Convergence behavior of iterative source-channel decoding

Whenever digital communication is subject to transmission errors, the utilization of reliability information at the receiver is beneficial. Soft-channel decoding techniques improve the error correcting capabilities and in addition, softbit source decoding performs error concealment. While the reliability gain of channel decoding is based on artificial diversities explicitly introduced at the transmitter, softbit source decoding exploits implicit redundancies remaining in the bitstream after state-of-the-art source encoding. In an iterative source-channel decoding scheme the reliability gains of both are exchanged iteratively in order to enhance the common error resistance. However, first investigations have shown that the number of profitable iterations is limited to small values. In this paper, the convergence behavior is analyzed using a modern tool called EXIT charts. EXIT charts permit us to predict the convergence behavior by studying the individual components. As a novelty, EXIT characteristics are specified for softbit source decoding. The convergence analysis is confirmed by simulation.