Maybe not completely—but some proactive work can help identify when PIM problems are likely
Interference from passive intermodulation often doesn’t show up as a major issue until a new site starts taking on significant amounts of traffic, or perhaps when a popular new device adds new spectrum support. But PIM really shouldn’t come as a surprise, 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.
“I refer to dealing with PIM as equivalent to the seven stages of grieving—and it always starts with denial. ‘How can this be?’ ” he says jokingly. He goes on to say that the industry has a process for site builds that it has been following, largely unchanged, for decades. “But what we’re finding is that practices that were used to build sites historically, aren’t good enough any more. And that means there’s a lot of change required in the organizations to adapt,” he said. Identifying PIM and its cause at any particular site is actually the easy part, Bell says, adding, “The hard part is affecting the change within the organization to say, let’s proactively stop building sites the way that we’ve done historically.”
He points out that PIM signals are predictable and calculable based on the originating frequencies, and there are multiple good, free PIM analysis tools available that will tell network planners whether, given current and new spectrum blocks, whether they should expect intermodulation products in their uplinks. But, Bell says, use of such tools is often an afterthought: Operators will turn up spectrum, find there is a major noise issue and then try to identify if it is PIM, rather then doing the analysis ahead of time to see if they should expect PIM to be present.
“Every time an operator adds a new frequency band, the probability of getting a third-order intermod falling in your uplink increases dramatically,” Bell said. “Quite honestly, it’s only a matter of time. … Spectrum is the most valuable asset an operator has, and so every time spectrum becomes available, operators are going to buy more—whether it’s good spectrum for them or not.
“You always find a way to make it work,” he acknowledges, but the turn-up of new spectrum—no matter how badly needed for capacity purposes—can result in abrupt PIM issues in the uplink that need to be remediated quickly.
In this context, then, network design processes that have stood the test of time in preceding generations of cellular technology may not stand the test of PIM now. Bell argues that, just as the telecom industry has adopted new standards for construction, connectors and PIM-tested components, it should also reexamine how its underlying system of network design and planning and prioritizing both deployment speed and lowest-cost contractors contribute to interference problems that then need to be fixed.
Bell gives a few examples: Not allowing antenna position to be skewed in relation to the frame, which ends up directing energy from the main beam into adjacent antennas and causing PIM; or skew with respect to the building edge for rooftop sites. When rooftop sites are placed far back from the building edge and illuminate yards and yards of rooftop, it illuminates more roof surface and increases the likelihood of PIM. These are site design issues, Bell says—and solving them (and the resulting PIM) means addressing some fundamental issues in the industry’s historical approach and constant pressure to do more with less. For example, a four-sector rooftop site design with 90-degrees of coverage for each sector would provide a better match with building edges where antennas could be pushed closer to roof edges and minimize illumination—but it would also be more expensive to add that fourth antenna.
Still, he says, the industry has made big changes before in order to address RF problems. Operators have raised their standards on construction and installation quality when it became clear that sloppy work was causing site performance issues, and factory-tested components for reduced PIM have also become more widely available and required by operators. Those efforts have gone a long way toward reducing internal system issues. Nothing is perfect, though. “The sheer number of internal problems is lower, but has not gone away,” Bell says. “That last guy who tightens that connection with the wrench is still always going to be a human, and humans can make mistakes—so that’s always going to be there.”
That’s also the reality going on now, with heightened build-out and densification activity that increases the risk of new PIM sources. “People going back to sites and touching the hardware is just terrible, because you know no matter what they do, they’re going to bump things, they’re going to add new equipment. Every time a human touches the site, you’ve got an opportunity to generate new PIM sources,” Bell says ruefully. “You’ve got lots and lots of metal-to-metal connections and so having well-designed connections, to minimize the probability of them becoming loose, is important,” he goes on. “But even if it’s perfectly designed, you still have humans putting it together, and things are going to happen. And things are going to change over time, and temperature, and wind, and everything else,” he concluded. “The best you can do is, use robust designs that are just less inherent to have the ability to come loose. That’s important.”
For additional insights on identifying and mitigating PIM in 5G systems, check out this free editorial report from RCR Wireless News: “More Spectrum, New Challenges: PIM in the Age of 5G.”
