Intermodulation Effects in the Transmission of Voice-Activated SCPC/FM Carriers Through a Nonlinear Repeater

When several voice-activated SCPC/FM carriers are transmitted through a nonlinear satellite repeater, a large number of intermodulation products appear at the output. An analysis of the baseband effect of these products is presented, taking into account the random on-off pattern followed by each carrier, the random nature of the level of each modulating speech signal, and a precise definition of the carrier power spectral density which is conditioned on the fact that the carrier is on, assuming that this is the circumstance under which we want to examine interference effects on any particular carrier. The analysis shows that computing intermodulation baseband noise, simply by introducing a penalty factor into the noise that would result by treating intermodulation as if it were thermal noise, may lead to very conservative estimates of intermodulation effects. The paper is restricted to the situation in which all carriers have equal amplitudes, but the general analysis is still valid when this is not so. The extension is straightforward except for the difficulties in counting the number of products of each type.