Polyspectral techniques for nonlinear system modeling and distortion compensation

The major approach used to address the power issue in communication equipment is to nonlinearly operate transmitter and transponder high-power amplifiers (HPAs), whether solid-state power amplifiers (SSPAs) or traveling-wave tube amplifiers (TWTAs). This, however, leads to unwanted distortion that must itself be compensated. The computer-aided design (CAD) and evaluation of such systems is crucial to their expedient and successful development. A polyspectral approach is described here using an accurate baseband time-domain measurement system (BTDMS). The basic polyspectral model architecture consists of parallel signal paths, one linear, and the remainder nonlinear with memory, consisting of either a filter-nonlinearity (FN) or nonlinearity-filter (NF) configuration, with each path representing a term in the operator series. A complex baseband variant of the NF-type model is described that is especially suitable for application to HPAs. An example is given using a 20-GHz helix development TWTA, with a bandwidth in the order of 6 GHz, and the results are discussed.