Skepticism about whether a "spectrum commons" could work most likely springs from the way weve been trained to think about "spectrum." A hundred years of careless talk has led many to think spectrum is a thing. Worse, a hundred years of careless talk has led most people to think that when radios suffer "interference" it is because the radio waves have, in some sense, collided. Both notions are simply wrong. There is no such thing as "spectrum" that gets "used" the way a pasture gets used. Spectrum is not a thing. And what we think of as "interference" is not an issue of radio waves; its an issue in the receiver. Clarifying these two misconceptions will go a long way toward a greater understanding of a spectrum commons.
Think about a conversation in the middle of a party. Lots of people are talking around you; there could be plenty of other noise in the room. The TV could be blaring or a police siren could be wailing outside. But despite all this "noise" you are still quite likely able to carry on a conversation. And thats because (1) humans are intelligent receivers and (2) sound waves (like radio waves) dont collide and fall to the ground; they essentially pass through each other without any "damage." Even if two people utter exactly the same sounds at exactly the same moment, people can hear the two speakers and distinguish their messages.
Older radio technology would not produce such efficiency in an analogous situation. If two transmitters tried to transmit on the same channel at the same time, then a receiver would not report two different transmissions. The receiver would instead report interference, but not because of any flaw in the radio transmissions. The flaw is in the receivers.
For most of our history, radio receivers were "stupid." They distinguished one transmission from another by picking out the stronger of the two. If two transmissions were on the same wavelength, then the receiver wouldnt know which transmission to focus on. That confusion is what we hear as "interference."
Progress in radio technology has made radios smarter; new technologies for transmitting radio signals effectively allow two transmitters to use the same channels at the same time. There are too many of these technologies for me to describe comprehensively. But consider spread spectrum technology as just one example.
The idea behind spread spectrum technology is very similar to the idea behind the Internet. "Data" is broken into chunks that are then transmitted on many different frequencies at essentially the same time. Each chunk is marked with a code that the receiver is able to detect. The receiver listens for all of the transmissions with that code and then collects them to "receive" the message.
Fast computers make it possible to scramble data like this and receive it intact. Assuming the processing capacity, spreading the transmission this way allows many different users to use the same swath of spectrum to transmit content at the same time. And because the receiver knows what it is listening for, the transmission need not be so powerful as to drown out every other transmission. Instead, like two people on opposite sides of a football stadium who have agreed on a common code of signals for communicating with a flashlight, small bursts of data are enough to get large amounts of content across any distance.