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Airlines’ faulty altimeters spur FCC plan to regulate wireless receivers

Digitally generated image of glowing circles that form a data tunnel

Getty Images | Andriy Onufriyenko

The Federal Communications Commission unanimously voted to launch an inquiry into poorly designed wireless devices that receive transmissions from outside their allotted frequencies. The Notice of Inquiry (NOI) approved Thursday could result in new receiver regulations and is the first major step in the FCC’s quest to prevent future conflicts like the high-profile battle between the aviation and cellular industries, in which a 5G rollout was delayed because airplane altimeters receive transmissions from the wrong spectrum band.

The FCC said it will “explore options for promoting improvements in radio frequency (RF) receiver performance, including through use of incentives, industry-led voluntary approaches, commission policy and guidance, or regulatory requirements.” The inquiry will also “gather up-to-date information on receiver performance, advances in receiver technologies, and various approaches for promoting development and adoption of more interference-resilient receivers while fostering innovation in the marketplace.”

In her statement before the vote, FCC Chairwoman Jessica Rosenworcel said, “Receivers that are not sufficiently resilient [to interference] can make it more difficult to introduce additional services in the same or adjacent airwaves [and thus] diminish the spectral environment and shut out new uses before they even begin.”

Initial comments will be due 45 days after the Notice of Inquiry is published in the Federal Register, and reply comments will be due 75 days after publication. The docket can be found here.

Airplane altimeters receive signals from other bands

As we’ve reported in numerous articles, some of the altimeters used in airplanes to measure altitude are apparently incapable of filtering out transmissions from frequencies assigned to wireless carriers for 5G. The altimeters’ poor performance led the FCC to set aside a 220 MHz guard band between the wireless carriers’ C-Band spectrum (3.7 to 3.98 GHz) and the frequencies allotted to altimeters (4.2 GHz to 4.4 GHz).

The 220 MHz guard band is actually 400 MHz in practice this year because AT&T and Verizon have not yet deployed above 3.8 GHz. Despite that unusually large buffer, the Federal Aviation Administration said that altimeters couldn’t avoid receiving 5G transmissions and forced the carriers to delay the rollout originally scheduled for December 2021. The FCC had urged the aviation industry to fix their altimeters in February 2020 when the commission approved the spectrum for cellular use. The carriers’ C-Band rollout eventually moved ahead but with additional temporary restrictions near airports.

The FCC requires transmitters to broadcast only in their assigned frequencies but has been too lenient on receiver performance, Rosenworcel said at yesterday’s meeting. “To avoid harmful interference, we typically have rules about how and when transmitters can operate,” Rosenworcel said. “But wireless communications systems involve transmitters and receivers… so we need to rethink our approach to spectrum policy and move beyond just transmitters and consider receivers, too.”

The Notice of Inquiry mentioned both the C-Band fight and the Ligado/GPS controversy that involved a much smaller guard band of 23 MHz. “In these cases, the ability of incumbent service receivers to reject signals outside their intended band has been directly relevant to the timing and scope of the introduction of new services,” the FCC said.

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