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How NTIA Research Powered a Groundbreaking Spectrum Sharing Effort

May 1, 2019

How can we get more use out of the radio spectrum? One way is by sharing radio bands between users who have never shared before. Consider radio frequencies near 3.5 GHz. Until recently, that part of the spectrum was used almost entirely by U.S. government radars, many of them on Navy aircraft carriers, enabling the same kind of air traffic control for the carriers as radars on land do at airports.

Now, we are preparing for new arrivals in the 3.5 GHz spectrum: communication systems such as cell phones. Operating on land, the new systems will be sharing frequencies with the Navy radars at sea.

More than decade ago, NTIA helped to blaze the trail for this kind of spectrum sharing when it did studies on exactly what it takes to share frequencies between regular radios and radars. Now NTIA has applied that knowledge to the new 3.5 GHz spectrum sharing. Industry and government, including NTIA, have developed a strategy to share 3.5 GHz without any interference from the new shore-based radios to the Navy’s aircraft carrier traffic control radars.

Here’s how it will work: When industry builds out a new 3.5 GHz network on shore, it will also build a network of shoreline radar detectors designed to see the Navy radar’s signal. When one of the Navy’s radars sails within about 120 miles of a detector, the station will see the radar’s signal, note its frequency, and alert the local on-shore communication network to immediately vacate that frequency.

So, how does the government know that these industry-designed and industry-built radar detectors will work? First, the detectors have been designed to meet the government requirement of radar receiver noise level for detection sensitivity.

Second, the government has tested the radar detectors itself. For this job, NTIA’s Institute for Telecommunication Sciences (ITS) in Boulder, Colorado, teamed with several companies who plan to build and operate the radar detectors. Late last fall, ITS engineers worked with industry engineers for several weeks in Boulder to test the new radar detector designs. For these tests, ITS designed its own automated system that exposed each radar detector to thousands of different sorts of radar pulses at the low detection level required to protect the Navy’s current and future radars.

The testing was technically grueling. Not only did each radar detector have to see thousands of low-power radar pulses successfully, but the detectors also had to keep functioning in the presence of high-power radar pulses that were equivalent to what you would see if you were just 6 miles from a radar transmitting a billion watts of power.

In the end, the radar detectors passed all of their tests, and the companies sent those results to the FCC, along with their requests to use these radar detectors all along all of America’s coasts. On Monday, the FCC approved three detectors, called Environmental Sensing Capabilities (ESCs), for use in the 3.5 GHz band.

With the detectors in place, Americans on land will be able to use the 3.5 GHz spectrum for cellular connections, while U.S. Navy aviators off-shore will continue to land on aircraft carriers with assurance that their controller’s radars are following them every inch of their way.