What is Passive Interference Module (PIM) on 5G network and how can it impact user experience?

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What is PIM and importance of 5G PIM in telecom?

Passive Intermodulation (PIM) is a growing concern among mobile network operators looking for maximizing their network’s capacity, data rate, reliability and Return on Investment (ROI) with the deployment of 5G wireless networks. But what is exactly PIM? When two or more frequencies are mixed in a non-linear way in a passive circuit, an interference is generated and is known as PIM. The performance of a network can be adversely affected if the interference collides with uplink receive frequencies of an LTE or 5G network, generating -100dBm or higher intermodulation signals. Lowering the level of PIM, operators can achieve better capacity and network coverage.

Symptoms of passive intermodulation interference in communication systems

  • Imbalance in receiving noise floor diversity

  • Elevated noise floor

  • Average call duration becomes shorter

  • Higher drop in call rates

  • Data rates are low – uplink frequency or downlink

  • Low volume in calling signals

Potential causes of passive intermodulation interference in communication systems

  • Low quality materials that can be easily exposed to rust

  • Contamination of other sites or shavings of metals

  • Adjacent sources of frequency (e.g. power lines)

  • Contaminated plating

  • One-to-one metal contacts are very loose

  • Unsmooth RF path comprising base for stealth enclosures

  • Increase in number of licensed spectrum bands has become crowded leading to strong occurrence of PIM

  • Vibration and shock

How can 5G PIM impact user experience?

We must have experienced distortion while having a mobile conversation or at most dropping of calls. With the demand for higher frequencies and bandwidth, telcos are facilitating with high-band and wide-band in LTE and 5G networks. However, they generate high-powered signals and such application of wide-band and multi-band in LTE and 5G networks cause more wireless interferences and are easily susceptible to LTE and 5G PIM.

Execute drive test by locking on to a specific cell. A case in point, 4G LTE cell in their test location and choose the bandwidth as you want namely LTE 40, LTE 25, and so on. You can perform cell lock / band lock feature on your Android smartphone and test with RantCell solution. Click on ‘Download’ for the procedure:

Procedure for cell lock/band lock

Significance of PIM testing

Studies have found that reduction in coverage can be greater than 10 percent and experienced as uplink sensitivity is dropped by 1dB in the wake of PIM within a tower. Provided highly sensitive current cell tower equipments used in 4G and 5G networks, PIM with even low values are enough to exceedingly deteriorate performance of the entire mobile network.

Identification of PIM on networks is not a one-time task and is a continuous battle for telcos. They are constantly looking for best processes to identify PIM on network and have to have best practiced during installation of a cell tower site before it goes on air and is made available for users. Hence, RF engineers need to perform PIM testing, mitigate issues and make strategic improvements in the mobile network, enhancing overall mobile network experience to users.

What are PIM measurement procedures?

PIM-identification-process

Significant presence of passive intermodulation is masked at a low power within the network. So, it is ideal to perform PIM testing at base station radio power level to measure KPIs. Mobile network operators face critical challenge during 5G PIM testing due to extreme sensitivity of PIM levels for testing equipment setup and adjacent surroundings. Nearby metal objects to the device placed under assessment and the use of damaged test adapter may increase PIM and can result in false failures (Elevated noise floor).

RF engineers have to deploy PIM testing antennas as testing equipment and select RF carrier frequencies required for PIM testing and perform certain tests to measure performance and identify PIM problems. Such equipment enables operators to detect and discard non-linear junctions at the site of the cell and improve performance of the site. High power test signals are transmitted by the testing equipment in the line or the device under test. If the test signals navigate through a non-linear junction, mixing occurs leading to production of PIM frequencies. These signals travel in every direction from its production point. In a coaxial system, they travel in and out of the PIM testing antennas. With this, operators can achieve PIM signal’s magnitude as measured from the produced test signals.

How to identify PIM on networks?

There are basically two types of procedures to identify/measure PIM on networks:

By monitoring performance data such as cell-level performance KPIs during busy hours:

  • Monitoring UL data throughput

  • PRB heatmap

  • Interference trends

  • Interference imbalance

  • Number of connected users during busy hours

  • Higher call drop rates – monitoring call setup success rates (CSSR)

  • Call setup success rate gets impacted

  • High congestion during busy hours

  • Interference between antennas on the cell sites increase (Loose connectors and bad jumpers)

  • Impact on UL PRB utilization

  • Number of RRC connected users at cell level

  • 4G UL/DL Data Volume

  • Number of VOLTE Calls

  • UL SINR

  • UL QPSK%

  • UL NAK%

  • UL THROUGHPUT

  • RRC Success Rate

  • Data Drop rate

By visiting physical cell tower site:

  • To conduct test on a different range of frequencies via walk testing.

  • Measuring signal losses and reflections over a specific frequency band on PM testing antenna sites termed as line sweeping.

  • Assists in detecting poor connection which in turn can translate into PIMs, though it cannot detect whether PIM is currently present or not.

Mobile network operators working on deployment of 5G networks are in need for 5G PIM testing solutions at an affordable cost. At RantCell, we have developed a solution to point out issues from end-user perspective where presence of 5G PIM can be identified by monitoring uplink throughput KPIs and voice call KPIs.

Conclusion

5G is garnering a lot of attention with huge demands in every sector, and PIM is going to be a continuous battle for mobile carriers. Mobile network operators are closely working to deploy 5G cell towers on a large-scale. 5G PIM testing antennas are being deployed to check network performance on RF paths (specific frequency bands are selected) based on performance KPIs listed above. Upon successful completion of PIM testing and certain improvements, mobile network operators can provide improved data connectivity like increased speed, no disruption in voice calls etc. Telcos need to perform such operations rigorously so that 5G PIM is controlled; they should adopt the best tools and processes to ultimately upgrade user QoE of 4G/5G network.