September 10, 2019
As cellular network density increases, passive intermodulation (PIM) effects are becoming more profound…and potentially more disastrous, especially when it comes to 5G. Though PIM is predominantly seen in high-power cell sites, it’s also present at lower transmit powers used in distributed antenna systems (DAS). It will become a greater issue as the spectrum continues to become more crowded. This is particularly significant in 5G with the growth of seamless integration of multiple base station technologies to service user needs.
Traditionally, PIM has been viewed as an installation problem. Today, however, PIM is more likely to occur due to changes in frequency content or the physical environment around the cell site. Common examples include RF bands added to existing sites, physical structures constructed in close proximity to a cell site, or connectors corroding or loosening over time.
5G and other modern mobile communication systems have wide transmit signals. These signals can mix with themselves to generate wide intermodulation signals centered at the same frequencies and predicted by the intermodulation equations. These patterns can overlap with other intermodulation signals created from the other intermodulation orders (figure 1). To prevent these interference issues, field engineers and technicians use PIM test equipment to detect, locate and eliminate PIM sources.
PIM Test Methods
Field technicians typically use one of the following approaches to measure PIM:
- RF-based PIM testers – Portable test solutions, such as the Anritsu PIM Master™ MW82119B, generate two 40W RF tones at set RF frequencies (F1 and F2) to measure the effects of PIM.
- Spectral analysis over CPRI – This method uses spectral analysis of I/Q data on the fiber interface between the baseband unit (BBU) and remote radio head (RRH) to view the PIM profile.
- Noise rise monitoring – Field technicians use installed cellular equipment to set Orthogonal Channel Noise Simulator (OCNS) / Air Interface Load Generator (AILG) on all carriers under test and measure Receive Signal Strength Indicator (RSSI) per carrier/resource block in every UL that may be impacted.
PIM in 5G
These methods have been effective tools in reducing PIM but have shown to be limited when it comes to emerging high-speed, high bandwidth 4G and 5G networks. A new method, PIM over CPRI, has been developed that adds insight into PIM by providing six key benefits:
- A more powerful view into the scenarios above, including how to measure all the IM products in the UL and all harmonic scenarios using a single measurement
- Monitor live traffic remotely without disturbing cell site RF equipment
- Accurately identify on-site/off-site PIM location
- Precisely measure PIM level near or below the RRH noise floor and/or in the presence of a busy UL
- Measure PIM for single-band or harmonic scenarios
- Predict the viability of using third-party/OEM PIM mitigation solutions
This test is effective and efficient because the CPRI link carries complex baseband data representing the downlink (DL) and UL signals as transmitted and received at RF by the remote radio unit. Data on the CPRI UL will indicate signals received from user equipment (UE) and any interference suffered by the RF signal.
Tapping into the CPRI link allows the transmitted and received signals to be monitored in real time on a live cell site. Observing the received spectrum gives only partial insight into the issues, however, because it is difficult to tell the difference between the wanted UE signals and PIM, even if the interference is severe. Some network OEMs will ask UEs to lower the transmit power to reduce the noise floor, while others will increase transmit power to overcome a high noise floor. There are tradeoffs, though, as a reduction of capacity and/or coverage area will be realized.
Saving Time and Money
PIM over CPRI is a better solution. It is an excellent initial step to determine if a site has PIM issues before calling in a PIM mitigation team. Unlike traditional RF PIM testing, PIM over CPRI allows testing at ground level without turning down service and disconnecting any antenna connection, saving carriers capital and expenditure expense costs.
PIM over CPRI utilizes an algorithm that differentiates PIM from wanted UE signals. The algorithm uses DL signal knowledge to build a detailed mathematical model of the PIM affecting the UL. Correlating the modelled PIM with the received UL signal simplifies determining if PIM exists in the UL.
Another benefit is that the algorithm works on the baseband signals, meaning the test instrument is band agnostic. To configure the mathematical model, knowledge of the center frequency as well as the DL and UL carrier bandwidths are necessary. The signals used for the mathematical model are flexible, so inter-band and cross-sector PIM can be measured.
The algorithm measures PIM by modelling all the MIMO DL transmissions in real time against the individual MIMO UL signals to isolate PIM created from each antenna or a combination. A full MIMO model is also essential to identify if the primary PIM source lies within the antenna line or external.
Benefits of CPRI PIM Measurements
A major advantage of CPRI PIM detection is that a non-invasive measurement can be made on a cell site that is in service. As the PIM source is illuminated by real cellular traffic, the PIM measured is directly applicable to UL desensitization and the actual PIM impact on the network.
The algorithm explained above requires a minimum DL power level to create the mathematical model. If the DL traffic doesn’t generate enough power for the algorithm, a method that adjusts the sector under test can be employed. The best approach for ensuring adequate DL activity is to enable OCNS/AILG (or equivalent) on the system to populate any empty resource blocks (RBs). This method does not affect active users on the system. The measurement also provides a check on the transmit signal strength to insure minimum signal level.
Figures 2 and 3 show measurements taken of a Band 13 2T2R RRH with LTE10 DL at 751 MHz and UL at 782 MHz using an Anritsu BTS Master MT8220T base station analyzer. As seen in the results in yellow highlighted box in figure 3, UL1 is showing a significant internal PIM issue. A repair on the Port 1 antenna line should be considered based on this display.
The residual correlated PIM is over 17 dB below thermal noise at -64.4 dBFS and the desensitization is negligible based on the figure 3 display. The result indicates that UL2 falls within the acceptable threshold and, therefore, passes.
As wireless systems continue to add RF bands, more complex modulation, carrier aggregation, increased cell density, and increased RF power, PIM will increasingly affect wireless network Key Performance Indicators (KPIs). By using non-intrusive solutions, such as the BTS Master MT8220T with the PIM over CPRI option, far greater insight into the nature of PIM in the network and how to correct it can be achieved.
To learn more about PIM over CPRI, download A New Technique for Measuring Passive Intermodulation Over Fiber Optic Cables white paper from Anritsu.