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What is C Band 5G?

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Jan 14, 2022, 3:06 PM   by Rich Brome
updated Mar 10, 2022, 5:53 PM

There's a whole new kind of 5G being launched and talked about in the US in 2022: C Band. It's much faster than existing "nationwide" 5G, yet has better coverage than hard-to-find mmWave 5G. For AT&T and Verizon, it represents a huge leap forward in the 5G race. For their customers, it will mean the first time many of them will experience a significant difference between 4G and 5G. So what is C Band? Is it just hype or something to actually get excited about? Is "C Band" the best term for this? Where did this band come from? Does it pose a danger to planes? And where does T-Mobile fit into all this? We answer all those questions and more in this in-depth guide to everything C-Band.



To even talk about C Band, we have to translate the carrier branding around this new 5G service. All major US carriers have come up with their own brand for their faster flavor of 5G. AT&T calls it "5G+". Verizon calls it "Ultra Wideband", sometimes abbreviated "UW".

T-Mobile's equivalent brand is "Ultra Capacity" or "UC". It's technically not C Band, and not new, but it does include mid-band, which — as we'll explain further down — means it's roughly equivalent to C Band and should be part of the same discussion.

All three brands are a bit tricky because they include both C Band / mid-band and mmWave. mmWave is a very different frequency range with very different characteristics. But in most cases going forward, if you get noticeably faster 5G, it's probably going to be C Band / mid-band, because it has such broader coverage compared to mmWave.

Radio Frequency Bands

First of all, C Band is — as the name might imply — just a radio frequency band. It may seem like a whole new 5G technology since the speeds and coverage can be so different from the 5G you may be used to. But it's just the same 5G tech in a new set of radio frequencies.

Different radio frequencies have different characteristics due to basic physics. Low frequencies are inherently great for coverage (extending vast distances and through walls, etc.) but have limited ability to carry data at high speeds. High frequencies are the opposite: they don't reach as far, but are great for high-speed data.

Before 2022, AT&T and Verizon only offered 5G in two flavors: low-band and mmWave.

Low-band 5G offers "nationwide" coverage, but data speeds only slightly better than 4G. There are three reasons the data speeds of low-band 5G haven't been very impressive:

  1. Low-band 5G uses the same exact frequency bands that have been used for 4G (and 3G, and even 2G), so physics applies the same basic constraints on capacity to carry data.
  2. Since it's using the same bands, 5G technology must share those bands with 4G, a situation that's generally not ideal with most radio technologies.
  3. 5G isn't a radically new technology compared to 4G. It's better, more modern, and a necessary step forward technologically... but 5G is heavily based on 4G; you could think of it more like 4.5 G.

Then there's mmWave 5G, which is very fast. However, it operates at so much higher of a frequency range that it has trouble reaching very far or going through walls (or sometimes even windows). It's great for stadiums, arenas, train stations, and certain downtown areas of major cities, but it's not practical for anything approaching broad coverage.

Very high frequencies are only part of the reason mmWave can offer such fast data speeds; there are two other key reasons. First, the mmWave bands are dedicated to 5G and not shared with 4G.

Second, the mmWave bands are very large in terms of bandwidth. This refers to the difference between the upper and lower bounds of the band. A wider band can be thought of like a larger "pipe" that can carry more data.

C Band combines the best of both low-band and mmWave. The frequencies are closer to low-band than they are to mmWave, so coverage is more like low-band. But the frequencies are higher than low-band, and so a bit better-suited to fast data.

But more importantly than the frequency, C Band is a wide band, that is exclusive to 5G. That means a lot of capacity for very fast data, which is why it's such a big deal. For example, Verizon says customers should expect speeds up to 10 times faster compared to 4G.


C Band is actually better referred to as "mid-band", a broader term that also includes T-Mobile's band 41. C Band is a higher frequency than band 41, but it's close enough in frequency, bandwidth (both are very wide), and 5G implementation to be in the same category. C Band and band 41 also share most of the same differences compared to other bands, so it makes a lot of sense to think of them together as the "mid-band" category.

C Band ...the rest of band 77 band 41 600 MHz 1 GHz 2 GHz 3 GHz 4 GHz 30 GHz 40 GHz mmWave* note difference in scale mid-band low-band

Another reason "C Band" isn't an ideal term is that the FCC has also just auctioned off a key band just below C Band that may effectively merge with C Band to be part of the same band (band 77) when those frequencies are deployed for 5G networks over the next 2–3 years.

