Wednesday, July 14, 2004

Software radios with unlimited bandwidth

Marginal Revolution has a good post regarding some cool upcoming technologies:
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.


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