Passive intermodulation (PIM) is a pernicious and destructive phenomenon for cellular network operators. PIM shows up as a set of unwanted signals created by mixing two or more strong RF signals. Mixing generally produces the sum and difference frequencies of the two transmit signals F1 and F2. These signals are F1 + F2 and F1 – F2. Sum and difference signals are also formed with the harmonics of the transmitter signals. Lower-order PIM is more disruptive, but higher-order PIM covers more bandwidth. The main effect is on the receiver with its high sensitivity. Interfering signals can raise the noise floor and block desired signals. Interfering signals can also reduce receiver sensitivity.
PIM limits the quality of service (QoS) and capacity of mobile phone systems such as LTE and 5G. PIM can result in dropped calls and reduced uplink and downlink throughputs, which, in turn, results in a reduction in QoS. Lower QoS can negatively impact carrier competitiveness and increase subscriber churn. In short, PIM can be expensive.
PIM can be caused by design limitations, assembly variations, and environmental factors. PIM results from signal degradation effects from passive system elements such as antennas, cables, filters, loose or corroded connectors, or nearby metal or rust, causing signals from multiple high-power transmitters to interact, creating nonlinear effects that interfere with the uplink signal and severely degrades throughput. Other names for PIM include the rusty bolt effect and the diode effect.
Although PIM is typically described as a mixing of two carrier frequencies, third-order products can also be generated from three carrier frequencies, fifth-order from up to five carriers, and so on. Multicarrier PIM products are more numerous and stronger due to the additional carrier power involved, exacerbating the PIM problem. And they are becoming more common as tower sharing and antenna sharing become more common.
PIM has been a problem for mobile networks for many years, but it isn’t easy to identify. It may not be discovered as a problem until site monitoring identifies specific performance issues. And PIM can be hard to fix; finding the root cause of a PIM problem can be challenging.
Causes of PIM
PIM problems are on the rise. As antenna-sharing schemes become common and the spectrum is increasingly crowded, the possibility of generating PIM rises from the interactions between different carriers. Adding to the problem, the use of digital modulation schemes such as OFDM/CDMA means that the system’s peak power is also increasing, adding to the PIM challenges. PIM can arise from a variety of sources, including:
Design PIM – passive components such as filters, circulators, duplexers, and switches together with connectors, cables, and transmission lines can add to PIM. System designers can trade-off between lower cost, smaller size or lower system performance, and higher PIM levels. Component manufacturers often specify PIM performance for certain devices.
Assembly/Aging PIM – even if the system is properly designed and operates within specified limits when first installed, performance can decline over time as the components age and begin to exhibit non-linear behavior. This effect can result from weathering or substandard installation practices. Connectors, cable assemblies, and waveguide assemblies are particularly susceptible to this type of PIM.
Environmental (Rusty Bolt) PIM – can be caused by external environmental factors. This can be a particularly troublesome form of PIM since the environment surrounding a site can change unexpectedly over time. It is often caused by dirty or corroded parts (such as rusty bolts). Corroded materials on antennas, or structural elements, can act as one or more diodes. This type of PIM is sometimes referred to as the “diode effect.” The PIM is generated after the signals have left the transmission antenna, with the PIM reflecting back into the receiver. Over time, rust can build up, and new metal elements may be added to the signal path, making this type of PIM a constant possibility, even in sites with well-designed equipment that has been properly installed.
