By analytical derivation, supported by simulation, the authors develop a closed form theory of the error floor probability of differentially detected MSK in a slowly fading Rayleigh channel with small delay spread. As the major error source they identify the phase jitter generated by group delay bursts coinciding with the frequency deviation of the modulated signal, rather than the timing jitter following the instantaneous mean delay. Such errors turn up in the fades even when the channel is short-term invariant and the receiver is stationary. For experimental verification they apply controlled delay spreads (over the air) in a DECT-like test bed. The results confirm that the error probability is, to first order, a function of the delay spread to bit-length ratio only and varies with the square of this ratio.
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