PIM is a form of signal interference. It is becoming more common as wireless networks become increasingly complex with multiple technologies at a single site. The signals combine to generate undesired interference, which impacts the LTE uplink signal. Antennas, cables, and dirty or loose connectors can be sources of PIM, and so can damaged RF equipment or metal objects near or at a distance from the cell site. Here is an infograph from API Technologies that explains the importance of PIM.
