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Passive Intermodulation (PIM): DAS & Signal Booster RF Interference

Dec 28, 2017

Passive Intermodulation (PIM): DAS & Signal Booster RF Interference

Interference becomes a real challenge when 2G and 3G wireless network infrastructures are overlaid with 4G LTE networks. Although PIM has always been an issue for RF communications involving more than one frequency, LTE is more sensitive to these effects. Cell phone signal booster systems and passive or active distributd antenna systems (DAS) are particularly vulnerable to this interference while they try to amplify available weak signals.

PIM is the interference that results from mixing frequencies in a passive circuit in a nonlinear fashion. Network throughput and performance can be crippled if the interference is the same as the network’s base receiving frequencies. PIM can be generated by water infiltrating transmission equipment, damaged cables or poor connections. It could also be caused by objects outside the signal path, including buried conduit, light posts, site equipment or fences.

Increased PIM levels are a significant risk to LTE network's profitability and operational efficiency. At high levels, receive channels can be overpowered by PIM from several transmit signals. In this case, base stations could easily interpret the signal distortion as a channel that is in use and refuse to assign that channel. This will cause the system to lose channel capacity, reducing both airtime and revenue. Extremely low levels of PIM can severely degrade system performance due to the highly sensitive nature of modern equipment. If an uplink's sensitivity drops by as little as 1dB due to PIM, it could reduce coverage by as much as 11%.

The first deployments where PIM issues had to be resolved were outdoor macro sites. Reliable, high data throughputs are even more important in DAS systems, as many components in the RF path could generate PIM. With passive components including directional couplers, hybrid couplers and splitters are placed close to signal sources in those systems, it is crucial for the PIM specification for the devices be as high as possible.

As PIM levels are very sensitive to surroundings and test equipment, conducting accurate PIM testing in the field is extremely difficult. Metal objects that are present close to the device under test, and using worn test adapters can both increase PIM, which would result in inaccurate readings being taken.

Seek to ensure connectors, cables and other low-PIM passive components installed are periodically inspected to avoid such effects as corrosion. As the component deteriorates without proper inspection PIM will be elevated.

The most important thing is to stick to low-PIM interior antennas, low-PIM RF Splitters, as well as cables, connectors, couplers and other passive components under our Premium Accessories category. Low-PIM solutions will eliminate the costly requirement of upgrading these components, ensuring your cell phone signal is never interrupted by high-PIM effects.

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