Passive intermodulation (PIM) is interference that is generated when two or more simultaneous signals interact in a nonlinear environment, where passive components (as opposed to transmitting ones) can cause new, unwanted signal noise to be generated that impacts the performance of the RF system.
Why is PIM taking on a new life in 5G? There are a number of reasons, explains Tom Bell. Bell is senior director of interference products at ConcealFab, which has been working since 2016 specifically on developing methods to identify and mitigate passive intermodulation. Those reasons include:
1) New midband spectrum that is designated for 5G deployments around the world is TDD spectrum, meaning that timing and synchronization is crucial to network performance. If neighboring (particularly co-located) cell transmissions are out of sync and “step on” one another, this results in intermodulation interference, says Bell. “If you don’t synchronize properly, you are absolutely dropping a third-order intermod directly into the adjacent channel,” said Bell. “So synchronization is absolutely crucial.”
Simply adding new spectrum bands—in this case, many spectrum bands, including low-, mid- and high-band airwaves—increases the chance of second- and third-order signals that will fall somewhere in spectrum actively being used at a site. In North America, Bell adds, there is also already PIM interference happening due to lower frequencies interacting and creating second- and third-order intermodulation products in the 3.7 GHz uplink.
2) Macro deployments will generally see more PIM issues. With a new network technology deployment, carriers focus first on gaining as much coverage as possible. This means a focus on macro-site deployments at full power. PIM is highly correlated with power levels. While small cells may also have PIM problems, the issue is more significant at macro sites (the first and easiest sites that are sought for coverage) because of the higher power levels.
3) New deployments can mean newly noticeable PIM. There’s a bit of an “if a tree falls in a forest” situation with PIM interference, where the severity of the problem isn’t necessarily evident until a new network is up and operating. Noise may be generated in the band(s) at issue, but it may not be obvious and impact performance until there is a turned-up network with active devices—because another characteristic of PIM is that as traffic increases, so does the level of interference.
4) Deployment activities themselves are likely to generate PIM. Every time a human touches a tower, they may do something that increases the risk of PIM: Bumping or knocking something loose, adding new metal to the site, leaving behind metal trash that could generate PIM. With the scale and speed being demanded for 5G deployment, this increases both site activity and the possibility of installation mistakes that can result in PIM.
Looking for more information on PIM and mitigating it in 5G systems? Download the new RCR Wireless News editorial report, available here.
