A method for trellis coding a logarithmic distribution of constellation points

摘要

A method for trellis coding voltage levels of signals of a pulse code modulation modem using trellis coding whereby only lowered level points in which the intervals between adjacent levels are equal to or less than a predetermined value are trellis coded. A Bit-to-Index generator (2) generates an index (In) from an input bit stream on an input line (3) which is compared in block (6) with a maximum trellis code index (Imaxt). Points corresponding to the generated index (In) when the generated index (In) is greater than the maximum trellis code index (Imaxt) are passed through without trellis coding. While those points corresponding to the generated index (In) which is less than or equal to the maximum trellis code index (Imaxt) are trellis coded. Trellis coding is carried out in a convolutional coder (1) where three bits are taken from the input bit stream when the generation index (In) is less than or equal to the maximum trellis code index (Imaxt) and two 2-dimensional subsets or four 1-dimensional reflections are outputted to a reflection block (9). The 1-dimensional reflections are applied to a transformed number (Pn) which is derived from the generated index (In) in a converter (4), where Pn=2.In+1-2(2.In+1).(In modulo 2) for generating a selector number (Qn) which is used for selecting the output level to be transmitted from a look-up table.