External PIM Modeling in FDD MIMO

In this paper, a physical approach is proposed to describe external (air-induced) passive intermodulation (PIM) for a frequency-division duplexing (FDD) multiple-input multiple-output (MIMO) system with an arbitrary number of transceiver chains. It is caused by metallic objects with nonlinear characteristics (“rusty bolt” effect) located near the base station (fence, billboard). PIM penetrates the antenna receiving chain and degrades the uplink signal. Generally, PIM modeling in MIMO systems represents a complicated task that traditionally requires numerous hardware experiments in numerous environment setups. We present a novel approach for replacing experiments with full-cycle physical modeling of external PIM in the MIMO system. The proposed model includes all steps starting from the generation of transmitting signals through the electromagnetic modeling of the antenna array and open space to a set of variable nonlinear sources and back to the receivers. Its performance was validated with synthetic signals equivalent to the measured data in the literature. Thus, this work contributes to more efficient PIM generation by decreasing the number of hardware experiments while increasing the diversity of PIM scenarios, especially for large MIMO systems and multiple PIM sources of any nature.