Soft-output MLSE for IS-136 TDMA

An inner estimator for concatenated maximum a posteriori decoding of convolutionally encoded DQPSK affected by time- and frequency-selective fading is derived and applied to IS-136 TDMA. The estimator is asymptotically optimal in the sense that the generated soft-decision metric constitutes a sufficient statistic for maximum a posteriori decoding of the convolutionally encoded data in the limit as SNR→∞. By imposing a fixed-delay constraint, the estimator may be implemented as a straightforward generalization of the Viterbi algorithm (VA). The simulation results confirm that gains of over 4 dB with respect to the hard-decision case can be obtained at high speeds and with significant delay spread