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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.
David Reed is not being completely honest, he is being overly optimistic, IMHO, and hasn't demostrated with actual experiments his claims.
Based on stories of 802.11b (Wi-Fi) and/or Bluetooth suffering from interference either from like-protocoled devices being operated by other parties, or cross-protocol interference which results in the one or both protocols not being effective in their data transmissions, and these are supposed to be advanced intelligent devices which don't suffer from interference due to their use of Spread Spectrum technology, and intelligent software controlled radios (which may or may not be software defined radio - SDR).
So unless he can demostrate experimental evidence, I'm a scepetic.
The big different between RF and optical receives is that RF receivers (radios) are usually fairly omnidirectional, whereas optical receivers (eyes) are usually pretty directional. In part, this derives from the physics of the things - longer waves go turn more round obstacles, and tend to broadcast wide angle if their wavelength is similar in size to their aerial.
/. a few months ago. These could very well lock onto the signal from a particular direction, and ignore signals on the same frequencey from a different direction.
The way we use radio takes advantage of this - we don't have to aim the antenna for our car radio, and we prefer it that way so we can listen as we drive. This leads to a promiscuous sort of receiver, which is subject to interference. I think it is going a bit far to say thai is because of the legislative environment or technological background - it is because it is the way we *want* it to be.
At optical weavelengths, we *want* a directional, even a focussed, image - and our eyes produce it. In between, we tend to use directional transmissions with point-to-point microwave dishes.
However, the simple reflector style lens, depending upon newtoinian optics to fouca an image of the transmitter onto the receiver, is not the only way to receive a signal. People are already working on multi-aerial systems which take a "holographic" approach to reconstructing the signal. There was an article about one of them on
I think the frequencxy hopping bit is actually somewhat of a red herring. It doesn't generate new spectrum, it meakes better use of the spctrum we have. It gets rid of the wastage caused by blank safety space betwenn radio stations both in geographical space and in spectum space.
Consciousness is an illusion caused by an excess of self consciousness.
However, far from being revolutionary, his 'discovery' is a well known fact, which is already in wide use by now:
Also, his analogy breaks when you compare wavelengths: light having much shorter waves is much more directional (allowing for the pinhole camera phenomenon) whereas radio need much bigger spatial separation to avoid interference. While you can put several green objects into one room, and still distinguish them, you need much larger cells for RF.
I'm not sure how you can consider the article complete bunk if you've had a sufficient college physics class that covered the particle-wave duality of electromagnetic waves.
In your example, it's true that your eyes can't discern the difference between the signals and this is classically how we've viewed radio detectors. However, the information in the signals is not lost - you're ability to detect between them is altered, but the photons themselves are unaltered.
If you switch to a different type of sensor or encoding scheme - for example, utilize frequency hopping (aka spread spectrum) then you could easily broadcast the two signals over the same range of frequencies (colors).
Overall the article has a lot of merit in providing a different and, in my mind, compelling metaphor of bandwidth as colors as opposed to the classical bandwidth as land. As to his ideas of limitless bandwidth being true, the idea is beyond my ability to see how this is feasible, but that does not detract from his idea that we could actually be communicating a LOT more over the current spectrum than we are today.
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.
What the guy in the article is talking about is using spread spectrum techniques.
This is done by spreading your signal over a large spectrum with a pseudo random key. The number of possible keys is still limited (There has to be a certain difference between two keys for it two work) and thus you still have a maximum number of users although things like roaming are a lot easier since you are limited by keys overlapping and not range overlapping.
This is what is being done in CDMA cellphones, Wireless Lan, Bluetooth etc. It is nothing new, already happening and you still need regulation to make sure the spectrum doesn't get completly unusable.
Jeroen
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What I'm unclear about is what he proposes we use all these radios for. Is he talking about making cellular networks more open and inexpensive? Is he talking about making radio and TV licenses cheaper and easier to acquire? Is he talking about replacements for Bluetooth and 802.11b/a/g? I guess he's talking about all of the above and more. Having spectrum open to such a wide array of uses with "autonegotiation" will result in huge drops in throughput. The article suggests that autonegotiation is used in frequency hopping systems,
Um..the TX and RX aren't negotiating -- they're following a very strict prescribed pattern of frequencies to which they hop. Same is true in cell networks, 802.11, Bluetooth..doesn't matter if it's frequency hopping or direct sequence spread spectrum, everything is planned out.
Where I work we've been doing preliminary work on software-definable radios for a couple of years now. The two biggest problems we foresee are: (a) how to justify the cost to customers up front, and (b) how to justify (to our company) selling someone a radio they will (conceivably) never have to replace. We're struggling to make money through software upgrades, and we've already seen that it's really hard to displace an existing, working system with a new, better system (just look at UMTS adoption).
The posts here give a nice insight in the problems between physicians and electrical engineers.
The author of this paper is right! There is no interference in a spectrum (besides the modulation of the signal to broadcast, but that is an effect of no importance here). This is mathematically and physically true.
However I can understand that electrical engineers have problems with this, because they notice interference every day. This has however to do with the _implementation_ of the radio signals, not the theory.
