DPSK vs. pilot-aided PSK MAP equalization for fast-fading channels

Abstract

We extend our recent work in maximum a posteriori (MAP) equalization for frequency-selective fast-fading channels to the case of differentially encoded transmitted symbols. The equalizer employs an expanded trellis for the purpose of joint channel estimation and equalization. Channel estimation is performed using minimum mean square error techniques coupled with the hypothesis trellis of the MAP algorithm. Differential encoding avoids phase ambiguities associated with coherent detection of phase-shift keying (PSK) symbols. A MAP equalizer employed in an absolutely encoded PSK system avoids coherent detection, but requires additional pilot symbols in order to resolve the phase ambiguity. The pilot symbols result in lower power and bandwidth efficiencies. This paper concludes that a pilot-aided absolutely encoded binary PSK system has superior BER performance, even after the penalty in signal to noise ratio (SNR) has been take into account. However, at low SNR, the benefit in BER is not as significant, and higher pilot symbol rates are required. Thus DPSK is competitive only at low SNR.