性能之考量下可使空-時傳輸字碼最佳化，以同時達到高頻譜效益與高鏈結品質(Link Quality, LQ)的通訊。在上述二種多輸入多輸出系統架構下之接收機亦可基於排序式漸次干擾消除技術發展設計。藉由利用正交空-時編碼所具有的強大代數性質，經驗證後，上述二系統所考量基於次序式連續干擾消除技術的偵測器可以群組(Group-Wise)偵測方式實現。此一群組式偵測性質為使用正交碼情況下的一種獨特結果，它可大大地改善信號分離的能力。此外，吾人更進一步利用潛藏於通道矩陣中豐富且特殊的結構，發展出一套具有計算量效益的遞迴式偵測器。

綜言之，本論文所提出之接收機與既存之接收架構比較，其主要的優點為具備較低實現複雜度、較快的收斂速率及較低的位元錯誤率。此外，本論文對於多輸入多輸出無線通訊收發機的設計也提供一種具智慧及強健性能的解決方案以適當地配合其通道狀況及系統需求。藉由數學分析與模擬評估，吾人證實所提出之新型收發機架構可有效提升系統之傳輸鏈結品質及獲取高頻譜效益，並有效抑制干擾及雜訊。

Recently, there has been a substantial increase in the development of multi-input multi-output (MIMO) technologies for evolving wireless mobile Internet services and next-generation cellular systems. These technologies must be able to cope with the challenging wireless environment, and antenna systems in the form of adaptive array or smart antennas can provide an effective and promising solution while achieving reliable and high data-rate transmission. Research and development in this area have significantly increased, and many commercial products are now readily available for wireless communication systems.

This thesis is concerned with the use of multiple antennas at the transmitter and receiver in wireless communication systems. We attempt to provide an overview of all these antenna systems for wireless communications and introduce some of the important issues surrounding them. The key points are that the system must be adaptive and consist of multiple antennas. Several aspects of space-time (ST) processing related to the ST signaling and interference suppression for MIMO based wireless communications will be discussed. In this thesis, we first focus on developing the interference suppression scheme based on an ST receiver. We study an ordered successive interference cancellation (OSIC) based MIMO equalizer over the frequency selective multipath channels. The MIMO equalizer is developed as a reduced-rank (RR) realization of the conventional minimum mean square error (MMSE) decision feedback equalizer (DFE). The MMSE weight vectors at each stage of the OSIC are computed based on the "generalized sidelobe canceller (GSC)" technique and RR processing is incorporated by using the "conjugate gradient (CG)" algorithm for reduced complexity implementation. After that, we then work on the design of MIMO transceiver over the frequency flat channels. In this part, we first study a general MU dual-signaling system, in which each user's data stream is either orthogonal ST block encoded for transmit diversity (TD) or spatially multiplexed for high spectral efficiency (SE) based on its own channel condition. Second we develop an efficient MIMO transceiver architecture (ST encoding/decoding design) with a slight amount of feedback information that optimizes the ST codeword with respect to the BER performance for a grouped orthogonal ST block coded system to achieve both the high SE and link quality (LQ). In particular, the receiver in the two MIMO systems is also designed based on the OSIC scheme. By exploiting the algebraic structure of orthogonal codes, it is shown that the OSIC based detector in the above considered two systems allows for an attractive "group-wise" implementation. The group-wise detection property, resulting uniquely from the use of orthogonal codes, potentially improves the signal separation efficiency. Moreover, the imbedded structure of the channel matrix is also exploited for deriving a computationally efficient "recursive based" detector implementation.

In summary, the main advantages of the proposed ST receivers over the popular existing ones lie in its lower implementation complexity, much faster convergence behavior and better BER performance. On the other hand, we provide a smart and robust solution for transmitter and receiver designs better matched to the channel condition and system requirement in MIMO wireless communications. Mathematical analysis and computer simulations show that the proposed methods can achieve high LQ and high SE and offer excellent immunity to interference and noise.