Postdetection diversity is attractive for narrowband digital FM signal reception because a cophasing function that may be difficult to realize in a fast Rayleigh fading environment, is not required. The combining scheme evaluated in this paper weighs each frequency detector (FD) output in proportion to the vth power of the received signal envelope of that branch. Maximum diversity improvement can be obtained when v = 2 (this combiner is referred to as postdetection maximal ratio combiner (MRC)in this paper). This paper presents experimental results of postdetection diversity reception in the Gaussian minimum shift keying (GMSK) signal transmission system. Diversity combining and FD-decision algorithms (decision feedback equalizer (DFE) and maximum likelihood sequence estimator (MLSE)) are performed by software on a computer using the data of the sampled FD output and received signal envelope obtained from a laboratory transmission system. It will be shown that the MRC can attain about a 1-dB larger diversity gain than the selection combiner (SC) when two-branch diversity is used. The degradations of two-branch diversity improvement caused by the differences between FD sensitivities and between received signal envelope detector gains are evaluated.
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