What’s igniting Spark? A look inside Sprint’s super-LTE network

Sprint(s s) didn’t just launch a new 4G service today when it announced Spark, it also launched two new LTE networks, one of which has been many years in the making and should give Sprint’s nationwide competitors cause for concern.

That network is the 2.5 GHz time-division LTE (TD-LTE) deployment started by Clearwire back in 2011. It uses a funky of version of LTE that no other carrier in the U.S. utilizes, and it runs on a spectrum band most radio network engineers consider atrocious for widespread and indoor coverage. But it more than makes up for those faults in sheer quantity of frequencies it uses.

cell phone tower / cellphone tower / antennaSprint has more than 100 MHz of 2.5GHz in most cities, and while its not using all of the bandwidth for LTE today, it’s strung enough of those frequencies together to produce a system averaging 50 Mbps to 60 Mbps on the downlink — at least according to Sprint’s marketing materials. To put that in perspective, the other two plain-old LTE networks making up Spark use 10 MHz each and have theoretical ceilings of 37 Mbps.

Sprint is also balancing out coverage and capacity, using its LTE networks at 1900 MHz and 800 MHz (which occupies Nextel’s former spectrum) for coverage, while packing the weaker TD-LTE cells into dense urban areas where capacity is most needed.

A new speed king?

Several U.S. LTE networks today are capable of breeching the 50 Mbps mark, though most of them are averaging 10 Mbps to 20 Mbps. If Sprint can really deliver average throughput of 50 Mbps it will most certainly take the speed crown away from AT&T. As of now, though, Spark’s full capabilities are only in five cities — New York, Los Angeles, Tampa, Miami and Chicago — and its footprint in those cities is limited at that. It appears it will grow slowly as well, covering 100 million people by the end of 2014.

Number 1 foam hands rankingsAnd Sprint will soon face a challenger in Verizon Wireless(s vz)(s vod), which is set to launch a new LTE network of its own using 40 MHz of spectrum in many cities. Early sightings of that new network in New York City are showing speeds of 80 Mbps, but once launched commercially it may actually be much faster.

But Sprint has a lot of bandwidth to work with. In lab tests, Sprint demoed today an LTE link of 1 Gbps, speeds it said it could eventually boost to 2 Gbps. It’s accomplishing this not only by patching together all of its available spectrum, but also using new LTE-Advanced techniques like carrier aggregation and multiple pairs of antennas. It’s unlikely that any user in the real world would ever see a 1-2 Gbps mobile connection, but there’s a lot more to this demo than mere bragging rights.

Those kinds of lab connection speeds speak to the enormous pent-up capacity in Sprint’s airwaves. As I’ve written before, Sprint has long talked about building the mother of all 4G networks, but it’s never delivered. Now it seems as if it’s ready to make good on its promises.

Nokia asserts itself

Sprint’s TD-LTE networks were built by Samsung, Alcatel-Lucent(s alu) and Nokia(s nok). While Samsung and Alcatel aren’t surprises given their work on Sprint’s other CDMA and LTE systems, Nokia is a shocker. It actually started work on Sprint and Clearwire’s original WiMAX networks — the predecessors to today’s TD-LTE systems — before getting kicked off the contract for failing to deliver its equipment on time.

This is a big deal for Nokia. Though a global powerhouse in LTE, it’s big weak spot has always been the U.S. Before the Sprint deal, it’s only LTE contract of note was with T-Mobile(s tmus). Nokia has long talked up its TD-LTE technology, implying it could use the new LTE variant to outmatch its archrival Ericsson(s eric). It appears that was more than just talk. Ericsson has won a piece of every major LTE contract in the U.S. except this one, even though it’s currently Sprint’s largest equipment vendor.

Tower image courtesy of Shutterstock user Serhat Akavci; No. 1 image courtesy of Shutterstock user Kittisak