Clearwire isn't taking the standard approach to LTE.

It's rolling out a version that runs in unpaired spectrum, versus the paired-spectrum flavor being used by every other major operator in the country.

It's using the 2.5 GHz band instead of the 700 MHz band.

And it's not aiming for ubiquitous coverage – it plans to concentrate service in congested urban areas where the network will be used to supplement other operators' capacity, a strategy that will require devices that can move seamlessly between Clearwire's TD-LTE network and its customers’ FD-LTE networks.

Making sure the device side of the equation is running smoothly is the aim of a new effort between Clearwire and Sequans, which made chips for Clearwire's WiMAX network and is an established player in the burgeoning TD-LTE market. The two companies plan to collaborate on performance testing, certification, standards development and devices that support both flavors of LTE.

John Saw, chief technology officer at Clearwire, says that while the issue of TDD and FDD interworking is handled in 3GPP's Release 9 standard, "the devil is in the details. Sequans will have to figure out how to implement that... It's one of the key motivations to work closely with Sequans, to make sure that we take the next steps from a locke- down spec to realizing devices our customers can use."

Sequans already has chips ready – it has shipped TD-LTE units to China Mobile and operators in Brazil, India and Australia, is already sampling the second generation of its TD-LTE product and unveiled a dual-mode TD-LTE/FD-LTE chip last fall. The work now is to make sure devices turning to Clearwire when they run into congestion have a seamless handoff between the primary FD-LTE network and the TD-LTE network.

"The challenge is having the device able to operate at the same time in TDD mode and FDD mode. You need to plan for this," Sequans CEO Georges Karam says.

Clearwire's first 5,000 TD-LTE base stations will be on line by the end of June 2013. Sprint, who plans to use Clearwire's TD-LTE network to augment its own LTE service, says it will come out with dual-mode LTE devices next year. Leap, Clearwire's other LTE wholesale customer, has not yet said when it will come out with devices.

"Part of our job is not just to build the wholesale network but to make sure all the pieces that we need to be successful are available for our customers," Saw says.

Saw declined to comment on whether Clearwire would ever offer LTE service through its retail division, which currently sells data-only WiMAX devices.

Saw also shied away from discussing exact speeds for Clearwire's TD-LTE network, which will leverage carrier aggregation technology to bond two 20 MHz channels into a fat 40 MHz pipe. Sequans doesn't yet have a chip that supports carrier aggregation technology, but Karam says the company "has a program going to implement carrier aggregation on the generation shipping next year."

The important thing now, Karam says, is to "show to the world" that Clearwire's novel approach to LTE will work as planned.