正交分頻多工通信系統之導引訊號輔助式通道估測與等化設計

Abstract

在本篇論文中，我們研究了正交分頻多工通信系統的通道估測及等化。在論文的第一部分，根據通道頻譜響應的特性和轉換域內插的概念，我們提出了兩個以離散餘弦轉換為基礎的導引訊號輔助式通道估測器，降低了以離散傅立葉轉換為基礎的通道估測器在通道有非取樣週期倍數的路徑延遲時的失真錯誤，並且這兩個估測器能夠利用成熟的快速離散餘弦演算法和架構來實現。在論文的第二部分，我們研究了快速衰減通道對正交分頻多工通信系統的影響。通道在一個正交分頻多工通信符元的時間內改變會造成正交性的喪失，而且頻道之間的干擾會降低系統的效能。我們根據現有的通道估測方式，提出了一個修正方法，加強了時變通道估測的準確度。除此之外，我們還提出了一個降低都卜勒效應引起頻道干擾的方法，降低了頻道干擾在快速衰減環境所造成的影響。

In this thesis, channel estimation and equalization for the Orthogonal Frequency Division Multiplexing (OFDM) system are investigated. In the first part of the thesis, based on the property of channel frequency response and the concept of interpolation in transform domain, we propose two discrete cosine transform (DCT)-based pilot-symbol-aided channel estimators, which can mitigate the aliasing error of direct discrete Fourier transform (DFT)-based channel estimator when the multipath fading channels have non-sample-spaced path delays. Furthermore, in implementation, the DCT-based estimators have the advantages of utilizing mature fast DCT algorithms and architectures. In the second part of this thesis, we investigate the effect of the fast fading channel for the OFDM system. The channel variation over an OFDM symbol will cause the loss of orthogonality between subcarriers and the corresponding inter-channel interference (ICI) will reduce the system performance. Based on an existing channel estimation method, we propose a modification that can enhance the estimation accuracy for the time-varying channel. In addition, we also propose a Doppler-induced ICI reduction method which can reduce the effect of ICI and the error floor of symbol error rate (SER) in fast fading environments.