AT&T, trade group CTIA and even officials with the U.S. Federal Communications Commission have talked frequently about a coming spectrum crunch, as mobile customers move to data-sucking smartphones and tablets. Smartphones use 24 times the spectrum compared to standard mobile phones, and tablets use 120 times the spectrum, FCC Chairman Julius Genachowski said in a speech on Tuesday.

"The spectrum crunch is the single biggest threat to one of the most promising parts of our economy," Genachowski said.

But Citigroup analysts Jason Bazinet and Michael Rollins questioned what has become the convention wisdom in the mobile industry. The U.S. has plenty of spectrum for mobile broadband, but much of it is in the wrong hands, they said.

U.S. carriers have 538 MHz of spectrum dedicated to mobile data and voice and are only using 192 MHz, the two analysts said in a . Major carriers control 422 MHz of spectrum, Bazinet and Rollins estimated.

"Too much spectrum is controlled by companies that are not planning on rolling out services or face business and financial challenges," the report said. "And, larger carriers cannot readily convert a substantial portion of their spectrum to 4G services, because most existing spectrum provides 2G-3.5G services to current users."