(Also, there isn't global consensus on the boundaries of the C Band. The US FCC defines it as 3.7 – 4.2 GHz, while the rest of the world defines it as 4.0 GHz – 8.0 GHz.)

While "C Band" is in the news today, it's actually a legacy term that makes more sense in the context of the satellite industry, the original owner of these frequencies. It's not the best term to use going forward when talking about 5G. That's why we'll mostly stick to "mid-band" for the remainder of this article.

Sub-6 and Other Terms

If you've heard the term "sub-6", that's a slightly shorter way of saying sub-6-GHz, which simply means all frequency bands below 6 GHz. Therefore sub-6 includes both low-band and mid-band. It's essentially another way of saying "not mmWave". When a company needs to convey that a phone doesn't support mmWave, it sounds better to say "supports sub-6 5G" instead of the negative.

That doesn't necessarily mean that "supports sub-6 5G" means a phone supports mid-band; many older 5G phones do not.

The C Band is also sometimes referred to as the 3.7 GHz band, and the two bands just below it are sometimes referred to as the 3.45 and 3.5 GHz bands.

The 3.45 GHz band was just auctioned off by the FCC. AT&T and Dish bought a ton of it, covering the whole lower 48. T-Mobile bought some, as well. We can expect these companies (especially AT&T) to launch 5G in this band in the next year or two, expanding their mid-band 5G service beyond bands like C Band and band 41.

The 3.5 GHz band is also called CBRS, and it's already been deployed by Verizon using 4G LTE technology. For this, they use band 48, which is a subset of band 77. So if you're with Verizon, you may already be using mid-band if your phone supports LTE 48, it's just not 5G. But according to Verizon, "there are plans to use CBRS spectrum for 5G." This would further expand Verizon's mid-band 5G network beyond C Band.

All of these together (C Band, 3.45 Ghz, and CBRS) are part of the larger band 77, which itself is sometimes referred to as the 3.5 GHz band (because that's roughly where it starts) or the 3.7 GHz band (since 3.7 GHz is close to its center).

Band 77

As I mentioned above, what matters isn't so much "C Band" but "mid-band", and all you really need to know about mid-band if you're with Verizon or AT&T is band 77. This is the international designation for the single frequency band that covers C Band and the related bands just below it (3.45 GHz and CBRS), as they relate to 5G networks. In other words, band 77 includes all of the frequencies used for most new mid-band 5G launches in the US in 2022 – 2024.

(In other parts of the world, band 78 covers similar frequencies.)

As I mentioned, with T-Mobile, mid-band means band 41... for now. Band 41 is separate from band 77, but band 77 will also apply to T-Mobile soon. T-Mobile also participated in the FCC's recent mid-band auctions and plans to deploy 5G in band 77 around the end of 2023 (or potentially even sooner in the 3.45 GHz band), adding even more mid-band bandwidth to their 5G network.

Phone Compatibility

If a phone supports band 77 — also referred to in phone specs as n77, 5G 77, or NR 77 — then it should support these new, faster C Band 5G networks being launched now by Verizon and AT&T (and later on, T-Mobile).

Band 77 support in phones may also cover the new 3.45 GHz band that was just auctioned off by the FCC, to be deployed in the next few years by AT&T and probably T-Mobile. But, not all phones that "support band 77" have actually been cleared to use the whole band. Some are only configured and tested to use the upper, C Band part. Ditto for when Verizon deploys 5G in the CBRS (3.5 GHz) band: some current phones with band 77 might support that as well. Unfortunately, the industry hasn't yet come up with a good way to communicate this in phone specs.

You can check which specific phones support which bands, including band 77, (but again, with the above caveat,) by looking up phone specs here on Phone Scoop. Generally, phones started supporting band 77 (for C Band, at least) in late 2020, and most new 5G phones released by AT&T and Verizon in 2021 already support band 77. (The only exceptions are some of the very cheapest models, like the AT&T Radiant Max.) We expect essentially all new 5G phones for these carriers in 2022 to support band 77.

(Compare this to mmWave, which — unlike mid-band — adds not-insignificant cost to a phone. That's why not all 5G phones support mmWave, and we expect that to continue to be the case in 2022.)

