As for a digital transmission system for transmitting data using convolution codes and performing error correction using Viterbi decoding at a receiving end, there has been a well-known technique in which a code sequence is terminated with a certain termination code so that a plurality of data to be coded, which are not related to each other at all, are connected to be treated as one code sequence, thereby enabling continuous decoding by a single decoder.
Hereinafter, a signal that is convolutionally coded by a convolution encoder 201 shown in FIG. 2(a) is regarded as a target. As shown in FIG. 2(a), the convolution encoder 201 stores successively inputted information bits I into shift registers D1˜D6, respectively, and two code bits C0 and C1 are generated from the previously inputted six information bits and the currently inputted information bit I. To be specific,C0=D6⊕D2⊕D1⊕IC1=D6⊕D5⊕D2⊕Iare generated. These are expressed by a trellis diagram 202 shown in FIG. 2(b). Hereinafter, the values of D1˜D6 are denoted by STATE{D1,D2,D3,D4,D5,D6}.
Generally, among bit sequences inputted to an encoder, a code sequence of a specific order is treated as termination bits to be used for a termination process. Hereinafter, a description will be given of a case where termination is performed with a code sequence in which termination bits are inputted in order of 01000111 as shown in FIG. 3.
It is assumed that the termination bits 01000111 are sequentially inputted to the convolution encoder 201 shown in FIG. 2(a). Since the bits before and after the termination bits can be 0 and 1, these bits are represented by X.
When 0 at the top of the termination bits 01000111 is inputted to the encoder, the STATE is {XXXXXX}. When the termination bits 01000111 are sequentially inputted from this state, as shown in FIG. 3, the value of the STATE transits as follows:{XXXXXX}→(1){1XXXXX}→(2){10XXXX}→(3){010XXX}→(4){0010XX}→(5){00010X}→(6){100010}→(7){110001}→(8){→111000}→(9){X11100}→(10){XX1110}→(11){XXX111}→(12){XXXX11}→(13){XXXXX1}→(14){XXXXXX}→. . .and convolution codes are generated by the respective values of STATE and the values of the input bits.
Presuming that thus encoded codes are inputted to a decoder without being affected by a transmission path, nodes of a trellis diagram in traceback at decoding should transit in the order of (13)˜(1) that is reverse to the order at coding.
Based on such characteristics, there have been proposed several techniques for preventing deterioration in error correcting capability before and after the termination process. As a typical prior art, there is a technique of controlling a value of a path metric corresponding to STATE of a peculiar node to 0 or MAX at a timing of any of the above-mentioned STATE (6)˜(8) on the basis of positional information of a termination code in an ACS (Add Compare Select) circuit, thereby to improve error correcting capability (for example, refer to Japanese Published Patent Application No. 2000-183756 (Patent Document 1)).