PIM interference in C-Band networks
C-band is the designation given by the IEEE for radio frequency (RF) spectrum in the 4 GHz to 8 GHz frequency range. Historically, this spectrum has been used for satellite communications with uplink frequencies between 5.925 GHz and 6.425 GHz and downlink frequencies between 3.7 GHz and 4.2 GHz. A portion of the C-band downlink spectrum has now been re-purposed in the United States for 5G fixed and mobile services via Auction 107. The first 120 MHz of this new spectrum will be cleared and available for auction winners to use in December 2021 with the remaining spectrum available in December 2023.
Each time a new frequency band is added to a cell site the probability of PIM interference increases. C-band is no exception. Analysis shows that that just about every MHz of this new spectrum is subject to “low order” PIM generated by the existing 600 MHz, 700 MHz, 850 MHz, 1900 MHz and 2100 MHz services deployed. This presents a major challenge for operators since the magnitude of these low order PIM products is thousands of times higher than the traditional IM7 or IM9 products typically encountered with the existing services.
Since most of these low order products are created by signals coming from different antennas at the site, External PIM sources (PIM sources located beyond the antenna) will be a higher concern than PIM sources inside the feed system. The high radiated power area in front of antennas as well as the reactive near field region behind / beside antennas will be the locations of highest concern. These regions have come to be known as the “High-Risk PIM Zone” and special care must be taken to eliminate non-linear objects in this zone to reduce PIM.
The good news for operators is that testing conducted by ConcealFab indicates that the area of concern in front of antennas will likely be limited to PIM sources within 30 FT of the antenna. This means that power lines or billboards across the street from a site will likely not be a problem. Metal flashing and hidden fasteners securing membranes at rooftop sites will likely be the biggest issues. These can be mitigated by moving sectors closer to the building edge or by installing RF barrier material over a relatively small area in front of the antennas.
The PIM sources found in the reactive near field close to antennas can be eliminated by performing “PIM Hygiene.” This involves replacing known PIM sources (hose clamps, metal snap-in cable hangers, etc.) with low PIM alternatives. ConcealFab offers a wide variety of low PIM cable support solutions that can be found in the Interference Mitigation products catalog.
Regardless of site type, it will also be important for operators to reduce or eliminate antenna skew wherever possible. Rotating an antenna only 15 degrees on the antenna frame doubles the amount of energy arriving at the hardware behind an adjacent antenna! Doubling the power increases the PIM generated by a PIM source by 9 dB (almost 8 times higher magnitude.) Definitely something to avoid if possible.
A new white paper is available from ConcealFab that provides greater detail on the PIM issues associated with C-band spectrum. With proactive planning, many potential PIM sources can be avoided, resulting in faster data rates and better coverage from new C-band 5G networks.