Diversity Schemes and Coherent Combining in Digital Receivers

This paper is focused on the aspects of diversity schemes and reception methods that were rare or never discussed in the technical literature. It clarifies ambiguities and corrects inaccuracies related to them, examines the influence of digital technology, and analyzes the theory and realization of maximum ratio combining (MRC) as well as related methods in digital radios, some of which are software defined and cognitive ones (SDRs and CRs). Among other subjects, it identifies areas where spread-spectrum (SS) signals are preferable in diversity schemes; explains similarity and differences between MRC in space diversity and adaptive beamforming and nulling as well as between MRC and optimal linear filtering; present examples of MRC realization; shows that classical MRC algorithm has to be generalized when the estimation of characteristics of signals and non-white noise is required; and describes joint use of MRC and optimal linear filtering for the reception of SS signals.