Where do U.S. top-tier operators sit in the race to 4G?

The past few months have been a hubbub of activity for network providers and infrastructure vendors as the market gears up for the race to 4G. Driven by smartphone successes and increasing demands for bandwidth, the nation's top-tier carriers are in the midst of defining their stance on the race from 3G to 4G. FierceWireless put together a snapshot of the current mobile broadband spread in the United States. Check it out here [1].