AT&T’s new souped-up LTE network is live in Chicago, but you’ll have to wait to use it

AT&T’s(s t) LTE network in the Windy City recently got a lot more powerful. Ma Bell is the first carrier in the U.S. to use a new LTE-Advanced technique called carrier aggregation to bond together two 4G networks, the end result being a big boost in speed to the device.

The new network configuration has gone live in several markets, AT&T SVP of Network Technologies Kris Rinne told me in a recent interview, but the only specific city she would identify was my hometown of Chicago. AT&T actually isn’t doing much to tout the network upgrade, and that’s likely due to the fact that only a handful of its customers can actually take advantage of it.

Rooftop cellular tower radios in a mobile network

Currently, AT&T has only one device that can support LTE carrier aggregation, a mobile hotspot called the Unite that it started selling late last year. Device makers have been waiting on a new generation of radio chipsets that support the technology, and now that those are available, they’re adding the feature to their smartphones as mobile operators bring these new souped-up networks online – starting in Asia. AT&T may be the first U.S. provider to use the technology, but all of the major operators have the technology in their roadmaps. Next up will be Sprint(s s) (there have been reports that Sprint is already using carrier aggregation in its Spark network, but that’s not the case).

The first smartphone we see that can tap AT&T’s upgraded network will most likely be Samsung’s new Galaxy S 5. At Mobile World Congress, Samsung confirmed that the S5 would support carrier aggregation on some U.S. networks when it goes on sale in April. AT&T is almost certainly on that list. More Android(s goog) devices with the technology will follow, though we’ll have to wait for a future generation of iPhone and iPad before we see it on an Apple(s aapl) device.

Samsung Galaxy S5 in blueSo what will these new aggregated networks do? In Chicago AT&T is basically running two LTE systems on two very disparate bands: 700 MHz and the 2100 MHz Advanced Wireless Services (AWS) frequencies. By aggregating those channels AT&T will be able to send data over a 15 MHz-wide downlink, increasing theoretical download speeds to around 110 Mbps. Though the overall capacity of AT&T’s network technically doesn’t change – it’s basically merging two roads into a single highway two-lane highway – it does gain some efficiency from running one big fat pipe rather than two smaller ones.

Though Rinne didn’t reveal what other cities have received the upgrade, she said that as AT&T brings online its second LTE network in new metro areas it will also begin the process of aggregating them. So it’s not too hard to guess where these upgrades are taking place. AT&T is building new LTE systems in its old 2G and 3G airwaves in Baltimore, Dallas, New York City, Philadelphia, San Francisco and Washington, D.C. And there are many other cities like Chicago where it owns clear AWS spectrum.

Verizon(s vz), T-Mobile(s tmus) and Sprint(s s) have broad swathes of contiguous spectrum, which they’re using to double the speeds of their networks. AT&T doesn’t have that luxury. As I wrote in my recent comparison of 4G networks in the U.S., AT&T has more of a patchwork of frequencies. But by using new LTE-Advanced technologies, AT&T is able to knit those spectral patches together. Ma Bell’s new network won’t be as a fast out of the gate compared to the monster systems T-Mobile and Verizon are currently launching, but the approach gives AT&T a lot of flexibility.

AT&T's LTE coverage March 2014

AT&T’s network coverage with LTE cities marked in black

As Ma Bell gains more spectrum through acquisition (such as its bid to buy Leap Wireless) and traffic on its HSPA networks wanes, it can gradually shift that bandwidth over to LTE, layering on more capacity and boost speeds. Chicago is good example of that strategy. When AT&T’s LTE service first launched here back in 2011, it offered pretty paltry 4G speeds, even when uncongested. But over the last three years, through the addition of more spectrum and now carrier aggregation, that network has turned into quite the powerhouse.

Rooftop cell photo courtesy of FlySi via Compfight cc