Error floors of digital FM in simulcast and Rayleigh fading

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. Results obtained both in the lab, using a radio channel simulator, and in the field show this performance. Observation of signal waveforms indicates that the bit-error rate (BER) floor is a result of FM clicks during deep fades. We present a mathematical model that can be used to calculate the BER floors. The model is based on the calculation of the click shape as a function of amplitude imbalance, frequency offset, and postdetection filter impulse response. The Rayleigh fading is approximated with a two-signal simulcast by substituting the RMS Doppler spread for the simulcast frequency offset. A good match between calculated and measured results is obtained. It is found that the symbol errors caused by simulcast and Rayleigh fading can be treated as independent, and their effect is cumulative.