On the detection of a class of fast frequency-hopped multiple access signals

The capacity of a fast frequency-hopped multiple access (FHMA) system is dictated by two major related design concerns: the hoping pattern and the receiver structure. This paper studies the impact of these two factors. We present a maximum likelihood (ML) diversity combiner for detecting asynchronous FHMA multilevel FSK (MFSK) signals in Rician fading channels and analyze the performance of a close approximation of the ML receiver. We compare systems using random hopping patterns and those using optimal hopping patterns of Einarsson. Performance comparisons between chip-synchronous and chip-asynchronous systems are made as well. We propose and examine the effectiveness of a two-stage multiuser detector, in which the first stage makes an initial decision while the second stage tries to reduce multiple access interference (MAI) and resolve the ambiguity left by the first stage detector. The MAI caused by undesired users is constituted by a cochannel interference (CCI) contribution and an interchannel interference (ICI) contribution. This detector is of modest complexity and is capable of removing most of the CCI and part of the ICI. Two methods for mitigating MAI are also examined.