AT&T is hitting the Windy City like a hurricane. At several sites in downtown Chicago, we tasted AT&T's new LAA cellular network and got download speeds up to a scorching 537Mbps with a Samsung Galaxy S9.

We used a regular retail phone, standing on an ordinary street using the public network. This isn't a test site. Our AT&T experience makes for a wicked followup to our visit to a single T-Mobile LAA site in midtown Manhattan, and it shows that high-speed competition is heating up in major US cities.

LAA, or Licensed Assisted Access, lets wireless carriers use 5GHz unlicensed spectrum (considered by most people to be Wi-Fi spectrum) as part of their LTE networks. It's being installed by AT&T, T-Mobile, and Verizon, and slowly spreading across the most congested parts of major cities.

This is an urban solution, because LAA spectrum doesn't travel very far. The four cells we checked out in Chicago had radiuses of about 150 feet to 400 feet, which is on par with some of the big public Wi-Fi solutions we've seen. Also, like Wi-Fi, it isn't very good at penetrating walls.

AT&T is rolling out LAA and gigabit-class 4G technologies in several cities. LA, San Francisco, and Indianapolis are also getting the LAA treatment right now, and AT&T has previously said it intends to launch LAA in at least 24 cities in 2018.

We achieved our smashing result by aggregating 15MHz of LTE Band 2 spectrum, the old 1900MHz band, with 60MHz (three 20MHz carriers) of LAA.

The LAA small cell sites in downtown Chicago (above) fit onto light poles. Made primarily by Ericsson, they consist of a black cylinder at the top of the light pole with the antennas, and a larger black box about halfway down holding the radio equipment.

The four sites in Chicago showed different performance, based on their level of congestion and that of surrounding sites. According to AT&T, they're part of a C-RAN setup, which links together a bunch of small cell sites and gives them one, consolidated internet backhaul connection. That's much easier to set up than if you had to give every site its own connection to the internet, but it can mean the effects of congestion get spread across a whole C-RAN unit.

At Kinzie and Dearborn in River North, we averaged 273Mbps down. At the second site, by the Merchandise Mart a little ways away, we hit an average of 424.2Mbps. The third site, in the South Loop, averaged 468.6Mbps (including that one 537Mbps result), and the final site, also in the South Loop area, showed 290Mbps.

Above: Our Ookla Speedtest.net results on our Galaxy S9; a service mode screen showing the LAA Band 46 connection.

LAA improves downloads a lot. Uploads still run on the dedicated LTE spectrum: we got between 33 and 43Mbps down on all of the sites. The LAA sites also showed better latency than most LTE sites, by a few milliseconds.

Who needs 537Mbps on their cell phone? No one. In this case, speeds are a proxy for capacity. If our single device could get 537Mbps, that means 10 devices will chug along at a happy 50Mbps each without choking up the cell or the spectrum.

You'll need an LAA-compatible phone to run in this fast lane. That means a Samsung Galaxy S8, Note 8, or S9, their variants, a Motorola Z2 Force Edition, or an LG V30 ($799.99 at AT&T Internet) . Most notably, no iPhone yet has LAA.

Speedtest results on an iPhone X.

With iPhones traditionally strong at AT&T, we compared an unlocked (Qualcomm model) iPhone X in the same locations. We got 92Mbps, 93Mbps, 201Mbps, and 173Mbps downloads, respectively. The fastest single result was 205Mbps. That's all pretty fast, but it's a fraction of the speed and capacity LAA brings. So as these sites build out, iPhones without LAA may appear to choke up on the network more quickly than LAA-compatible phones. We expect this year's upcoming iPhones will support LAA.

We'll be looking for LAA as we drive around with Galaxy S9 phones on our Fastest Mobile Networks trip in May, which will take in several of AT&T's target cities.

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