Avoiding PIM

This is the fifth post in a new blog series based on the

The CommScope North American Band/Block PIM Calculator available on our website has proven to be a valuable planning tool. It was designed to help RF design engineers avoid combining downlink (transmit) bands/blocks that cause harmful interference to the companion uplink (receive) frequencies. For all modulation schemes – but especially LTE – this interference reduces the signal to noise ratio and thus degrades the all-important throughput.

If you haven’t downloaded this calculator recently, then you do not have all the updated options available in this latest version. These options include the:

• Addition of the recently auctioned AWS-3 blocks
• Ability to check for not only second- and third-order passive intermodulation (PIM) but now also for fifth-order PIM.

For many operators, the addition of AWS-3 block(s) when combined with existing AWS block(s) can create new PIM challenges – especially if the AWS/AWS-3 band is diplexed with other bands like 850 or 1900 MHz. If there are plans to diplex two or more ground-mounted radios onto common RF paths, then inputting the specific bands/blocks and checking for second-, third- and fifth-order PIM will determine if there are any concerns.

Since the industry testing is focused on third-order PIM testing, one may ask why we are adding the fifth-order calculation capability to the Calculator. It has to do with the RF path PIM values that operators consider acceptable.

Especially for ground-mounted radios, we have seen acceptable values as low as -140 dBc for third-order PIM. When testing with two 20 watt (+43 dBm) carriers, this results in a third-order product at -97 dBm – definitely not acceptable for LTE receivers having sensitivities as low as -120 dBm. Since testing has shown that the difference between third- and fifth-order PIM can range between 10 and 20 dB depending on the characteristics of the PIM source, the fifth-order values would be in the -107 to –117 dBm range – still unacceptable for LTE.

With the expanded bandwidth being offered on many Remote Radio Units (RRUs), it is now possible for one RRU to generate LTE frequencies in multiple PCS 1900 MHz blocks. Some combinations can cause third-order PIM (Blocks A & F or G & B) while others can cause fifth-order PIM (A2 & B1, A & E). Designers should be aware of these combinations that can cause degradation of the LTE uplink and thus reduce the throughput of their network.

For more information and links to resources about avoiding PIM, see my chapter in the LTE Best Practices ebook. If you have a question about any of the CommScope PIM Calculators, leave me a comment. And if you haven’t heard about the innovative Optical PIM Tester we debuted at Mobile World Congress, see Malik Ishak’s blog post. With the Optical PIM Tester, a single technician connects the tester to the baseband unit via the Common Public Radio Interface (CPRI), all from the safety of the ground.