All messages or signals sent without wires must modulate a frequency somewhere in the electromagnetic spectrum. Unfortunately, only a small portion of the spectrum can be used in a practical manner—no one, after all, would want to receive a cell-phone call via X ray! This usable range, known as the radio-frequency spectrum, extends from about 3 kilohertz to 300 gigahertz, but even this span is further limited by physical and practical barriers.
Radio waves are attenuated by rain, fog, and even the water and oxygen molecules in the air. This attenuation increases significantly as the frequency increases. In fact, the atmosphere becomes almost opaque in the region right around the oxygen-molecule absorption lines (around 60 gigahertz). While communication is possible at frequencies greater than 60 gigahertz, the penalty is extreme in terms of higher power needs and equipment costs.
The demand is greatest for spectrum below 3 gigahertz—the "beachfront property" of the spectrum-allocation world. Users of this frequency range can get by with smaller antennas and lower-power transmitters. Simpler equipment is obviously of great value to soldiers in foxholes and in aircraft, where mass and power needs are demanding—but it's also of great value to commercial cell-phone service providers. The ability to use smaller and simpler hardware is also important because the cost of putting a kilogram of equipment in orbit is tens of thousands of dollars.
Most of this prime real estate below 3 gigahertz has already been taken. Now, the challenge is how to prioritize and share this very limited and highly valuable resource.