PIM is an escalating issue in 5G midband networks deployed in TDD spectrum
Passive intermodulation 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.
PIM isn’t a new issue in cellular networks, but it is becoming a more common cause of interference because of the increasing density of cellular networks, the increased bandwidth and use of more sensitive, higher-order signal modulation and new, midband Time Division Duplex spectrum that is being deployed, particularly in the C-Band.
When it comes to identifying, tracking down PIM sources and mitigating them, the process has some relatively new strategies as well as the advantage of knowledge built up over years of dealing with PIM issues, according to Tom Bell, who is senior director of interference products at ConcealFab. The company has been working since 2016 specifically on developing methods to identify and mitigate passive intermodulation.
Mobile network operators’ internal systems are intelligent enough to flag when a key performance indicator is off—high Received Signal Strength Indicator (RSSI) or Received Total Wideband Power (RTWP) indicating elevated uplink noise that is impacting performance in a cell sector, Bell says. At that point, there are several potential causes for the problem: PIM, external interference, or possibly something like incorrect set-up of the sector that could give a false RSSI reading. Bell says that now, operators will often try to narrow down the possibilities by taking advantage of remote network control and running artificial traffic at the site. “Does the RSSI increase as the traffic level increases? Is it consistent? If that’s the case, that generally points them back to, it’s a PIM problem,” Bell says.
The traditional next step has been to send a team out with a PIM analyzer and a PIM probe to go hunting within the sector. But Bell says that an even easier place to start is with what he calls “PIM hygiene,” which relies on the expertise that people within the industry have built up over years of dealing with PIM. ConcealFab and test equipment company EXFO have worked together to build a new solution for PIM source identification, one that uses a combination of RF over CPRI analysis to determine if interference at a cell sector is internal or external, plus a PIM-blocking “blanket” that can be used to cover suspected PIM sources in order to quickly isolate the problem and over-the-air spectrum analysis to get real-time confirmation and feedback on whether a given action mitigates the interference at the site. A crew armed with those tools and hand-tools can go in and fix loose brackets or pick up metal trash from previous installations, then proceed with covering radios or adjacent materials to see if they are PIM sources. “A lot of time, they’re able to get enough improvement by doing that, that you don’t have to go any further,” Bell added. With artificial traffic running, a crew can test to see if covering, say, the parapet wall in front of a rooftop antenna reduces the level of interference. “If you see a noise reduction, well, you’ve just found the PIM source,” Bell says. “Sometimes [crews] are able to get 10, 12 db of reduction, just by using their intuition and covering obvious things.”
The RF over CPRI and real-time spectrum analysis cut down on the need to wait for an off-site network technician to run commands on the radios themselves—lag time that adds to the cost of the endeavor. “While you’re doing those tests, your feedback is immediate, and so it makes those crews much more effective,” Bell explained. They can cover a potential PIM source or even wrap pads around the antenna itself, and get immediate confirmation of whether the noise in the uplink was reduced or not, so that a crew can successfully mitigate interference without necessarily having an RF engineer in the field with them.
“There are a lot of changes in the workforce, and fewer and fewer RF experts,” says Danny Sleiman, business development manager for EXFO, who said that the company’s approach is meant to both reduce the time involved in PIM interference finding as well as make it easier for people with less expertise to successfully find and mitigate PIM sources. The combined analysis and RF absorber system that EXFO put together with ConcealFab helps identify quickly whether the interference at a site is external—originating beyond the antenna—or an internal issue, and whether it’s PIM or external RF interference. Sleiman explained that with a PIM analyzer-based approach, multiple ports have to be tested, since PIM can have uneven effects across antenna ports—but with RF over CPRI, EXFO is able to allow viewing of effects on each of the “branches,” or antenna ports.
Figuring out if the interference at a site is internal or external is the basis for helping to determine who is responsible for fixing the issue, as well as mitigation tactics. “As simple of a thing as that is, it’s actually very powerful” for determining whether a contractor needs to revisit a site or if a crew needs to address external PIM, Sleiman said.
Bell chimed in that in the event that PIM is external, the RF absorber kit also provides a target for a site’s noise floor—the point at which mitigation is as good as it’s going to get. He said that ConcealFab tells customers that they should look at RSSI levels when the antenna is completely enclosed and target within 3 db of that evel, with the site at 100% loading. “You know, if you can get to within 3 dB of the theoretical best-case at 100% loading in real-world conditions, that means you’re not going to see measurable interference in the network,” he explained.
For more comprehensive information about PIM in 5G systems, read this free report from RCR Wireless News.
