There’s a big advantage that you missed, btw. Check out, for example, the “gnu radio” open source software radio project. Then consider this: the FCC can pass any rule that it likes saying you can’t copy digital television off the air or what have you, but anyone who decodes HDTV with a software defined radio (SDR) can just change the software to ignore the desires of the regulators, and open source SDRs are already here. This is yet another instance where general purpose computers make regulation very difficult.
Really, though, SDR isn’t that interesting until it is coupled with another major advance that is creeping up — phased array active antenna technologies. These have the capacity to literally make the amount of bandwidth available in radio unlimited. I do not use the word “literally” figuratively here — I mean “literally”. With phased arrays, you and I can broadcast as much as we want on exactly the same frequency, and so long as we’re slightly spatially separated no one will care.
Consider the way radio works now. Two stations can’t be broadcasting at the same frequency unless they are so far apart that receivers can only hear one of them at a time. Bring them too close together and the
primitive sensors we use for electromagnetic radiation will get both signals and become confused.
Now consider the situation with another sort of electromagnetic sensor — the human eye. There is no rule that says you and your friend can’t wear the same color shirt in the same room for fear that people will
be unable to distinguish the two of you. The eye has no trouble seeing multiple sources of the same electromagnetic frequency and distinguishing them. You can easily focus on any one of several sources of the same electromagnetic frequency even though you are receiving others.
What’s the difference between your eye and your FM radio? Entirely that your eye is directionally sensitive and the radio is not. The radio has much the same view of the world that a non-directional photocell with a color filter would have — it can tell that a particular frequency has arrived but not from where and it can’t distinguish multiple signals directionally. Imagine if you had to see the world that way.
Well, as it turns out, you don’t have to see the world that way. There’s a technology out there (the details aren’t important) called “phased arrays” that allows you to broadcast a signal very directionally (like shining a light at one and only one person) or to focus very directionally on a single signal (so that you can listen to
one FM broadcast on 99.5 and not hear another originating one mile away at the exact same frequency.)
This is not a theoretical technology — it has been in use in military radar systems (such as the one used on the very expensive Aegis ships the navy has) for decades. However, it depends on doing very complicated signal correlations, and until now that has been very expensive. However, if you do the work in software, it turns out to be pretty straightforward and the expense falls by 50% every year.
There are already companies playing this game — look at Vivato (www.vivato.com) for example, which is making phased array WiFi equipment.
With phased arrays, there’s no reason why everyone can’t be using the same spectrum at once. Couple phased arrays, spread spectrum (a modern technology for reducing interference) and SDR and suddenly the way we use radio today looks totally silly.