PIM Mitigation

Passive Intermodulation (PIM) Mitigation

Passive intermodulation (PIM) occurs when two downlink signals at a cell site combine due to a non-linear object (or objects) in the RF path. Mixed signals generate new frequencies that can degrade the overall performance of the system and raise the noise floor. This causes multiple problems for network operators.

PIM interference is a growing challenge for all wireless networks, and in broadband-reliant cellular networks in particular. PIM is not a new problem for cellular networks. Industry leaders have seen the issue grow for years and attempted to accommodate it. However, consumer demand has outpaced wireless infrastructure. Networks have worked quickly to establish new cell sites which frequently increase PIM even as they grow capacity.

New frequency bands have only increased the issue. Operators no longer have the option to shift overages to less crowded frequencies. Those frequencies have now been auctioned for expanded use with the advent of 5G.

The increase in interference is driving a critical, industry-wide need for PIM mitigation/PIM hygiene. PIM hygiene includes actions such as site clean-up. Crews can replace components with poor PIM performance with alternative options: low PIM unistruts, straps, support brackets and other hardware.

In this article, you will learn the common forms of PIM and explore the negative impact PIM has on system performance. You will also learn methods for detecting and mitigating PIM to improve performance and create better outcomes for network operators and the systems they manage.

Common Forms of PIM Interference

There are an extensive number of objects that can cause PIM interference. Items such as metal snap-in cable hangers and stainless-steel hose clamps frequently generate service-impacting PIM. This is especially true when cables and other equipment are installed near antennas.

Companies deploy many modern cell sites on structures or buildings built with parts that are not optimized for current PIM standards. These factors create the perfect conditions for PIM to occur. The list below identifies the most common causes of external PIM in cell sites.

Common Building Materials

Multiple construction components contribute to external PIM. The stainless-steel brackets that secure cables at the cell site are one of the most common causes. While the brackets are sturdy, inexpensive and impervious to rust, they can cause several issues.

For example, if the clamps are loose, PIM will occur in all areas where metal surfaces touch. Additionally, the free end of the clamp can touch metal, including itself and generate PIM. Finally, anywhere that two metals with differing electrochemical potentials touch can create pockets of galvanic corrosion. Corrosion can cause extreme PIM, even when the RF energy is low.

Roofing Materials

Roofing material create challenges for detecting PIM interference because the materials that cause it are often hidden below protective barriers. These materials include overlapping deck members, sheet metal, screws, plates, flashing, air conditioning units, antennas, mounting components, rust on metal, lighting and certain types of rooftop signage.


As noted in the description for common building materials, corrosion is problematic for small cell sites. This is particularly true in the case of bimetallic corrosion that occurs due to electrochemical incompatibility. Other forms of corrosion include oxidized connections, chemical corrosion caused by other site materials and rust caused by environmental influences. Corrosion can cause a tremendous amount of PIM when it is situated near base station antennas at small cell sites.

Outdated Components with Poor PIM Performance

There is an increased need for small cell sites in areas with high population density. This means that many sites are established on buildings or in areas that were not optimized for PIM during their construction. Even modern construction can utilize materials with poor PIM performance, including that of new cell sites. Outdated cable mounts, poorly insulated connections, bad interfaces and metal mismatches are frequent culprits in PIM interference.

Metal Objects

In addition to the common metal building components such as screws, brackets, flashing, harnesses and connectors, other metal objects can also affect PIM performance. Metal scraps left behind on the job site, nearby fencing and even construction equipment can cause issues with PIM performance.

Power Lines and Digital Billboards

Power lines and digital billboards are an inevitable part of any community. These structures present issues with PIM hygiene because they are outside of the control of the cell site. When these items create PIM, it requires mitigation within the site itself.

These are the common PIM examples, but what problems does PIM actually cause? The next section will outline the negative impact of PIM on network operators, cellular companies and broadband users.

