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No 'A-ha' Moment

Preliminary Testing of C-Band 5G/Altimeter Not Showing Harmful Interference: NTIA

Preliminary federal testing of possible incompatibility issues between airborne radar altimeter receivers in the 4200-4400 MHz band and 5G transmitters operating in the 3700-3980 MHz band isn't showing interference from the C-band 5G signals, Frank Sanders, senior technical fellow at NTIA's Institute for Telecommunication Sciences, told a connected aviation conference Wednesday in Reston, Virginia. He said if further testing does find interference, a technical fix appears to be available via filtering of the altimeters and/or the 5G transmitters.

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The work of the Joint Interagency 5G Radar Altimeter Interference Group has looked at 5G radios from AT&T and Verizon, with a third manufacturer's equipment to come, he said. Measurements are scheduled for June 9 for that 5G radio model, meaning there should be good airborne field strength data and wideband emission spectra for all three in the 3700-3980 MHz band by later this month, per NTIA. The data, including anonymized identification of particular receivers and 5G radio transmitters, should be released by August, it said.

Emission spectra collected show a sharp cut-off filtering above 4 GHz for both makers' 5G radios, and if the third shows the same "5G emissions above 4 GHz seem to not be a big problem," Sanders said. The group is still analyzing data, but there has been "no overt, 'a-ha, oh my God' moment," he said. The Interference Group includes the FCC, DOD, Homeland Security, Department of Transportation, NTIA, various cellular carriers, radar altimeter makers, aircraft makers and airlines, NTIA said.

If unwanted 5G emissions are found to cause harmful interference to the altimeter receivers at 4300 MHz, better 4200 MHz filters on the transmitters appear to be a fix to the problem, Sanders said. He said if 5G emissions below 4200 MHz cause interference to some receivers, those receivers apparently can work with more effective filtering below 3980 MHz, such as with better RF front-end filter, or they can be replaced.

The testing involved transmission of 5G signals at power levels of up to 50 kilowatts coming from a base station, with both fixed-wing aircraft and helicopters flying over or by. Tests done in March and April at Utah's Hill Air Force Base and Texas' Majors featured a variety of aircraft including a Boeing 737, 777, a UH-60 Black Hawk helicopter, an FA-18 jet and a C-17 cargo aircraft, NTIA said. In those tests, the aircraft flew a route repeatedly with the 5G transmitter off and then on, with the aim of seeing if there were altimeter data stream differences, the agency said. The distance from the aircraft to the 5G transmitter was from 200 feet for helicopter flights to 1,300 feet for the fixed-wing aircraft, it said. Separate testing at the Table Mountain Radio Quiet Zone in Colorado had a Black Hawk and Raven R-44 helicopter carrying radar altimeters, as well as differential GPS and laser altimeter equipment for independent elevation data. The helicopters, at heights of a few feet above ground level to 1,300 feet, flew around, above and past the 5G arrays again at lateral distances of 200 to 1,300 feet. There also were laboratory bench tests of mostly military model altimeter receivers with 5G-type signals, it said.

Sanders said if harmful interference is detected, one next step is determining the power level that harms not-so-robust receivers and then calculating the distance at which the effective isotropic radiated power on a 5G tower triggers that inference threshold. If that distance is a few feet, it's not a problem because aircraft flying that close to the tower have a bigger imminent threat of colliding with the tower, but a much larger distance poses a big problem.

Ideally, spurious 5G emissions are filtered above 4 GHz, while altimeter receivers' RF front-end responses get effectively filtered blow 4 GHz, NTIA said. Altimeter receivers often use minimal RF front-end filtering below 4 GHz, NTIA said. They can be prone to slowly rejecting power until the filter frequency is below 3700 MHz -- that slow "roll-off" being partly a legacy of the C-band having been historically quiet, the agency said.

The lack of reports of altimeter interference in other countries where the C band has been used for years in wireless networks, including seven countries that allow use at higher power levels than the FCC authorized, "makes one wonder," Rysavy Research President Peter Rysavy said. The 220 MHz guard band is "unprecedented" and should be more than enough to protect any well-designed altimeter system, though some are less well designed with minimal filtering, he said. He said the real-world analysis being done now should have been started years ago since the World Radiocommunication Conference identified the C band for future use in 2003.

Odds are there could be further spectrum-related conflicts between aviation and terrestrial mobile interests, experts said. Anthony Rios, president of navigation equipment company Freeflight Systems, said there was an assumption in the aviation community some frequency bands near safety-of-flight operations were well protected and walled off from other uses. "Going forward, that's not true" because demands for bandwidth for internet connectivity will surely further encroach, Rios said, citing Freeflight work on an altimeter using digital signal processing that would identify interference more precisely.

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