On the achievable extrinsic information of inner decoders in serial concatenation

In this paper we address the extrinsic information transfer functions of inner decoders for a serially concatenated coding scheme. For the case of an AWGN channel, we give a universal proof for the fact that only inner encoders yielding an infinite output weight for a weight-one input sequence, such as recursive convolutional encoders, lead to perfect extrinsic information at the output of the corresponding SISO decoder. As an example we consider bit-interleaved coded modulation with iterative demapping (BICM-ID) and insert an additional recursive precoder prior to the mapping operation. Simulation results show that the proposed system does not suffer from an error floor and thus significantly outperforms BICM-ID systems that solely use mappings as inner encodings, even when they are optimized