The Effect of Frequency Selective Fading on Intermodulation Distortion and Subcarrier Phase Stability in Frequency Modulation Systems

Time varying multipath channels produce a form of fading known as frequency selective fading. The presence of frequency selective fading on the transmission channel causes intermodulation distortion to appear at the base-band output of frequency modulation systems. In addition this frequency selective fading causes phase and amplitude fluctuations to appear on demodulated subcarriers. This paper derives for the first time probability density functions and distribution functions for intermodulation distortion and subcarrier phase fluctuations as a function of the delay power density spectrum of the channel. The derivations involve the use of a canonical channel model called the power series model which is particularly useful when the transmitted signal bandwidth is less than the correlation bandwidth of the channel. Another novel feature of the analysis is the inclusion of the effects of diversity combining.