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摘要

进入21世纪,随着信息社会的到来,人们对电信业务的需求日益增大,逐渐由单一的业务发展到数据,图像,多媒体等综合业务.非对称数字用户环路(ADSL)作为接入网宽带过程中的过渡技术,得到日益广泛的应用.其核心技术是离散多音频调制(DMT),它比正交调幅调制(QAM)和无载波振幅相位调制(CAP)具有更优越的性能.

DMT调制把信道分成若干个子信道,每个子信道有一对正交载波,对数据流进行QAM调制,其中采用的网格编码调制(TCM)编码实际上是在QAM的基础上进行卷积编码以实现纠错功能,解调时采用"最大似然维特比(Viterbi)软判决"进行译码.

卷积码可以看作是将发送的信息序列通过一个线性的,有限状态的移位寄存器而产生的码,描述卷积码的方法之一是给出其生成矩阵,其编码是通过将输入信息序列与表示寄存器和模2加法器相连关系的生成矩阵进行卷积得出.维特比译码算法是卷积码的最大似然译码,通过在网格图上寻找与接收序列有最大度量的路径得以实现,根据其采用的不同度量可分为硬判决和软判决算法.

本文模拟实现了简单卷积编码和译码的过程,从理论分析和仿真图形两个角度比较了首次差错事件概率,比特差错概率这两种描述译码性能的方法,并给出了不同译码方法时的编码增益,比较得出:在的范围内,软判决译码比硬判决译码有2～3dB的增益.使用卷积编码的系统较之未编码系统的编码增益取决于码率Rc和最小自由距离.

关 键 词 :ADSL,DMT,卷积码,维特比算法

ABSTRACT

In the 21 century, with the development of munication, the requirement of the tele operation goes increasingly greater. It has gradually grown from the telephone service to the integrated operations include the high speeds data tranission, image, multimedia and so on. Asymmetric Digital Subscriber Line(ADSL) as a transition technology in the processing of Access Network bandwidth is being more and more popular today. It is based on the existing twisted pair loop. Its core technology is Discrete Multi-tone Modulation (DMT), which is superior to Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude Phase Muduation (CAP).

DMT divides the ailable bandwidth into a large number of bands. The information sequence is split into parallel bitstreams that are used to QAM-modulation by a couple of quadrature carriers in each band. The Trellis Code Modulation (TCM) used in DMT is the bination of convolutional encoding and QAM-modulation, using Viterbi algorithm to decode when demodulation.

Passing the information sequence to be tranitted through a linear finite-state shift register generates a convolutional code. One method of describing a convolutional code is to give its generator matrix that contains the connections of the encoder to that modulo-2 adder. The Viterbi Algorithm is the Maximum Likelihood Decoder of convolutional code. The Viterbi algorithm is a solution to the problem of "finding a route in trellis which has the maximum metric". It is called soft-decision and hard-decision rest with the metric in trellis it uses.

This paper implements the process of convolutional encoding and Viterbi decoding. Analysis the performance of Viterbi algorithm in theory and imitate figure. In pare the performance between soft-decision and hard-decision decoding, note that the difference obtained from the upper bound is approximately 2-3dB for . The coding gain achieved by a convolutional code over an uncoded system is depends on the date rate Rc and the minimum free distance .

Keywords:ADSL, DMT, Convolutional code, Viterbi Algorithm

目录

第一章 ADSL简介1

1.1 接入网1

1.2 宽带接入网1

1.3 ADSL概述3

1.4 ADSL调制方式5

第二章 纠错码的基本概念8

2.1 数字通信系统的组成及信道模型8

2.2 调制和解调11

2.3 最大似然译码12

2.4 信道编码定理14

第三章 卷积码16

3.1 卷积码概述16

3.2 卷积码的编码16

3.3 卷积码的图解表示19

3.4 卷积码的转移函数22

第四章 维特比算法27

4.1 维特比算法原理27

4.2 译码算法实现28

4.3 软判决译码32

4.4 软判决译码的差错概率34

4.5 硬判决译码的差错概率41

4.6 性能分析结论45

参考文献50

附录:仿真源程序51

致 谢64

第四章 维特比算法

4.1 维特比算法原理

卷积码的译码基本上可划分为两大类型:代数译码和概率译码.在

.

.

.

在大信噪比的情况下,卷积编码的软判决译码相对于不编码的二相PSK系统而言的编码增益是

(dB)(4.40)

.

.

.

图4-12 软判决和硬判决比较

表4-2 软判决维特比译码时的编码增益/dB

Pb未编码Rc等于1/3Rc等于1/2Rc等于2/3Rc等于3/4L等于7L等于8L等于5L等于6L等于7L等于6L等于8L等于6L等于910-36.84.24.43.33.53.82.93.12.62.610-59.65.75.94.34.65.14.24.63.64.210-711.36.26.54.95.35.84.75.23.94.8参考文献

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[2] [美]Dennis J R. ADSL/VDSL 原理. 杨威 王巧燕 译,人民邮电出版社,2001:120-126

[3] 冯建如,王岚. ADSL宽带接入技术及应用.人民邮电出版社,2002:21-23

[4] Forney G D. The Viterbi Algorithm. IEEE Proc.,1973,61(3):268-278

[5] Lin S and Costello D J. Error Control Coding: Fundamental and Applications. Prentice-Hall, 1983

[6] Cioffi J M. MMSE Decision-feedback Equalizers and Coding-Part 1: Equalization Results. IEEE Trans. Commun.,1995,43(10):2582-2594

[7] 王新梅, 肖国镇. 纠错码——原理与方法(修订版). 西安电子科技大学出版社, 1996

[8] [美]John G P. 数字通信(第四版). 张力军, 张宗橙, 郑宝玉等译,电子工业出版社,2003:340-354

[9] 曹志刚, 钱亚生. 现代通信原理. 清华大学出版,1991:371-393

[10] 吴伟陵. 信息处理与编码. 人民邮电出版社,1999:404-424

[11] 冯景锋,门爱东,全子一. DMT调制技术在xDSL中的应用. 中国数据通信,2002.1

[12] 易志强, 徐元欣, 齐子初. Viterbi译码原理及参数性能分析. 杭州电子工业学院学报, 2000

[13] 周筑, 叶楠, 韩声栋等. 针对DMT调制技术的自适应均衡技术. 通信技术,1997(3)

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论文模板说明

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摘要

ABSTRACT

ABSTRACT

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目 录

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第四章维特比算法

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参考文献

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