Cellular communications towers contain a wide range of antennas, cable assemblies, and other components that can contribute to the system's passive intermodulation (PIM) distortion. Typically, PIM distortion is caused by problems with metal-to-metal junctions in the RF signal path, which are aggravated by the advancing age of components and systems in the field. Although it sounds routine to control, minimizing PIM in wireless communications systems can be quite a challenge. The secret to avoiding problematic PIM is extremely careful installation combined with parts that are designed to minimize distortion in the first place. Fortunately, a growing number of portable tools can help measure PIM, and an expanding number of components are being developed with improved PIM performance.

What really happens? PIM is a form of intermodulation distortion that occurs in components normally thought of as linear, such as cables, connectors, and antennas. However, when subjected to high RF powers found in cellular systems, these devices can generate intermodulation (IM) signals at -80 dBm or higher. Such IM signals are generated late in the signal path and, therefore, cannot be filtered out. This means they may cause more harm than the stronger, but filtered, IM products from active components.

PIM does not discriminate by frequency band, but it does greatly affect the receive band and, consequently, decrease network capacity. It is aggravated by higher operating power levels, so it is a growing problem as service operators share base stations, operate antennas across multiple bands to avoid tower loading limitations, and install multi-band feed systems.

Local ordinances and zoning restrictions are making it difficult or impossible for service operators to add antennas to towers when they expand their systems to handle more frequencies. As a result, networks are constrained to a limited number of antenna ports. This means that PIM generated by the transmit tones at the cell site are in the same feed line used for reception.

More specifically, the original cellular tower transmitted on one antenna is received on another completely separate antenna, with more than 40-dB isolation between the two, so that the PIM generated by transmit signals would be attenuated by 40 dB before entering the receive lines. Now that extra isolation no longer exists, it greatly aggravates the problem.

One way around all of these PIM issues would be to use frequency combinations where PIM does not fall in the range of the receive signal path. However, wideband systems, such as third-generation/fourth-generation (3G/4G) cellular long-term evolution (LTE), make this approach essentially impossible.

By far the biggest development for PIM avoidance in the last few years has been the availability of portable equipment to test for PIM distortion in the field. Useful are test instruments with range-to-fault technology, or solutions designed for use at cellular sites. They have been on the market for years.

They help locate sources of PIM distortion, their distance from the test equipment, and facilitate accurate evaluation of PIM levels at customer-specified test-signal frequencies.

To avoid PIM, the mobile operator needs to ensure that PIM signals remain below the base station receiver's sensitivity. The amplitude of these undesired signals is directly influenced by the fidelity of the transmission line path, including all components and junctions along that path, which might introduce a nonlinear effect to the signals passing through them.

Engineers may need to use a spectrum analyzer to compare spectrum responses between sectors. This makes it possible to discriminate between PIM energy and external interference signals coming from outside the antenna, such as from adjacent cell sites, old TV transmitters, or metallic structures nearby.

Since PIM is power sensitive (the more power in the tower, the more likely there will be distortion), it is important to use a test solution with real-world output power.

PIM testing enables network operators to find and eliminate non-linear junctions at the cell site in order to improve site performance. The testing equipment transmits two high-power signals into the line or device under test. If the test signals run into a non-linear junction, mixing occurs causing the PIM frequencies to be generated. The PIM signals travel in all directions from the point of generation. In a coaxial system, this means they travel out toward the antenna as well as back in the direction of the PIM test equipment. The PIM test equipment measures the magnitude of the PIM signal generated by test signals and displays this information to the test operator.

The 3rd order, IM3 is used to characterize PIM performance both in the factory and in the field. The IM3 signal generated by a non-linear junction is usually of a higher magnitude than the other PIM products enabling higher measurement accuracy.

The higher-order products (IM5, IM7, IM9, etc.) typically drop in magnitude by 5 to 10 dB for each successive PIM product. By controlling IM3 to a specified level, the higher-order products are held well below that level, often by 10's of dB's.

As more field testing takes place, component vendors are learning what the critical failures are for deployed parts, and can work on addressing these issues in next-generation parts.

Moving forward, portable equipment manufacturers must continue to optimize equipment for precision as well as ruggedness and weight. We can expect these portable testers to evolve toward including more functionality and diagnosis capability in single units. And as QoS degrades, we can expect carriers to start putting concerns about PIM distortion front and center.

Arjun Sharma

Sr. Manager-Business Development,

Fastech Telecommunications (I) Pvt. Ltd.

Mobile networks are getting increasingly dense and complex with multiple technologies sharing the same bands with higher bandwidths. In many cases the same frequency band is used for 2G, 3G and 4G signals by different cellular service providers. Further, the breakneck speed of deployment of new technologies and sites poses the risk of field personnel installing misconfigured base stations, without judicious and systematic checking of frequency plans. The resulting RF interference affects all operators in the vicinity, creating "havoc in the air", and ruining service quality to demanding subscribers. There is an urgent need for mobile operators to detect, locate and eliminate RF interference and protect the spectrum they have purchased at astronomical costs.

Field usable Interference hunting and troubleshooting tools are the answer to handle these interference issues. Such handheld interference and direction finding devices are available now with very sensitive RF signal detection (DANL of -167dBm), fast signal detection (12GHz/second sweep rates) and real-time IQ analysis characterization and localization of the interference signals. Since interfering signals are often intermittent, it is important for such tools to detect all RF activity without any gap. Conventional spectrum analysers sweep through the frequency spectrum, making them blind to most of the spectrum for large time intervals. Analysers with gapless IQ analysis tools continuously monitor the spectrum of interest and detect even minor variations in RF activity.

These handheld instruments are quick in localizing suspected signals, interferers and RF leaks. With the help of the inbuilt GPS and active directional antenna with built in electronic compass, these tools can quickly and effectively locate the interference suspect/source and display on a map view, even under difficult conditions or with complex signals such as broadband communications or pulsed and sporadic signals.

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