I recently ran into a long-time friend of mine who mentioned he was beginning to look into DARPA's XG project. Not having heard about it, I asked for more details. Apparently, DARPA is seeking to enable dynamic access to radio frequency spectrum. My friend explained that one application scenario would be the following: a US traveler arrives in London, exits the plane and turns on his mobile phone. His mobile phone connects with the local base station and determines the "policies" for that area such as frequency usage, bandwidth, etc. When the traveler returns home, and exits the airplane in the USA, his mobile phone connects to the local base station which communicates the "policies" to the phone -- making it ready for use in that locale. You can see more about this project by clicking this link
This application is going to drive further the need for software-defined radios which allows one to decouple policy (spectrum usage, cost of use, etc.) from the engineering (designing/building the mobile phone).
As the usable spectrum becomes more crowded and users clamor for their application, the communications agencies are going to need more flexible solutions to meet customer demands. Software-defined radio is one potential solution. It is on the horizon and approaching.
If you are working with Software-defined radios, I would like to hear from you. Please email me at firstname.lastname@example.org