A simulation approach to relating S/D to C/I for multiple FM voice signals through a nonlinearity

This paper describes the derivation of a numerical relation between carrier-to-intermodulation noise ratio (C/I) and signal-to-distortion ratio (S/D) of a voice signal when a representative signal of 20 FM carriers is input to a nonlinear amplifier. Three different transfer characteristics for the amplifier are assumed. Values of S/D and C/I for a 4-carrier input to one amplifier are also given. For the cases considered, intermodulation noise is about 1 to 3 dB worse than thermal noise. The numerical results show that there is a threshold effect due to intermodulation noise. Also, the results show that the signal quality, S/D, is not very sensitive to the particular type of nonlinear amplifier but depends mainly on the value of C/I. Numerical values of S/D for a given amount of co-channel interference due to a single interferer are also derived. These numerical results should be useful for the design of multiple FM voice satellite communications systems.