Effects of multipath interference on an FM data subcarrier

There is renewed interest in utilizing the upper portion of the baseband spectrum of an FM broadcast channel for a digital data service. This part of the baseband spectrum was previously unused by the audio signal. In the presence of multipath interference, the instantaneous frequency of the received FM signal is a nonlinear combination of the instantaneous frequency of the transmitted signal and its time delayed versions. Due to this nonlinearity, the audio and data signals are no longer confined to separate parts of the baseband spectrum. Correct reception of the data signal is then limited by the effects of multipath interference from the audio signals. In this paper we formulate the multipath interference problem, and present some preliminary analysis and simulation results.