Channel decoding using inter- and intra-correlation of source encoded frames

The goal of source controlled channel decoding is to improve the performance of the channel decoder by using the residual redundancy of the source encoded data. The original approach exploits only the interframe correlation between bits and a few simple schemes have been devised in order to use the intraframe correlation. In this paper, we present an efficient method to exploit the intraframe correlation during the channel decoding process. We estimate an a priori soft value of the bit being decoded from the bits already decoded along the trellis path. This estimate is then used together with the estimate obtained from interframe correlation in the Viterbi metric. We also introduce an adaptive estimator of bit correlation that takes into account the reliability of the bits at the decoder output. Our channel decoding algorithm is applied to the full-rate and enhanced full-rate speech codecs in the GSM system and results in a noticeable improvement of the bit error rate.