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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?
Interference is a fact of life. Sure, the technology can improve and allow us to do the same things with less of the spectrum, and other things like spread-spectrum can come along and lessen the interference problem, but spectrum is still a limited resource.
The FCC is currently forcing the switch to digital communications all over, which is shrinking the required spectrum. I'm sure when other technologies mature, they will make use of those as well to further free-up the spectrum.
Slashdot gets worse every day... Pipedot: News for nerds, without the corporate slant
From the article:
Pantone may own the standard numbers by which digital designers refer to colors, but only the FCC can give you an exclusive license to a color itself.
So I could patent the wavelength of a colour of my choosing, and claim royalties every time someone uses a colour that matches my wavelength? Now there's a way to get rich quick...
Except people wearing clothes using your colour could run away from you really quickly and cause red shift:
"See? It's not the same as your colour. It's very slightly more red. You can't sue me!"
If Reed is right, nearly a century of government policy on how to best administer the airwaves needs to be reconfigured, from the bottom up.
Based on the power that Television companies hold, does anybody really think this is going to happen? We have a hard enough time with the record labels, now they want to go up against people like NBC?
Great idea. Unfortunatly, it would never happen without serious reform within the Gov itself.
Not that I don't like making waves, but one step at a time.
Just my humble opinion,
SirLantos
The flying hamster of DOOM rains coconuts on your pitiful city.
This article is complete bunk. Yes, its true that radio frequencies are like colors. So imagine this scenario: you are receiving signals from someone who is using 'green'. They are flashing a huge green light, and you can pick up the pulses they are sending by being bathed in the green light. Now someone else comes along and also starts flashing a huge green light. You can't read the signal any more, because there are now two huge green lights bathing you with their signals. How can you tell which pulse is coming from which light? You can't! That's interference.
All is Number -Pythagoras.
Perhaps, I'm not the most knowledgeable guy on RF interface, but I went to The University of Texas at Austin, got my degree in electrical engineering (studying electromagnetics), worked at Ericsson designed cellular systems and RF planning, worked at a company making "smart antennas" for cellular systems. From my experience, I had a hard time understanding what he was talking about. "Spectrum is more like the colors of the rainbow"? Of course it is, that's how the radio spectrum works. But then he goes off on, "There's no scarcity of spectrum any more than there's a scarcity of the color green." Which makes little sense to me.
It's not that using a radio frequency somehow "depletes" a resource -- it means that if you put a green object in a green room with green lights, after a point you won't be able to see the object any more, kind of like how camouflage works. The problem is when you have a lot of signaling broadcasting in an area, the noise level can increase to the point that no single signal can be resolved. The classic example is how it's very difficult to understand a particular conversation in a noisy room. And that's why you have to generally parcel out radio spectrum and define limits on how it can be used (signal strength, bandwidth characteristics, noise levels, coverage patterns, etc)
That guy's nutty analogy makes me think he's a leftover of the dotcom era -- when eyeballs was more important than revenue and other silly things. Admittedly, I should read the whole article, but the first few paragraphs made me feel like I'm talking to a crazy guy on the bus.
Insert simplistic political, ideological, or personal proselytization here.
Radio's basic signal function defined in software? Sure, "Maximize your bandwidth with our new RadioBooster!!!" (at the cost of your neighbors).
While this guy might have a point - the current FCC policies on RF spectrum might be a bit outdated, I would be careful with deregulation here.
Large radio broadcasters love to claim this when there is a threat of a new station being added in their market. Not because there is a possibility of interference if the frequencies are close - they're scared of competition.
Well made and tuned equipment can eliminate any chance of interference and allow for more radio stations within an area. However, organizations like NAB (www.nab,org) and now, the FCC stonewall any attempts to open up the airwaves. At one time, there was a proposal to allow low power broadcasters to operate, unlicensed, if they could prove they weren't interferring and accept the interference from other channels. It was approved but still puts the "little guy" at a disadvantage: http://www.fcc.gov/mb/audio/lpfm/.
If there ever was an "ol' boy network", it's broadcasting. If you want to broadcast legally, you're looking at dropping half a million in legal and license fees alone before you buy your first piece of equipment.
Sit back and relax as Windows 98 installs on your computer.
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.
Uhm - no. The reduction in radio frequency usage is due to the adoption of compression of the video stream. These are still going to be multi-MegaWatt Xmitters because of the frequency(UHF), and the distance they want to cover. Put two of these on the same frequency, close enough, and you have inteference at the receiver. PERIOD.