The Negative Impact of PIM Interference

Cellular network operators are understandably concerned about the quality of their service. The negative impact of PIM interference on reliable service cannot be understated. Providers need to add more coverage and more data while increasing capacity to meet customer demands. This has created a razor thin margin of error.

The most common issues PIM causes are dropped calls and poor voice quality. In addition to being incredibly frustrating for cellular customers, it can also be dangerous. Customers rely on their phones for emergency services, roadside assistance and a variety of other support. They also expect to use their service wherever they are. Users are left in a bind when PIM limits the coverage area or causes dropped or garbled calls.

PIM also slows down data transmission rates. The large number of users who rely on their smartphones on a daily basis make it easy to see why this is a pressing issue. Users expect high speed data and even higher performance for their dollar. Consistent failure will have a negative impact on customer retention and brand loyalty.

No provider wants to be the leader in unreliable coverage, dragging data and dropped calls. That is why finding and eliminating PIM is of critical importance.

Detecting the Source(s) of PIM

Detecting the source of PIM is no easy task. The cell site alone presents numerous opportunities for interference. Unfortunately, it is typically caused by the very mechanical components that make up the site’s infrastructure. Additionally, many sources of PIM can change depending on factors such as temperature, humidity or activity around the site.

For example, PIM may be more present in the winter at some sites due to heavy snow. At other sites, it may be more present in the summer due to dust storms or high humidity. However, that does not mean that companies should deploy a large-scale search operation immediately. There are alternative steps that can be taken before advancing to more aggressive options.

The first option is locating PIM through a thorough inspection.

Perform a PIM Inspection

Utilize a crew to perform PIM hygiene by seeking out and mitigating obvious sources of interference first. Mitigation will be discussed in more detail in the next section. The following details steps to take during a PIM inspection.

  1. Check for visibly obvious damage to parts and structures. Are there broken or cut lines? Is equipment damaged? Is there any sign of environmental PIM like flooding, mud or debris interfering with the site?
  2. Look for corrosion/rust on components and nearby parts and structures (rusty bolt effect). This extends to structures beyond the site, like nearby fencing, water towers, metal roofing or resting equipment.
  3. Check for metallic contact. As noted previously, this is one of the more common causes of PIM. Check for broken hose clamps and loose support brackets. Check for signs of corrosion, which can signal issues between two different metals.
  4. Check for debris or materials around the site that may cause disruption. Construction can leave scraps of metal that seem innocuous but cause big issues for cell site performance. Even shavings of metal or pooled solder can create interference.
  5. Are there ferromagnetic materials present? Ferromagnetic materials like iron, aluminum, nickel, certain alloys and cobalt contribute to PIM. Even if they are not part of the cell site, their existence nearby can create PIM.
  6. Are there ferrimagnetic materials present? Used in microwave parts, these materials may be created with low PIM in mind. However, they can still generate PIM and parts that contain them should be checked for damage.
  7. Are there loose connections? While this might seem to be an easy search, it can be quite difficult, depending on the site layout. Ensure all connections are tight and that they are secured to prevent them from being moved or separated.
  8. Check all cables. The protective material that encloses wiring can be damaged by rodents, weather or persistent movement. Even if the wiring is not damaged directly, wear on the coating can allow water to penetrate the cable. It can also allow interference from nearby metals.
  9. Examine area for common disruptors such as poles, antenna, etc. This search can help technicians determine next steps for PIM mitigation. Are there antennas or signage on the roof or nearby? Are there power lines, light poles or other structures that could be contributing?

Perform a PIM Hunt Utilizing PIM Testing Equipment

The PIM testing procedure utilizes handheld spectrum analyzers in conjunction with a PIM probe and other testing equipment. Testing is performed at the site using specialized equipment. Technicians undergo PIM mitigation training both in the classroom and in the field. Instruction teaches technicians how to accurately follow PIM measurement procedures to detect it at the source.

