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Blind Equalizations for OFDM-CDMA Communications in Multipath Channels
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|摘要:||隨著通訊系統的演進，分碼多工存取(CDMA)的傳輸速率已漸漸不敷使用，更高的資料傳輸量成為新一代通訊系統所需面對的最重要挑戰。正交分頻多工分碼多工存取(OFDM-CDMA)系統，又稱為多載波分碼多工存取系統(MC-CDMA)，結合了正交分頻多工(OFDM)與分碼多工存取(CDMA)技術的優點，能夠有效的抵抗干擾及提供較大的傳輸速率，適用於無線高速數據傳輸，因此被視為第四代無線通訊系統重要的選擇之一。然而，正交分頻多工分碼多工存取系統對載波頻率偏移非常敏感，在應用上多重存取干擾(MAI)及碼間干擾(ISI)都是必須解決的問題。在接收器端等化器扮演非常重要的角色，它可以補償通道在傳輸時對訊號造成的影響，使接收器能夠較正確的解出訊號。等化器分為訓練等化器及盲蔽等化器兩類，訓練等化器資料傳輸前一傳送和接收端皆已知的訓練序列會被傳送，藉由此來推斷通道並加以補償。不同於訓練等化器，盲蔽等化器不需要訓練序列，而是由接收的訊號中來運算並等化通道。Sato最早提出盲蔽等化器概念，而Godard恒模數演算法(CMA)是最被廣泛被使用的演算法。另外，恒模數根基演算法(CMA-based)、改良型恒模數演算法(MCMA) 和改良型恒模數根基演算法(Modified CMA-based)分別被提出。改良型恒模數根基演算法結合了MCMA及CMA-based的優點，能避免因為模數選擇不當對各系統的影響。最後，以電腦模擬上述所提及的演算法，並比較各演算法在4-QAM及16-QAM還有不同通道下系統錯誤收斂情形。|
With the progress of communication systems, the transmission rate for CDMA system is not enough anymore. To increase the transmission rate is the most important challenge for new generation of communication systems. OFDM-CDMA (also known as MC-CDMA) combines the benefits of OFDM and CDMA. It is robust to interference and provides higher transmission rate. The system is suitable for high-data-rate wireless communication and has become one of the most important candidates for 4G wireless communication system. However, OFDM-CDMA system is very sensitive to carrier frequency shift. Mitigating multiple access interference (MAI) and intersymbol interference (ISI) should be done in application. Equalizer plays a very important role in receiver. It can compensate the effect caused by passing through channels and make the receivers recover the transmitted signals more correctly. There are two types of equalizers. One is trained equalizer and the other is blind equalizer. In trained equalizer, a training sequence known by both transmitter and receiver is sent. By calculating the received signal in receiver side, it can deduce and compensate the effect cause by channel. Unlike trained equalizer, training sequence is not required for blind equalizer. The equalization could be done by calculating the signal in receiver. Sato is the first person who proposed adaptive blind equalization, and the Godard or constant modulus algorithm (CMA) is the most popular scheme for blind equalization. Besides, CMA-based, Modified-CMA (MCMA) and Modified CMA-based have been proposed respectively . Modified CMA-based combined the benefits of CMA-based and Modified-CMA. It can effectively avoid the influence of incorrect modulus. Finally, computer simulations for above algorithms are performed. The difference between 4-QAM and 16-QAM and the convergence of MSE of different channels are showed.
|Appears in Collections:||電機工程學系所|
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