A major part of communications theory is issues dealing with bit-error rates, and interference. It is a reality. Now we can move to things like "spread spectrum" but even this is no panacea. Fact - for a given bit errror rate, bandwidth, and communications path conditions - there are a finite number of spread spectrum transmitters than can coexist in the same band before the bit-error rate is exceeded!
How do I know? Well I've been a ham for 25 years giving me practical experience, and I'm a EE as well.
Have you compiled your kernel today??
...just took place earlier this month. There's a lot of good information here. An audio/video archive of the conference will be available on the 17th for those who didn't catch the webcast.
The idea that Spectrum doesn't need to be regulated is quite old, and it seems more and more likely to be valid. In any case, the idea that it needs to be controlled by government interests is less and less likely.
-R
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.
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.
with improved transceivers we could open the spectrum up to high-quality broadcasts by anyone
While this is *techniclly* correct, On could also say that A knife could be built that can cut a loaf of bread into infinite pieces, if we could design it to cut sub-elementary particles. Why are we not making knives that can do this? Because the technology isn't there, and if it was it would probably be cost prohibitive.
There are many concepts that, if tweaked to the current technology, could be greatly improved. However, keeping old technology also has it's merits: Firstly, it's proven technology so all quicks are known or resolved; New technology undoubtedly has more problems. Even the threat that new technology has more problems, people will not use it. Also, changing to a new kind of technology require huge investments. New technology has to be pretty profitable if it is to overcome the investments made in the old one.
This principle is part of human nature: People get used to some kind of technology/ideas and stick to it. Even when these concepts stop to be meaningful. I refer to the Querty-effect: Old typewriters had little pins with letters on them which hit an ink-soaked ribbon and presses it onto the paper. To prevent these pins from hitting eachother (which happened a lot), the qwerty keyboard was invented. The most abundant letters in English were as far apart as possible to prevent collisions. But a computer doen's have pins, so why do we still use a qwerty keybaord?
But also think of buttons in programmes: You press buttons in real life, why show them on a screen and press them with a virtual hand (the mouse cursor)? There are many more examples; the radio/TV frequency story if Mr. Reed being one of them.
The problem usually isn't the technology, it's the ideas that need to be changed. But sometimes technology improvements do get through, e.g. the DVD is nothing than an up-to date CD. MP3-player replacing the old walkman. Telefones replacing the telegraph.
Things change, ideas change. Some want to accellerate it, some want to slow it down. In the end, things just change at the rate they do and, as harsh as it sounds, there's nothing you can do about it. It just takes a little time...
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
Secure messaging: http://quickmsg.vreeken.net/
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).
This guy isn't quite a crackpot. Before you skip this comment you should know that I do have a Masters in Electrical Engineering where I specialized in methods to reduce RF interference.
The jist of the article is that RF waves do not "interfere" with each other. By this he means that two RF waves will not affect each other as they pass by each other in space. This is correct. The two waves will simply pass through each other. The problem is when you try to receive the signal.
When you receive a signal you get ALL the radio waves from the entire spectrum (not quite this simple, but it will do). Then the signal is amplified and the spectrum you don't want is filtered off. The problem is that if your antenna is receiving two RF waves in the same spectrum they will be superimposed.
What he's trying to say is that an intellegent receiver will be able to pick out one of these waves while rejecting the other. Much like when you pick out one conversation in a noisy room. Much easier said than done.
There are currently some schemes to do this, such as CDMA phones which work on a spread spectrum. Each of them transmit and receive on the same spectrum at the same time using what are called "codes" (Code Division Multiple Access). However there is still a capacity issue. When too many phones come into the same area, the noise floor comes up and nobody can receive information. To prevent this the cellular phone comany will limit the number of active cell phones in a given cell and drop any new calls over the limit.
There are more advanced methods, but as many people in this field know, the signal processing that your brain does to pick out only one conversation is mind blowing.
To sum up, he's technically correct. His use of the word "interference" is confusing to say the least. RF engineers talk about interference as the superposition of singnals as you receive them. He talks about interference as the interaction of signals in space.
Karma: Abstruse (Mostly as a result of using words nobody understands)
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."
The essential claim of "unlimited spectrum" that this fool is waving around is really, just barely sensible enough to fool someone who hasn't studied information theory. Take any finite dimensional span, like a foot-long ruler. You have, in theory, an infinite number of possible subdivisions of that 12-inch length--you can have arbitrarily many divisions, if you make them all small enough.
In short:
YOU CAN'T TRANSMIT AN ARBITRARILY LARGE AMOUNT OF DATA/SECOND ON A FINITE AMOUNT OF BANDWIDTH. No matter how good your equipment, or how clever your signaling patterns, you will never be able to increase your data rate above the amount determined by Shannon's equations.