Advanced testing equipment can detect the source of PIM with incredible accuracy. PIM differs from procedures like line sweeping. Line sweeping can detect issues like limited connectivity but it cannot define the interruption as passive intermodulation.

PIM test signals detect PIM in the RF path by transmitting two signals into the line in question. These signals must be high-powered so they do not accidentally cover up PIM. Once the two signals are transmitted into the line, the technician can observe their field device and measure the PIM signal.

The PIM probe is connected to a spectrum analyzer. This helps the technician pinpoint the location of the source causing the interference. The spectrum analyzer provides the inspector with a “path” to follow while the probe alerts the technician to the proximity of the source. Once the source is identified, the crew can be dispatched to resolve the problem.

The Downside of PIM Hunting

Providers can decide that a PIM hunt with testing and measurement equipment is an option. This often occurs if the crew is having difficulty locating external PIM sources. However, it should be noted that a PIM hunt can be time consuming and expensive:

The more cost-effective option is to perform an inspection. Crews should look for common forms of PIM interference and replace parts with low PIM hardware. For example, replace existing struts with low PIM struts or utilize a low PIM cable mounting system instead of metal mounts. The remaining sources can be identified by a PIM hunting crew if hygiene does not drop interference to an acceptable level.

Conducting PIM Mitigation to Enhance PIM Performance

After detecting the cause of PIM, it is time to launch a mitigation strategy and reduce PIM across the site. Depending on the findings, PIM hygiene can be as simple as clearing up construction debris or cleaning corroded parts.

Alternatively, it can require a shift to more rigorous options. These include replacing numerous pieces of hardware with components built for low PIM performance. Some basic steps to take include.

Mitigation crews can take other steps to reduce PIM as well.

  1. Replace site hardware with low PIM components. Hardware made of stainless-steel is sturdy and affordable. However, it presents a continuous potential for PIM. Install low PIM angle adapters, low PIM cable support brackets and other components to reduce existing interference. These changes can also help prevent future PIM in those updated areas.
  2. Inspect and repair corrosion: Eliminate the rusty bolt effect by cleaning up corroded areas. Remove rust and other corrosion carefully, ensuring the area is clean after the task is complete. If possible, correct metal discrepancies or insulate/protect the area to prevent further corrosion.
  3. Remove dust and construction debris, metal in particular. Inspect the area for metal shavings, solder spills, stray screws, metal discs, washers and nails and remove them.
  4. Adapt the site to account for nearby PIM interference caused by immovable objects or necessary hardware. PIM foil can cover large areas such as rooftops, as can PIM shield tape.

Once these changes are completed, cell site performance should improve. However, it is important to inspect and maintain equipment regularly. PIM can reoccur as the cell site ages or as exterior factors change.

PIM Mitigation Summary

PIM presents multiple challenges for cellular network providers attempting to grow cellular access and increase wireless capabilities. PIM can slow data, block calls, drop calls and squeeze capacity, seriously compromising service in the affected area. The prevalence of potential sources for PIM interference, such as site hardware, weather, aging equipment or nearby structures compounds these issues.

Combatting PIM in small cell sites requires regular inspection of the equipment. Whenever possible, construction of the site should follow best practices for preventing PIM before it begins. This means utilizing components such as low PIM brackets, low PIM support straps and other materials commonly found to combat PIM.

When PIM occurs, operators can begin to troubleshoot by inspecting the site and conducting PIM hygiene. Technicians should examine the site for issues such as corrosion, loose connections, metal incompatibilities, damaged cables and nearby objects. They should then repair or replace where necessary.

If the mitigation crew’s PIM hygiene fails to detect the source, operators can deploy a testing crew. The crew can utilize advanced detection equipment to find it. Once the source is detected, the mitigation team can get to work. Mitigation includes making repairs, cleaning corrosion, insulating parts and replacing hardware known to increase PIM.

Following this strategy for PIM detection and mitigation will ensure that providers can limit interference even as small cell site saturation continues to increase and demands on the network continue to grow.