Offset frequency allocation to reduce intermodulation noise

Abstract

In SCPC satellite systems intermodulation (IM) noise is generated due to transponder nonlinearity. It is known that this noise can be reduced by proper signal frequency allocation. In the conventional method the signal frequencies are allocated at integral multiples of a basic interval, that is, the minimum signal frequency separation. This paper is concerned with offset frequency allocation in which the signal frequencies are not necessarily integral multiples of a basic interval. Specifically, the IM noise power spectrum is analyzed for 2‐phase PSK signals and studies are made for the IM power reduction characteristics after passing the receiving filter, specific signal frequency allocations with reduced IM noise and IM noise improvement over equal‐interval frequency allocation. Also, starting with the Babcock allocation, offset frequency allocations are derived and their IM characteristics and frequency bandwidth are evaluated.