August 11, 2020
Wireless network operators and field technicians have been dealing with passive intermodulation (PIM) for decades. Though it has always been an issue with single-band towers, PIM and its ability to adversely affect network performance is more prominent with the proliferation of multi-band base stations. The rollout of Band 14 for the FirstNet public safety network and 5G New Radio (NR) exasperates the problem, making the need to locate and remove PIM a high priority for all wireless operators.
Most towers now support at least two or three bands. In fact, a single-band site is so rare in today’s wireless networks that some treat it like a Big Foot sighting. When you also consider the mission-critical nature of FirstNet and some 5G applications, eliminating PIM so networks achieve Key Performance Indicators (KPIs) is critical.
Multi-band transmitters, which are being installed on towers to accommodate the ever-crowded RF spectrum, create an RF environment that significantly increases the chances of experiencing PIM noise. Those responsible for cell site performance must answer three key questions to effectively locate and remove PIM in today’s networks:
- Which downlinks (DL) are the main contributors to the PIM problem on the uplink (UL)?
- Which frequencies need to be troubleshooted?
- Where is the source of the PIM located? It can be internal or external to the system. If the source is external, how far away from my antenna is it?
To answer these questions, field technicians must collect and analyze data from the base stations. In additional to the standard KPI reports, the RF spectrum, transmit power, and frequency of the DL must be gathered. This is important because the transmitter plays a pivotal role in PIM creation and prevention. The RF spectrum of the UL must also be examined to help decipher the nature of the PIM problem.
Both the DL and UL data must be acquired simultaneously when measuring a multi-band base station. While this can be a daunting task using conventional PIM testing techniques, it is not with new test tools, such as the IQ Fiber Master MT2780A (figure 1). This solution uses the CPRI link to analyze RF data and identify PIM levels and location, and conduct RF spectrum analysis. It provides RF engineers with important diagnostic information that helps debug and hunt PIM.
Acquiring data digitally using the site’s CPRI links has three key advantages:
- Measures individual carrier signals – While each carrier sharing a cell site uses the same fiber lines, every operator has its own designated CPRI links. More accurate testing and evaluation can be done with this approach, helping to find the cause more quickly.
- Cell site stays live – By using a solution such as the IQ Fiber Master, PIM is monitored using live traffic. This eliminates the need to turn off a tower, which affects network quality of service (QoS).
- Reduces tower climbs – Field technicians can conduct these tests from the ground, eliminating the need to climb the tower at the onset. This is another potentially time- and cost-saving benefit.
Efficient PIM Diagnosis Method
Proper diagnosis of PIM requires a specific process to be followed. The first step is to make sure the problem is PIM-related and not due to RF interference. A visual evaluation may be possible by analyzing a KPI report, which occasionally can present the heat signature of PRVs that are representative of PIM. Another option is to monitor the spectrum across live traffic to see if any performance spikes or dips occur.
The aforementioned PIM over CPRI method is a more effective approach. It gives a more definitive answer by determining the sensitivity of PIM. The IQ Fiber Master has two unique features to aid in this analysis:
- PIM Level calculates the PIM noise via a signal processing algorithm to create a model using the baseband DL power and analyze the amount of noise in the UL channel.
- RX Desense uses another patented algorithm to determine the rise in the UL noise floor due to PIM. It is an indication of the reduction in receive sensitivity of the receiving antenna.
Figure 2 is an example of the PIM over CPRI results display. It presents a clear Pass/Fail scenario for the antennas. As shown in the display, antenna 1 failed because of external PIM and has excessive UL noise of 6.2 dB. Antenna 2, with a PIM level of -108.9 dBm and UL noise of 0.8 dB, passed.
Another key element when testing multi-band towers is the ability to determine the transmitter creating the most PIM. This will help gain an understanding of the frequencies causing the problem, enabling more efficient troubleshooting. Figure 3 shows a heatmap function that indicates Tx antenna ports 739 #1 and 763 #1 are the causes of PIM.
Solving PIM Issues
In the cases shown in the figures above, the site’s performance is being impeded by external PIM. To correct the problem, a Distance-to-PIM™ (DTP) measurement must be made to help locate the PIM source. DTP shows the location of PIM problems within the antenna system, as well as distance to external PIM sources outside the antenna system. While the aforementioned IQ Fiber Master can calculate DTP, the problem will often require a more detailed troubleshooting exercise.
A valuable tool in this scenario is the PIM Hunter. This patent-pending technology allows a test technician to pinpoint the location of external PIM beyond the antenna. By walking along the arc of the DTP distance from the antenna with PIM Hunter, technicians can find the source of the PIM typically within a few centimeters.
Want to learn more about how to diagnose PIM problems on a multi-band tower? Watch this How To Analyze PIM Using Live Traffic Without Climbing the Tower webinar given by Senior Product Manager Roger Paje.
