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Given that President Obama delivered his State of the Union address on Tuesday, I figured this was as good a time as any for an update on the state of LTE in the U.S.

Since 4G technologies arrived early here, LTE probably seems like old news. New 4G market launches are hardly cause for headlines and LTE is pretty much standard in any new smartphone or connected tablet. But the carriers have been busy in other ways over the last year.

They’ve expanded their networks into new spectrum and implemented new technologies that squeeze more capacity, speed and range out of their existing towers. While it might be easy to view LTE as simply a status marker — either you’ve got it or you don’t — the reality is that no two LTE networks are the same.

This is my attempt to explain what the four nationwide carriers have done to differentiate their networks in the last year. I’m not trying to make a judgment about who has the “best” network per se, and I’m not delving too much into speed tests or other benchmarks. But I’ll describe what exactly each of these carriers has built and what those technologies will mean for the consumer mobile subscriber.

Verizon’s new network beast

Verizon Wireless(s vz)(s vod) was the first major operator to launch LTE, and last year it started showing its age. Its speeds started slowing down as more than 40 million LTE devices began connecting to its networks. In some major markets, the LTE network got so overloaded that it began kicking people down to 3G.

But right after Verizon installed the last base station of 4G network No. 1 in June, it started building a second, much more impressive one. While the first network was focused on coverage, No. 2 is focused on creating capacity in major cities where mobile data demand is highest.

In many major cities like New York, Chicago, Boston and Seattle, Verizon has built new 40MHz systems, tripling its capacity. In some cities in the western U.S. — notably San Francisco and Los Angeles — Verizon was only able to field 20MHz or 30MHz systems, but even those deployments equate to at least a doubling of capacity.

What this means is Verizon is now able to stack tens of millions more LTE devices onto its networks without overloading them. And for the speed-obsessed out there, Verizon’s network has become the one to latch onto. Early tests clocked downlink connections of 80 Mbps, though the network is sure to slow down as it fills up with subscribers.

Verizon is uncharacteristically quiet about the rollout, and that’s likely because it will be a while before all of its customers can access it. To connect to the new network, subscribers need a Verizon device that support the 1700 MHz/2100 MHz Advanced Wireless Service (AWS) band. About 20 percent of Verizon’s smartphone base — including the latest iPhones and iPads, Motorola’s(s goog) new Droids and the latest Samsung smartphones — can today, but nearly all new devices going forward will have the necessary radios.

T-Mobile’s LTE miracle

T-Mobile has been boasting a lot about its 4G networks of late, and in truth it has a lot to brag about. The country’s smallest national carrier went from zero to nationwide LTE in just eight months, and in November it doubled the capacity of its network in at least 40 major cities.

T-Mobile’s 20MHz network is pretty much on par with AT&T’s network as well as Verizon’s first generation network. But because of T-Mobile’s small size, its fewer 4G customers have a lot more bandwidth to play with. The result is T-Mobile’s current customers are starting to see consistently higher speeds than the other carriers.

That led T-Mobile CEO John Legere to claim that T-Mobile has the fastest network in the country “bar none,” which is a bit misleading. T-Mobile’s numbers are based on Ookla speed tests across the current installed base of smartphones, not the newest capabilities of today’s devices and networks. Here’s how that translates: if you compare two older iPhones on both networks today, T-Mobile’s will most likely be the fastest. If you’re buying a new iPhone 5s, Verizon probably will give you far faster speeds.

Legere’s bravado aside, T-Mobile has an intense network, and it’s only going to get more powerful in the next year. The “Un-carrier” plans to double its network capacity once again in several dozen cities, which will give it an LTE monster to match Verizon’s.

The big knock on T-Mobile has always been its lack of mobile broadband coverage outside of the big cities. But if you happen to live in one of those cities, you’re set when it comes to 4G. For the foreseeable future T-Mobile will have capacity out the wazoo, allowing it to pile millions more subscribers onto its networks while maintaining impressive speeds. And by the end of the year, it truly will have the fastest network in the country.

AT&T plays Dr. Frankenstein

To be honest, AT&T’s days of making big 4G headlines are probably over. Ma Bell had a big run over the summer after several independent studies all crowned AT&T the 4G speed king, but since then it’s been forced to back down from those claims.

AT&T simply lacks the big hunks of contiguous airwaves necessary to deploy the 40MHz behemoths that Verizon and T-Mobile are building. But that doesn’t mean AT&T lacks resources. It has spectrum in bands all over the map, and it’s using every megahertz it can. It’s cannibalizing old 2G and 3G capacity for LTE in the 1900MHz PCS band and it’s tapping into the virgin bandwidth locked up in its AWS licenses. It has boutique airwaves at 2.3GHz and it’s eyeballing the cellular band as well.

By stitching all of those frequencies into the same system, AT&T won’t get the speediest of networks, but it will get a lot more bandwidth to divide among its customers. Basically AT&T is adding many more lanes to its mobile broadband highway even though the speed limit will remain the same. What’s more, AT&T is delving into new technologies like small cells, which will surgically insert bandwidth into the highest-demand niches of its footprint, and self-optimizing networks (SON), which will allow cells to literally grow and shrink, following subscribers as they move through the network.

In its marketing, AT&T has stopped focusing on speed and is now hyping its network reliability, which makes sense given its situation. Sure, the speed demons among us should look to T-Mobile and Verizon, but for many subscribers the more important thing is maintaining a consistent, resilient connection at a respectable speed. And that’s where AT&T’s network is heading.

Sprint fans its Spark

Spark is an apt name for Sprint’s new 4G network. Spark could just as easily explode into a roaring blaze as it might fizzle into nothingness. Sprint has been making a big deal about the new tri-band system it launched last fall as an overlay to its existing LTE network, but at least for now, Spark is nothing to brag about.

Sprint is advertising Spark as a revolution in mobile broadband, combining three disparate bands in a kind of super-LTE system. First off, Spark isn’t really accessing three LTE bands, rather just two: its main LTE network in the PCS band and the 2.5 GHz airwaves it got when it took over Clearwire. The third band — the 800 MHz band it cannibalized from its old Nextel networks — won’t come online until later this year.

Second, Spark isn’t splicing together those bands (a technique known as carrier aggregation) it’s just running them as separate networks similar to the frequency patchwork AT&T is stitching together. Devices either access one network or the other, not both simultaneously.

Finally, Sprint is advertising 50 Mbps speeds on Spark, but those are really peak speeds. When pressed, Sprint said it expects average speeds of 5-12 Mbps. Spark is basically an average network by U.S. standards, and that average network is available only in 12 cities. The primary LTE network Sprint is building uses only 10MHz of spectrum, supporting half the capacity of any of its competitors, and it’s not even nationwide (Sprint has yet to launch LTE in San Francisco).

The sad thing is Sprint has the spectrum resources to build the biggest, baddest LTE network in the country. It just refuses to do so. As of now, Sprint’s mobile broadband service is the slowest in the country, it has the least amount of capacity and it has the smallest coverage footprint. Maybe one day Sprint will truly live up to it’s 4G potential, but I doubt its customers will wait that long.

LTE image courtesy of Shutterstock user Inq; Frankenstein photo courtesy of Flickr user annabellaphoto