Sprint that it has agreed to work with Clearwire to make the two carriers' LTE (Long-Term Evolution) networks work together. Sprint will turn to Clearwire for additional capacity to fill growing mobile data demand, probably beginning in 2013. Because the two carriers plan to use different types of LTE, they will need to make the two versions work together smoothly, a development that may have impact beyond the two U.S. partners.

Sprint and Clearwire currently share a WiMax network, with Clearwire as the operator and Sprint reselling services for its 4G phones and data devices. Both are moving on to LTE, the 4G technology with the most international acceptance, but they plan to do so separately. Clearwire plans to use TD (time-division) LTE, which uses the same band to send and receive signals, while Sprint will use FD (frequency-division) LTE, which uses two bands.

With a view to letting subscribers use both networks, the two companies have completed a non-binding memorandum of understanding to cooperate on technical issues such as what chips to use in devices, when and where to set up base stations and how to smoothly hand off data sessions. For subscribers, the two networks will have to look like one.

"It shouldn't be too much of a challenge, but still, it's not something that's ever really been tried before," said Monica Paolini, an analyst at Senza Fili Consulting.

The carriers' cooperation is likely to be good news for users of TD-LTE, which has strong momentum in China and India but not in the developed markets of North America and Europe, Paolini said. European carriers are looking to TD-LTE for future supplemental capacity after their current FD-LTE systems start to strain under growing demand, she said.