Performance of a turbo coded system with DPSK modulation using enhanced decoding metrics and matched channel side information

The classical turbo decoding algorithm is based on the assumption that the input sequence to the decoder has been disturbed by a noise process that has a Gaussian distribution. For a DPSK waveform, the output statistics of a differentially coherent demodulator is nonGaussian. When this sequence is fed directly into the turbo decoder, particularly under a fading environment, the mismatch between decoder algorithm assumptions and actual input statistics can result in significant decoder performance degradation. This paper investigates the bit error performance for such a mismatch for the DPSK waveform. Techniques for forming enhanced turbo-decoding metrics with biased channel side information (CSI) are proposed. Using these new metrics, error performance in fading conditions is shown to improve significantly.