適用於正交分頻多工行動系統之都普勒偏移估測與干擾消除技術

Abstract

近年來，正交分頻多工（Orthogonal Frequency Division Multiplexing, OFDM）系統在無線寬頻通訊之應用上日漸普及，許多系統與規格均採納此技術，例如數位電視、數位廣播、IEEE 802.11 a/g/n、IEEE 802.16等。此外，移動傳輸將是未來無線通訊系統的趨勢之ㄧ，但正交分頻多工系統在移動的環境下將面臨都普勒偏移（Doppler Shift）的問題。都普勒偏移使得接收端的子載波間失去正交性，因而導致子載波間的相互干擾，並限制了系統的效能。為了解決此問題，在本篇論文中，吾人提出一個最大都普勒偏移的估測方法，此方法可良好地運作在低信噪比（Signal-to-Noise ratio, SNR）的環境下。且利用此估測出的都普勒資訊，吾人可使用一個適應性的干擾消除方法，選擇合適的等化器（Equalizer）來消去子載波的干擾，而不會浪費運算量。吾人也藉由電腦模擬來驗證所提方法可良好運作在正交分頻多工系統中。

In recent years, orthogonal frequency division multiplexing (OFDM) becomes a popular technique in wireless broadband communications. There are many systems adopting OFDM techniques, such as digital video broadcasting (DVB), digital audio broadcasting (DAB), IEEE 802.11 a/g/n, IEEE 802.16, etc. On the other hand, mobile transmission is a trend in future wireless communications. However, in mobile OFDM systems, Doppler shift destroys the orthogonality between subcarriers, and this causes the intercarrier interference (ICI) and limits the performance of the receiver. To deal with this problem, we proposed a maximum Doppler shift estimation method for mobile OFDM systems, and the proposed method can work well at low SNR. Then, with the help of the Doppler information, an adaptive ICI cancellation scheme can be employed, which can choose an appropriate equalizer to cancel ICI without needing unnecessary computation. Finally, computer simulation results for mobile OFDM systems are included to confirm the proposed method.