A New Iterative Decoder for Turbo Codes on the Nonlinear Channel with Non-uniform 16QAM Modulation

In conventional bit interleaved coded modulation with iterative decoding (BICM - ID), a turbo decoder is employed where extrinsic information of the data bits is exchanged between two soft input soft output (SISO) devices. For a fixed size constellation, the probability distribution and distance among signal symbols are usually assumed to be uniform. Performance of BICM-ID is determined by the code structure, the signal mapping and the iteration scheduling between the soft-demapper and the decoder. In this paper, we propose a new scheme for the turbo receiver where an extra soft-demapper is placed between the two SISO decoders. Extrinsic information is exchanged for both data and parity bits. Also, a new non-uniform probability distribution technique for 16QAM is used with this new iterative decoding structure. We simulated a parallel concatenated convolutional code (PCCC) scheme with 16QAM modulation. Over a linear channel, this new iterative decoding scheme with non-uniform 16QAM offers a gain of 0.9 dB at a bit error rate (BER) of 10(exp -5). On a nonlinear satellite channel, we observed a gain of 1.2 dB compared to uniform 16QAM, also at a BER of 10(exp -5).