正交分頻多工系統之I-Q不平衡補償與ISI消除

Abstract

正交分頻多工(Orthogonal Frequency Division Multiplexing，OFDM)是一種高速資料傳輸的技術，在正交分頻多工系統中常會遇到的一個問題是I-Q通道間存在著振幅不平衡以及相位不平衡。I-Q不平衡的問題會嚴重地影響系統效能，許多補償的方法已被提出；然而，在有載波頻率偏移的情況下，絕大部分的I-Q不平衡補償方法都將變得無效。在本論文中，我們提出一新方法來解決此問題，此方法在載波頻率偏移存在的情況下，依然能夠有效地預估出振幅及相位不平衡的偏差量。 此外，我們也將探討正交分頻多工系統中之ISI問題，解決此問題的作法一般是利用時域等化器(Time-domain equalizer)來縮短通道響應的長度。然而，時域等化器的方法並無法有效適用於複雜的無線通道環境中，因此，我們提出一決策迴授等化的新方法來有效地移除ISI，同時提升系統的效能。最後將以歐規數位電視地面廣播(DVB-T)之規格為標準進行實際模擬，藉以驗證本論文提出之演算法的有效性。

Orthogonal frequency division multiplexing (OFDM) is a promising transmission technology for high data rate communications. One problem arising in the OFDM system is the gain and the phase mismatch between I-Q oscillators. It has been shown that the I-Q imbalance problem can significantly affect the performance of OFDM systems. Many I-Q imbalance compensation methods have been proposed; however, most of them are not valid in the presence of frequency offset. In this thesis, we propose a novel method to solve the problem. This method can effectively estimate the gain and phase mismatch disregarding the frequency offset. We also investigate the ISI problem in OFDM systems. A common remedy for this problem is to apply a time-domain equalizer (TEQ) such that the channel impulse response can be shortened. However, the TEQ approach is not effective for complex wireless channels. We then propose a decision feedback equalization method that can effectively remove ISI and at the same time enhance the detection performance. Finally, computer simulations based on DVB-T specification are conducted to demonstrate the effectiveness of the proposed algorithms.