If you're with T-Mobile, you may want a phone that supports band 77 starting in December 2023. So if you're shopping now for a phone that you plan to keep for two or more years, keep that in mind.

Launch & Expansion Plans

As of December 2021, the FCC is allowing AT&T and Verizon to launch C Band 5G in 46 of the top 50 metro areas in the US. By agreement with the FAA, the actual launch date will be on or around January 19th, 2022. Washington DC, Baltimore, Atlanta, Denver, and most rural areas aren't included in this phase. Those areas will have to wait until December 2023 for C Band, although 5G could launch in other mid-band frequencies (that are part of band 77) before then.

Verizon has announced that its new mid-band 5G network will cover 90 million people at launch, and plans to expand that to 175 million over the next two years. After December 2023, Verizon plans to expand this coverage to 250 million people in 2024.

AT&T launched its mid-band 5G service alongside Verizon on January 19th, in "limited parts of" Dallas/Fort Worth, Houston, Austin, Chicago, Detroit, Jacksonville, Orlando, and Miami. They promise to have a mid-band footprint of "200 million people by the end of 2023". They also "plan to deploy more C-Band than anyone else in the U.S. by the end of 2023." So it's clear they intend to keep up (or perhaps catch up) in the mid-band 5G race over the next two years.

T-Mobile has offered mid-band service for some time. They recently announced that their mid-band 5G now covers 200 million people, and should cover 250 million by the end of 2022. It's too early to say what their C Band deployment will look like around the end of 2023, but they already lead in mid-band 5G, and C Band can only make it better.

It's also too early to say exactly when AT&T (and perhaps T-Mobile) will deploy 5G in the new 3.45 GHz band. It could be before the end of 2022, or it could take a little longer.


Forget the term "C Band". "Mid-band" is the term you should pay attention to. For most people, mid-band 5G means faster 5G with broader coverage. If you expected 5G to be faster, this is when it might finally meet those expectations.

Phones that support band 77 support AT&T and Verizon's new mid-band 5G networks launching in January 2022, as well as additional mid-band 5G launches over the next 2–3 years. Phones that support 5G in band 41 (AKA n41 or NR 41) support T-Mobile's mid-band 5G, which launched a while ago. T-Mobile will also add band 77 to its mid-band 5G network around the end of 2023.

Verizon brands their mid-band 5G network "Ultra Wideband", while AT&T calls it "5G+" and T-Mobile calls it "Ultra Capacity". All three also use these brands to refer to mmWave 5G, which is even faster, but has limited coverage and fewer phones support it. This is all compared to low-band 5G (AKA "nationwide" or "extended range" 5G), which is not much faster than 4G and therefore doesn't get the special branding.

Those are the basics. There are three additional sections of this article if you want to learn even more about mid-band. The Details section gets into the weeds about specific radio frequencies and the layout of mid-band, including different parts of the C Band and the bands just below it. In the History section, we answer the question "where did C Band come from?", and explain how the FCC is dealing with the fact that none of these frequency bands are empty. In Aviation Safety, we address the issues raised by the FAA that have been in the news lately. And if you still have more questions about mid-band, please don't hesitate to ask in the comments.


What radio frequencies are we actually talking about, here? We'll focus on mid-band, but let's be thorough and start with low-band for context.


Low-band includes a large number of different bands, many of which have been in use for cellular networks for a very long time. The range in the US is around 600 – 2,400 MHz, or 0.6 – 2.4 GHz. (If 2.4 GHz sounds familiar, it's because low-band ends around where Bluetooth and Wi-Fi frequencies begin.) But that range includes AT&T's band 30, a relatively small (narrow), high-frequency outlier. Band 30 aside, the bulk of low-band in the US is 0.6 – 2.2 GHz, with a significant gap between 0.9 and 1.7 GHz.

When you see phones that support 4G LTE in bands 2, 4, 5, 12, 13, 30, 66, and 71, that's all low-band. And 5G has been deployed in low-band in the US as well, in bands 2, 5, 66, 25, 66, and 71. That's what carriers have thus far referred to as "nationwide" or "extended range" 5G; it's not a lot faster than 4G, but it is 5G.


Mid-band in the US currently means 2.5 – 4.0 GHz. Within that, the C Band as it applies to 5G is 3.7 – 3.98 GHz, which the FCC auctioned off in Auction 107.

