Optimum detection of differentially-encoded MPSK in a dispersive aeronautical channel

Several different waveforms have been proposed for use in aeronautical data links in the next-generation National Airspace System (NAS). Reliability of the links, expressed in terms of the assurance of a very low probability of message error, is crucial. Hence, optimal detection schemes are of interest. The performance of the proposed waveforms in multipath channels has not been thoroughly studied to date, and this work aims to begin filling the gap for a specific candidate modulation scheme: DPSK. This modulation scheme is the one that will be used for two of the modes of the VHF digital link family. The use of multiple symbol differential detection has long been known to improve the performance of differentially-encoded PSK on the additive white Gaussian noise (AWGN) channel, but its performance has not been thoroughly studied on dispersive channels. Related work has addressed the more complicated case of Rayleigh fading for generally very dispersive channels. The aeronautical channel typically lies between the AWGN and these "random" dispersive channels in that the rate of change of the channel is slow and essentially deterministic, and the dispersion is typically limited to two multipath "rays". Optimum noncoherent detection rules for PSK have not been derived for this moderately difficult but important special ease. Using a simple channel model for the aeronautical environment at VHF, we re-visit the derivation of the optimum sequence metric for this dispersive channel, and obtain a new sequence metric for use in detection. Performance of the new detectors is compared via simulations.