MAP equalization for DQPSK in multi-pass demodulation

This paper investigates the effect of pilot bits on the performance of a MAP equalizer for DQPSK modulation. This study pertains to IS-136 and the ACELP vocoder error control coding scheme. We present analysis and simulation results that explain the impact of pilot bits in detail. In multi-pass demodulation (MPD), a number of re-encoded bits are available as pilot bits to a 2nd pass demodulator. Previous studies indicate that pilots helps both the channel tracking process and the bit detection process. Here, we assume perfect knowledge of the channel, and we concentrate on the effect of pilots on the decision process. Specifically, we use a MAP equalizer, which is the optimal bit-by-bit decision process. We find that the Class 2 bit error rate (BER) with pilots improves by about 1.5 dB over the baseline with no pilots. This is the case in a non-dispersive channel as well as in a dispersive channel. Since the channel is known, this gain is fully attributable to the decision process. The main application is to improve the performance of IS-136 terminals using MPD