Moore's Law, Cooper's Law, Zoning Law

Spectrum efficiency typically doubles about every 30 months, where Moore's Law suggests computing or storage capacity doubles about every 18 months.

Ability to place new cell towers does not double every 30 months, according to a rule of thumb called Cooper's Law.

For reasons of zoning rules, neighborhood objections and money, the ability to build new towers to increase capacity without additional spectrum is limited.

There is no Moore's Law for steel towers or electricity or construction costs. Some estimate that the cost to each national mobile provider of additional capacity, gained by shrinking cell sizes, without adding spectrum, is as much as $40 billion.

To be sure, spectrum costs money, and new towers would have to build at some locations, even with new spectrum allocations (networks operating at different frequencies require different topologies, all other things being equal). So there is no way to avoid additional investment.

All that explains why the Federal Communications Commission believes it is imperative to license new spectrum, and why AT&T thinks it has to move now to acquire T-Mobile. It's a real estate deal, more than anything else.