The C Band isn't just one lump of spectrum, though, it has sub-bands within it. Satellite operators are still using some of it, and are vacating different parts of the band in phases, a process called clearing.

C Band (B & C), auction 107Dec. 2023 C Band (A), auction 107Dec. 2021 band 48, auction 105Mar. 2021 3.45 GHz band, auction 110 600 MHz 3.5 GHz 4.0 GHz band 77

The first, lower-frequency section of C Band cleared in December 2021, opening up 3.7 – 3.8 GHz for use by AT&T and Verizon in 46 of the top 50 areas (cities). (This section of the C Band is the "A" blocks. Read more about "blocks" below, and there's more detail on the clearing process and how it relates to satellite operators in the History section.)

December 2023 is the next big date. That's when C Band will first be available for 5G in more rural areas, as well as full availability in Washington/Baltimore, Atlanta, and Denver. That's also when the upper part of the band — 3.8 – 3.98 GHz — will clear, opening up even more bandwidth for carriers. It's also when T-Mobile will join the C Band party, since the licenses they won in Auction 107 are in this upper part of the C Band.

But as we touched on earlier, the C Band described above is only the upper part of the larger band 77, and band 77 is what will end up mattering in the end. The FCC is auctioning off the lower parts of band 77 in auctions 105 and 110.

Auction 105 took place in 2020 and covers 3.55 – 3.65 GHz. (This band technically extends up to 3.7 GHz, but licenses giving companies priority access were only made available up to 3.65 GHz.) In most key areas, Verizon bought up as much of this as they were allowed. Licenses were issued in March 2021, and Verizon has already deployed it for 4G LTE using band 48, a narrower subset of band 77. Verizon also plans to deploy 5G in this band in the future. When that happens, this band will complement C Band and further improve Verizon's mid-band 5G service.

Dish (Boost) was also a big player in Auction 105. When they launch their planned standalone 5G network, this band will presumably play an important role.

Auction 110 concluded in January 2022, and covers 3.45 – 3.55 GHz. AT&T and Dish won licenses nationwide, while T-Mobile snapped up some key areas around the country. This will likely be used by AT&T to add more capacity to its mid-band 5G network nationwide, and by T-Mobile to beef up capacity in key areas where its mid-band 5G network might otherwise be relatively weak. 5G could potentially be deployed in this band before the end of 2022, giving AT&T a way to expand its mid-band 5G coverage before December 2023. (Or it could take longer.)

Then there's T-Mobile's band 41, which spans around 2.5 – 2.7 GHz. It's been around for a while, first used for Sprint's ill-fated WiMAX network, then re-deployed for LTE and now re-deployed again for 5G by T-Mobile. It qualifies as mid-band as well. (For a visual, the graphic on the previous page shows band 41.)

T-Mobile owns a lot of band 41, but they don't own it all. A good amount of it is unclaimed, and the FCC will be auctioning off the rest of it in the upcoming Auction 108. The dates for that auction should be announced soon. It will be interesting to see if another major company deploys 5G in band 41.

Blocks and Regions

As with most radio bands allocated for mobile networks, these bands are sub-divided into blocks. When the FCC auctions off bands, they actually auction off specific blocks. And the bands are also divvied up geographically. Some bands/blocks are auctioned off in relatively large regions, while other bands are divided up into much smaller geographic divisions.

Some bands for mobile networks are "paired", with separate frequencies for transmitting vs. receiving. This is not the case with mid-band; all mid-band frequencies in the US are unpaired, meaning they are simple, singular chunks of radio spectrum. This type of band is typically used with TDD technology, which means the network tower and mobile device "take turns" sending and receiving.

Auction 107 for the C Band is evenly divided into 14 20-MHz blocks. The lowest five of those (the ones launching this month) are blocks A1 – A5, spanning 100 MHz (3.7 – 3.8 GHz). The remainder (clearing in Dec. 2023) are blocks B1 – B5 and C1 – C4, spanning 3.8 – 3.98 GHz.

The satellite operators have actually cleared the lower 120 MHz of the C Band, but block B1 (3.80 – 3.82 Ghz) will serve as an empty "guard band" to prevent interference between 5G and satellite operations between now and the end of 2023.

