ADSL收發機之信號處理與編解碼技術之研究

Abstract

非對稱數位用戶線路 (Asymmetric Digital Subscriber Line, ADSL) 是一種利用傳統電話雙絞線 (twisted-pair telephone line) 來 提供高速資料傳輸的技術。ADSL 不需額外架構新的電纜線路, 就可以在 下鏈(從中央交換機到用戶端)通道(downstream channel)上傳送高達 8 Mbps 的資料量。藉由先進的數位信號處理(DSP) 及多重載波調變(DMT)技 術, ADSL可以改善傳輸線路的使用效率, 並在不影響原本電話服務的情況 下從事資料傳遞。雖然其間曾因隨選視訊(Video on Demand) 的應用不被 看好而遭忽略, 近來卻又拜網際網路(Internet)的盛行而再度興起, 並有 取代語音頻帶數據機 (Voice-band modem)的趨勢。 本論文旨在研究 ADSL相關的信號處理與編解技術, 探討在現有標準下, 接收端可以改善性 能的地方與方法。文中首先概述目前ADSL的系統, 並介紹幾種在雙絞線傳 輸環境所要考慮的雜訊, 包括: 串話(crosstalks), 白雜訊(white noise),脈衝雜訊(impulse noise)。接著, 我們說明了ADSL所用的特殊多 重載波調變技術與為提昇性能而使用的編碼調變(Trellis-coded Modulation)。這種調變在脈衝雜訊下的性能我們同時作了模擬與理論分 析。此外, 連結編碼調變及里德-所羅門碼(Reed-Solomon Code)而成的串 接碼 (concatenated code) 在ADSL上之解碼方式及相關的性能分析與模 擬也是討論的重點。最後, 我們設計一種新的渦輪編碼調變(turbo TCM) 並探討其在ADSL上的應用及可能的系統性能改進。 ADSL (Asymmetric Digital Subscriber Line) is a digital transmission technologythat provides high speed data communications over the existing nonloadedtwisted-pair telephone lines. It is capable of delivering data rates higherthan 8 Mbps in downstream transmission channels without disturbing the original telephony service on the same line. The purpose of this thesis is to study the signal processing and codec techniques of the DMT-based ADSL system. These techniques are capable ofenhancing the system performance within the current standard ADSL framework.We evaluate the baseline performance of the standard DMT-ADSL system and present numerical behavior associated with various receiving algorithms.We begin with an overview of the ADSL system model according to ANSI T1.413-1995 standard. Various noise impairments of ADSL access channels are discussed.Several useful models of the impulse noise are presented as well. Performanceof a basic discrete multitone (DMT) modulation system, the line code of ADSLis evaluated first, followed by that of Wei's 4-dimensional 16-state trelliscode, which is used as an optional inner code in ADSL. To exploit potential performance enhancement, we consider first a concatenatedcoding scheme that uses an inner trellis code and a outer interleavedReed-Solomon code. Our investigation then extends to a new class of turbo TCMscheme. Numerical results show that significant performance improvement can beobtained.