Passive intermodulation (PIM)
What is passive intermodulation (PIM)? PIM is the generation of interfering signals caused by non-linearities in the passive components of a wireless system. Two signals mix together, in what is known as amplitude modulation, to produce different signals. Ultimately these new signals can reside within the same band which creates interference. With PIM, and when the signals are wideband, the noise level or the noise floor is raised which can block desired signals and reduce receiver sensitivity. This can lead to issues such as drop calls, decreased system capacity and decreased data rates.
There are two types of PIM. Internal PIM is caused by the internal radio frequency (RF) elements in the radiating infrastructure, such as loose connectors, damaged cables and connectors, and faulty elements in the antennas. These issues typically occur between the transmitter and the antenna. Since internal PIM issues reside at the radio itself, a tower crew must climb to the top of the cell tower to perform troubleshooting. This can be extremely costly and time consuming. External PIM differs from internal PIM as the former is caused by objects located near cell sites. Examples include metallic objects (usually rusty) close to the antenna. With external PIM, technicians must hunt down and pinpoint the source (or sources) causing the interference.
Troubleshooting either internal or external PIM can be an expensive, manual process. When the issue involves internal PIM, a costly tower climb is usually required. Since there’s no guarantee the issue will be solved with an initial tower climb, a second climb may be required. In addition to being expensive, the entire process of finding and resolving an internal PIM issue can take weeks to complete. This is due in part to the required analysis, which is typically a 90% manual effort.
When PIM is external, the process is similar to RF interference hunting, where a technician is dispatched onsite and uses an RF spectrum analyzer to pinpoint the source. The process of interference hunting is highly manual and typically requires expertise in the area of RF.
Infographic: RF spectrum in today’s 5G world
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