The U.S. Department of Defense (DoD) recently agreed to share 100 MHz of mid-band spectrum with the private sector, in a move that analysts believe could be a sign of things to come. 

Sharing Spectrum in 5G and 6G

“It will most likely happen in the U.S. more and more because it’s an easier way than to fully clear something,” said Roger Entner, lead analyst at Recon Analytics, who nevertheless believes there’s room for improvement, likening the situation to inconveniently having to split amenities with a roommate.

“You still have to share the kitchen and bathroom and all that. It’s not going to make anybody happy, right?” he asked. “When we look forward to 6G, one of the priorities would be to create a standard that allows coexistence of multiple spectrum owners so that existing users can continue to use the spectrum and we can dynamically overlay it.”

Dynamic spectrum sharing takes different forms that may not be all that related. For example, several telecom companies, like Verizon, operate their 4G and 5G networks in the same band at the same time, as they roll out the latter. Meanwhile, the Citizens Broadband Radio Service (CBRS) is a spectrum sharing plan on more of a macro level. As Entner is suggesting, there’s potential room for improvement to make it dynamic in its own right.

FCC Authorizes Full Commercial Deployment of CBRS

The Federal Communications Commission (FCC) authorized full commercial deployment of the CBRS, a 150 MHz portion of the 3.5 GHz band, early this year. While the CBRS is in its relative infancy in that regard, the same is true for 5G, having its initial deployment in 2019. Initially poised to benefit 4G, the CBRS should help with the roll out and development of 5G in the months and years ahead.

The DOD’s 100 MHz (3.45-3.55 GHz) is in some ways an extension of the recently concluded CBRS auction (3.55-3.65 GHz) in its intended use. Researcher Alan Weissberger points out how, altogether, a great deal of mid-band spectrum is being released to 5G network providers, combined with the highly anticipated C-band auction in December (3.7-3.98 GHz). Furthermore, the Wireless Internet Service Providers Association (WISPA) believes the CBRS makes for a good framework for how the DoD’s 100 MHz should be used.

The issue is as follows: The DoD’s 100 MHz, which is expected to be auctioned off in 2021 and released under a spectrum-sharing framework, is also used by defense radar services. Meanwhile, U.S. Navy radar systems separately use CBRS frequencies. Even if the Navy only does so on the coast or at sea, allowing the use of the frequencies elsewhere in the country, Entner says it will be interesting to see how the situation develops.

“… More than a half of Americans live within a hundred miles of the coast, right? And then current 5G standards don’t allow for a dynamic spectrum sharing situation with somebody like the Navy. And so it might be time-based,” he said, arguing the situation is less than ideal, as 5G relies on exclusive spectrum use as do other Commercial Mobile Radio Service (CMRS) technologies.

The Trick to Effective Spectrum Sharing Co-Existence

Time is indeed one of many factors, which administrators of the in-development Spectrum Access System (SAS) are looking into, for effectively managing shared spectrum. Google, Sony, CommScope, Federated Wireless, and Amdocs have been approved by the FCC to undertake the role. An SAS can be considered a sort of “spectrum Airbnb,” according to Jennifer McCarthy, the VP of legal advocacy at Federated Wireless.

To illustrate, once the Priority Access Licenses (PALs) they bid on are issued, the CBRS auction winners can transfer them when they’re not in use. An entity can log on to the SAS and choose between whatever spectrum is available. With regard to the Navy, Environmental Sensing Capability (ESC) sensors along the coast alert admins when CBRS spectrum is in use, so no conflicts ensue.

In theory, it makes for efficient and effective co-existence. In practice, according to Entner, dynamic spectrum sharing is a possibility too, whether it’s with regard to the CBRS or the spectrum the DoD is sharing. For example, technology used in soldiers’ tactical communication gear may serve as a model.

“… When they invade a foreign country, that country isn’t usually clearing spectrum for soldiers to make it nice and convenient for them, but basically these systems grab whatever piece of spectrum is not utilized at the time… spectrum hop so that they can’t be intercepted and whenever they get jammed or whenever there is a band that is being heavily used, there’s no difference between heavy using and jamming,” he said.

“These systems simply avoid these spectrum bands. If we can use that same kind of technology in 6G, our spectrum allocation process will become significantly easier. And the bands that we can use will become by magnitudes bigger.”

Whatever the FCC ultimately decides, spectrum sharing in some form is the end goal. After all, the FCC’s plan is literally to Facilitate America’s Superiority in 5G Technology (FAST). Getting as much spectrum as possible into the hands of market players is the first of three key components to the plan. The other two are to update infrastructure policy and modernize outdated regulations. Dynamic spectrum sharing, at least in principle, addresses all three.