One of the keys to enabling the FCC's plan for spectrum sharing in the new 3.5 GHz Citizens Broadband Radio Service (CBRS) is the spectrum database, which will dynamically manage spectrum allocations on the fly, based on preset policies and spectrum availability to protect against interference that might negatively impact incumbents and priority users.
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In March, the FCC announced its plan to apply spectrum-sharing techniques to frequencies between 3550 MHz and 3650 MHz, (and possibly up to 3700 MHz) which some mobile operators hope to use for small cell deployments. The commission is proposing a three-tiered access and sharing model comprised of federal and non-federal incumbents, priority access licensees (PALs) and general authorized access (GAA) users, with the latter enabling unlicensed service for the public.
In order to have all three groups coexist without interference, a database--or what the FCC calls a "Spectrum Access System" or "SAS"--must be developed and launched.
Shahid Ahmed, managing director and North America communications industry and network practice lead at Accenture, said the 3.5 GHz database, or databases, will have to track and enable spectrum sharing based on three dimensions: geolocation, time and spectrum frequency. Devices will need to contact the SAS, provide their geolocation coordinates, request transmit approval and get assigned a usable frequency.
"I think the database technology's there. It's just a matter of transitioning to this model and getting all of the players together to agree on some of the policies," Ahmed said. He noted that transitioning incumbents to the new model will likely pose challenges.
Further, there remain lots of questions regarding the proposed 3.5 GHz database, including how it should be architected, how it will be managed and who should manage it, said Ahmed. The SAS "is going to be more or less like an HLR (home location register). It's going to be very dynamic, and it's going to have policies that it has to manage on a real-time basis," he noted.
Incumbents would be the top dogs, enjoying protection from everyone else. PALs would be protected from the general access users. However, "critical facilities" such as hospitals, public-safety organizations and local governments will be allowed to reserve up to 20 MHz of GAA spectrum for indoor use. "It could get very complicated very quickly," Ahmed noted.
The FCC has some experience overseeing spectrum sharing via the TV white-space (TVWS) spectrum that sits between TV broadcast channels. However, TVWS spectrum databases are relatively simple because they are quite static, while a 3.5 GHz database must be dynamic.
"White space tells us a lot about databases but not a lot about dynamic access and moving in and out of bands," observed Lynnette Luna, senior analyst at Current Analysis. She noted that the TVWS spectrum also is relatively uncrowded, making database management that much easier.
Yet despite the technological challenges, much of the industry is excited about the prospect of gaining access to the 3.5 GHz spectrum and proving that spectrum sharing can be used to free up for frequencies.
Enabling three tiers of access at 3.5 GHz provides "an important opportunity to test out the technical capabilities and business strategies of spectrum sharing," Luna said. "This is the way the FCC wants to go, but there are a lot of moving parts to it."
Spectrum scarcity is driving international interest in spectrum sharing, and all eyes are on the FCC's efforts. "There's a lot of spectrum to be used if you can share it," Luna said.
"We think spectrum sharing is the future," Ahmed said. "We don't need to stay in a model where everybody has specific access to specific frequencies."
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