PIM interference in 3.45 GHz spectrum
Auction 110 concluded in January 2022 with bidders spending $22.5B to acquire 100 MHz of mid-band spectrum in the 3.45 GHz to 3.55 GHz frequency range. This band is referred to as the Department of Defense (DoD) band since use of this spectrum was previously limited to “Federal” applications. In total, there is now 530 MHz of contiguous mid-band spectrum available to US operators spanning across the 3.45 GHz (DoD band), 3.5 GHz (CBRS band) and 3.7 GHz (C-band) spectrum.
ConcealFab published a White Paper in 2021 that discussed Passive Intermodulation (PIM) issues impacting operators deploying networks in the 3.7 GHz (C-band) end of this spectrum. Analysis showed that just about every MHz of the C-band spectrum is subject to “low order” intermodulation products generated by the existing 600 MHz, 700 MHz, 850 MHz, 1900 MHz and 2100 MHz systems installed at sites. These low-order intermodulation products can elevate the noise floor, reducing the speed and coverage of a new C-band networks. I am pleased to say that several US operators have taken this threat seriously and are proactively requiring PIM Hygiene while on site deploying new C-band radio equipment to reduce the potential for PIM interference.
So, what about the new 3.45 GHz (DoD band) spectrum? Is it also going to have the same PIM concerns as C-band? ConcealFab performed an analysis looking at multiple combinations of lower frequency bands deployed by US operators and has concluded that “YES”, the 3.45 GHz spectrum will have similar PIM interference issues as C-Band. Figure 1 shows multiple lower frequency band combinations able to create IM3 in the 3.45 GHz to 3.55 GHz band.
The good news is that the same solutions presented in ConcealFab’s C-Band PIM White Paper also apply to the 3.45 GHz band. Operators can proactively and cost-effectively address potential PIM problems during mid-band spectrum deployment rather than building in problems for someone else to repair later. Changing BOMs to require low PIM cable support hardware plus spending a little extra time to perform PIM Hygiene while on site are simple steps that can help maximizing the performance of new mid-band networks.