C-Band Spectrum Rollout for 5G and Aviation Altimeters

Robert D. Atkinson November 9, 2021
November 9, 2021

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One key to effective government is effective regulatory policy, and in many cases that requires thoughtful interagency coordination and collaboration. On that measure, the United States often appears to be dysfunctional. Case in point is a recent safety bulletin in which the Federal Aviation Administration (FAA) warned that a Federal Communications Commission (FCC) decision to allow wireless carriers to start using C-band spectrum in the 3.7 to 3.98 GHz range could potentially cause problems with airplane altimeters.

The FCC started the process of opening up an adjacent band for mobile broadband in 2011 and for this specific band in May 2018. It has reviewed the implications of opening up this band since 2017 and has received comments from a host of stakeholders, including the aviation industry and the FAA. As a result of that process, the FCC decided to authorize the use of the 3.7 to 3.98 GHz band in phases, starting in 46 markets on December 5, 2021, with operations limited to the lower portion of the band—from 3700 to 3800 MHz.

Yet, based on last-minute input from some in the aircraft industry the FAA issued its bulletin on November 2 expressing concerns about possible interference with airplane radio altimeters, which operate between 4200 and 4400 MHz. The bulletin called on aircraft manufacturers, radio altimeter manufacturers, operators, and pilots to take certain actions, mostly involving testing their equipment and reporting technical information to appropriate civil aviation authorities. In other words, the FAA (and industry groups) has had more than 3 years in which to study this issue and warn stakeholders if it found a problem with its planned deployment. Because of the FAA’s last-second scrambling, wireless broadband carriers have agreed to delay use of the band from December 2021 to January 2022.

It is important to note that whenever the FCC opens up bands of wireless spectrum for new uses, it carefully studies potential interference. For example, it undertook that process when it opened up the 3.55 to 3.7 band (just below the C-band), and it did so again for the 3.7 to 3.98 GHz band. Indeed, in its C-band order, the FCC wrote:

…we are providing a 220-megaherz guard band between new services in the lower C-band and radio altimeters and Wireless Avionics Intra-Communications services operating in the 4.2-4.4 GHz band. This is double the minimum guard band supported in initial comments by Boeing and [Aviation Spectrum Resources, Inc.].

The reason the FCC initially limited rollout to the lower end of the 3.7 to 3.98 GHz band was to minimize any interference and to ensure that if problems did occur they would be detected early. Moreover, it’s important to note that even when a user of spectrum operates within their assigned band, users of adjacent bands can experience interference if their “radios” are deficient and do not effectively limit “listening” to the band they are assigned to.

In these cases, the fault is not with the new operator (assuming that they are staying in their band, which wireless operators do); it is with the adjacent user. As such, any responsibility for upgrading deficient older altimeters lies with their owners. The analogy would be to two houses in a county with a noise ordinance that has been set at a certain level assuming houses have modern double-pane windows. If one of the houses has older single-pane windows and they hear music from a next-door neighbor that is within the country noise ordinance, that’s their fault. The other neighbor still should have the right to play music in accordance with county law.

To continue this analogy, it is all too often the case that the neighbor with the broken windows complains to the government that their neighbors’ music is too loud—even though it meets the requisite decibel standards—so the house playing the music should have to pay for them to install new windows. In the case of C-band spectrum, the broken windows are the older altimeters that may pick up interference from 5G operators in the adjacent band—and like neighbors with a single-pane window. In this case, it’s the responsibility of the owners of older, less accurate altimeters to fix them so that they operate in the band they were licensed for.

It’s also important to note that more than 40 nations around the world have opened up the 3.7 to 3.98 GHz band for 5G wireless—and according to the FAA, “There have not yet been proven reports of harmful interference due to wireless broadband operations internationally.” Likewise, commercial deployment in this range started in September 2019, and the FAA reports that there are “no known issues for altimeters to date.”

Finally, the FAA announced that it is “conducting a risk assessment to ascertain whether further mitigation is warranted.” Given the fact that the United States is competing globally in the 5G technology race, especially with China, is it is incumbent upon the FAA and FCC to wrap up this process as quickly as possible. While a one-month delay in rolling out C-band equipment is not the end of the world, additional delays will be more problematic. This is key because 5G wireless systems are expected to provide the connective tissue for many emerging technologies critical to productivity, innovation, and national competitiveness. It would be ironic and regrettable if, just as Congress passed a $1.2 billion infrastructure package, the Biden administration slows down the deployment of the wireless technology that represents the most important infrastructure for the country’s near-term future.