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What is C Band 5G?

Basics Details History Aviation Safety Comments  1  

The Issue

The FAA and the airline industry have raised some concern that the new C Band mobile networks could interfere with radio altimeters in airplanes. Radio altimeters measure the distance between the plane and the ground, and can be critical instruments when planes land in poor weather. So if there is a chance of interference, that would be a legitimate safety issue.

Is this because radio altimeters operate within the C Band? No. They operate in a different but somewhat nearby band: 4.2 – 4.4 GHz. So how could there be interference? Well, nearly all radio equipment uses components called filters that allow the radio to "tune into" the correct frequencies and ignore others. Some radio equipment has better filters than others.

Apparently the radio altimeters in some planes have not-great filters. AT&T and Verizon refer to these in a letter to the FAA as "obsolete altimeters that, in the view of some aviation interests, do an abnormally poor job of filtering signals".

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The thing is, even the highest frequencies being deployed for mid-band 5G in the US are quite far from the lowest used for radio altimeters. The FCC considered that gap and the possibility of interference before deciding which new frequencies could be used for 5G, and even held back an extra "guard band", as an extra precaution. With the guard band, the total separation is 220 MHz, which is quite a lot in radio terms. In fact, for the part of C Band being deployed in 2022, the separation is over 400 MHz, which is huge.

On top of that, the mobile industry early on agreed to special restrictions, such as limiting power levels in some situations, as an additional precaution.

Over 40 countries around the world have already deployed 5G in these frequencies, with no evidence to date of interference with airplane systems. That includes counties serviced by US airlines.

The Battle

Nonetheless, the FAA raised a series of last-minute objections, asking to delay the launch of C Band 5G networks. Carriers agreed to some delays, but were frustrated that the FAA waited until the very last minute and was not proposing a reasonable path toward a firm launch date.

It's the last-minute part that seemed to bother AT&T and Verizon the most, and it's easy to understand why. Here is an excerpt from their letter to the FCC in the middle of the dispute:

Relying on the FCC's comprehensive rulemaking process and the C-Band Order, AT&T, Verizon, and others bid more than $80 billion on C-Band spectrum. ... AT&T and Verizon spent most of 2021 preparing to put the C-Band spectrum into service. ...

Amid all this activity, we were told for the first time late last year that the Federal Aviation Administration (FAA) and parts of the aviation community had concerns about the timing of our use of C-Band under the FCC's February 2020 order. The aviation community participated in the C-Band proceeding, and the FCC considered all their input and found that the use of the spectrum would cause no harmful interference to altimeters. Nonetheless, the FCC encouraged the aviation community to use the nearly two years before C-Band deployment to upgrade any altimeters that might not be properly designed to filter out frequencies far removed from the 4.2-4.4 GHz altimeter band. Inexplicably, the FAA and the aviation industry apparently did nothing following the February 2020 order or even after the C-Band auction closed in January 2021. In fact, it was not until November 2, 2021 that the FAA even issued a notice to begin collecting data about altimeters from the aviation industry.

(emphasis ours)

The initial carrier proposal to appease the FAA was for limited power levels (up to 50% lower than the FCC otherwise allows) below the horizon, and even lower power levels above the horizon. Even stricter power limits were proposed within a box extending over 1/2 mile from any part of an airport runway, and over one mile in the "final approach box" where radio altimeter readings are most critical. These voluntary restrictions would remain in place until July 5th, 2022.

The Solution

After some last-minute back-and-forth, the revised carrier proposal adopts even stricter rules equivalent to those in place in France. For six months, at 50 airports designated by the FAA, carriers will operate effectively zero C Band within a half-mile of runways. An additional zone of low-power restrictions will extend almost one mile from the sides of runways and over 1.5 miles from the ends of runways.

After two delays – from early December 2021 to early January 2022, then again to mid-January — the FAA finally agreed to this revised proposal on January 3rd, and promised no more delays. The new mid-band 5G networks will launch on or around January 19th.

But the FAA will still enforce new rules that limit flight activities that rely on the radio altimeter, which generally relates to limited visibility.

That seemed like the end of it, until major US airlines — struggling to catch up with new rules imposed by the FAA at the last minute — decided that complying with those now rules could result in over 1,000 flights canceled on some days. And Boeing — also struggling with the last-minute rules — was unable to guarantee that the altimeters on some of its most popular planes would meet new FAA standards.

This resulted in even more last-minute chaos. Major airlines around the world briefly cancelled flights to the US that used Boeing 777 aircraft. US airlines demanded a larger 5G-free zone around airports (extending up to two miles from runways) that might let them avoid the new rules. AT&T and Verizon relented, which means much less C Band 5G coverage in cities with airports near downtown.

The situation is still very fluid. The current restrictions on 5G deployment in the C Band are temporary, but it's unclear when they might lift, or what the final rules will be. The primary remedy is for specific models of radio altimeters on specific planes to be tested and certified as immune to this interference, and thus exempt from the stricter rules. This process is finally underway; on January 18th, 45% of commercial planes in the US had been cleared. By the next day, that number climbed to 62%, and then to 78% the day after that. 13 radio altimeter models have now been cleared, covering all major wide-body planes in the US and most narrow-body planes, including the popular Boeing 737 and Airbus A320. Theoretically, some smaller and/or older planes may be found deficient, and would need better radio filters installed before they can land in low visibility at an airport covered by C Band 5G.

Hopefully, the FAA can work out a plan to ease the restrictions in a timely way.

Going Forward

That covers the specific mid-band launches in January 2022, but what about the rest of mid-band?

First of all, T-Mobile's band 41 is at a much lower frequency that the FAA has no issue with. In fact, it's been in use for many, many years.

The lowest part of band 77, spanning 3.45 – 3.55 GHz, should also be a non-issue, since these are lower frequencies than the C Band, and lower than band 48 (3.55 – 3.65 GHz), which Verizon has been using for almost a year without issue. As lower frequencies, both of those bands are even farther away from the frequencies used by radio altimeters, so the FAA has no reason to be concerned about them.

But we may run into this issue again in late 2023 when the upper part of the C band is slated to become available for 5G. These frequencies — from 3.8 – 3.98 GHz — are 220 MHz away from the frequencies used by radio altimeters, instead of the 400 MHz separation of the frequencies launching in January 2022.

Hopefully, the FAA and the airline industry will take the opportunity over the coming months to settle the issue once and for all, ensuring planes are ready for not just July 2022, but also December 2023.

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