Some large U.S. carriers may face a spectrum shortage down the road, but much of the U.S. spectrum available for mobile broadband is in the hands of companies slow to move forward with business plans, Rollins said. "It's the control of spectrum, not the availability, that's the real constraint," he said.

Cash-strapped Clearwire has 133 MHz of spectrum, but provides 4G service to markets covering only about 42 percent of the U.S. population. Startup LightSquared controls 59 MHz of spectrum, but its plans to build a nationwide 4G and satellite network are raised by GPS users.

Dish Network controls 47 MHz of undeveloped spectrum, with the company asking the FCC in August if they can use it to offer LTE (Long Term Evolution) service. The Citigroup report counts the Dish holdings as spectrum available in the future, not currently available spectrum.

In addition, some U.S. carriers have significant amounts of spectrum tied up in 2G and 3G services, Rollins said. He and Bazinet estimated that 147 MHz of the 192 MHz of spectrum now in use for mobile services is currently dedicated to 2G and 3G services.

"Their pace to 4G is dampened by the needs of converting spectrum over to 4G," Rollins said. "Because there's so many [2G and 3G] users on that, they need extra space, they need extra room, to be able to manage this conversion. That's where some specific players could use more spectrum, but there's a lot already licensed in the industry, so it's a question of how they get it."