Maximum likelihood combining for noncoherent and differentially coherent frequency-hopping multiple-access systems

The frequency-hopping multiple-access (FHMA) approach to digital mobile radio provides diversity gain against frequency selective fading. Optimum combining methods, in the maximum likelihood sense, are derived for FHMA systems with either noncoherent or differentially coherent detection. In the optimum combining structure, the detector outputs are first passed through a nonlinearity and then a linear combiner.