Why 5G operators should worry about PIM, and how to find it

Author: Giuseppe Lipari - Solution Specialist, EXFO 

Wireless networks are transforming at a rapid pace, bringing new 5G technologies into play. As additional frequency bands are added into the mix, the impact of RF interference and passive intermodulation (PIM) increases. This is especially true at the lower- (450 MHz to 1GHz) and mid- (1GHz to 6 GHz) spectrum bands which are increasingly crowded. In addition, more cell sites are experiencing interference issues due to power lines, street lamps and electronic boards. Interference can cause big problems for 5G networks. To avoid problems, interference issues need to be solved quickly.

Old issue, new challenges
Interference comes from many sources and takes many forms. Passive intermodulation (PIM) has existed since the day radio frequency (RF) communications have involved more than one frequency and coaxial cable. However, as LTE and 5G networks have become more complex, they have also become especially sensitive to the effects of PIM. As a result, mobile network operators (MNOs) are faced with a new set of challenges around quickly diagnosing the growing number of interference issues across their 5G networks. So, let’s dig a little deeper into what is PIM.

PIM is the generation of interfering signals caused by non-linearities in the passive components of a wireless system. Two signals mix together, in what is known as amplitude modulation, to produce different signals. Ultimately these new signals can reside within the same band which creates interference. With PIM, and when the signals are wideband, the noise level or the noise floor is raised which can block desired signals and reduce receiver sensitivity. This can lead to issues such as drop calls, decreased system capacity and decreased data rates.

There are two types of PIM. Internal PIM is caused by the internal RF elements in the radiating infrastructure, such as loose connectors, damaged cables and connectors, and faulty elements in the antennas. These issues typically occur between the transmitter and the antenna. Since internal PIM issues reside at the radio itself, a tower crew must climb to the top of the cell tower to perform troubleshooting. This can be extremely costly and time consuming. External PIM differs from internal PIM as the former is caused by objects located near cell sites. Examples include metallic objects (usually rusty) close to the antenna. With external PIM, technicians must hunt down and pinpoint the source (or sources) causing the interference.

Troubleshooting PIM
Troubleshooting either internal or external PIM can be an expensive, manual process. When the issue involves internal PIM, a costly tower climb is usually required. Since there’s no guarantee the issue will be solved with an initial tower climb, a second climb may be required. In addition to being expensive, the entire process of finding and resolving an internal PIM issue can take weeks to complete. This is due in part to the required analysis, which is typically a 90% manual effort.

When PIM is external, the process is similar to RF interference hunting, where a technician is dispatched onsite and uses a spectrum analyzer to pinpoint the source. The process of interference hunting is highly manual and typically requires expertise in the area of RF.

Same technicians, many more responsibilities
When technicians are dispatched to troubleshoot interference issues, they often lack details about the problem or how to solve it. In addition, they may lack the skills and/or the best-practice tools required to perform interference hunting.

These same teams troubleshooting interference (and other issues) in existing 4G and 5G networks are also out in the field to deploy new 5G cell sites. To get everything done, they need expertise in multiple domains, including fiber, RF, timing and Ethernet. In order to be successful, today’s multi-tasking technicians must be equipped with the latest test tools and best-practice techniques.   

EXFO’s PIM and RF interference hunting solutions
EXFO uses automation to speed up processes by building expertise into its solutions, allowing technicians of varying skill levels to perform interference hunting activities that previously required experts to complete. EXFO’s FTB 5GPro solution features an intelligent RF over CPRI application, suitable for both 4G and 5G networks. Its one-button test provides a complete analysis and pass/fail verdict of the RF spectrum, and identifies whether the site is experiencing internal or external PIM. The application features an automated configuration of the CPRI rate, so no CPRI links or rates need to be manually entered. It has automated detection of vendor mapping and bandwidth, so no manual manipulation is required.

For handling RF interference or external PIM issues, the FTB 5GPro solution includes the 5GPro Spectrum Analyzer. An over-the-air spectrum analyzer, it helps pinpoint the source of external PIM and covers both FR1(both low and mid bands) as well as FR2 (mmWave). It includes a real-time spectrum analyzer as well as a traditional spectrum analyzer with analysis bandwidth to 100 MHz. The 5GPro Spectrum Analyzer also offers 5G NR and 4G LTE demodulation with beam and signal analysis, as well as TDD support.