Spectrum sharing in the mid-bands beyond CBRS

While there’s a lot of focus on CBRS at the moment, there are significant spectrum machinations at a federal level that will impact the future of other valuable chunks of mid-band spectrum. This started with the MOBILE NOW Act, which directs the Secretary of Commerce to work with the NTIA and other impacted agencies to provide the Federal Communications Commision by March 23, 2020, with “a report evaluating the feasibility of allowing commercial wireless services, licensed or unlicensed, to share use of the frequencies between 3100 megahertz and 3550 megahertz.” 

3110 MHz to 3550 MHz

There are approximately 445 assignments in the the 450 megahertz with users including the Department of Energy, Department of Homeland Security, the Air Force, Army, Navy, Marine Corps and Department of Commerce. The spectrum is used for things like airborne and shipborne radiolocation–the Navy’s Aegis Combat System, which tracks and guides missiles, for instance.

The FCC today (Dec. 12) will take up a notice of proposed rulemaking regarding this spectrum. Potential action items are focused on clearing the spectrum of non-federal users, secondary radiolocationa and amateur allocations, and protecting incumbent federal users while also setting the stage for future shared use. 

From the NPRM: “By proposing to delete the existing non-federal secondary allocations from the 3.3-3.55 GHz band…we are taking an important initial step towards satisfying Congress’s directives and making as much as 250 megahertz of spectrum from this band potentially available for advanced wireless services, including 5G.”

3700 MHz to 4200 MHz (C-Band)

In the Federal Communications Commission’s quest to free up more mid-band spectrum for 5G wireless services, the debate around the C-Band is one of the most contentious and complicated. 

The C-Band spectrum at issue is 500 megahertz between 3.7-4.2 GHz. The FCC has been exploring the potential repurposing of the C-Band in recent years. The agency issued a notice of proposed rulemaking last summer to gather information on the possibility of transitioning all or part of the C-Band to terrestrial wireless broadband use, possibly under the auspices of an auction. A previous proposed rulemaking in 2017 had sought information about whether the spectrum could be opened up for flexible use. The commission has made opening up additional mid-band spectrum a priority (along with the auction of millimeter wave spectrum) in its efforts to support U.S. deployments of 5G.

Major satellite operators have proposed a private auction of a portion of the spectrum; the C-Band Alliance, formed last year and made up of satellite providers, would conduct that auction. The satellite operators say they could rapidly clear up to 200 megahertz of spectrum (180 megahertz with a 20 megahertz guard band) for wireless terrestrial use, with some available as soon as 18 months after an auction.

Co-channel sharing of satellite C-Band spectrum is feasible with far smaller geographic protection areas than are currently in place, study published earlier this year— and trimming down the exclusion zones around satellite earth stations and enabling spectrum sharing could open up high-speed wireless broadband service for 80 million Americans, the study’s authors concluded.

The results of the study, which was backed by Google, Microsoft and the Wireless Internet Service Providers Association, were presented at a July event in Washington, D.C. Claude Aiken, president of WISPA, said that the 500 megahertz of C-Band spectrum is “currently quite underutilized as a result of overprotective policy” and that the study shows that sharing is feasible. Aiken said that if the C-Band spectrum were repacked and shared with satellite operations, it would result in multiple operators capable of providing gigabit or near-gigabit speeds to serve rural areas “nearly overnight.”

The lead author on the study, Professor Jeff Reed, founded the wireless research group at Virginia Tech and is a co-founder of Federated Wireless. Andy Clegg of Google noted during his introduction of Reed that the spectrum sharing in the CBRS band also involves coordination with fixed satellite service earth stations, as the FSS stations are one of the incumbent types in the band.

Clegg added that the FSS stations are the “poster child” for “super-sized”, overprotective geographic zones in which other uses of the spectrum are not permitted due to concerns about interference. Those zones, he said, have a radius of 150 kilometers or about 70,000 square miles around each station — which he said means that for every one FSS station, the protection zone is larger than 10 of the 50 U.S. states, under rules which were put in place decades ago and have not been revisited.

Federal Communications Commission Chair Ajit Pai said in a letter to Congress that a C-Band auction, tentatively set for December 2020, will be public and conducted by the FCC, dashing satellite operators’ hopes that they might be able to auction 300 megahertz of prime mid-band spectrum for terrestrial 5G services in an FCC-like auction with minimal involvement from the FCC.

Pai’s Nov. 18 letter came on the same day that U.S. Senators Roger Wicker (R-Miss.), who is chairman of the Senate Committee on Commerce, Science, and Transportation, and John Thune (R-S.D.), chairman of the Subcommittee on Communications, Technology, Innovation, and the Internet, introduced new legislation to require that the C-Band spectrum be auctioned publicly and at least 50% of the auction revenues go to taxpayers in the form of a payment to the U.S. Treasury.

In his letter, Pai outlined the four principles that the FCC should be guided by in the C-Band decision: free up significant spectrum for 5G; do it quickly; generate revenue for the federal government; and “protect the services that are currently delivered using the C-band so they can continue to be delivered to the American people.”

5925 MHz to 7125 MHz (6 GHz Band)

There’s a lot going on in the 6 GHz band. Incumbent uses include fixed satellite services, transmission from the Radio Astronomy Service, some WISPs, pipeline operators, local television transmission, power companies and public safety, among others–”criticals operations,” as CommScope’s Gibson put it. 

At the same time, a number of companies have drawn the FCC’s interest in extending Wi-Fi into the 6 GHz band. Facebook, Apple, Microsoft and Google has thrown its support behind providing additional room for Wi-Fi to grow, writing in a recent filing, that “Wi-Fi has become the single most important wireless technology for American consumers and businesses,” and that “moving forward and opening up the 6 GHz band is critical. It has been more than twenty years since new mid-band spectrum was made available for Wi-Fi and other unlicensed uses, causing a severe shortage for a wireless technology that handles 75% of mobile data traffic.”

In addition to the four tech giants, other companies which support 6 GHz for unlicensed use included chip companies such as Qualcomm, Broadcom, Intel, Marvell and Qorvo; cable companies including Charter Communications; the Wireless Internet Service Providers Association (WISPA); and other industry players such as Boingo Wireless, Cisco, Extreme Networks, HP and Hewlett Packard Enterprise, Philips and Sony.

However, another recent filing lays out some of the opposition to unlicensed use at 6 GHz from incumbents, who worry that the microwave links on which their networks depend will be at risk. That filing, from 11 associations including the Utilities Technology Council, International Association of Fire Chiefs and trade associations for gas, petroleum, railroads and rural electric cooperatives and nearly 60 individual utility companies, says that because of the risk to interference with their mission-critical networks, “unlicensed operations should only be permitted in the 6 GHz band if the Commission adopts more stringent interference protections for co-channel and adjacent channel microwave systems, including proven technology to mitigate the risk of interference by prior coordination of unlicensed operations.”

CommScope’s Mark Gibson noted the FCC “is still working on the rulemaking,” but he expects some movement in January or February 2020. “The proposal that’s on the table now is to chunk it into four chunks…with two types of operation for unlicensed,” standard power and low-power indoor. “It would be under the control of the automated frequency coordination system.”

Per work from the Wi-Fi Alliance, there are three potential AFC configurations. A third-party operator could control–”like what we’re doing with CBRS,” Gibson says; there could be an embedded AFC using on-device firmware and communicating with a database; or there could be a private cloud operating the whole thing. 

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