Auction 107 used PEAs (partial economic areas) as its geographical divisions. 406 PEAs cover the contiguous states and DC. These frequencies will not be available for 5G in Hawaii, Alaska, nor US territories. Only 46 of the top 50 PEAs cleared in Dec. 2021. Everyone that lives in the other 360 (mostly more rural) PEAs will have to wait until late 2023 to see C Band 5G from AT&T and Verizon.

Why only 46 of the top 50? Well, the satellite industry has to get out of the way before these frequencies can be used for 5G, and that's trickier where there are facilities on the ground that essentially control the old C Band satellites still in use. Unfortunately, the exempted cities are a few major ones: the Washington DC / Baltimore area, Atlanta, and Denver. If you live in these areas, you might have to wait until the end of 2023 for C Band 5G from AT&T and Verizon, although 5G could potentially launch before then in other mid-band frequencies (from auctions 110 and 105). Update: Verizon has found a way to launch 5G in C Band in those cities in 2022.

Auction 110, spanning 3.45 – 3.55 GHz, is also split up into the same 406 PEAs. Again, states and territories outside of the contiguous 48 states are left out of this one. The blocks are different, though. Here, the band is split into ten equal blocks of ten MHz each (designated blocks A – J). 10 MHz is relatively small for 5G, so carriers can combine several into one larger span.

AT&T bought four licenses (the maximum allowed by the FCC for this auction) in all 406 PEAs, giving them 40 MHz of new bandwidth nationwide, which should significantly boost their mid-band 5G network when deployed. Dish bought closer to three licenses per PEA in Auction 110.

Auction 105, covering 3.55 –3.65 GHz (band 48 / CBRS), was also divided up into ten equal blocks of 10 MHz each for priority access licenses, but the FCC only auctioned off seven blocks in any given area. And those areas are much smaller: auction 105 was divided up by county, not PEA. Verizon bought the maximum of four blocks in most key places.

Auction 108 should take place later in 2022, auctioning off the unused parts of band 41. The way this band is split up into blocks is rather complex. It was originally divided up into small, oddly-defined blocks, with a separate band called BRS wedged in the middle. In Auction 108, it's split into three larger channels. "New Channel 1" and "New Channel 2" take up the bottom portion of the band, spanning 2.5 – 2.6 GHz. They're each around 50 MHz wide. Oddball "New Channel 3" is smaller and broken into two parts, to weave around that BRS band. Geographically, this whole band is split up by county, like the CBRS band from Auction 105.


Finally, for complete context, we should mention mmWave, which in the US currently means 27.5 – 40 GHz. Now you might be thinking "whoa, that's a big jump from the other frequencies!" Well, yes... yes it is. And that's why mmWave has such different properties, like range measured in feet instead of miles, and why it has difficulty penetrating glass in some cases. The band numbers for mmWave in the US are 260 and 261.


Where did the C Band come from? Radio frequency bands cannot simply be created out of thin air. (Well, radio waves are just air, in a sense, but....) Nearly all useful frequencies were allocated to something specific many decades ago. So when mobile networks need more bandwidth to handle more devices, using more data, the FCC has to find frequency bands that can be re-allocated from something else.

C Band

In the case of C Band, that something was satellite communications, mainly satellite TV.

The steady evolution of television technology has been a huge benefit to the mobile industry over the last decade. In the 4G era, it was the death of analog broadcast TV (the kind you picked up with "bunny ears" or even a large antenna on your roof, if you're old enough to remember such things) that made room for "new" frequency bands allocated to 4G networks. Most people had moved on to cable, satellite, or streaming. And for the over-the-air TV signals still being broadcast, new digital signals were more efficient and took up less of the airwaves. That's what made room for bands 12, 13, and 71. Those bands are now the foundation of 4G (and in some cases, low-band 5G) networks in the US.

As it was with analog broadcast TV, the same has now happened with satellite TV. C Band is the original, old-school band for TV broadcasts from space. To receive it, you needed your own massive, 10-foot dish. (Hey, kids: Yes, this was a thing!) But fewer people are using satellite TV services. What remains of the satellite TV industry has moved on to modern digital signals and/or the Ku band, which uses much smaller dishes on the ground. So the C Band was sitting there under-utilized, and the frequencies were ideal for new mobile networks.

But "under-utilized" does not mean abandoned. Satellite companies still owned FCC licenses for C Band frequencies — a legal contract — and they are still using them. Crucially, they had satellite hardware in orbit that was designed to use those specific frequencies and none other.

