  In this paper, the physical approach to model external (air-induced) passive
intermodulation (PIM) is presented in a frequency-division duplexing (FDD)
multiple-input multiple-output (MIMO) system with an arbitrary number of
transceiver chains. The external PIM is a special case of intermodulation
distortion (IMD), mainly generated by metallic objects possessing nonlinear
properties ("rusty bolt" effect). Typically, such sources are located in the
near-field or transition region of the antenna array. PIM products may fall
into the receiver band of the FDD system, negatively affecting the uplink
signal. In contrast to other works, this one directly simulates the physical
external PIM. The system includes models of a point-source external PIM, a
finite-length dipole antenna, a MIMO antenna array, and a baseband multicarrier
5G NR OFDM signal. The Channel coefficients method for multi-PIM-source
compensation is replicated to verify the proposed external PIM modelling
approach. Simulation results of artificially generated PIM cancellation show
similar performance as real-life experiments. Therefore, the proposed approach
allows testing PIM compensation algorithms on large systems with many antennas
and arbitrary array structures. This eliminates the need for experiments with
real hardware at the development stage of the PIM cancellation algorithm.
