Less than 300 MHz and the antennas needed are too big to carry, and even 300 MHz might be too big for a mobile phone and might only work in a tablet device or notebook computer.
Higher than 2.5 GHz and it gets more difficult to get the signal to propagate.
Beyond that, virtually all spectrum available in the United States already has been designated for particular applications and particular users. Re-allocating spectrum always is a messy, expensive process. Existing users must be convinced to move, and typically have to be paid off. And the Federal Communications Commission is looking at a huge swath of new spectrum for mobile, ranging from 300 megaHertz to 500 MHz. Keep in mind that most currently active mobile networks use 10 MHz to 20 MHz in each direction.
Aside from the costs of reallocating spectrum, there is the additional matter of the amount of capital new owners will have to consider spending. In the long term, lots of new spectrum might be made available. In the short term, spectrum suitable for mobile use will continue to be quite scarce, and therefore valuable.
That doesn't mean spectrum owners automatically "win" when placing big bets on acquiring spectrum. In fact, lots of entrepreneurs have lost hundreds of millions on failed spectrum-related ventures, and others have narrowly escaped failure. At other times, the new industries or applications the spectrum is supposed to support simply fail to develop.
The blocks of spectrum now used to support Sprint and Clearwire 4G networks once were set aside for non-profit educational broadcasting, for example, then envisioned as the foundation of a fixed wireless TV delivery business, and finally wound up being used for mobile communications.
The big issue at the moment is reallocation of TV broadcasters from spectrum bands originally used for analog TV broadcasting, so the spectrum can be used for mobile services. But that involves such a huge block of spectrum that protracted sparring is inevitable.
