RF spectrum sharing between U.S. military, public wireless users to be put to the test

The U.S. Federal Communications Com­mission (FCC), with help from the National Institute of Standards and Technology (NIST) and other agencies, is about to open up a 150 MHz-wide swath of radio frequency () spectrum traditionally reserved for the military and allow it to be shared with mobile operators.

Long coveted by mobile operators and considered to be a valuable yet underused asset, the 3.5 GHz band of the electromagnetic spectrum, a.k.a. the “Innovation Band,” will now be shared with commercial users when the U.S. Navy’s not using it for mission-critical offshore radar operations.

This move marks the first time that commercial broadband users will be able to “share spectrum dynamically with government users and, if it works, the FCC may allocate other currently protected RF bands for shared use,” says Michael Souryal, lead for the spectrum sharing support project within NIST’s Communications Technology Laboratory. “More spectrum sharing could provide less-congested wireless channels for densely populated areas and more reliable connections for advanced communications needs such as 5G wireless and Internet of Things [IoT] applications.”

Wireless communications and mobile operations place an enormous drain on the spectrum, which is a finite resource that’s largely already allocated. As users continue to embrace and use more mobile devices, the spectrum will eventually run out of space.

For years, long-term evolution () equipment vendors and service providers such as AT&T, Eriksson, Google, Nokia, Qualcomm, Sony, and Verizon have lobbied heavily for access to this band – between 3550 and 3700 MHz – because they want the opportunity to expand product markets and give end users better coverage and higher data-rate speeds in a variety of environments in which service is traditionally weak.

The rules of the FCC-regulated 3.5 GHz Citizens’ Broadband Radio Service (CBRS) state that the Navy retains the first right to the band and private use will only occur during its downtimes. Others will be granted access using a three-tier priority allocation structure: 1) incumbent users such as the U.S. Navy; 2) LTE providers and other organizations which will pay license fees for the right to share; 3) and general users.

In February 2018, the Wireless Innovation Forum’s Spectrum Sharing Committee (WINNF SSC) approved 10 standards for operating the service. “The completion of the CBRS Baseline Standards represents an important milestone for spectrum sharing – unleashing the band’s potential for innovation,” says Paul Challoner, vice president of Network Product Solutions for Ericsson Inc. USA.

One important factor behind what’s making all of this possible is an algorithm that NIST developed to protect military incumbent users; a NIST-designed computer reference model of that algorithm will be an integral part of the certification process.

For example, the NIST model simulates 45,000 LTE smaller-size networks (known as small cells) using the 3.5 GHz band in the northeastern U.S. In response to a simulated need for the band by an offshore Navy vessel, the model calculates which small cells must be shut down and those that can continue to transmit. These simulations, along with others that model wireless networks in other U.S. coastal regions, will allow the FCC to test and evaluate how effectively commercial LTE providers can actually share the band with the Navy. (Figure 1.)

The RF spectrum will continue to be a fascinating space to watch. Other researchers, including several at the Data Science Institute (DSI) at Columbia University, recently received a grant from the National Science Foundation to develop energy-efficient sensors that will allow mobile and wireless device users to tap into, unused channels within the RF spectrum and enable future communications systems to share the spectrum.

“We’ll use all of the data-science tools we possess – machine learning, neural networks, algorithms, and advanced computation techniques, in conjunction with new hardware devices – to sense pieces of the RF spectrum as they become available,” says John Wright, a DSI affiliate and electrical engineering professor.

Peter Kinget, an electrical engineering professor at Columbia who specializes in and RF integrated circuits, will design circuits that can create snapshots of a large portion of the spectrum. Wright will then design algorithms to reconstruct the spectrum and help design a more energy-efficient sensor.

“It will be exciting to see how much more progress we can make using new algorithms built on the latest insights in ,” says Kinget.

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Figure 1: NIST simulation shows wireless users of shared RF spectrum in coastal Mass. Colored markers identify users that can continue operating (blue) or must be shut off (red) to accommodate a priority need for wireless band by naval vessels within an offshore “designated protection area” (gray). Credit: Michael Souryal/NIST.

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