Research on the Orthogonal Frequency Division Multiplexing-Code Division Multiple Access Technology

[[abstract]]正交分頻多工（Orthogonal Frequency Division Multiplexing；OFDM）是一種多載波調變技術，可以有效地解決通訊傳輸中的頻率干擾、高頻譜效率及多重路徑衰落等問題。分碼多重接取（Code Division Multiple Access；CDMA）是一種展頻通訊技術，能有效抵抗外來雜訊干擾與來自其他使用者間信號之相互干擾，除了保密外，且具有比傳統多工更高容量的特性。 OFDM-CDMA是一種結合正交分頻多工（OFDM）與分碼多重接取（CDMA）的混合技術，它具有OFDM能有效對抗多重路徑衰落的優點，以及CDMA所能提供比傳統多工更高容量的特性，使其具有高頻譜效率、高容量、低位元錯誤率(BER)等優點，所以正被各研究單位熱烈的探討著，期望能成為第四代行動通訊的傳輸技術。 本論文主要在探討OFDM-CDMA系統，並評估其在多重路徑衰落通道下之效能。首先，我們建立一個多重路徑衰落通道，針對屬多載波系統的OFDM做分析，了解OFDM在此通道下傳輸之性能，然後再探討OFDM-CDMA，並將模擬結果與直序展頻分碼多重接取（DS-CDMA）做比較，主要探討的重點為多重路徑衰落、剪截效應、不同型式的保護區間、延遲色散、正交性及系統容量等問題。[[abstract]]The OFDM is a multi-carrier modulation technique. It inherits the robustness against the inter-symbol interference（ISI）effect, high spectrum efficiency and enhanced anti-multipath fading ability. The CDMA is a spread spectrum communication technique. It inherits the anti-interference capability and against the inter-channel interference（ICI）. Except the encryption, it although has the higher user capacity than other conventional multiple access techniques. OFDM-CDMA is a technology that combines OFDM and CDMA. It inherits the anti-multipath fading ability of OFDM technology as well as the high user capacity of CDMA. Since OFDM-CDMA owns a lot of advantages, such as high spectrum efficiency, high capacity and low bit-error-rate (BER), and so on, many researchers have devoted their efforts on this new technology. And it is expected to be the possible solution of the fourth-generation mobile communication systems. In this thesis our discussion is focused on the OFDM-CDMA system, and evaluate the performance in multi-path fading environment. The multi-carrier OFDM system is first analyzed over a multi-path fading channel. And then the performance of OFDM-CDMA is evaluated. The simulation result is compared with that of a direct-sequence (DS) CDMA system. The major emphasis of our investigation includes multi-path fading, clipping effect, different types of guard, delay spread, orthogonality and system capacity