Higher-order differential modulation techniques

We consider differential phase modulation techniques to be used over channels in which frequency uncertainties are the major impairment to performance rather than additive white Gaussian noise (AWGN). For example, in low Earth orbiting (LEO) satellite communication systems, both Doppler and Doppler rate terms contribute to a phase process difference between the received signal and the receiver's local reference signal. We consider techniques that mitigate the harmful effects of frequency uncertainty without the need for automatic phase or frequency control loops. Specifically, we consider n-th order nonadjacent M-ary differential phase shift keying (NA-MD/sup n/PSK), in which the data are encoded into the signal phase as a nonadjacent n-th order difference of the transmitted phase angles. We show that NA-MD/sup n/PSK signals can be acquired in 2/sup n/ symbol times. We present the performance of NA-MD/sup n/PSK signals in the presence of both frequency uncertainty and AWGN. We also present simulation results to show that the performance can be improved with signal processing at the receiver.