Sunday, August 30, 2009

What "interference" with radio signals really means, and its implications for property rights

A common confusion often arises: people talk of the "interference" with radio tower transmissions, without understanding what physical process the term refers to. This misunderstanding makes it hard to see the logic in my analogy between intellectual property and rights to radio frequencies.

In a recent debate on intellectual property, I finally decided to set the record straight, and what follows in this post borrows heavily from what I said in the debate.

First, how does radio communication actually work? I'll admit that I don't know the answer all the way down to the nuts-and-bolts level. But I can explain it from the perspective of information theory.

Radio communication works, to the extent that it works, because a listener can perform a measurement, and thereby learn something about the source, i.e. the message transmitted. (This "something" they learn is called the "mutual information" between the two points, and is equivalent to so-called "Bayesian evidence".)

And when it comes to your radio, what is that actual measurement? Setting a dial on it that changes a circuit's properties so that it resonates when the surrounding area is filled with electromagnetic (EM) waves around a certain frequency. And when it resonates, an electrical signal in the radio follows a certain pattern that's correlated to the signal the radio tower is sending. Your radio then converts the circuit's electrical signal into sound that is meaningful to you.

All of this functioning relies on an assumption: that by performing the measurement, you do in fact learn something about the source. That assumption is violated when more than one tower transmits with enough intensity at the frequency you perform a measurement on. In this case, no measurement result tells you anything about either source: the transmitted waves overlap each other, coming across a gibberish on your radio. (In the lingo, there's no "mutual information" between you and either source.)

So whenever you talk about "interference" with radio communication, what you really mean is "violation of an assumption some parties were using to communicate which, when violated, makes them unable to communicate."

To understand the significance of using the term "interference" in this way, let's look at a more practical, intuitive example with the same dynamic, but unrelated to the EM spectrum.

An Illustrative Example

Let's say that I live in a small village where I have a few friends. I want an easy way to communicate to them that I expect a rainstorm today. So, I work out an "encoding scheme" with them in advance: if they hear me hit my gong before 8 am, I predict rain. If they don't hear me hit my gong, I don't predict rain. So, instead of having to tell them all individually, I can just hit the gong. They'll hear it, and they'll get a message from me. By "measuring" the sound they hear before 8 am, they learn the "signal" I'm sending.

So far, so good.

But there's a little snag: my friends will hear a gong sound as long as anyone hits a gong not just me! So, our communication scheme only works as long as we can rely on no one else hitting a gong before 8 am. If we can't rely on that, I can't send them the message, at least not as reliably. Because when they hear a gong, sure, it could be me, but it could also be anyone else with a gong. Hearing the gong sound is no longer a reliable sign that I think it will rain.

So there you see it: our communication system can be defeated by "interference" from other people, either because they're trying to set up their own similar system, or because they just like being mean. But this "interference" simply means: violating an assumption that we, rightly or wrongly, thought we could rely on.

And how does this relate to radio communication? Simple: the existence of the gong sound before 8 am is just like a radio signal within a given frequency range: it can provide information to others, but only if others don't try to use the same means to communicate.


So do you think people should be able to "homestead" such "communication assumptions" like that? Should I be able to assert rights as "the only one who can hit a gong in this area before 8 am"? (Or, to be less greedy, the right to hit a gong in this area in a certain five-minute window, with a certain rhythm.) Your answer to that question tells you a lot about how you should look at other issues.

For example, how about asserting rights as "the only one who can broadcast radio waves in this area within a particular frequency band"? How about asserting rights as "the only one who can distribute books containing Harry Potter stories"?

Hey! That last one kinda sounds like intellectual property rights...