Discover the science behind Passive Intermodulation (PIM) 

Also known as the “rusty bolt effect,” passive intermodulation (PIM) is a serious issue in the wireless industry, and one that has been growing over time. Constant installation of new equipment along with higher density of modulation symbols compound into difficulties caused by PIM. Explore the science of PIM to take a closer look and discover why it matters, and what you can do to address it.

Why is External PIM a challenge?

Passive Intermodulation (PIM) can have a major impact on network performance and capacity. And while PIM has been known in the wireless industry for a long time, its effect on limiting network performance has been increasingly severe, and now it has become a pressing issue.

The growing demand for wireless communication has increased spectrum utilization, which in turn has created new challenges in signal interference and noise – including additional opportunities for higher order PIM (and Harmonics) to interfere with the operation of the new bands. These new challenges need new solutions to effectively address them. 

What exactly is PIM?

PIM is a form of intermodulation distortion (IMD), which in its most basic form is an interaction between two or more simultaneous signals at different frequencies, interacting in a nonlinear environment. When these signals pass through a non-linear system or device, they generate numerous new, undesired signals at frequencies determined by the original signal frequencies (see below).

IMD can be broken down into classes: Active and Passive Intermodulation. Active Intermodulation (IM) is generated by active (powered) devices in a system, such as tower-mounted amplifiers, receivers, and transmitters. Active IM tends to be associated with design issues and is fairly easy to identify and correct within the system.

In contrast, PIM occurs in passive, unpowered components that would otherwise be expected to operate in a linear fashion – that is, their output current should be directly proportional to their input voltage – such as cables, connectors, and fasteners. However, when two or more frequencies mix in passive devices that are nonlinear (see below), they can produce intermodulation signals which, if those signals fall in sensitive frequency bands (e.g. receive bands) can degrade the site and network performance.    

Different combinations of signals/interference

Where does PIM come from?

As mentioned above, PIM is produced by passive elements with nonlinear properties. This nonlinearity is caused by a variety of factors including the presence of ferromagnetic materials in regions with high magnetic fields, cold or cracked solder joints, junctions of dissimilar metals, oxidation, rust and more – which is why it’s often called the “rusty bolt effect.”

PIM sources can be found within and outside of the antenna system – making them much more difficult to pinpoint. External PIM sources are particularly challenging, as they can occur anywhere outside the signal path. As mobile wireless networks become more complex and rooftops and towers become more crowded, with more antennas and equipment co-located at the same site, the possibility of external PIM sources dramatically increases.

Common PIM Sources

  • Damaged cables
  • Corrosion/rust
  • Dirt/moisture/oxidation/contamination
  • Poor solder joints
  • Loose/overtightened connections
  • Dissimilar metallic junctions
  • Ferromagnetic materials (iron, nickel, cobalt…)
  • Nearby metal objects (guy wires, anchors, roof flashing, pipes…)

In addition to these virtually endless sources, new PIM sources can emerge over time. Environmental factors such as temperature variations, air quality and vibration, as well as age can take a toll on a system. As systems and their components age, they’re more likely to develop PIM-causing issues, like rust. Add in the growing number of carriers staking their claim on small footprints, and you get a recipe for increasing PIM.

What does PIM affect?

PIM often limits the reliability, capacity, and data rates of wireless systems. In addition to a Tx channel in a FDD antenna affecting its own Rx channel, PIM can also effect other bands and operators, further degrading overall system performance. Interfering signals can reduce receiver sensitivity to the point of drastically reducing download speeds or even dropping connections. Even slight increases in PIM can significantly decrease network performance.

5G and 4G/LTE networks are more vulnerable to PIM effects than legacy 2G and 3G technology due to increased bandwidth and their need to pack more information into each symbol to support higher order modulation schemes, meaning that there is less phase space between adjacent symbol values and therefore less tolerance for noise. Particularly as customers demand strong, high-quality signals and the spectrum becomes more crowded, these issues simply can’t be ignored.

Why it’s important to address PIM

In our connected age, addressing PIM is vital. As the density of wireless networks has increased, so has prevalence of shared sites with multiple operators, each deploying new frequency bands and more complex systems than in the past. “And as mentioned above, higher order modulation schemes are more sensitive to interference than those used in the past.”

This translates into an increasing prevalence of PIM-related issues over time.It’s important to keep in mind that PIM doesn’t just reduce performance – it can also reduce a cell site’s coverage footprint, value and return on investment.

Thankfully, as these issues have evolved, so has technology. There are multiple resources to identify and address PIM on the market, including the 3M™ PIM Reduction Kit 1000.

PIM is a formidable challenge, particularly in an increasingly wireless world. However, by taking the time to educate yourself on the issue, how to identify it and its sources, you can set yourself up for a better network.

Don’t be passive about passive intermodulation – take action and protect your network! Get more information about the 3M™ PIM Reduction Kit 1000.