Coherent MAP detection of DQPSK signals in non-ISI channels

Both IS-136 and PDC digital cellular systems employ forward error correction (FEC) encoding followed by a form of DQPSK modulation. To optimize FEC decoder performance at the receiver, soft information from the DQPSK demodulator is required. For frequency nonselective fading channels, which occur often in these systems, coherent demodulation optimizes performance. In this paper, we derive the maximum a posteriori (MAP) bit estimate for DQPSK modulation in non-ISI channels, assuming knowledge of the channel response. The MAP estimator forms a bit log-likelihood ratio, which provides the optimal "soft information" for MLSE or MAP convolutional decoding. Both single and multiantenna receivers are considered. MAP estimation requires exponentiation and logarithm operations, as well as knowledge of the noise covariance. To reduce complexity, two approximate forms are developed which avoid the exponentiation and logarithm operations. If the noise has the same power on each antenna and is uncorrelated between antennas, then knowledge of the noise covariance is also not needed. Performance of the optimal and approximate schemes is evaluated via simulation for single and multi-channel receivers under static and frequency non-selective Rayleigh fading conditions. The approximate approaches are shown to be within 0.5 dB of the optimal approach and to outperform existing approaches.