PIM testing in DAS Networks
By: Tom Bell, Senior Director, Interference Product
I recently was invited to join a call to discuss PIM test methods that were being used to certify distributed antenna systems (DAS) performance. The parties involved were using an IWPC PIM testing best practices document I helped co-author in 2014 to guide their testing methodology. The subject of this call was related to power levels being used while performing PIM tests in their DAS networks.
Reading the IWPC document 8 years later, I think it is still a very good reference! I am writing this document to address specific questions raised during the call as well as discuss the evolution of PIM test specifications over the years.
PIM Test Specifications:
Factory PIM test equipment became commercially available in 1996. The first international PIM test specification (IEC 62037-1:1999) was published 3 years later to define test equipment requirements and to provide guidance on testing best practices. A key objective of this specification was to establish a common test methodology to allow engineers to compare the linearity of different RF components produced by different manufacturers. To meet this objective, the IEC specification recommended measuring the IM3 level (vs. IM5, IM7 or some other product) and recommended using 20W (+43 dBm) test tones for components used in mobile communications systems. The current edition of the PIM test specification (IEC 62037-1:2021 ED3) still maintains the same recommendation, but refines the language a bit to make it clear that higher or lower test powers may be appropriate. Figure 1 below shows the language related to test power levels in the current edition of the IEC 62037-1 specification:
Why was the IWPC document published?
Portable PIM test equipment became commercially available in 2005. By 2014, the equipment was being used by many network operators around the world to test the linearity of antenna feed systems in both indoor and outdoor networks. As stated in the Introduction of the IWPC best practices document, the IEC specifications did a good job defining procedures for PIM testing individual components in a factory environment but did not provide guidance for field PIM tests. The purpose of the IWPC document was to quickly “fill the gap” and provide guidelines that mobile operators could use to address the unique challenges encountered when testing both indoor and outdoor networks.
It only took 7 years, but the IEC just published its Part 7 specification titled “Field measurements of passive intermodulation.” The new specification can be purchased on the IEC website through the
following link IEC 62037-7: 2022. Given that many of the same authors are responsible for both documents, there are no big surprises in the new IEC specification. The new IEC specification addresses the primary technical concerns related to field PIM testing but does not specifically address best practices for PIM testing indoor networks. At this time, the IWPC document remains the best industry reference available for PIM testing in DAS networks.
PIM test power levels
In the conference call that initiated the writing of this document, a concern was raised that some people interpret the test power recommendations in both the IWPC and the IEC documents to mean that “PIM tests are not valid unless they are conducted using 20W test tones.” This could not be farther from the truth! Table 2 of the IEC document states that other test powers may be used as does Section 2.1 of the IWPC document which states that “Lower test power may be required for Small cell and Distributed Antenna System (DAS) testing.”
Remember, PIM tests are a linearity test designed to characterize the linearity of a system. It is up to those specifying the test to determine what level of linearity is “good enough” to prevent PIM interference from impacting network performance. From a theoretical standpoint, PIM is only a problem when the noise generated is higher than the receiver noise floor. From a practical standpoint, PIM noise may only start impacting customers when the level is many dB’s above the receiver noise floor. Each operator needs to determine what level of PIM noise is acceptable and design tests to assure that this level of PIM noise is not exceeded.
PIM test levels in a DAS network
If an operator has done the work to establish an acceptable linearity level at the radio input, appropriate linearity tests can be determined for other test points within that DAS based on the loss between the radio input and the new test point. To explain this, let’s break it down a bit, starting with a typical system level PIM test as shown in Figure 2.
As shown in Figure 2, site radios transmit signals into a system of RF devices that produce PIM. The linearity of that system is “good enough” when the system produces IM3 noise no greater than a predetermined acceptable value when CW test tones of a specified magnitude are injected into that system. In this case, the predetermined acceptable value is -97 dBm when two 20W (+43 dBm) test tones are injected.
Let’s now assume there is a point within the system where we can measure both the power of our test tones and the IM3 generated by the system as shown in Figure 3. In this system, there is 10 dB internal loss between our new test point and the system input. Starting with the IM3 returning to the PIM analyzer, PIM measured at the new test point could be as bad as -87 dBm and still produce a passing result (-97 dBm) when it reaches the system input due to the 10 dB of internal loss. Similarly, the power of the test tones used to generate this -87 dBm IM3 signal are +33 dBm, since the test tones are also attenuated by 10 dB before reaching the new test point. If we could inject our PIM analyzer at this point in the system and perform a PIM test, we would want to test using +33 dBm test tones with a pass fail level of -87 dBm. This would assure an appropriate level of linearity at the new test point that would meet the system level criteria when the IM3 signals return to the system input.
This same logic can be applied to PIM tests performed within a DAS network. Losses inside the DAS feed network attenuate both the transmit signals arriving at a PIM source as well as the IM3 signals returning to the DAS input. If an acceptable linearity pass / fail level at the system input is determined to be an IM3 level of -97 dBm with 2x 43 dBm test tones, appropriate test criteria can be calculated for floor, branch, and antenna level PIM tests. Using the splitter, tapper and cable losses shown in Figure 4, the appropriate PIM test levels at each location within this particular DAS are shown in the table in Figure 5. Obviously, appropriate PIM test levels will be different for each DAS based on the actual feed system losses for that DAS.
I generally stick to dBm when describing IM3 levels but in this case, I included the dBc levels in the table in Figure 5 for those who prefer to think in dBc’s. The acceptable IM3 level measured at new test points inside the DAS changes by 1x the loss between the new test point and the system input. Since the test power level is also reduced by the network loss, the IM3 level relative to the test power level (dBc) changes by 2x the loss between the new test point and the system input.
Applying system level PIM test criteria at points inside a DAS, without accounting for network loss results in significant over-testing of the DAS. Requiring higher than necessary linearity at points inside the system can be very expensive and may be impossible to achieve. One exception that is often made during DAS construction is floor or branch level cable system tests where all outputs are terminated into low PIM loads rather than antennas. Since the PIM performance of cable system components (spitters, tappers, combiners, cable assemblies, etc.) is very good (typically better than -110 dBm with 2x 20 W test tones) an “over-test” is often used during DAS assembly to identify marginal RF connections. The “over-test” in this case validates that the workmanship at RF connections is of the highest standard to prevent return visits to service marginal connections.
Hopefully this document dispels the thought that PIM tests should only be performed using 20 W (+43 dBm) test tones. Additionally, this document should provide clear guidance on how to reduce PIM test power levels and increase the acceptable IM3 limits for PIM tests performed within a DAS to achieve appropriate linearity tests that assure system level linearity is met.
The IEC 62037 family of PIM test specifications are managed by Technical Committee 46, Working Group 6 within the IEC. A project has just been opened within the working group to consider updates the IEC 62037-1 specification. We welcome the knowledge and experience of people working in the field performing PIM tests to participate in the specification development. Please consider reaching out to your national committee to learn how you can become a member and join the IEC TC46 / WG6 team.