PIM standards and testing
IEC 62037 applies to the general requirements and measuring methods for intermodulation (IM) level measurement of passive RF and microwave components, which can be caused by the presence of two or more transmitting signals. The standard specifies using two +43 dBm (20W) tones for the test signals for PIM testing. IEC 62037 addresses the measurement of PIM but does not cover the long-term reliability of a product with reference to its performance. The standard includes six sections:
- IEC 62037-1 – Part 1: General requirements and measuring methods
- IEC 62037-2 – Part 2: Measurement of passive intermodulation in coaxial cable assemblies
- IEC 62037-3 – Part 3: Measurement of passive intermodulation in coaxial connectors
- IEC 62037-4 – Part 4: Measurement of passive intermodulation in coaxial cables
- IEC 62037-5 – Part 5: Measurement of passive intermodulation in filters
- IEC 62037-6 – Part 6: Measurement of passive intermodulation in antennas
Continuous-wave (CW) signal transmission test techniques have been the primary methodology for PIM measurement for frequency division duplexing (FDD) testing. More recently, low duty cycle (LDC) PIM testing has been offered as an alternative to CW PIM testing. LDC involves transmitting two high-power test tones for roughly 5 ms to 10 ms for each measurement cycle, translating to about 3% of the TX transmit time, depending on the number of measurements taken per second. While the transmission time is decreased with LDC, the transmitted signal is still a CW signal.
The choice of using a CW test or an LDC test is not necessarily simple. Both can be used to measure PIM; factors to consider when selecting a testing procedure include:
- A CW test may be more stringent because the longer time constant of power can generate more heat and expose non-linearities in one or more devices.
- On the other hand, actual network operations do not use unmodulated CW signals. GSM, OFDM WCDMA, etc., all have a periodic on/off transmission signal. LDC may be a better representation of actual operating conditions.
Regardless of the PIM analyzer’s measurement technique, CW or LDC, the only traceable device available to independently validate a PIM analyzer’s measurement accuracy is a PIM standard. A PIM standard generates a known PIM level (commonly –80 dBm) when stimulated by two 43 dBm (20 W) test tones. PIM standards provide a specified accuracy of ± 3 dB for comparative tests.
PIM standards are adapters which generate intermodulation products of a certain preset level. They are used to verify intermodulation test benches and for instant and/or long-term level stability monitoring. If the intermodulation value displayed by the test instrument deviates from the specified value of the intermodulation standard, it indicates a general measurement uncertainty that should be addressed.
Line Sweep and PIM Testing
Line sweep testing and PIM testing are important measures of a cell site’s ability to provide service and perform optimally. Line sweeping measures the signal losses and reflections of the transmission system. PIM testing is a measure of design and construction quality and the potential for self-interference.
PIM testing performance measurements are not relevant unless accompanied by comprehensive line sweep tests. PIM test measurements performed on a transmission system with poor microwave performance are not necessarily useful indicators of the transmission system’s performance.
Good PIM performance requires both low system loss and good return loss. If PIM testing is done before line sweep testing, the operator may not have the needed detailed information about the transmission line’s characteristics. PIM test signals are attenuated by high insertion losses, preventing the interference energy from reaching the components that need testing. And poor return loss can report a false positive by reflecting a portion of the PIM test signals back into the test set, causing some signal cancellation.
Performing a line sweep test before PIM testing ensures that the insertion loss and return loss parameters are at acceptable levels. That, in turn, ensures that the PIM test produces an accurate indicator of PIM performance.
Summary
PIM results from signal degradation effects from passive system elements such as antennas, cables, filters, loose or corroded connectors, or nearby metal or rust, causing signals from multiple high-power transmitters to interact, creating nonlinear effects that interfere with the uplink signal and severely degrades throughput. PIM can arise from various sources, including design problems, poor assembly of cell sites, component aging, and environmental concerns. It can result in dropped calls and reduced uplink and downlink throughputs, which, in turn, results in a reduction in QoS. Various methods have been developed to test for PIM and to mitigate its impact.
References
BTS System Line Sweep and PIM Testing, Anritsu
Comparing CW and Low Duty Cycle Passive Intermodulation (PIM) measurements, Rohde & Schwarz
Multiband Passive Intermodulation Testing, Keysight Technologies
Passive Intermodulation (PIM), Anritsu
Passive Intermodulation (PIM) Effects in Base Stations: Understanding the Challenges and Solutions, Analog Devices
