Rollins defended the report's methodology. He and Bazinet started with 2010 numbers, but updated them, he said. The two used averages to come up with spectrum use estimates; if a carrier has a 10 MHz nationwide block, but is only delivering service to half the U.S. population, the report considers that 5 MHz of used spectrum, Rollins said.

The numbers are based on ongoing research, including discussions with carriers and information from the FCC, Rollins said. Citigroup did not give an explanation of the BRS and EBS numbers it used in the report.

CTIA also questioned why the National Association of Broadcasters sent the report to journalists. The NAB has concerns about an FCC plan to take back 120 MHz of spectrum from U.S. TV stations, with the stations that volunteer to give up spectrum sharing in the auction proceeds. The NAB pointed to the Citigroup report as evidence the broadcast spectrum isn't needed.

But the report reaches its conclusion about spectrum needs based in part on the FCC's plan to make available 300 MHz of "high-value" spectrum for mobile broadband, including 120 MHz from TV stations, over the next 10 years. The FCC's national broadband plan, released in early 2010, calls for a total of 500 MHz to be made available in 10 years, with 300 MHz suitable for mobile broadband.

"If [the report's] conclusion is, if we bring 300 MHz to market, and there's not a short-term spectrum shortage, I wouldn't be that troubled by that, especially if we can get that in the next couple of years," Guttman-McCabe said.