Insights

All about passive intermodulation distortion in cellular telecommunication networks

By Rajeev Gandhi, Head of Technology, Telco Network Engineering

The telecommunications industry is typically an early adopter of new technologies and digital transformation. A case in point is with 5G cellular networks. The industry is transitioning to 5G rapidly because of the many highly skilled technology innovators in the telecommunications industry. Consumers are another reason for the rapid migration to 5G. Whether for work or personal reasons, most people now have an insatiable demand for connectivity, and they find that smartphones and high-speed network services are a critical part of their daily lives.

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The potential threat of PIM distortion

As telecom companies strive to grow their customer base and scale network infrastructure, and in parallel, consumers want faster transmission speeds, wireless providers must do their best to anticipate what future developments in the industry might bring and how this evolution will positively or negatively impact their businesses. In particular, telecom companies must pay particular attention to potential threats to service reliability and network traffic capacity.

Wireless network operators should be aware of passive intermodulation (PIM) distortion as a possible obstacle to seamless cellular network functionality. According to this article on everythingrf.com, “PIM is an issue for almost every wireless system, but it is most noticeable in cellular applications (CDMA, HSPA, and LTE). This is because the frequency bands used by these technologies are very close to each other.”

5G uses similar, and even higher, frequency bands compared to CDMA, HSPA, and LTE. This makes it even more susceptible to PIM issues due to intermodulation between its own carriers or with other nearby signals.

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Understanding PIM

With that brief explanation above, let’s dig a bit deeper to understand PIM better. It is a type of distortion caused by intermodulation, which are currents in an electrical device at two different frequencies. PIM occurs when passive, nonlinear devices take on two or more signals. As LTE, 5G, and Wi-Fi networks become ubiquitous, the modulated carrier frequency prone to nonlinear passive devices using different source metals can generate more self-interference signal distortion.

These unwanted additional frequencies can interfere with the uplink or downlink frequencies, which reduce the receiving cell sensitivity and disrupt communications. The everythingrf.com article also says, “PIM lowers the reliability, capacity, and data rate of wireless systems by limiting the receive sensitivity. End results may include dropped calls, decreased system capacity, and decreased data rates.”

Most cellular operators address distortion issues caused by active devices. However, many wireless providers ignore distortions caused by passive network components, such as aging or dirty cables, connectors, and antennas. With spectrum use and antenna-sharing activities growing exponentially, network operators must understand the causes of PIM and add testing, identification, and resolution processes to their network maintenance strategy.

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Sources of PIM distortion

Wireless providers have traditionally used frequency planning to avoid most PIM issues. However, PIM has become a much more serious issue for telecom carriers. In general, aging equipment, installing new equipment that overlays old antenna runs, or co-locating two or more carrier’s antennas in a cell tower can lead to significant PIM distortion. With that said, here are some of the most common technical causes of PIM in cellular network systems:

• Faulty electrical connections—Surfaces that are not smooth can cause high current density in contact areas. This scenario can lead to high temperatures in the conduction path. If you don’t tighten connections with the right amount of torque, a resistance change may occur, which can cause PIM signal distortion.
• Ferromagnetic materials—A non-magnetic substance that becomes permanently magnetic is ferromagnetic. Although only a few materials have ferromagnetic properties, iron can become ferromagnetic over time. This natural phenomenon occurs when a substance generates many unpaired electrons. Since iron is often used in cellular systems, it can become ferromagnetic if it is exposed to elevated levels of radio frequency (RF) waves. Ferromagnetic iron can cause high levels of PIM distortion. Aging antennas and cable connections are examples of possible iron-based ferromagnetic elements in a wireless network.

• Oxide—Metal surfaces on connectors, cables, duplexers, or antennas typically have a thin layer of oxide. This thin oxide layer can become oxidized or corroded over time as rust builds up with prolonged exposure to oxygen, water, and high humidity. As an oxide layer slowly increases in thickness, thermal conductivity decreases, which can lead to PIM distortion. The oxide layer on metals can also cause the Schottky effect. In this scenario, the oxide layer on metal surfaces can create a hill between two surfaces. The Schottky effect occurs when electrons have enough energy to jump over the hill, which can cause significant PIM distortion.

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Impacts of PIM

PIM can have considerable negative impacts on the performance of a cellular system. Network coverage decreases when receiver cells become less sensitive to weak signals. This weak signal issue leads to dropped calls, significant interference, and disrupted communications services. In the case of data transfers, PIM tries to resend more error-protection bits, leading to even lower data transfer rates. PIM disruptions have even caused some cellular operators to experience a complete shutdown of their receiver sector.

Wireless network operators need to pay close attention to common indicators of PIM, such as:

• High noise floors
• Imbalances in receive-noise-floor-diversity
• Decreases in the average call duration
• Increases in dropped calls
• Low data rates
• Reduced call volumes.

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Reducing the effects of PIM distortion

If your network isn’t as stable, reliable, or high-quality with as wide of a coverage area as you’d like to provide to your customers, or you want to avoid PIM distortion proactively, we recommend these strategies:

• Design and implement PIM testing and detection processes—to continuously monitor equipment for indications of PIM with thresholds that pinpoint when PIM may be occurring, so your technicians can quickly identify root causes to resolve PIM.
• Augment your cellular network with advanced PIM cancellation algorithms—so you can conduct periodic inspections and cleanings of the active and passive elements of your system to reduce or eliminate PIM.
• Maintain adequate torque for the connectors—to make sure connectors are clean and secure from the point of installation and any bad RF connections are removed.

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Proactively avoid PIM

While you may think PIM distortion is a trivial matter or something you can manage down the road, it could have a very serious negative impact on your network services over time. Our telecommunications experts are here to help you analyze your potential for PIM distortion and deploy strategies to reduce or avoid PIM with advanced, automated testing, detection, and resolution services to ensure your wireless network is as reliable as possible. Please visit us at ust.com.