Progressive joint source-channel coding in feedback channels

It is well known that Shannon's separation result does not hold under finite computation or finite delay constraints, thus joint source-channel coding is of great interest for practical reasons. For progressive source-channel coding systems, efficient codes have been proposed for feedforward channels and the important problem of rate allocation between the source and channel codes has been solved. For memoryless channels with feedback, the rate allocation problem was studied by Chande et al. (1998). In this paper, we consider the case of fading channels with feedback. Feedback routes are provided in many existing standard wireless channels, making rate allocation with feedback a problem of considerable practical importance. We address the question of rate allocation between the source and channel codes in the forward channel, in the presence of feedback information and under a distortion cost function. We show that the presence of feedback shifts the optimal rate allocation point, resulting in higher rates for error-correcting codes and smaller overall distortion. Simulations on both memoryless and fading channels show that the presence of feedback allows up to 1 dB improvement in PSNR compared to the similarly optimized feedforward scheme.