Iterative demapping and decoding for multilevel modulation

Iterative decoding algorithms for spectrally efficient modulation have become a vital field of research in digital communications. We show how iterative demapping reduces the bit error rate in conventional communication systems applying multilevel modulation combined with simple channel coding. For this the soft demapping has to be modified in order to accept a priori information. The system can be regarded as a serially concatenated iterative decoding scheme whereby the inner decoder is replaced by the soft demapping device. A design rule based on mutual information to find the 'best' mapping is proposed. Numerical results are presented for 4-PAM and 16-QAM examples.