OFDM 是一種應用在數位通訊的新技術,它的特點是可以有效對抗多重路徑的通道衰退｡ CDMA 為分碼多重存取,它能有效地提升使用者的人數及容量｡ 若結合OFDM 與CDMA 便可提高在抵抗多重路徑下多工存取的效能｡

　　在本論文中,首先利用最小平方法(Least-Square Fitting Channel Estimation)通道估測器來估測下行(Downlink) 的通道,以偵測出OFDM-CDMA 信號｡ 並利用不同的偵測技術,如單人偵測(Single-User Detection, SUD) 與多人偵測(Multi-User Detection, MUD) ,來比較彼此的偵測效能｡ 並利用部份等化技術去簡化接收端的偵測技術,或利用線性MMSE(LMMSE) 來得到更好的效能｡

　　最後,由基本的架構延伸到多根發射天線,以探討OFDM-CDMA 訊號在多根發射天線下該如何偵測訊號｡ 利用OFDM-CDMA 信號調變技術結合時空方塊碼(Space-Time Block Code) ,應用在分集(Diversity) 的環境下,並比較不同的偵測技術所得到的效能｡

　　另外,我們提出範圍性的部分等化技術(Range-Partial Equalization, R-PE) ,使接收端能以快速簡便的偵測技術,得到良好的效能｡ 並且由模擬結果可看出, 範圍性的部分等化技術效能接近最佳MMSE 偵測技術效能｡ 但又由於最大可能性(Maximum-Likelihood, ML) 演算法在MUD 偵測下具有高複雜性,因此利用一種低複雜的最大可能性序列偵測器(Maximum-Likelihood Sequence Detection, MLSD)搭配各種SUD 偵測技術,以期找到低複雜性MUD 的偵測技術｡

　　在第五章,我們找出在Uplink 常用的兩種MUD 的偵測技術, MMSE 與MMSE-PIC(Parallel Interference Cancellation) ,並比較這兩種偵測器在多人上傳資訊位元的效能｡

　　In 1993, a novel transmission technique employing a combination of Orthogonal Frequency Division
Multiplexing(OFDM) and Code Division Multiple Access(CDMA) was proposed[1]. This combination is called MC-CDMA or OFDM-CDMA. In radio link, multi-path fading distorts the transmitted signal. To overcome the multi-path fading and achieve a good performance, we may use an OFDM transmission scheme. OFDM is based on a parallel data transmission scheme that reduces the effects of multi-path fading and makes complex equalizers unnecessary. The OFDM system transforms a wide-band signal into an array of parallel narrow-band signals that are more insusceptible to multi-path fading. CDMA based digital wireless communication systems are of increasing significance today both in science and practice.

　　As we are dealing with the down-link of a cellular radio system where the computation is done at the mobile unit, it is important to have a detection scheme of low complexity and having the capability to provide reliable performance. Thus, we can use simple detection techniques at the mobile unit, such as MRC, EGC, and ORC. In this thesis OFDM-CDMA detection based on single-user detection(SUD), and multi-user detection(MUD) algorithms, such as maximum likelihood(ML) detection and maximum likelihood sequence detection(MLSD), are analyzed in the multiple user environment. From the aspect of SUD, partial equalization(PE) can be used
to reduce the complicated receiver, and linear minimal mean square error(LMMSE) can provide
the better performance than minimal mean square error(MMSE) combining in multi-path fading
channels.

　　We also use different space-time block codes(STBCs) in the diversity environment, such as two,three, four, and five transmit antennas. Then we combine STBC and OFDM modulation system in multiple antennas environment, namely diversity environment, and compare the performances of different number of transmit antennas .

　　Finally, we apply different STBCs in OFDM-CDMA modulation systems, and compare the performance of different number of antennas. The equalizations (MRC, EGC, ORC, MMSE, and PE) use the simple detection techniques to compensate the channel response. We also propose the range-partial equalizer(R-PE) to equalize the received signal in multiple antennas environment.

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