Interference mitigation with partially coherent demodulation in a slow frequency-hopping spread-spectrum system

Iterative signal processing has gained in popularity over the last decade as a way of reducing the effects of interference in a communication system. In previous work, the expectation maximization (EM) algorithm was used to estimate the parameters and data of colliding transmissions in a slow frequency-hopping spread-spectrum (FHSS) system using binary frequency-shift keying (BFSK). As in many FHSS systems, noncoherent demodulation was used. In this paper, we propose a refinement to the EM algorithm to allow the phase estimator in each iteration to be characterized as a Tikhonov random variable. We then use partially coherent detection for the desired user's signal. The parameters of the Tikhonov distribution are refined at each iteration, which results in the demodulator transitioning from fully noncoherent operation in the first iteration to nearly coherent detection in later iterations for many dwell intervals. We show that this approach significantly improves the multiple-access capability of the system.