Improved Channel Estimation Techniques for TDS-OFDM Communication Systems

[[abstract]]本篇論文中，我們在時域同步正交分頻多工（TDS-OFDM）通信系統下提出一種改良式離散傅立葉變換（DFT）通道估測技術。在使用循環字首（CP）的OFDM系統下，傳統的DFT-based通道估測技術使用導航信號（pilots）估計通道的頻率響應，並且在DFT端透過對雜訊補零來提升效能。最近在CP-OFDM系統下，一種基於DFT-based的顯著通道檢測（SCTD）技術已經被提出。它可以藉由挑選顯著的通道路徑訊息提高系統的效能。然而，由於不同估測段長度的差異，原始的SCTD法是不適用於TDS-OFDM系統在較大延遲擴散的通道環境。 本論文的主要貢獻是，在SCTD法的概念下提出適用於TDS-OFDM系統的通道估測技術，且有效地提高了系統的性能。為了避免符號間干擾估測段造成估測精準度降低，我們使用雙偽雜訊序列填充（DPNP）的框架結構。由於在較大延遲擴散通道環境中，原始SCTD法會使門檻值估計錯誤造成損失路徑信息使得效能退步。在此，我們利用兩個連續的估測段計算其相關性大致預測其平均雜訊功率作為門檻值，利用門檻值選擇顯著通道路徑估測原始通道脈衝響應。一些比較模擬顯示，提出的通道估測技術與傳統的最小平方估測技術相比更具有改善效能的潛力。[[abstract]]In this thesis, an improved discrete Fourier transforms (DFT)-based channel estimation technique is proposed for time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) communication systems. The conventional DFT-based channel estimation techniques for cyclic prefix (CP)-OFDM systems employ pilots to estimate the channel frequency response and suppress the noise effect in the DFT by using the zero insertion. Recently, the DFT-based channel estimation called significant channel tap detector (SCTD) has been developed for CP-OFDM systems. It could improve the system performance by deciding significant channel taps adaptively without requiring any channel statistical information. However, the original SCTD scheme is not suitable to apply for the TDS-OFDM systems in large delay spread channel environment. The main contribution of this thesis is that the proposed channel estimation technique based on the concept of SCTD scheme could effectively improve the system performance of TDS-OFDM systems. The frame structure with dual pseudo noise sequence padding is considered in order to avoid inter-symbol interference. The correlation of two successive preambles is employed to estimate the average noise power as the threshold for obtaining the SCTD threshold estimation error and loss path information resulting in performance degradation in large delay spread channel environment. The proposed estimation scheme could also roughly predict the noise power in order to choose the significant channel taps to estimate the channel impulse response. Some comparative simulations are given to show that the proposed channel estimation technique has potentials to improve the bit error rate performance compared with the conventional least-squares channel estimation.[[note]]碩