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The Myth of Radio Spectrum Interference
Selanit writes "Just came across a fascinating article on Salon about a technologist who claims that there is no such thing as "interference" in the radio spectrum. He argues that interference is a symptom of inadequate equipment, not a fact of nature, and that with improved transceivers we could open the spectrum up to high-quality broadcasts by anyone. Reference is made to the GNU Radio Project. Neat stuff." We've posted other stories about this. I wonder if the "color" meme will catch on.
I know a physicist who claims that pi is in fact rational. He claims that the only reason we don't realize it yet is because of the current limitations of our circle measuring devices.
Kan jeg få en pils, vær så snill?
David Reed is many things, but crackpot is not one of them. He was a professor of computer science at MIT, then chief scientist at Software Arts during its VisiCalc days, and then the chief scientist at Lotus during its 1-2-3 days. But he is probably best known as a coauthor of the paper that got the Internet's architecture right: "End-to-End Arguments in System Design."
thank you for reading the article.
Reed's article is based on the observation that Maxwell's Equations are linear (for most materials) and that, therefore the waves pass through each other without modification (again, unless you're in pretty exotic environments --- early universe, etc.) The problem with interference arises because of imperfect spectral content and non ideal antenna response for both transmitters and receivers. Interference is like being at a party: There are a lot of people talking, and your ears hear in all directions, so you have to be near the person you're trying to talk to.
... the reason that it works is that your eyes have very fine angular sensitivity (high quality antennas) compared to your ears.
For a variation on this theme, there's an interesting moment in a movie (Frankie and Johnnie?) where there's a terrific racket in a diner, impossible to understand anything, but a cook and a clerk are communicating easily --- by sign language. Consider also those occasional TV images of the Wall Street pit traders flinging gang signs at each other
Spectral purity and antenna quality limitations can be overcome --- by money. You can build higher quality receivers and transmitters, bigger antenna installations but it costs money and space in fairly unavoidable ways.
Reed is also wrong from a regulatory level. It's not just the FCC that you'd have to work with, but the ITU. Those pesky radio waves have this interesting habit of leaking over borders on the ground, and pretty much everywhere down here from satellites.
There are pretty good reasons to pick on modern broadcasting: crappy content, media concentration --- but "broadcasting" is not one of them. Those great big transmitters permit the use of very dumb receivers with poor sensitivy. The very simplicity and asymmetry of broadcast provides tremendous economic and technical appeal, and I'd be amazed if it ever went away.
Far more interesting is the glacial progress of DTV in broadcast.
Radio Astronomers have a hard enough time keeping the important wavebands free of interference without the radio spectrum being unregulated. Lots of useful, hard science is being done by the radio telescopes around the world observing the machinations of galaxies out in the distant universe. One of the key problems is that these signals are amazingly faint. The standard unit used in radio measurements is the Jansky - thats 10^(-26) Joules per second per square metre - which should give you some indication as to how faint. Lift that coke can off the floor onto the table and you've just used up more energy than has been received from distant galaxies by ALL the radio telescopes on the surface of the planet.
Terestrial radio transmitters are so many orders of magnitude stronger than these signals that any sideband transmissions even 90db below peak transmission still totally swamps the surrounding spectrum. And very few transmitters are truely 'perfect'. It's not as though a transmitter broadcasting at frequency X with HWHM waveband Y can't be detected at X +/- 8 Y. Yes - better quality receivers allow you to separate out signals at close frequencies, but a very strong signal next to a very weak signal will drown out it's neighbours.
Cheers,
Toby haynes
Anything I post is strictly my own thoughts and doesn't necessarily have anything to do with the opinions of IBM.
I have a Ph.D. in applied mathematics and am an expert in numerical methods for wave propagation, so I do know something about waves. Yes, one can imagine a different technology such as directional antennae or spead-spectrum, but how much more complex do your receivers have to be?
Clearly there is no such thing as limitless bandwidth; Shannon's theory tells us there is maximum amount of information that can be transmitted over any one channel, and simple physics tells us that there are a limited number of channels, no matter how you slice it.
All is Number -Pythagoras.
There are some very commonplace phenomena, such as the colors on a soap bubble or oil slick, which are the manifestation of interference of light. There are more fundamental experiments that can be done with lasers or radio waves to demonstrate interference.
Actually, if you do the experiment, there is a specific pinhole size at which you get the best image. Make the pinhole any smaller and the image starts getting blurrier because of diffraction effects which, loosely speaking, are due to the photons interfering with each other.
From his misunderstandings of the nature of light so far, it's impossible for him to have any real understanding of the quantum nature of light. He wouldn't know Schrodinger's equation if it walked up to him and smacked him upside the head, seeing as how Schrodinger's equation is a wave equation and predicts all sorts of interference phenomena.
The most fundamental problem is that he admits the notion of frequency, which is intrinsicly tied to the wave nature of light and radio. If he admits the wave nature of light, then he also has to admit interference of light as a natural phenomenon and not as a detection artifact, at which point all of his theories crumble.
"It take 9 months to bear a child, no matter how many women you assign to the job."
Unfortunately, this is not true.
Suppose a city has two stations, one on 1600 kHz and one on 900 kHz. Let's add a station on 700 kHz, ok? Let's put him near the 1600 kHz station, since we don't want these damn antennas cluttering up the whole city. No problem with "well made equipment", right?
Now consider that near to both the 1600 and 700 antennas is a large, old, steel-framed building, containing tens of thousands of rivets and metal-to-metal joints. Some of these joints have some corrosion. Consider that there may be several such buildings. Why is this a problem?
Each joint is a potential non-linearity. Each joint is capable of taking the 1600 and 700 signal and creating the sum and difference signals and re-rediating them. The sum is 2300 kHz, outside the AM broadcast band. The difference is ... 900 kHz. The same frequency as an existing station.
Now consider if you live inside one of these buildings. You used to listen to the station on 900 kHz. Now you hear a wonderful mixed babbling of both the 1600 and 700 kHz stations -- and your radio has nothing to do with creating the problem.
Let's go one step further. These same non-linear conductors will cause sum and difference issues with single-frequency signals, too. The new station on 700 kHz will sum with itself and cause a signal on 1400 kHz. And it's even worse. The actual result will be signals on every multiple of 700 kHz well up into the shortwave bands. (If the non-linearity created a perfect square wave, you'd get only the odd harmonics, but these aren't perfect and you get even harmonics, too.)
Can't happen, you say? Yes, it can, and does. I've lived with this problem for the last 4 years from two nearby stations. It has finally gone away, since one of them moved their antenna location a mile further away, but before they did that, they made a lot of the spectrum useless here.