在現今的無線通訊架構中，由於系統的傳輸率要求愈來愈高，因此愈來愈多的傳輸規格皆採用正交分頻多工(Orthogonal Frequency Division Multiplexing, OFDM)作為調變的技術，其優點是可以提高頻譜的使用效率，進而提供更高的傳輸速率(data rate)。
除此之外，近年的研究中指出，理論上，利用多重天線傳輸的系統架構，由於獨立傳輸路徑數(或稱空間分集數)的增加，可提高通道容積(channel capacity)，這麼一來便突破了以往單一天線對傳的效能限制；因此多重輸入多重輸出(Multi-input Multi-output, MIMO)傳輸方式對通訊系統的效能增進亦備受矚目，所衍生的主題不僅在傳輸架構上，MIMO系統中的特殊編碼－時空碼(Space-Time Code, STC)的編碼方式也常被研究，因為時空碼便是在如何讓空間達最大分集(spatial diversity)上扮演極重要的角色。
綜合上述兩種技術的優點，因此結合OFDM及MIMO的系統，在近期的許多新傳輸標準中皆被採用，例如：IEEE 802.11n、IEEE 802.16-2004及IEEE 802.16e。
然而值得注意的是，無論是正交分頻多工系統中的通道等化器(channel equalizer)，或是多重輸入多重輸出系統中的時空解碼器(space time decoder)，通道估測(channel estimation)都扮演著重要的角色，不過由於它複雜的電路運算，因此一些相關研究論文中，便對於如何去簡化或避免通道估測提出了新的架構或演算法。
針對相同的問題，在此篇論文中，我們以IEEE802.16-2004標準定義的實體層(Physical layer)為基礎，提供了一個分頻雙工(Frequency Division Duplex, FDD)模式的多輸入多輸出正交分頻多工(MIMO-OFDM)系統架構，並在下行線路(downlink)中採用展頻(spread spectrum)的技術，以達成簡化用戶端運算電路的目標。

As wireless communication systems demand for higher and higher transmission rate, there are more and more transmission standards adopt OFDM (Orthogonal Frequency Division Multiplexing) technique as the modulation process because it can improve the efficiency of bandwidth usage. Besides, some recent researches have proved that the channel capacity can be enhanced when transmission systems use multiple antennas, due to the increase of independent transmission paths (or called spatial diversity). It breaks the limit of the performance in the single antenna transmission systems. Recently, many researches exploring the improvements of the MIMO (Multiple Input Multiple Output) system have been conducted, not only on the architecture, but also on the specific coding algorithms in the MIMO system, especially the Space-Time Code (STC) because the STC plays an important role on maximizing the spatial diversity. According to the above description, the system combines OFDM and MIMO techniques would exhibit both high data rate and high performance properties. Hence, there are many new transmission standards which have adopted both techniques. For example, IEEE 802.11n、IEEE 802.16-2004 and IEEE 802.16e.
In the MIMO-OFDM system, the channel estimation has played a critical role in determining the performances of either the channel equalizer in an OFDM or the space-time decoder in a MIMO. But, the channel estimation needs a complex circuit operation. Hence, there are some related researches which have proposed new architectures or algorithms to reduce the complexity of channel estimation. In this thesis, we propose a novel MIMO-OFDM architecture for achieving such purpose of high performance and low circuit complexity. The proposed architecture is based on the physical layer in IEEE 802.16-2004 standard and operates in FDD mode transmission. Moreover, we use the spread spectrum technique in the downlink to further reduce the complexity of the processing circuit in Customer Premises Equipment.

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