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What Is C-Band, and What Does It Mean for the Future of 5G?

For 5G to offer an experience that’s noticeably better than 4G, it needs broad, dedicated channels, ideally 50MHz or wider. That’s where C-band comes in — the frequency that might save 5G in the US.

By Sascha Segan

It came. You saw. Planes did not fall out of the sky.

AT&T and Verizon recently turned on their C-band 5G networks after months of dispute with the airlines and FAA. And, knock on wood, so far we’ve seen neither major flight disruptions nor any danger to air safety.

After two years of AT&T’s and Verizon’s “nationwide 5G” networks feeling much like 4G, C-band could finally majorly multiply speeds. But what is C-band, and what does it mean for 5G? Do you need a C-band phone? Is C-band a new frequency? Should you be scared of C-band? We can explain.

Recovering the Satellites

According to wireless testing firm Rohde and Schwarz, the C-band is all frequencies between 4 and 8GHz. When US wireless geeks talk about C-band, though, they’re talking about 3.7 to 4.2GHz—and specifically, in this case, the range from 3.7 to 3.98GHz.

This frequency had been used for satellite TV since the 1970s, but as C-band satellite reception requires “big, ugly dishes” up to 10 feet in diameter, it got largely replaced by more flexible systems with smaller dishes on the Ku band, such as Dish and DirecTV. C-band is currently used for the “satellite downlink” for broadcast television distribution.

With more advanced methods of digital encoding than they had in the 1970s, the satellite companies can now “repack” their broadcasts into the upper portion of the C-band, leaving the lower portion available for cellular companies to use.

This chart shows the primary wireless carrier user of each band; it does not include minor users.

C-band sits between the two Wi-Fi bands, which are at 2.4GHz and 5GHz. It’s slightly above and very similar to the 2.6GHz band that Clearwire and then Sprint used for 4G starting in 2007, and which T-Mobile currently uses for mid-band 5G. And it sits immediately above CBRS, a band from 3.55 to 3.7GHz that’s currently being deployed for 4G. So its transmission characteristics are very well known, and its safety is well established.

Most of the rest of the world started to use C-band already; in terms of the number of countries, it’s probably the most popular 5G band in the world. The International Telecommunication Union (ITU) chopped C-band up into three chunks: band n77, band n78, and band n79. Most European and Asian countries currently use n78, which stretches from 3.3 to 3.8GHz. We in the US will use n77, a larger band from 3.3 to 4.2GHz. Japan also already uses n77, so equipment and phones are out there.

Mid-band, including C-band, is the world’s most popular form of 5G.

Why Do We Need C-Band?

The state of 5G in the US is pretty rough right now. For 5G to offer an experience that’s noticeably better than 4G, it needs broad, dedicated channels, ideally 50MHz or wider. For 5G to cover entire cities, it needs to be on a frequency below about 6GHz, so it can get decent range from towers. Right now, AT&T and Verizon aren’t using any airwaves that fit these bills. Verizon may be able to pull something together using the new CBRS airwaves just below the C-band, but the jury’s still out on that. So by and large, AT&T and Verizon are delivering 5G that’s either no faster than LTE would be on the same frequencies or has poor range.

C-band can fix this. The government auctioned off 280MHz of airwaves that are likely to go up to about a half-mile from each tower, so plenty of bandwidth for several different wireless carriers to have solid 5G using mostly existing cell sites.

Verizon’s C-band network showed decent range in testing in a dense part of Queens, NY.

Cell distance is a tricky thing. This paper from 2006 estimates that 3.5GHz networks can go up to 1.2 miles from each site in an urban area and up to 6.2 miles in a rural area. According to CellMapper.net, there are up to 2.5 miles between T-Mobile’s 2.6GHz sites in suburban Dallas, but only 0.6 miles between sites where I live in Queens, New York. So I’m saying a half-mile for the 3.5GHz networks to be safe.

Our first tests of Verizon’s C-band show about a 0.37-mile range in very dense Queens, NY. That seems to be limited not by the power of the airwaves, though, but by Verizon not wanting its cell sites to interfere with each other. So half a mile in much of the country still feels good to me.

Who Will Use C-Band?

Generally, AT&T and Verizon.

The C-band frequencies will become available in two chunks. The first 100MHz are now available in 46 major markets across the US, covering about 60% of the US population. Verizon and AT&T split those “A block” airwaves 60/40. So Verizon will be just above, and AT&T just under, the 50MHz rule of thumb I’ve been using for real 5G differentiation.

Whether you’ll be able to use it yourself requires them to build out their cell sites, but they’ll have the spectrum.

The other 180MHz in the C-band is only scheduled to be cleared by the end of 2023. That includes more Verizon and AT&T spectrum, along with airwaves purchased by T-Mobile, US Cellular, and some local phone companies. At that point, Verizon will end up with an average of 160MHz of spectrum and AT&T with an average of 80MHz—both able to deliver a truly different 5G experience.

