Steps to Conduct PIM Over CPRI Measurements

November 6, 2018

As we discussed in our last post, PIM over CPRI is a new method that incorporates live traffic to measure and detect passive intermodulation (PIM) over the CPRI link in a MIMO system. It serves as mobile operators’ first line of defense, as it more efficiently determines if a tower crew needs to be called to fix PIM problems at the top of a tower or roof. If that is the case, RF PIM instruments, such as the PIM Master™ MW82119B and a Spectrum Master™ handheld spectrum analyzer or Site Master™ cable and antenna analyzer with a PIM Hunter™ probe, can be used for more comprehensive testing to find the root cause of the issue.   

PIM over CPRI can save operating expenses (OpEx) and capital expenses (CapEx) for operators via a straightforward and simple technique. To perform a single- or multi-carrier PIM over CPRI measurement, the field technician has to properly configure the BTS Master™ MT8220T handheld analyzer with the CPRI interface. This is done by following the steps below:

• Install the appropriate small form pluggable (SFP) transceiver for each slot of the BTS Master analyzer (note: the selected SFPs must support both the optical fiber mode and CPRI line rate being used by the base station installation)
• Connect the optical tap to the baseband unit (BBU) per the CPRI RF configuration diagrams
• Connect the duplex fiber connected to the BBU from the remote radio head (RRH) into port A on the optical tap
• Run a duplex fiber from port B on the optical tap(s) to the BBU
• Run a simplex cable from the optical tap(s) downlink (DL) to the SFP 1 Rx port to establish a downlink monitor connection
• Run a simplex cable from the optical tap(s) uplink (UL) to the SFP 2 Rx port to create an uplink monitor connection

A visual representation of the setup is shown in Figure 1.

Figure 1

After the connections are made, the BTS Master MT8220T analyzer must be switched from the default CPRI Spectrum mode to PIM over CPRI. The capability is launched by pressing the PIM Aid button found on the PIM over CPRI measurement menu. All PIM over CPRI measurement configurations can be made from the PIM Aid screen.

PIM Over CPRI: Making A Measurement

We will touch on the configurations in this post but more detailed explanations of each are published in this PIM Over CPRI application note. 

Site Configuration – First, recognize if the system is SISO, 2x2 MIMO, or 2x4 MIMO. This is based on the current system’s site configuration and tells the algorithm how many downlinks to use for the PIM measurement against each uplink signal.

Pass/Fail PIM Desensitization – PIM desensitization can be thought of as the amount of noise in the uplink signal caused by PIM. Field tests show that a PIM desensitization value of 10 dB will typically downgrade the uplink data throughput by 50%.

Downlink Configuration - The DL configuration establishes all the necessary connection and calculation values to make PIM over CPRI measurements.

Frequency - The PIM measurement algorithm needs the center frequency of the LTE DL and UL under test as one value that determines the IM product for the test.

LTE Carrier Bandwidth – This needs to be set at 5 MHz, 10 MHz, 15 MHz, or 20 MHz for both UL and DL. The PIM measurement algorithm needs the bandwidth of the LTE UL and DL under test to help determine the IM product for the test. This helps the algorithm to correctly make self-generated PIM and harmonic measurements.

AxC Configuration (UL and DL) - Configure the AxC container group for each MIMO carrier. The UL AxC configuration locates the individual uplink MIMO carriers on the CPRI link needed for the measurement.

Line Rate - By pressing the appropriate button, the correct line rate of the CPRI link connected will be automatically detected. A green light will appear at the top of the screen to show a sound CPRI connection.

Radio Preset - The radio preset has the default CPRI RF configuration settings needed to connect to Alcatel/Nokia, Huawei, or Samsung RRHs/BBUs.

View DL/UL Configuration - Once all settings are configured, press the View DL Configuration and UL Configurations buttons, respectively, to review the spectrum traces and verify they are configured correctly. A spectrum view of the LTE MIMO carriers will be shown for each. DL carriers are identified by the rise and fall of the signal being rounded, whereas UL carrier signals have sharp, vertical rises and falls.

Save and Measure - Once everything is configured and verified, press the Save and Measure softkey. This brings up the PIM over CPRI measurement screen with all saved configurations and will automatically start the measurement. The first measurement for each uplink will take approximately one minute, and each subsequent test takes about six seconds. It will then cycle through all Uls to automatically measure each configured uplink for PIM until the measurement is completed.

Generate Report - Once the measurement has finished, the PIM Over CPRI capability will automatically gather all the JPGs and measurement files of the Uls tested and create a report that is saved in the PIM over CPRI folder within the instrument internal file system. Reports include:

Figure 2 shows a sample screen of the PIM over CPRI results table and graph.

Figure 2

Determining PIM Cause

If PIM is found outside the pass/fail established limits, it is important to determine the PIM source. If it is within the antenna system, a tower crew is typically called to troubleshoot the issue with an RF-based PIM tester, such as the PIM Master.

A team is called to search for the external culprit, if PIM is caused by something outside the antenna system. This can be accomplished with a fast-scanning spectrum or cable and antenna analyzer along with a specifically designed probe to detect PIM, such as the PIM Hunter. The field technician “probes” the area around the antenna at a distance defined by the PIM Master Distance-to-PIM™ (DTP) measurement. PIM Hunter generates a response on the spectrum analyzer only when in close proximity to a PIM source, such as a rusty guard rail or loose metal contact. Once the issues are found, there are numerous products to absorb or deflect RF signals to reduce PIM back into the antenna system.

Your work is not completed once all major contributors to PIM are detected and fixed. The final step is to retest to validate the PIM mitigation has been successful.

To learn more about how to conduct PIM Over CPRI measurements, download our free application note.