A lot of comments here deal with issues which are quite off-topic, such as what antenna (omni or not, size) you use. This has nothing to do with the spectrum or interference, the direction is an extra design parameter for a system, which can be used to pick up a certain frequency, but there is no coherence with the interference topic; a a certain spectral component stays the same in the air, no matter what antenna you use.
However I don't find this artical inspiring, because it contains nothing new. Let the electrical engineers deal with the problems, they are more experienced with the implementation..
[Disclaimer: I have phys. degree]
So...he's talking about using the spectrum more efficiently.
But more than that, I think. Consider that the spectrum itself is not quantized. We quantize it with different radio stations, but this is not really absolutely necessary. If our recievers/transmitters where all spread spectrum, and they could all recieve/transmit at nearly any frequency we wanted, then there really wouldn't be much problem with interference. Sure, you might get signal degradation in one frequency band because someone else was using it, but you'd get less in another band that would make up for it.
To make sure that the spectrum doesn't become completely unusable wouldn't require government regulation of WHO uses it as much as it would require regulation on HOW they use it. If people used the spectrum the way that broadcasting companies do now, we would certainly have a problem.
But it is unlikely that anyone would be able to completely use all of the spectrum because of the unbelievable energy requirements that this would need.
In short, with the appropriate scheme, there really is enough bandwidth for everybody (that is, bandwidth would be limited by power, not by regulation).
Mod me down and I will become more powerful than you can possibly imagine!
If we were starting our broadcasting systems today, he'd be right. There are many better ways to do it today.
However when radio and television began, there were no computers or even transistors, there were no phase-locked oscillators or QAM modulation, and there were only a handful of broadcasters.
Yes, some of the frequency hopping and CDMA type concepts have been around for a while, but only in the last 10 years available at a price that anyone but the government could afford.
Mr. Reed's ideas are insightful, but not very practical. Our entire telecommunications infrastructure relies on spectrum assignments. The technology does encounter interference. To simply point the finger at bad planning and blaming the decisionmakers from the 50s for not predicting the state of technology fifty years later is ludicrous.
Reasonable proposals to more forward with UWB that doesn't interfere with traditional infrastructures should be pushed. Eventually the old technologies will fade away like the telegraph.
But to simply rant that "It sucks. Cooler, better tech exists." doesn't do anything.
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Furthermore, allowing "substandard" receivers to exist is deliberate. We did this with the AM spectrum when FM came along, and we are doing it with other receiver technologies. AM can be received with a few cents worth of primitive electronic components and it is widely deployed, that's why we continue supporting it.
The division into bands also allows enforcement and specific power limits. Without that, people might broadcast over astronomical frequencies, or they might engage in RF shouting matches until they light up each other's fluorescent lightbulbs.
Basically, Reed's science is iffy, and his arguments are completely missing the point. Yes, we can open up spectrum (UWB is essentially trying to do just that), but let's not kid ourselves about the consequences, which will at the very least include the obsolescence of lots of radio equipment and probably a kind of arms race over the airwaves.
I'm currently 18 months through a PhD revolving around the assignment of frequencies in a frequency hopping spread spectrum network (more details here) so I know a bit about this stuff. And that article is not fantastically insightful.
Interference, as it says, is not a law of nature. It's what happens when you are trying to listen to, say, a 1.1Mhz signal coming from over there and someone over here is also transmitting on 1.1Mhz. How can the radio receiver tell the difference between those signals? As the article hints, it's an engineering issue; but it's a non-trivial one. Radio engineers all over the world will not read this article and rejoice. Reclassifying the problem in some bizarre colour analogy has not magically solved it.
Now as for the politics of spectrum allocation and the potential improvements of a free spectrum policy: now that's a more interesting issue, but one the article doesn't address in any but the most superficial of ways.
Bah, I say to it.
You win again, gravity!
The processor in your Palm Pilot is completely different than the DSPs found in many digital radios, etc. The Palm Pilot is a general-purpose CPU, which means it requires much more hardware to do what a dedicated DSP designed around signal processing does. (DSPs are often HEAVILY pipelined to maximize throughput because decisions rarely have to be made so branch mispredictions are a non-issue. If you thought the P4 had a deep pipeline, check out some DSPs...) Also, many "software" radios aren't really software - More appropriately a lot of them are "reconfigurable hardware" - Essentially using FPGAs to implement custom dedicated logic. (Once the domain of ASICs, but for small runs FPGAs are much cheaper, and for anything where you might expect to change the logic around later FPGAs are a must.) An FPGA can do things that a 2.4 GHz P4 could barely dream about, while costing not much more than the CPU in your Palm Pilot, simply because it's dedicated to the task.
Note that the GNU Radio project recently achieved ATSC (US digital TV) demodulation.
Using $1000+ worth of hardware
40x slower than realtime.
Compare that to the MyHD HDTV tuner card, which can do realtime demodulation, MPEG decoding, and display scaling for $300. Why? Because it's designed for the task. It's somewhat reconfigurable, but you can't take a Palm Pilot and turn it into a software-defined radio.
retrorocket.o not found, launch anyway?