The mobile network space is an ever-changing landscape, and AT&T is trying to keep up with the crowd by officially launching LTE-Advanced in at least one U.S. market. According to an interview with GigaOM, AT&T said it has officially launched a new LTE-Advanced network in Chicago by making use of a technology called "carrier aggregation." That sounds fancy, but it isn't an entirely new concept in mobile networks — the basic idea is that if a carrier doesn't have a large swath of spectrum that is continuous, carrier aggregation lets it pull together separate pieces of spectrum to form a single, faster connection.
The new network will make use of both 700MHz and 2100MHz (AWS) spectrum, at least in Chicago, together to offer a 15MHz wide downlink —that means a theoretical download speed of 110mbps. It doens't make the overall capacity of AT&T's network in these new markets any higher, but it does increase efficiency when you have more customers on a single pipe.
AT&T is the first carrier in the U.S. to make use of carrier aggregation (though Sprint's Spark network is doing similar things) and launch LTE-Advanced, but it's more out of necessity than innovation. Verizon and T-Mobile have swapped and purchased spectrum to have large chunks of continuous waves to have their networks on, offering faster speeds without any fancy tactics.
While an AT&T representitive wouldn't comment on precisly which markets were ready to receive LTE-Advanced next, they did say that multiple markets are already live or in testing where AT&T has fired up the new LTE network equipment. It makes sense for the carrier to keep things quiet right now, though, as only a single mobile hotspot device called the Unite can actually make use of the network. Samsung announced that the Galaxy S5 would support carrier aggregation for LTE-Advanced on U.S. networks, but didn't specify whether or not AT&T was one of them — things are quiet on both sides right now.