It's been nine months since the FCC opened the 3.5GHz CBRS spectrum band to full commercial operations, enabling government entities and commercial interests to share valuable midband spectrum. And so far there appear to have been zero instances of commercial networks interfering with the US Navy or any other incumbent user of the spectrum band.

Meaning, the first-in-the-world spectrum-sharing technology used in the CBRS spectrum band appears to be working.

"We did not receive reports of interference from any federal or commercial incumbents," said Federated Wireless CTO Kurt Schaubach. Federated is one of several companies that operate the Spectrum Access System (SAS) that coordinates sharing in the band.

Dave Wright, who heads up spectrum policy and standards for CommScope and is president of the CBRS Alliance trade group, reported similar findings. "There have been no reports of interference to incumbents and we have no reason to believe they will be at any risk of interference as deployments increase," he said.

The FCC – the US government agency that oversees all commercial spectrum usage – did not dispute the assertions. However, an FCC representative noted in an emailed statement that the agency's job is not to review or act upon every report made to every SAS. Instead, its role is to require that every CBRS radio user work with one of the band's SAS providers.

Usage of the CBRS band is divided into three groups: Incumbent users are at the top, followed by those who have paid for Priority Access Licenses (PALs), and then those who use the spectrum without a license. The FCC allocates at least 80MHz of CBRS spectrum for General Authorized Access (unlicensed) use in every location, unless incumbents are already using the spectrum there.

Thus, it appears SAS vendors and others using the spectrum have been able to keep everyone in their lane. And that could pave the way for sharing in other spectrum bands.

Safety first

From a national security standpoint, the most important users of CBRS spectrum are US Navy radar operators. For that reason, every SAS must include access to an Environmental Sensing Capability (ESC), which comprises a set of sensors deployed along the US coastline to securely detect incumbent use. The ESC sends this information to SAS providers so that they can reroute users in the band if necessary. Every SAS must include an ESC as part of its solution, either owned or licensed. Google and CommScope are sharing an ESC; Federated has its own.

Companies using CBRS spectrum need to pay the SAS providers, usually on a per-radio basis. They generally pay less for CBRS Category A radios, meant for indoor use with a shorter range, than they do for longer-range Category B radios. Not all CBRS radios currently work with all Spectrum Access Systems, so to some extent the choice of radio vendor determines the choice of SAS.

The FCC recently conducted an auction of PAL CBRS licenses, with Verizon, Dish Network, Comcast and Charter walking away with the most valuable licenses.

— Martha DeGrasse, special to Light Reading. Follow her @mardegrasse