So the FCC had to broker a deal where the mobile companies would effectively pay the satellite companies to take over the lower part of the C Band, pay for it to happen sooner rather than later (an "accelerated" schedule), and, in some cases, pay for some of them to launch new, replacement satellites.

None of that was cheap. Intelsat and SES together are launching ten new satellites to space to make this happen. There are also a bunch of things that need to happen on the ground, such as replacing equipment and installing filters. The satellite companies need to coordinate these upgrades with their customers, a complex and expensive process.

Fortunately, the satellite companies were able to effectively stop using the lowest part of the C Band (at least in ways that would interfere with 5G in most populated areas) before the new satellites were fully operational. This is the first phase of the "clearing" process, and what allows AT&T and Verizon to start using the 3.7 – 3.8 GHz part of the C Band (the A blocks) in most metro areas now.

Most... but not all. As we mentioned, some key cities like DC/Baltimore, Atlanta, and Denver are exempted from this first phase. That's because this transition process needs to work around not only existing satellites in orbit, but the corresponding Telemetry, Tracking, and Command (TT&C) sites those satellites communicate with on earth (often via C Band frequencies).

The remainder of the C Band, 3.8 – 3.98 GHz (the B and C blocks) won't become available until the new satellites are up and running. And the TT&C sites need to be consolidated and/or reconfigured in order for those three metro areas to open up for 5G. The deadline for those things is December 2023.

At that point, the clearing process will be complete, and satellite companies still using the C Band will be restricted to 4.0 – 4.2 GHz. But with modern digital technology, that should be enough to do everything they did before with 3.7 – 4.2 GHz. That includes digital TV broadcasts and satellite data services.

Sharing the 3.5 GHz Bands

As for the lower portion of band 77 (below C Band), there were — and remain — a variety of existing, or "incumbent", users of these frequencies. These include the Department of Defense, some RADAR systems, some ground stations that receive signals from satellites in orbit, and more.

Some of these incumbent users are relocating to other frequencies. However, some incumbent users will continue using these frequencies, and wireless carriers will need to share these frequencies with those incumbents.

For the 3.55 – 3.65 GHz portion (AKA CBRS, from Auction 105), automated commercial clearinghouses called Spectrum Access Systems (SASs) coordinate this sharing, allowing Verizon to use to it for 4G LTE service (in band 48) — and soon, 5G — whenever it's not being used by an incumbent.

The process to commercialize the CBRS band started in 2012, but Auction 105 didn't conclude until August 2020, and Verizon wasn't permitted to deploy 4G in this band until March 2021.

Another, lower level of priority allows others to use this band without a license, although they must use an SAS to make sure they don't interfere with incumbents or licensees like Verizon, and SASs can charge fees.

The 3.45 GHz band (spanning 3.45 – 3.55 GHz) was enabled by the Beat China by Harnessing Important, National Airwaves (Beat CHINA) for 5G Act of 2020. It directed the federal government to clear federal users out of this band wherever possible, and to auction these frequencies off for 5G. The NTIA (National Telecommunications and Information Administration) and FCC did just that. The FCC concluded Auction 110 for this band in January 2022.

The FCC has declared that, in "most" areas of the country, license winners (AT&T was the biggest) will have unrestricted access to the 3.45 GHz band. However, some federal users — the Department of Defense in particular — will retain some priority access to the band. This will only apply to certain areas, such as near military bases, including San Diego. This will require carriers to defer to the DoD or share frequencies in part of the band and/or during certain time periods (expected to last 1-2 months each year). The process for this coordination will be more manual than the SAS system for CBRS.

Aviation Safety 

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".

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.

view article organized across multiple pages

About the author, Rich Brome:

Editor in Chief Rich became fascinated with cell phones in 1999, creating mobile web sites for phones with tiny black-and-white displays and obsessing over new phone models. Realizing a need for better info about phones, he started Phone Scoop in 2001, and has been helming the site ever since. Rich has spent two decades researching and covering every detail of the phone industry, traveling the world to tour factories, interview CEOs, and get every last spec and photo Phone Scoop readers have come to expect. As an industry veteran, Rich is a respected voice on phone technology of the past, present, and future.


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bones boy

Jan 19, 2022, 9:54 AM

Brilliant stuff Rich!

Thanks for posting this.
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