NTIA's 'go/no-go' proposal could open up more 3.5 GHz spectrum sharing

While FCC Chairman Tom Wheeler met with Assistant Secretary of Commerce for Communications and Information and NTIA Administrator Larry Strickling last week, NTIA released a report that outlines a possible roadmap to further sharing of the 3.5 GHz band.

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Click image to enlarge. Source: NTIA

It's all part of meeting the president's goal of making 500 megahertz of additional spectrum available for wireless broadband by 2020. On Friday, Wheeler and Strickling met to discuss the FCC and NTIA's collective work to identify and prioritize opportunities to increase spectrum availability, including for 5G and "other innovative uses," said FCC spokesman Neil Grace in a statement. "Both affirmed a continued commitment to taking the steps necessary to meet the President's goal of making 500 megahertz of additional spectrum available for wireless broadband by 2020," Grace said.

The same day, engineers from NTIA's Institute for Telecommunication Sciences (ITS) and Office of Spectrum Management (OSM) proposed what they describe as a simple and robust method to implement spectrum sharing between commercial communications systems and federal radar operations in the 3550-3650 MHz (3.5 GHz) band.

The report follows what they consider groundbreaking analysis and modeling techniques from ITS and OSM to shrink the exclusion/protection zones around government military sites. It also proposes a feasible methodology to meet the environmental sensing capability (ESC) requirements in the FCC rules. It calls for a potential approach to ESC monitor configuration that would send the associated Spectrum Access System (SAS) a simple "go/no-go" signal based on a predetermined threshold of detected radar power level. Once such a signal is sent, the SAS could adjust Citizens Broadband Radio Service Devices (CBSD) channel assignments to protect radar receivers.

The engineers say the method they're proposing will require further research, but it would allow for new commercial uses while protecting the security and integrity of military radars. "It also builds on our efforts to maximize use of spectrum while ensuring that federal users can continue to carry out vital missions for the American people," Keith Gremban, director of the ITS, said in a blog post.

It was a year ago this month that the FCC voted to adopt new spectrum sharing tools and policies to make 150 MHz of spectrum available for mobile broadband and other commercial uses in the 3.5 GHz band that previously was locked up by the U.S. Department of Defense (DoD). The FCC calls it the "Innovation Band" and it became possible after the FCC and DoD worked together and ultimately agreed to use advanced spectrum sharing technology.

The plan calls for the establishment of an ESC network of monitors working in concert with an SAS to protect offshore radar receivers from interference from the on-shore 3.5 GHz CBSD network.

"The need for ESC monitors and an associated SAS network controller to trigger actions for protection of offshore radar receivers begs the question of how the ESC/SAS would know that radars are present within potential interference zones of the terrestrial CBSD transmitters," the report states, noting that's assuming, per the conditions laid out in the FCC's public notice, that the radars will not provide the SAS with direct information that they are experiencing interference from CBSD devices. "One approach is to use a network of on-shore ESC monitors to sense signals coming from radar transmitters (co-located with radar receivers), and then infer, based on radar signal strength being received on-shore by ESC units, whether the radar receivers located off-shore may be receiving (or are about to receive) interference from the on-shore CBSDs that are in the vicinity of the ESCs."

The report shows a method to make that approach work. "The key is to understand that the propagation losses from any radar transmitter to any given ESC monitoring station will be equal to or less than the propagation losses in the reverse direction-- from terrestrial CBSD transmitters to the same radar receiver--provided that the ESCs are located near shorelines with sufficient antenna heights."

The report's authors – Frank Sanders, Edward Drocella and Robert Sole – say their approach will protect the radar receivers by making the SAS-controlled CBSD devices take mitigation action, such as possibly move channels, when necessary. They also recommend analyzing other ESC models and infrastructure outside the scope of their report.

For more:
- see this blog post

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