That calendar puts those networks on a late 2023 or 2024 launch. Those launches may happen earlier in 2023, though, if the precedent from the 600MHz band holds. T-Mobile took a lot of the 600MHz band, which was previously TV channels 14 to 55, in 2017. That had a clearance schedule that went out to July 2020. But T-Mobile worked with (and paid) broadcasters to clear early, getting New York City cleared more than a year before it was supposed to, for instance.

What About the Airlines?

The C-band networks were initially supposed to launch on Dec. 5, 2021, but the airline industry and the FAA argued that C-band signals would be picked up by airplane radio altimeters, which judge airplanes’ distance from the ground for landings in poor weather. Those altimeters aren’t designed to use the same band as the 5G network—the airlines’ frequency is 400MHz away from the 2022 launch networks—but some were designed not to filter out surrounding frequencies, because those frequencies weren’t being used as heavily at the time the altimeters were built.

The FAA and carriers agreed on “exclusion zones” two miles long around many airport runways where the carriers wouldn’t initially launch C-band. That caused Verizon to notch its initial projected population coverage from 100 million Americans down to 90 million, as it cut out parts of cities including Chicago, Dallas, New York, and San Diego that are too near the airports. Our tests show Verizon appears to be keeping the signal off well outside the two-mile boundary, at least in New York City.

The C-band exclusion zones extending from Midway Airport knock out some in-city Chicago neighborhoods. (Image: Rene Ramos)

The FAA is also working to verify and approve which altimeter models have the right filters, although some observers question why the agency couldn’t have done this a year ago when the spectrum was auctioned.

Once the networks launched on Jan. 19, the CEO of American Airlines said the conflict appeared to be past.

What About “C-Band Part Two?”

Call it C-band part two. There’s another 100MHz coming later in 2022, part of a separate auction of 3.45 to 3.55GHz frequencies very near what we’ve been calling C-band. Those airwaves were largely snapped up by AT&T, Dish, and T-Mobile, with US Cellular also grabbing some and Verizon sitting this one out.

There’s a lot of good news about 3.45GHz. It’s also in band n77, so the same phones will work with it as with the current C-band. It will be available nationwide in 2022, as opposed to just in 46 top metro areas. And it’s far enough away from those pesky altimeters that the airlines likely won’t have any complaints about it.

Because it’s a separate slice, it’s a little bit difficult to combine 3.45GHz with the C-band we’ve been otherwise talking about, in the same connection. What’s most likely to happen is that AT&T will use 3.45GHz to spread a C-band-like service nationwide outside those top 46 metro areas, into midsized and smaller cities that Verizon can’t turn on until 2024. Dish will use it as the backbone of its 5G network, if Dish ever launches a 5G network.

Otherwise, the characteristics of 3.45GHz are just like the other C-band, with similar devices, similar range, similar uses.

What Phones Support C-Band?

For a phone to support US C-band, it needs two components. Its hardware must support it, and it must be approved by the FCC to work on it. If the hardware supports it but it doesn’t yet have FCC approval, the phone’s maker can file for a “class 2 change” with the FCC, but they have to do it. Phone makers sometimes choose not to file for changes because they cost money; they’d rather just make a newer model and sell that. The key to look for on the spec sheet is whether it supports band n77, but the carriers haven’t certified all of those phones yet.

The iPhone 12 family was the first set of phones approved by the FCC for C-band. (Photo: Sascha Segan)

AT&T and Verizon have started by certifying the Samsung Galaxy S21 series, the Samsung Z Flip3 and Z Fold3, the iPhone 13 series, and the Google Pixel 6 and 6 Pro for C-band. AT&T has also added the Samsung Galaxy A13 to its list.

But many more popular phones can support C-band, if the carriers choose to enable it—most notably, the iPhone 12 series, the Google Pixel 5, and the Samsung Galaxy A32 5G. We’ll have to see whether the carriers are willing to support those devices with software updates, or if they just want consumers to buy new phones.

Do You Need a C-Band Phone?

Right now, deciding whether you need a C-band phone is about trying to foresee whether your carrier will launch a C-band network before you get a new phone.

  • AT&T users should think about their data performance now. If it’s fine, C-band isn’t urgent. If you want faster data in 2022 and live in one of the initial C-band markets, get a C-band phone.
  • Verizon is going in big on C-band. If you live in one of the 46 initial C-band markets and data performance is important to you, you should definitely look into getting a C-band phone.
  • T-Mobile doesn’t have any C-band airwaves that will be available before 2024. T-Mobile users shouldn’t concern themselves with C-band yet.
  • US Cellular is in the same boat as T-Mobile.

We’ll be tracking the C-band developments here on PCMag.com, and will tell you whether it’s available on every phone we review.

Originally published at https://www.pcmag.com.




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