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A digital FM system in the presence of simulcast interference and/or Rayleigh fading exhibits. performance floors, where increased signal strength does not reduce the error rate. The mechanism which generates FM clicks even in the region of high SNRs, in simulcast as well as in multipath, is suggested. It is shown that a simple model based on two unmodulated carriers is capable of not only qualitatively explaining the phenomenon, but also of providing quantitative estimates which match well with experimental results obtained both in laboratory and in field tests.The observed "equivalency" of the Doppler spread and the frequency offset in their effect on BER has been given an analytic form. Analysis applies to a narrowband M-FSK systems with FM discriminator and post-detection fdter.

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