V.90数字调制解调器解码算法的研究

A Study on the Decoding Algorithm of the V.90 Digital Modem

V.90数字modem的编码并不是真正意义上的纠错编码,其主要目的是对发送信号进行星座成形和频谱成形.频谱成形所需的运算量远大于星座成形.数字modem发送的PCM码字符号位的选择是在网格图中寻找最佳路径,因此可以在编码器中使用Viterbi算法.这就相当于把传统编解码问题中的解码过程放在了编码器中,因此,从逻辑上讲,V.90数字modem的解码过程应该是一个编码过程.从这一基本思想出发,把两状态网格图所描述的符号位修改规则用一个编码器表示,进而推导出V.90数字modem解码所需的各种关系,最后给出了V.90数字modem的解码实现算法.

The purpose of coding in the V.90 digital modem is to shape the transmitted signal spectrum and constellation instead of error correction. The computational load of spectrum shaping is much heavier than that of constellation shaping. The sign bits selection is a problem of searching the optimum path in the trellis, thus Viterbi algorithm can be used. This is equivalent to placing a conventional decoder in the (V.90) digital modem encoder. Logically speaking, the (V.90) digital modem decoder is like a traditional encoder. From the above basic idea, we have represented the modification rules constrained by 2-state trellis as a convolutional code, derived all the formula needed for decoding, and finally proposed a decoding algorithm for the V.90 digital modem.