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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.

Do you have any comments on this topic? Please share your thoughts below for other readers to read. Thank you.

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  • I see you mention 2G and 3G networks here. I know this article is older and I don’t even know if 2G and 3G networks are around so my question is whether passive intermodulation is a problem with LTE anymore.

    Kelly Heinz on
  • Two good questions here. Will it help and will it increase electromagnetic radiation. The best way to answer question one is to ask how often you have trouble hearing the person on the other end (or vice versa) and how often you get dropped calls. If it happens a lot, you may want one. The second question is like anything dealing with science. Do your research. I’ve looked into cell phone radiation and from what I’ve found, a weak signal makes your phone work harder, which leads to more radiation coming out.

    Jerome Keating on
  • How do I boost my cell phone signal at home? There are different ways I’ve read about on the blogs here (which I recommend you check out because this site has some excellent and helpful information). First, do a map of your home to see where you have weak and strong signals. Second, find out how far away the closest cell phone tower is. No doubt, you will find spots in your home that get weaker signals than others or even no signals (often in the basement). Check your phone’s battery too because a weak battery usually leads to poor call quality and it makes your phone work harder to get a good signal. Okay, you’re probably saying, what does this all mean? It means you can find areas of your home that are best to make calls in. There are also tricks to getting a better signal like switching to airplane mode then back to normal. In the long run, your best bet is to get a cell phone booster so you have strong signals throughout your home (even in the worst spots). As long as there’s a signal, your phone will get a boost and you should be amazed by the results.

    Kelly Tyler on
  • This is a nice technical article but I’m thinking a little closer to home (actually home). How do I boost my cell phone signal at home?

    Jonathon Creely on
  • Passive intermodulation interference in communication systems is apparently nothing to take lightly. Communication systems have become so commonplace that we take them for granted, but they often are delicately set up, which means you have to be aware of the small and large things that can interfere with them. While there can be problems, this blog shows how minor accessories like the proper cables and connectors can help.

    Jules Hoffman on
  • Good and understandable explanation of what is PIM RF. I know cell phone boosters are an excellent tool for improving your cell phone’s power, but I didn’t know there are issues with different networks 2G, 3G, etc. when they are overlaid with 4G’s. It sounds like passive interference can make things miserable, but it’s good to know there are fixes to the problem such as the accessories mentioned in your blog.

    Pedro Lopez on

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