Switched-diversity FSK in frequency-selective Rayleigh fading

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Abstract
This paper presents the measured performance of a 64 kbit/sec switched-diversity FSK receiver subjected to simulated frequency-selective Rayleigh fading. The single receiver input is switched between 2, 3 or 4 uncorrelated Rayleigh-fading signals whenever the instantaneous receiver output falls below a threshold. The optimum level of this threshold relative to the mean signal level is relatively insensitive to frequency selectivity and vehicle speed. A nearly-optimum threshold may be determined using an AGC amplifier and fixed comparator. Two-branch switched diversity can achieve 10-2BER with 6 dB lower SNR than that needed without diversity. In the absence of frequency selectivity 2-branch switched diversity can perform within 3 dB SNR of maximal-ratio diversity and within 1 dB of selection diversity. Switched diversity can achieve BERs lower than the irreducible single-channel BER produced by frequency selectivity. Four-branch switched diversity typically requires 4 dB lower SNR to achieve a given BER than does 2-branch switched diversity.