PIM lowers the reliability, capacity and data rate of cellular systems. It does this by limiting the receive sensitivity. In the past, RF engineers could select channel frequencies that would not produce PIM in the desired receive bands. However, as cellular usage grows, the licensed spectrum has become crowded. Engineers must often select less desirable RF carrier frequencies and accept potential PIM issues. Compounding this problem, existing antenna systems and infrastructure are aging, making any PIM that does occur stronger.
When PIM products fall within the receive band of a cell site radio, they make the receiver less sensitive to weak signals that limits receive coverage. This increases the bit-error-rate (BER)and creates more dropped calls. If the connection is for data, interference from PIM creates more error protection bits and resends, which causes a lower overall data rate. In some cases, PIM can even cause receiver blocking, shutting down the sector.
Signs of PIM problems include receive-noise-floor-diversity-imbalance and high noise floors. Other signs include shorter average call duration, higher dropped call rates, lower data rates, and lower call volume.
Unrepaired PIM problems can cause the base transceiver station (BTS) to assume a certain Rx noise level exists during calibration periods, causing gross Tx power and Rx gain figures to be utilized. These figures carry over into heavy traffic periods and then the BTS exhibits poor statistical performance. Any increase in the noise floor at the receiver input causes a decrease in the dynamic range of that receiver. If the Rx sensitivity is –107 dBm, but the real noise floor is –97 dBm, the call or connection will need to be terminated or handed off to another site/sector 10 dB earlier than the system would normally be designed to do. If 1 dB in Rx level was equal to 0.5 miles, this connection would hand off 5 miles early, in terms of distance. Sectors that do not always have a lot of margin between sites will end up with dead zones where the call will be lost. A PIM test and rectification of faults will restore the original performance that is required.
https://www.anritsu.com/en-US/test-measurement/technologies/pim