Analysis and simulation of multipath interference of FM subcarrier digital signals

We present analytical results characterizing the effects of multipath interference on an FM subcarrier data channel. A closed form expression for the FM discriminator output (or the instantaneous received frequency) is obtained when there are M reflected path signals in addition to the primary path signal. We derive the power spectral density when the multipath interference is weak, and determine the conditions under which multipath interference causes nonlinearities in the instantaneous frequency of the received signal. In order to verify our analytical results, we have built an SPW simulator of HSDS (the high speed data service) system, and we have observed close agreement between our analysis and simulations. The analytical techniques presented could potentially be applied to other FM subcarrier systems, as well as to digital wireless systems that adopt FSK or its variants. Hence, our results could be useful in understanding noncoherent reception of the GSM digital cellular system, for example