An incentive auction that will shift spectrum from broadcasters to cellular phone providers will go forward with limits on the amount of airwaves the biggest carriers can buy, the Federal Communications Commission ruled today.

There were two orders, one adopting rules for next year's Broadcast Television Incentive Auction, and another taking steps to promote competition in the wireless market. Both votes were 3-2, with the three Democratic commissioners voting in favor and the two Republican commissioners dissenting.

"Spectrum is a finite resource integral to providing mobile broadband service. No one or two providers should be able to dominate use of wireless airwaves," the FCC said in a fact sheet. "The two largest nationwide mobile wireless providers [AT&T and Verizon Wireless] hold a combined share of approximately 70 percent of all low-band spectrum licenses; the two other nationwide providers [Sprint and T-Mobile] hold a combined share of approximately 15 percent of all low-band spectrum licenses."

The broadcast auction includes airwaves in the 600 MHz range. The exact amount will depend on how much broadcasters are willing to sell. Providers that hold at least one-third of the low-band spectrum in any regional market will face bidding restrictions.

"To promote competition in the auction and in the marketplace, the rules establish a market-based reserve of up to 30 megahertz of spectrum targeted for providers that currently hold less than one-third of suitable and available low-band spectrum in a license area," the FCC said.

Non-nationwide providers won't face any limits.

Some of the 600MHz spectrum in each market will be unlicensed, allowing White Spaces broadband networks and other public uses.