The challenge is for the FCC to get current spectrum holders to give it up. The U.S. Department of Defense and other government agencies control huge swaths of spectrum -- the government has exclusive rights to more than 600MHz of spectrum, according to one recent study -- but agencies have resisted calls for them to turn over spectrum to the FCC.

PRTM's Hays questioned if the FCC will be able to find 500MHz to auction. The DOD holds a significant chunk of spectrum, and the use is classified, he said. "I actually think in some sense, this one may well be a pipe dream," he added. "It's completely unclear whether there's a group of willing sellers."

This is one way the FCC hopes to identify new spectrum, but it's been one of the most controversial pieces of the broadband plan so far. TV stations control nearly 300MHz of spectrum across the U.S., and in some markets, as little as 36MHz is used, Genachowski said recently. Even in the largest TV markets, only about half of that spectrum is currently being used, he said.

But the National Association of Broadcasters (NAB), representing TV stations, has panned proposals to reallocate spectrum. TV stations lost more than 100MHz of spectrum in the transition to digital TV completed in mid-2009, with the upper 700MHz band of spectrum sold in auctions that ended in early 2008. More spectrum reallocation could hurt over-the-air TV service, the NAB has said.