CBRS users learn the ins and outs of Spectrum Access Systems

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CBRS spectrum is driving interest in private wireless networks for a wide range of users, from utilities to schools to wireless internet service providers. In some ways CBRS is like a more powerful version of Wi-Fi, but there are a few important differences. One major difference is access — although some users can use the spectrum without paying for a license, all users need to pay for the software that “clears” the spectrum for them. Companies that are using CBRS say getting this part of the process right can be a challenge.

Spectrum Access Systems are the frequency coordination systems that manage spectrum sharing on a dynamic, as-needed basis for three sets of users: those who access generally available spectrum without a license, those who hold priority access licenses, and incumbent users (mostly operators of U.S. Naval radar systems.) Five companies operate Spectrum Access Systems (SAS): Federated Wireless, Google, CommScope, Amdocs, and Sony. A sixth, Key Bridge Wireless, has a system in field trials.

Every SAS provider must develop or license an Environmental Sensing Capability (ESC) to detect the presence of incumbent users in the spectrum bands. These are networks of sensors built along the U.S. coastlines to detect Naval radar transmissions. Federated Wireless and CommScope have been approved by the FCC to operate ESCs, and Key Bridge is working on certification. Google, which uses CommScope’s ESC for its SAS, has asked the FCC to consider abandoning the ESC system in favor of a system that would put the Department of Defense in charge of dynamically allocating the spectrum.

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For entities that are experimenting with CBRS, learning how to share the spectrum with the government can be one of the hardest parts of the process. Some companies rely on their radio or core network vendors to manage the SAS, while others are finding that a direct relationship with the SAS providers is needed.

CellTex’s experience

CellTex Networks, a wireless internet service provider (WISP) serving south central Texas, has transmitters close enough to the Gulf of Mexico to potentially interfere with the U.S. Navy. When the WISP decided to use unlicensed CBRS spectrum, it initially turned to its network equipment vendor to manage the relationship with SAS provider Federated Wireless. The vendor paid Federated and CellTex paid the vendor, but the WISP soon realized that it needed a direct relationship with a SAS provider. When the SAS took CellTex radios off the air to avoid interference, the WISP had no one to call for more information.

“If we get cut off we need to talk to someone,” said Greg Huber, manager at CellTex. CellTex has more than 3,000 subscribers, and currently about 1,000 of them are using CBRS spectrum for home internet access. The rest are served by the 5.8 GHz band.

CellTex decided to switch to Google, but still struggled to understand the reasons behind service interruptions caused by the SAS. Finally, CellTex moved back to Federated, this time through a direct relationship with the company. 

Greg Racino, VP of communication systems at Alcon DTS, is a certified CBRS installer and consultant to CellTex. He said the move to a direct relationship with Federated has made a huge difference. “Sometimes they call me and tell me I’ve got a problem before I even know it,” Racino said.

Federated Wireless CTO Kurt Schaubach said that technically there is little difference for customers who work directly with Federated versus those who go through an equipment provider. “They access the combination of the Spectrum Access System and the ESC through an API,” he explained. “That API is exposed to the customer directly through a business relationship that we have with the customer, or through their OEM.”

Although the technology is the same either way, the experience can be different for network operators. CellTex’s Huber said that when his company first started using CBRS spectrum, it learned about service interruptions through its OEM’s network management software, but the alerts came without explanation. Now, his team gets information about exactly when and why the SAS detects possible interference.

Some CBRS users will not experience significant impacts from their Spectrum Access Systems because they are far enough inland to naturally avoid interference with the U.S. Navy. But according to Schaubach, interference can be an issue up to 200 miles from the coast. He estimates that up to 80% of the U.S. population lives in areas where new CBRS users could potentially create interference with incumbents. So far, however, the Spectrum Access Systems have done their job. During the first nine months of operation, both Federated and Google said they received no reports of commercial CBRS networks interfering with incumbent users.

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