高速行動下多輸入多輸出正交分頻多工系統之子載波間干擾連續消除與遞迴式連續通道估計

Abstract

依照慣例來說，OFDM系統的通道響應都是假設為quasi-static，也就是說在一個OFDM符元裡是非時變的。但是在高速行動的環境下，這個假設不再成立。時變的通道會破壞子載波間的正交性，進而產生子載波間的干擾（intercarrier interference;ICI）。連續的ICI消除器（successive ICI cancellation;SIC）已經成功的運用在減輕這個問題上。然而它的效能會與通道估計的準確度有相當大的關係。在這篇論文裡，我們將會提出遞迴式的通道重新估計方法來加強SIC的效能。主要的想法是利用從SIC的輸出來當做額外的已知信號(pilot)並且遞迴地接續著通道重新估計。SIC的另一個問題，就是大量的處理上的時間延遲。為了解決這個問題，我們提出了一個windowing的設計，可以使得ICI的影響縮小，而且可以應用在較簡化的SIC上。在程式模擬上可見到，所提出的方法可以有效的減低由高速行動所產生的ICI問題。

Conventionally, the channel response for an OFDM system is assumed to be quasi-static, i.e., time-invariant in an OFDM symbol. In high-speed mobile environments, however, the assumption is no longer hold. The time-variant channel will destroy the orthogonality of subcarriers and result in intercarrier interference (ICI). Successive ICI cancellation (SIC) has been successfully applied to mitigate this problem. However, its performance greatly depends on the accuracy of channel estimation. In this thesis, we propose recursive channel re-estimation schemes to enhance the performance of SIC. The main idea is to use the successive outputs from SIC as additional pilots and conduct channel re-estimation recursively. One problem associated with the SIC is its large processing delay. To solve the problem, we then propose a windowing design such that the ICI effect can be reduced and a simplified SIC method can be applied. Simulations show the proposed methods can effectively mitigate the ICI effect induced by the high mobility problem.