Six Steps to Diagnose PIM Problems on CPRI-Based Systems

December 9, 2020

As more bands get turned up on wireless networks, the sub-6 GHz frequency spectrum gets increasingly crowded. Newly installed transmitters can mix with existing ones, making uplinks (UL) vulnerable to intermodulation products that reduce network performance. Passive intermodulation (PIM) is also exacerbated by densification, whereby more antennas are being added to the network to improve coverage. Consequently, more RF power is introduced into the environment, further increasing the chances of RF mixing with non-linearities.

Current PIM test solutions lack a method to simply diagnose a PIM problem quickly, particularly for a multi-band CPRI-based site, so identifying and eliminating PIM is a tedious (i.e. expensive) process. Fortunately, a new test solution (figure 1) has been developed to analyze PIM problems on a single or multi-carrier site in six easy steps.

Figure 1: The Anritsu IQ Fiber Master MT2780A test solution.

Step 1: Site Assessment

Field technicians need certain site information before conducting PIM analysis. This helps determine the physical connection as well as best understand the measurement results. Questions that must be answered are:

• Which sector needs to be analyzed?
• Who is the manufacturer of the base station and radio?
• What are the bands for the transmitters to be analyzed? This information should include the center frequency, bandwidth, and EARFCN.
• Do the carriers exist on separate CPRI links or are they concatenated onto a single links?

In most situations, answers to these important questions are available through operator documentation and/or polling the base station.

Step 2: Physical Connection

The second step is to connect the IQ Fiber Master™ MT2780A test solution to the system. Turn off the radio with the UL that has the suspected PIM problem using the network software that controls the baseband unit (BBU). We advise that the company’s operating instructions are reviewed to make sure it is done correctly.

With the radio locked down, insert the optical tap inline between the remote radio head (RRH) and BBU. Typically, this involves removing the fiber cable from the RRH and plugging it into the RRH1 port of the optical tap. An additional duplex fiber cable is then connected from the BBU1 port on the optical tap to the BBU (figure 2).

Figure 2: Test set-up for PIM analysis using the IQ Fiber Master.

When this is complete, turn on the radio. If there are problems, the optical tap and/or additional fiber cable needs to be checked for proper operation. Next, connect power to the sensor and attach the UL tap port (UL1) to the sensor SFP 1 using a single-plex fiber cable. Make sure to connect to the right side (input) of SFP in the sensor module. Repeat these steps for each downlink (DL) radio that need to be analyzed. Use the DL1, DL2, and DL3 ports of the IQ Fiber Master, as necessary.

Step 3: Define Site Parameters, Test Combinations

For the analysis, input the frequency bands used in the system and the network equipment manufacturer, so the software can account for certain vendors who use non-standard CPRI configurations. Most operators will have this information available.

For simple data entry, the test tool has an Auto-Detect feature that configures the network equipment manufacturer parameters and frequency band information. In instances where the information is not available, it must be input manually.

It is also important to prepare a test roster, which is a list of all the different transmitter combinations to be evaluated. In analyzing PIM, two transmitters must interact with a non-linearity in the RF path to produce PIM. IM1, IM3, and IM5 products are calculated for each scenario, so the field technician can gain insight into which ones may produce the highest PIM levels. Tests can be run either individually or as a complete set.

Step 4: Analyze the RF Spectrum

The spectrum is analyzed by simply clicking on the CPRI Analysis button at the top of the window of the screen. This display aligns with what the antenna recognizes for UL and DL. Any RF stream can be seen individually by clicking on its listing in the carrier tab.

Several functions are part of this analysis window to interpret the RF spectrum. Among the more common are peak hold, spectrogram, and persistence setting. Parameters and markers are adjusted, as needed, to meet the desired visualization of the RF spectrum.

A suspected PIM problem will often manifest itself in the RF spectrum. A slope or “shark fin” such as the one shown in figure 3 may serve as the alert to which UL can have possible PIM.

Figure 3: Example of a sloped RF noise floor, indicating a probable PIM problem.

Step 5: PIM Analysis

If PIM is suspected, an analysis will give insight into the degree and location of the problem. To maximize test result accuracy, the radio must remain in high-power mode, which is done by using the AILG, OCNS, or equivalent command for your base station.

The IQ Fiber Master MT2780A will initiate PIM analysis for each scenario defined in the test roster and produce results. Analysis results will report whether or not PIM is detected, PIM levels, receiver desensitization due to PIM, and if the PIM cause is internal or beyond the system.

If PIM is detected and the antenna is accessible, a distance-to-PIM (DTP) analysis can be performed to help locate the non-linearity(ies) in the RF path. DTP can improve troubleshooting and mitigation. A zero reference point is established by the test solution so accurate DTP measurements can be made.

Step 6: Interpreting the Results

Table 1 provides an example of the reports that provide details of the measurement results.

Table 1

To learn more about how to conduct PIM tests using the IQ Fiber Master MT2780A, download an application note, How to Use the IQ Fiber Master MT2780A to Diagnose PIM Problems on CPRI-Based Systems, published by Anritsu.