離散多載波傳收機之最佳通道傳輸率時域等化器

Abstract

在離散多頻帶(Discrete Multi-tone，DMT)接收機中，使用一時域通道等化器(Time-domain equalizer，TEQ)，藉此來縮短通道響應以達到降低符號間干擾(Intersymbol interference，ISI)的目的。近來有關TEQ之研究，不只在縮短通道以降低ISI，更以提升整體傳輸率為主要設計考量，其關鍵在於訊雜比(Signal-to-noise ration，SNR)的定義是否正確。要定義出正確的SNR，首先有關整體雜訊源的型態必須獲得確認。在DMT系統中，雜訊源主要分為兩個部分；一種為通道雜訊，另一種則為殘餘的ISI。近來，殘餘ISI的部份已獲得確認而提出了能達到最大傳輸率的TEQ設計。遺憾的是，此種方法並沒有考慮到通道雜訊的特殊影響。在本論文中，我們將要研究此一問題。首先將證明雜訊比預期的高，這部分可從雜訊不似傳送訊號具有前置週期(Cyclic Prefix，CP)的角度來解釋。如此一來，在TEQ之後各個頻帶上的SNR會被降低。我們所提出的方法將同時考慮殘餘ISI和通道雜訊的影響。經模擬之後可以看出，所提出的演算法確實優於現存所有的TEQ設計。

In a discrete multitone (DMT) receiver, a time-domain equalizer (TEQ) is used to shorten the channel impulse response. The general criterion for the TEQ design is to maximize bit loading. To do that, all noise sources have to be identified and taken into signal to noise ration (SNR) calculation. There are mainly two noise sources in a DMT system; one is the channel noise and the other is the residual intersymbol interference (ISI). Recently, the residual ISI has been identified as a noise source and a method to achieve maximum bit loading was proposed. Unfortunately, this method does not consider the special effect of channel noise. In this thesis, we intend to solve this problem. We first show that noise can be more damaging than it looks. This can be explained by the fact that the transmitted signal has a cyclic prefix (CP) while the noise signal does not. Thus, the SNR (for each tone) in the TEQ output is lower than that in its input. We then propose a TEQ design method that can simultaneously take the residual ISI and the channel noise into account. Simulations show that our method outperforms all existing TEQ design algorithms.