AT&T and Verizon finally fired up vital components of their 5G networks in the United States on Wednesday. Mostly.

The two companies had already agreed twice to delaying the activation of the parts of their networks that operated on the so-called C-band, because the U.S. Federal Aviation Administration (FAA) had raised concerns about the spectrum’s usage.

The C-band stretches between either 4 to 8 GHz or 3.7-4.2 GHz, depending on who you’re asking. (The IEEE considers it to be the former, while the U.S. Federal Communications Commission says it’s the latter). Regardless, the issue is that swath of spectrum hovering around either side of that 4 GHz mark. Above it are the frequencies that airplanes use and below it are frequencies opened up for use by wireless network operators to meet the growing bandwidth demands of their 5G networks.

What’s the problem with 5G and planes?

FAA has raised specific concerns over airplanes’ radio altimeters, which help planes (and their pilots) determine how far above the ground an aircraft is by bouncing a signal off the ground below and timing how long it takes to return to the plane. Such data are crucial when the plane is taking off and landing, particularly when visibility is low: at night and in fog or rain.

Therefore anything that potentially messes with radio altimeter signals could be bad news. If other entities are using the same frequencies (between 4.2 and 4.4 GHz), altimeters could be affected and return incorrect altitude measurements or worse, have their signals blocked entirely.

It’s important to note that the C-band frequencies being used by AT&T and Verizon are not in that same 4.2 to 4.4 GHz band. Both companies’ C-band allotments are between 3.7 and 3.98 GHz. The concern raised by the FAA is whether those frequencies are too close. Signals broadcast on frequencies that are similar, but not exact, to one another can still cause interference, although not as severe as if they were on the same frequency.

So what happens now that AT&T and Verizon are turning on their C-band radios?

For now, AT&T and Verizon aren’t turning on C-band radios close to airports, to avoid interfering with take offs and landings. After the companies indicated that they would not delay switching on their C-band spectrum for a third time, some airlines had begun to cancel flights. Most of those flights were rescheduled after AT&T and Verizon’s decision to limit C-band usage near airports. In the end, fewer than 200 flights were cancelled on the first day of C-band operation.

Meanwhile, the FAA is working to clear radio altimeters already in use. By the time AT&T and Verizon had turned on their radios, the agency had okayed five different altimeters used in certain Boeing and Airbus planes. In total, the FAA estimates that 62 percent of the U.S. commercial fleet can still safely land even in low-visibility situations where 5G C-band radios are operating. AT&T and Verizon agreed on Tuesday (the day before their C-band activation delay was set to expire) to keep C-band radios turned off near airports, to serve as an additional precaution while the FAA assesses the remaining radio altimeters in use.

The affected cell towers largely belong to Verizon, which has agreed to keep 5G radios on roughly 500 of its towers switched off—about 10 percent of its total C-band deployment. Those towers, and the smaller number owned by AT&T, will remain switched off until the two companies and the FAA arrive at a more permanent solution. It’s not clear yet how long that pause will last, or what a permanent solution might entail beyond the altimeter vetting the FAA is already conducting.

Wait a second—what about T-Mobile?

There are three big mobile operators in the United States, and the C-band kerfuffle has only involved AT&T and Verizon. That’s because T-Mobile lucked out.

In 2020, T-Mobile and Sprint completed a drawn-out merger process. When it was done, the newly merged T-Mobile had an abundance of so-called “mid-band spectrum.” This includes, but isn’t limited to, C-band spectrum.

One of the most important things to understand about 5G is that, to deliver the promised downlink speeds (up to 1 terabit per second!), it requires more bandwidth than 3G or 4G networks. Thus, cellular network radios have been creeping into higher and higher frequencies on the radio spectrum. These higher frequencies, although they don’t travel as far as the traditional frequencies used for cellular communications, can carry a lot more data per hertz.

As it so happens, T-Mobile had all the mid-band spectrum it needed to begin rolling out 5G networks courtesy of the merger. And that mid-band spectrum is nowhere near the frequencies used by radio altimeters—it’s centered around 2.5 GHz. AT&T and Verizon, meanwhile, acquired their problematic mid-band spectrum from FCC auctions, in part to compete with T-Mobile’s extensive existing mid-band spectrum.

Have airports in other countries faced this problem?

Nope. This has been a uniquely U.S. problem. That’s because different countries allocate radio spectrum in different ways for different uses. When 5G was being developed, and it became clear that mid-band frequencies would play an important role, the specific frequency bands that would be used were not defined in order to avoid selecting frequencies that might be available in some countries but already assigned for military or scientific or other uses in others.

In Europe, for example, 5G mid-band rollouts have proceeded without much concern for radio altimeters, because the spectrum allocated is at just slightly lower frequencies (3.4 to 3.8 GHz in Europe, as opposed to the mentioned 3.7 to 3.98 GHz in the United States). Meanwhile, countries like Canada have installed buffer zones like the ones AT&T and Verizon have agreed to. The Australian Communications and Media Authority has said it believes that a 200 MHz guard band (like the one in the U.S.) between 5G networks and radio altimeters is sufficient in itself.

And it remains to be seen whether the FAA is acting simply out of an abundance of caution. Currently, Ireland, Denmark, and Finland have operating 5G networks with mid-band signals that are more powerful than approved in the United States, with no effect on altimeters.

Source: IEEE Spectrum Telecom Channel