By: Gigaom
T-Mobile’s plan to supercharge LTE: A whole lot of antennas
Exclusive to GigaOM: Over the next 12 months, T-Mobile USA will bolt thousands of new LTE antennas to its cell towers, utilizing a technique called 4X2 MIMO. It's not LTE-Advanced, but it will create a faster and more resilient network.

T-Mobile USA has been talking some smack lately about how its brand-spanking-new LTE network gives it an edge over the competition. Being the last major U.S. carrier to launch LTE means T-Mobile is using the most up-to-date radio access gear and is thus better positioned to implement future LTE-Advanced techniques and other fancy next-generation network technologies.

T-Mobile, though, has been short on specifics, so far keeping mum on what particular tweaks it plans to make that will beat out its rivals. But talking to T-Mobile’s equipment vendors, GigaOM has learned some of those details of its network roadmap. The most impressive upgrade on its list is a plan to blanket its network with extra antennas in order to achieve significant performance gains.

speedometerThe smart antenna technique is called 4×2 MIMO (shorthand for Multiple Input-Multiple Output) and T-Mobile will be among the first if not the first global operator to implement it. Those of you familiar with 4G probably have already heard of 2X2 MIMO, which is used in all LTE networks today. It sends the same data transmission over parallel paths from two antennas at the tower, which are then picked up by two antennas at the receiver. 4X2 MIMO actually doubles the number of antennas — and thus the number of transmission paths — at the tower while the number of antennas in the device remains the same.

In English, that means there are a lot more signals flying at your smartphone, and there will be a lot more antennas at the tower to pick up your phone’s generally weaker return signals. That increases your chance of getting a decent link at the edge of a cell’s coverage zone where connection speeds tend to trail off. 4X2 MIMO won’t increase the maximum speed of the network beyond its 50-to 75-Mbps theoretical limits, but it will ensure that customers at the fringes of the network get much better connections.

How much better? Nokia Siemens Networks North American head of technology Petri Hautakangas said that in lab trials, T-Mobile and NSN are seeing speed gains at the cell edge as high as 100 percent on the uplink and anywhere from a 50 percent to 60 percent increase in downlink bandwidth. Simple geometry means overall network gains would be big (the further the distance from the tower the more space is covered). The end result is a big boost in the real-world capacity of the cell — it can support more simultaneous connections while making more of those connections faster and more resilient.

The best news is for T-Mobile’s accountants. Implementing 4X2 MIMO on T-Mobile’s network will require simple software upgrades to Ericsson and NSN’s base stations as well the installation and the mounting of new antennas on T-Mobile’s towers – many of which are already in place. Since 4X2 MIMO is already in the baseline LTE standard, most current generation handsets will automatically support the technique.

T-Mobile store logoAs for timing, Hautakangas had to be a little cagey when talking about a customer’s rollout plans. “I can say that in less than 12 months we’ll have a commercial 4X2 MIMO network rolled out with a major U.S. operator,” he said during an interview. NSN has only one Tier 1 radio infrastructure customer in the U.S., and that’s T-Mobile.

I talked to T-Mobile VP of radio network engineering Mark McDiarmid, and while he wouldn’t discuss the specifics of T-Mobile’s network blueprint, he did confirm that 4X2 MIMO was one of the multiple LTE and LTE-Advanced technologies T-Mobile was considering for future use.

“We have a very good handle on what 4X2 MIMO can do for us,” McDiarmid said. “And we’re one of the few that are in a position to use it.”

As for other technologies on T-Mobile’s roadmap, both Ericsson and NSN confirmed that their network gear will support the eventual upgrade to carrier aggregation, the first of a long list of LTE-Advanced techniques (though it’s still a far cry from being LTE-Advanced ready as T-Mobile likes to claim).

Carrier aggregation bonds two disparate LTE bands together creating a super-fast connection. T-Mobile already uses carrier aggregation in its HSPA+ network, which is how it achieves 42 Mbps speeds over what is technically a 3G network.

Again McDiarmid wouldn’t comment on T-Mobile’s specific plans, but he said T-Mobile is weighing the use carrier aggregation in two ways. First, it could glue together different parts of its current LTE network in the Advanced Wireless Service (AWS) band, giving it bigger channels in markets where it doesn’t have contiguous spectrum. Second, when it launches LTE in the PCS band, it could bind together two completely separate frequency bands, creating the mother of all mobile broadband connections.

T-Mobile image courtesy of Flickr user swruler9284


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