When two or more signals of different frequencies pass through a nonlinear system,
intermodulation distortion (IMD) occurs, resulting in the formation of spurious distortion signals.
IMD is most commonly found in active circuits of a radio system, but it can also be found in passive
wireless components such as lters, transmission lines, connectors, antennas, attenuators, and so
on, especially when transmit power is quite high. Passive intermodulation (PIM) distortion is the
name given to the IMD in the latter scenario. With the evolution of radio systems and the scarcity
of radio spectrum, PIM interference is being recognized as a potential stumbling block to a radio
network's maximum capacity.
This article classifies the PIM sources in BS radio systems into two categories, internal and
external sources. Internal sources are the radio's passive components such lters, transmission lines,
connections, antennas, and so on. External sources, on the other hand, are passive items that are
located outside of the BS antenna but inside the RF signal path, such as metallic and rusted objects
in the antenna near eld. The high power current flowing through such passive devices can cause
nonlinear behavior, resulting in IMD for both types of sources. Also, a review of PIM mitigation
techniques is presented in the article.