NTIA: 3450-3550 MHz “is a good candidate for potential spectrum sharing”
In its hunt for additional midband spectrum that could be cleared or shared to make more room for mobile networks, the National Telecommunications and Information Administration says has identified a “good candidate”: Spectrum from 3.45-3.55 GHz, adjacent to the Citizens Broadband Radio Service at 3.55-3.7 GHz.
NTIA, under a 2018 mandate to examine the possibility of spectrum-sharing from 3.1-3.55 GHz and interest in sharing the band dating back to a 2010 ten-year plan to open up 500 megahertz of spectrum for commercial use, concluded in a recently released feasibility report that “the 3450-3550 MHz portion of this band is a good candidate for potential spectrum sharing, including at the commercial system power levels sought by the wireless industry.” NTIA described 3.45-3.55 GHz as having “the highest probability of being able to accommodate sharing with commercial wireless services in a relatively short timeframe.”
The band, according to a recent feasibility report from NTIA, is “critical” to Department of Defense (DoD) and Department of Homeland Security (DHS) radar operations, with DHS operating fixed and transportable radars in the band and DoD operating high-powered defense radar systems that include fixed, mobile, shipborne, and airborne platforms for air defense, missile and gunfire control, bomb scoring, battlefield weapon locations, air traffic control, and range safety. In terms of geographic area, federal operations span the entire United States and its territories, with some airborne systems operating nationwide, more than 100 locations for ground-based radars and shipborne radars that operate in more than 20 ports as well as along the U.S. coasts.
The DoD conducted a related 3450-3550 MHz Technical Study considered all federal systems in the band, potential aggregate interference to those systems under two different hypothetical commercial deployments and three sets of power levels for each, including “both the relatively low-power operation currently permitted for commercial operations in the adjacent band above 3550 MHz and the higher power levels that industry representatives have indicated are optimal.” “Some sharing” might be feasible below 3450 MHz, it added, but that portion of the spectrum is more crowded and more analysis is needed of the band, potential sharing methods and potential for relocation of operations, which include both classified and unclassified federal operations.
While commercial systems at 3.45-3.55 GHz would generate interference with the existing ones, the study found, it “nonetheless concludes that, with a transition of nationwide aeronautical systems to alternative frequencies, proper interference mitigation mechanisms, and further study, spectrum sharing may be technically feasible for all or portions of the 3450-3550 MHz sub-band, including at all the power levels analyzed.” Further work is needed to figure out an actual sharing mechanism that would enable both uses to successfully coexist.
The 2010 ten-year plan that highlighted this spectrum as a potential sharing candidate also jump-started the development that led to the commercialization of the three-tiered sharing framework for CBRS. So why not just take the same approach that has been realized with CBRS? The NTIA report says that while the work done for CBRS “is directly applicable to the establishment of sharing between commercial wireless and shipborne radars in this band, it is less relevant to the development of appropriate systems to protect the other – ground-based and airborne – systems in the band. This is partly because industry-managed monitoring stations outside military installations are inherently problematic from an operational security perspective and many more would be required due to the large number of ground-based radar sites, and partly because there are unique technical challenges to monitoring airborne operations.”
NTIA will be looking at a couple of options: possibly moving some airborne systems out of the frequency entirely, so they can still operate at full power, and looking to an “an automated, real-time, incumbent-informing spectrum sharing system … that NTIA would operate in conjunction with DoD to notify commercial entities when the latter would need to cease operations.” In CBRS, private Spectrum Access System operators handle the access to the spectrum, rather than a government agency.
NTIA also recommended that the FCC “give serious consideration to setting tighter limits on out-of-band emissions of commercial operations,” saying that it would benefit all users in the band in terms of incumbent protection and spectrum efficiency; and that NTIA is also working with agencies that operate in the spectrum to consider what incumbents could be relocate out of the band.
