What to know about RF interference in 5G networks

As network operators navigate a multi-pronged transformation that encompasses 5G, fiber proliferation, increasingly cloud-based and virtualized networks, one of the challenges that they face is that of radio frequency interference.

RF interference is not a new issue in wireless networks, and in fact, some of the same sources of RF interference that affect 4G networks will also impact 5G networks. Here are some of the most important things to know about RF interference in 5G networks.

5G can be affected by the same RF interference as 4G because it may operate in the same frequency bands as 4G. 5G, explains Giuseppe Lipari, a product specialist with the transport and datacom business at test company EXFO, currently uses both new frequencies between 24-52 GHz (Frequency Range 2, commonly referred to as millimeter-wave) and existing bands below 6 GHz (Frequency Range 1, which encompasses both low- and mid-band spectrum).

“These bands, whether they be the low, the mid or the high-frequency bands, they can be affected by sources of interference—and these sources of interferences are increasing everywhere, and have really become a huge headache for network operations teams,” Lipari said. In the low and mid-bands where 3G, 4G and 5G may all be currently operating, he added, EXFO’s customers are seeing tens of thousands of sites impacted by RF interference and specifically, passive intermodulation or PIM.

Where does RF interference come from? Sources of interference can include:

-Broadcast television signals

-Power lines or electric fences

-Street lamps or light dimmers

-Lightening, or lightening suppression devices

-Wi-Fi devices

-Jumbo screens

-Arc welding

-Bakery ovens

-Elevator motors

-Poor network design, such as overlapping cells or neglecting the effects of objects which can reflect signals

-Poor cellular site installation, bad cable connectors, faulty radios or rust

-Intentional interference, such as from signal jammers

In the case of intermodulation, signals mix and create interference either within the originating frequency bands, or in different bands. Sources of interference range from parts of the cell site equipment itself, to sources that can be several miles away—making it a huge challenge to locate and mitigate.

5G NonStandalone networks depend on 4G, so the same RF interference that impacts 4G will affect 5G. While it may be tempting to think that 4G issues wouldn’t affect new 5G networks, Lipari points out that when it comes to 5G NonStandalone—which is how most operators began, and are still, deploying 5G—the 4G network is an integral part of 5G, with control traffic traveling over the 4G uplink.

“5G [NSA] deployments really begin with 4G,” he says. “So when we hear how we troubleshoot for 4G networks, PIM issues or RF interference, we think of RF over [Common Public Radio Interface] and we think it’s really for 4G only. But in reality, it’s a technology used for 5G deployments, and specifically for 5G NSA to troubleshoot RF interference and PIM issues.”

In 5G NSA, Lipari added, operators need to pay attention to uplink issues in particular, as reflected in Received Signal Strength Indicators (RSSI)—because 4G uplink issues will impact the 5G network.

RF interference impacts the utility of an operator’s spectrum investment, and customer experience. He also noted that as channel bandwidths increase, from 5-20 megahertz in 4G to 100-400 megahertz in 5G, the number of resource blocks increases as well. If a resource block is has interference, Lipari explains, a user most likely will not be able to able to connect or will have bad signal quality.

“At one site, having interference isn’t a big issue or isn’t the end of the world. But if interference is experienced at more than one site or and on numerous resourec blocks in a channel, that’s where where there’s capability for a big issue,” he adds. Operators spend billions of dollars on spectrum, and if it is impaired by interference, that reduces the utility of their investment. RF interference and PIM can create issues such as reducing the overall speed and capacity of a site, or it can rise to the level of causing unstable connections and dropped calls.

“These issues can lead to customers not being happy, and operators need to fix the issues as quickly and efficiently as possible,” Lipari said.

The process of finding and fixing RF interference and PIM is highly manual, but new tools are bringing automation. The typical RF troubleshooting process is usually kicked off by a combination of customer complaints and trouble tickets, perhaps bolstered by additional information from the carrier’s Operational Support System (OSS), Lipari explained. A RAN performance engineer will analyze the relevant KPIs and look at historical statistics to determine the location of the problem, and then a ticket is passed to a crew who will visit the site—usually meaning that a cell tech will go and take measurements that will hopefully illuminate the issue.

Depending on the results from those measurements, remedying the problem may be passed off to an interference-hunting team, a third-party fiber provider or the party responsible for maintaining a DAS. If the issue is PIM, that tends to arise either at or near the site, but often requires at least one visit from a tower crew who have to climb to the antennas and replace parts or tap on connectors to isolate the source of the PIM. RF interference hunting involves the often laborious process of tracking down sources that may be a significant distance from the site. The whole process of finding the source and addressing it can take weeks to complete, Lipari said, and it can be a very costly and is highly manual, requiring attention from experts in RF.

EXFO, he added, has sought to streamline the PIM mitigation process by bringing automation to some parts of it, such as a one-button, pass/fail analysis of RF spectrum by its intelligent RF over CPRI (iORF) application for 4G and 5G networks, with automatic configuration, detection and analysis in which the app deciphers whether the issue at the site is likely to be internal PIM, external PIM or some other type of RF interference and at what frequency that interference is occurring. Once the app identifies the issue, Lipari said, EXFO’s 5G Pro over-the-air FR1 and FR2 spectrum analyzer on the same platform can be used for digging further into the details in order to fix the issue.