5. Spectrum-sharing

Some experts say it's time to throw out the whole notion of allocating certain frequencies exclusively to commercial mobile services, or to any exclusive use. Instead, they advocate mobile operators sharing spectrum with current users, such as government agencies.

In a report issued earlier this year, the U.S. President's Council of Advisors on Science and Technology recommended that the secretary of commerce identify 1,000MHz of frequencies where commercial and federal users could coexist. "This study finds that today's apparent shortage of spectrum is in fact an illusion brought about because of the way spectrum is managed," the report said.

Advances in technology, including small cells and radio performance improvements, help make it possible for mobile networks to use the same frequencies as other services, as long as potentially interfering signals don't rise above a certain level, the report said. It cited the success of Wi-Fi in unlicensed spectrum as an example. Sharing could multiply the effective capacity of spectrum by a factor of 1,000, the group said.