Reverse Intermodulation in Multi-Tone Array Transmitters

The modern spectral, and thereby linearity, requirements force 5G phased-array transmitter systems to operate at reduced power efficiency, as they can no longer use voluminous filtering. To reduce the linearity requirements of the transmitter, we consider the case of an array consisting of closely spaced radiating elements operating at different frequencies. The coupled tones from one element to another create reverse intermodulation distortion (RIMD). We explain how RIMD is created within a power amplifier (PA), and derive an estimate for the power of the RIMD components. We provide a set of measurements for an X-Band GaAs PA and draw a direct comparison between RIMD and IMD. We show that RIMD has a third-order behaviour up to very high reverse power levels, opening up the perspective for higher output power operation as well as simpler and lower-power predistortion in multi-tone array systems such as 5G and radar.