正交分頻多工系統之通道響應重建與區塊干擾消除

Abstract

正交分頻多工技術以良好的頻譜效益且具有抵抗多路徑衰減通道效應能力，成為現今高速無線傳輸技術主流。正交分頻多工系統中，當多路徑衰落通道延遲擴散超出保護區間時，則造成相鄰區塊干擾(Inter-Block Interference，IBI)效應。因此正交分頻多工系統若欲於通道延遲擴散較大的環境運作，則須使用區塊干擾消除方法以避免嚴重性能衰減。本論文主要探討OFDM系統之區塊干擾消除議題，除了探討時域循環性旋積信號還原及頻域等化器區塊干擾消除方法之性能外，本論文亦提出頻域適應性通道預測法，此方法可用於重建多路徑衰落通道響應，且具有適用於不同延遲擴散通道環境之彈性，毋須事先猜測通道環境特性。經由電腦模擬結果顯示，此方法所重建之通道響應具有較小之歸一化均方差(NMSE)，在循環性旋積信號還原區塊干擾消除方法應用上亦有較佳之封包錯誤率(PER)。

Orthogonal Frequency Division Multiplexing (OFDM) is a promising technique for high data rate transmission. It has higher bandwidth efficiency and is more immune to the effect of multipath dispersion, as compared to the single carrier technique. However, as multipath dispersion exceeding the guard interval used in the system for multipath protection, inter-block interference (IBI) occurs and that results in significant degradation on system performance. In this thesis, the issue of IBI cancellation is investigated. Both time- and frequency-domain cancellation methods are studied and compared. A new frequency-domain channel prediction based on recursive least square (RLS) algorithm is proposed to accurately reconstruct the channel impulse response. Accurate channel reconstruction is a necessity for time-domain IBI cancellation to work properly. Computer simulations show that our new method of channel reconstruction can improve the cancellation performance very significantly.