Nonlinear Distortion Analysis of Multiport Amplifier

Abstract

This paper presents an analysis of nonlinear distortion generated in Multi-Port-Amplifier (MPA) systems that have attracted considerable attention for their potential use in multi-beam mobile satellite communications. The MPA configuration is a special one composed of a power divider, a power combiner and an array of high power amplifiers (HPAs). The HPA nonlinear characteristics degrade the MPA linearity. Intermodulation (IM) products generated by this nonlinearity cause interference with signals and degrade the signal transmission performance. We investigated the dominant third-order IM (IM3) performance of an MPA under HPA nonlinear operations by taking the specific properties of the MPA configuration into account and showed that it was possible to improve the carrier level to a third-order IM (C/IM3) ratio by properly assigning multiple carriers to multiple MPA ports without adding any additional circuits. Based on these results, we proposed an amplifier system immune to IM3 interference and a frequency assignment method effective in improving the C/IM3 ratio in an MPA system. The analysis and our proposed system will help make it possible to construct a power-efficient satellite system with a simple configuration not only for multibeam mobile communications but also for other uses.