Originally Posted by Mendocino
There is a lot of spectrum available, its just not in "popular" places. Check out "TV Whites Spaces" (TVWS) and dynamic spectrum access.
Not to mention that spectral efficiency will increase, just like it has for roughly the past century of RF communications. You've heard of Moore's Law on the doubling of computing power, there is an analog called Cooper's Law. He ran Motorola's mobile phone division for years. Anyway, he observed that spectral efficiency doubled every 30 months, IOW a given spectrum's capacity to carry information concurrently. This has held true since 1895 until today. So in Marconi's day it took MHz of spectrum to carry a single conversation where now billions of simultaneous conversations can occur in the same spectrum.
So when a cell phone carrier declares that they'll need 10 times the 'spectrum' in 15 years, they mean they'll need 10 times the capacity in 15 years. Then figure that in 15 years technology will allow existing spectrum to carry around 50 times (6 periods of doubling is 64 times the capacity) the information it does now. In Cooper's day they could never have imagined 64QAM and OFDMA that allows ~108Mbps in 1.4MHz of spectrum. Back when AMPS and PCS phones were leading edge 1.4MHz of spectrum might have carried a hundred phone calls and now it can handle a few thousand. This is without much change in total quality, both being approximately 15Kbps (or equivalent) sub-channels.