In a response to the PCAST report, CTIA, the main industry group for U.S. mobile operators, cast doubt on its conclusions. CTIA said cleared, exclusive spectrum is the "gold standard" for mobile service, and some technologies that PCAST cited aren't available yet.

6. Spectrum refarming

As carriers adopt LTE, which uses spectrum much more efficiently than earlier technologies, they plan to gradually migrate users off of their oldest networks and reuse those frequencies. Those moves ultimately will make a significant amount of additional spectrum available for high-speed data services, but it's not an overnight solution.

Earlier this month, AT&T announced it would phase out its 2G GSM and EDGE networks by the beginning of 2017, shifting customers to 3G and 4G services market by market. It estimated 12 percent of its subscribers use 2G phones. In May, Sprint Nextel to use spectrum in the 800MHz band, which is now used for its aging iDEN network and has been limited to narrowband technology, for LTE services that use wider bands.

7. Wi-Fi