New Solutions to Age-Old (PIM) Problems in RF Networks

Passive intermodulation—commonly referred to as PIM—has dragged down the performance of RF networks since their invention. PIM is the presence of interference in the RF path itself, most often caused by some kind of discontinuity in the path (e.g., improperly mated connectors, damaged cable, metallic surface corrosion or water infiltration). Though PIM is often generated internally by components of the RF path, PIM can also be caused by external sources, as we’ll explore below. Whatever its source, however, PIM’s effects degrade uplink performance in the RF link, increasing data error rates, reducing effective coverage distance and degrading other network key performance indicators (KPIs).

These effects can be categorized into two main PIM types:

·       Intra-band PIM, caused by multiple signals in the same band, generally an internal problem

·       Crossband PIM, caused by multiple signals in different bands, generally by external sources

As the largest and most advanced RF networks currently in use, cellular networks are keenly sensitive to the impacts of both kinds of PIM and the costs that come with them. Each component and connection in the network has the potential to be (or become) a source of internal, intra-band PIM interference, and co-siting different network technologies can introduce crossband PIM. The evolution to digital cellular networks (4G and 5G) has raised the stakes considerably because of the increased complexity of cell sites that comes with multiple network technologies.

One might think that by increasing signal transmission power, the impacts of PIM could be overcome or mitigated.  Unfortunately, since PIM impacts the uplink between the mobile device and the network, that implies that the mobile device transmission power would need to increase.  This is not generally practical, for a number of reasons, including device battery life and RF safety implications.

What can operators do?

Mobile network operators instead focus their efforts on PIM-mitigating measures and solutions that can be implemented cost effectively in their networks—simpler architectures with fewer connections, improved connector types less likely to develop PIM issues, non-metallic components where practical, and so forth. Such products, certified for low-PIM performance, can help with intra-band PIM. Crossband PIM, on the other hand, is much harder to isolate and control because it can include interactions with other bands coming from outside the RF path itself—situations too specific and unique for simple, universal solutions. For a technical look at this fascinating kind of PIM, check out this recent white paper.

For a deeper dive into the nature of PIM and strategies to combat its effects, I suggest chapter 6 of CommScope’s Understanding the RF Path eBook, one of our most-accessed pieces of content—now revised for its 14^th year.

PIM has been an unfortunate fact of life for RF networks since the beginning, and the campaign against PIM never ends. But thanks to some recent innovations from CommScope’s Outdoor Wireless Networks business, mobile network operators can look forward to some breakthrough improvements.

Advanced PIM-busting innovations can turn the tide

CommScope has been leading the charge against PIM for decades. We’ve made it our mission to mitigate this preventable problem in every RF solution we build, and we factory-test and validate PIM performance in our products to ensure mobile network operators get the performance they pay for. But in addition to these practices, designs and materials choices, we’re also advancing the technology with entirely new solution types designed to defeat PIM, preserve network performance and reduce TCO for our mobile network operator partners. 

Here are some of these exciting recent innovations:

• The Mosaic® antenna platform. Built to support both active and passive antenna infrastructure, the Mosaic platform is designed to provide mobile network operators with a simple, interference-free way to add 5G network to existing 4G sites—without compromising the performance of either technology by crossband PIM that could otherwise result from having both antennas in such close proximity. The Mosaic platform nests the active 5G antenna directly behind the passive 4G antenna, which is transparent to 5G bands. The result is multiple technologies operating seamlessly side by side, with one of the major causes of PIM virtually eliminated.

• PIM-Guard® accessories replace many of the metallic components associated with internal PIM with high-performance polymer equivalents. These accessories run the gamut from RRU mounts to standoff brackets and support clips. As an added bonus, they are also lighter and less expensive than the more PIM-prone components they replace. Our latest addition to the portfolio is BSA PIM-Guard isolation shields, which provide exceptional PIM defense while also reducing wind load up to 21 percent.

• SEED™ high-efficient antenna technology. These new base station antennas use an innovative, low-loss distribution feed network and advanced phase-shifter technology to boost the antenna’s radiated power relative to its input power. As a result, they dramatically increase antenna efficiency—and these devices also feature fewer solder points in construction, which diminishes a common source of internal PIM. SEED technology also helps operators achieve their own sustainability goals by enabling them to balance power and coverage as needed.

• CommScope® PIM calculators.  This easy-to-use tool analyzes and predicts real-world impacts of crossband PIM on a network’s coverage. Enter transmit and receive frequencies, define acceptable tolerance and the tool will calculate the various kinds of PIM that system is likely to encounter. We also offer a more advanced offline tool, as well as a proprietary 3D PIM Calculator for mobile network operator partners with the most complex infrastructure.

PIM still matters—and we’re still working on it

For more than a century, PIM has been a problem, but a preventable one. CommScope will never stop working to make it easier and more affordable to keep PIM at bay. Building on our decades of experience fighting the effects (and the costs) of PIM, these current innovations have moved the battle lines in favor of mobile network operators. While the battle isn’t yet won, CommScope is confident that the best solutions are yet to come.

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