Channel decoding of block codes with a priori information

In this paper we propose a complete transmission system for still images. This system comprises a VQ source encoder with a compression rate of 9 and the data is encoded using a BCH block code and mapped on a 16-QAM constellation. Two BCH codes are considered : BCH(29,24,3) and BCH(54,48,3). The parameters of the code book and channel encoder have been optimized for 16-QAM. We use a new joint source-channel decoding algorithm for block code. As compared to classical channel decoding algorithms (hard decoding and soft decoding), we obtain an improvement of 1dB over Gaussian channel and 3dB over Rayleigh channel, in terms of coding gain for a BER of 10(-5). The PSNR of the received image exhibits an improvements of 3dB as compared to a conventional system without joint source-channel decoding at low SNRs. The decoded image presents a good subjective quality even if the residual BER is high.