SOVA based decoding of double-binary turbo convolutional code

Abstract

The channel encoder is incorporated in communication system to mitigate the effects of distortion, attenuation and fading as may be experienced by the transmitted signal. In this article we have discussed about circular recursive systematic convolutional (CRSC) based turbo codes and their bi-directional soft output viterbi algorithm (SOVA) decoding approach. The bi-directional SOVA [10] is considered in view of its better performance and implementation complexity trade-off. Necessary changes in basic message passing equations of bi-directional SOVA has been introduced, when the double binary CRSC codes are considered instead of the classical binary turbo codes. Double binary CRSC code provides significant error-correcting performance while not suffering from the well-known bit error rate (BER) floor of classical binary turbo codes. Decoding of these circular codes requires a prologue decoder, prior to the actual trellis decoding, to estimate the initial state. A prologue-decoding algorithm for double-binary CRSC code has also been proposed. Prologue decoding plays a very important role in assessing the circulation state of the encoder for a CRSC codes [4]. Efficient determination of circulation state through prologue decoding helps in achieving impressive error performance. Insight into the process of prologue decoding is also useful in reducing average power dissipation in digital implementation of such turbo codecs. The design issues related to prologue decoding and their effects on error performance have also been discussed in this paper. It has been shown that, the performance of prologue decoder does not depend much on prologue length. Adequate simulation results have been included.