Sequential decoder

Abstract

A sequential decoder for decoding a systematic and convolutional code signal having a code rate greater than 1/2 and carrying out error correction coding of the code signal. A local most likely path in a plurality of possible paths for a newly received information bit is determined by calculating a branch metric indicating likelihood of each of the plurality of possible paths in accordance with a predetermined algorithm. The path decision is carried out by a construction including a two-path comparing path decision circuit which receives a pair of bits comprised of an information bit and a parity bit at one time and determining a local most likely path between two possible paths for the information bit. A four-path comparing path decision circuit receives a pair of information bits at one time and determines a local most likely path among four possible paths for the pair of information bits. A parity bit timing detecting circuit detects a timing of an input of the above pair of bits comprised of an information bit and a parity bit. A selecting circuit selects an output of the above two-path comparing path decision circuit at the input timing of the above pair of bits comprised of an information bit and a parity bit, and selects an output of the above four-path comparing path decision circuit at the timing of the input of the above pair of information bits.