The flaw in Reed's reasoning is that we're talking about subdivisions of frequency, and the amount of data that can be transmitted in a given wavelength band has an absolute upper limit. It's Shannon's rule about bandwidth. So yes, Reed can go around giving everybody a gnat's ball hair width of radio frequency to push their data, but each riny segment will only be able to transmit a piddle of bits per second.
This is like people who don't know Calculus, but who think they've disproved Special Relativity with a thought experiment. Anybody who's sat through a class on it, or read a book, will laugh and laugh, while everybody who hasn't had the benefit of learning will probably be suckered.
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]
While the article brings up spread spectrum the concept of non-interference is not spread spectrum. If you put two highly directional transmitters at one side of an X and two recievers at the other two sides, the two signals won't interfere at those receivers locations but if a receiver was placed at the center of the X the two signals will. If you can wall off the direction of one of the signals from recieving then the other signal will be clear at the center.
aspect of this article are total bunk. However, I do think we should rethink our spectrum.
High quality broadcasts for everyone is a pipe dream. Want to know how that works out? Check out our Citizens Band. Not pretty at all.
I am in the process of getting an amateur radio license again. HAMs do more with less spectrum than just about anybody. Doing this has made me rethink spectrum allocations and how they are wasted. The amateur bands have very reasonable band plans that allow for a number of uses and work well.
Our primary problem with spectrum use is the band planning, not the avaliable resource. (Which is limited no matter what this guy says.)
Commercial and military uses basically get what they ask for and they ask for everything they can.
Comes back to this really. We live in a competitive culture. We have given companies the same rights we have. They are better competetors than we are.
We lose.
Our fault.
Blogging because I can...
There must be some other explanation, but it seems like Dr. Reed is making a freshman-physics terminology mistake. When a physicist says that two waves "interfere", he/she doesn't mean that one wave knocks out the other or that they undergo some linked dance. The linearity of Maxwell's equations indeed does show that each wave "passes through" the other without reducing or amplifying it.
Nonetheless, they interfere -- because "interference" is the interaction of the waves at a given point in space, where the amplitudes add algebraically. Consider a given location x at a given time t. If at that moment wave A has ampitude 5 and wave B has amplitude -2, then a receiver will measure a disturbance of amplitude 3. It doesn't -- and can't -- know that there are two waves, because there is only one signal. If the content in wave A is uncorrelated with the content in wave B (for example, two different radio stations playing different songs), then their addition will be essentially random -- and hence sound like noise (because it is noise).
Dr. Reed's proposal doesn't really speak to this. He wants smarter receivers that can track a signal and so distinguish wave A from wave B. The technology is not here, not cheap, and certainly not universal. The system we have was not foisted on us by some big government conspiracy and it's not maintained by the pressures of a cartel. It's here because interference is a fact and that "overcoming" it -- which is really more like shuffling past it -- is expensive and unproven.
And you would still have to deal with the transition from legacy to newfangled
The Mongrel Dogs Who Teach
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.
nuclear iraq bioweapon encryption cocaine korea terrorist
I had always suspected that pi would be rational, if not in denary (decimal), then in another base.
Binary octal and hex don't appear to be too promising, but I now realize the answer:
Base pi
Heck, it might even work in base e or base i
You will need:
- A sheet of college-ruled paper
- A green marker
- A copy of Moby Dick
Open up the Moby Dick to the first page. Then, with the marker, start transcribing the text onto the sheet of paper -- "call me Ishmael" and all. When you run out of space, don't get more paper -- instead, just go back to the top of the sheet, and overwrite the text that's already there. When you are done with the entire Moby Dick, mail the sheet to Mr. Reed.Since there is no such thing as color spectrum interference, Mr. Reed should be able to read the entire Moby Dick just from the one sheet of paper.
This revolutionary discovery will surely eliminate waste, and save our rainforests... If only the paper-making companies didn't want to keep it under wraps !
>|<*:=
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?
While he may be correct in saying that radio signals, in and of themselves, don't "interfere" with each other he's neglecting to mention a critical point.
It's also true that two radio signals, each of a different frequency, will, when mixed together, produce an entirely different set of signals based on the sum and difference of the two frequencies.
This is the same principle that superheterodyne circuits (the type used in just about any kind of modern RF receiver) are dependent on. Example: You want to receive a signal on a carrier frequency of 146.5200 MHz, and your receiver has a 10.700 MHz IF.
OK, so the local oscillator (LO) in your receiver needs to produce a frequency of its own that will mix with the incoming 146.5200, and produce 10.7MHz as a result. That 10.7 signal will then be demodulated and turned back into audio.
Assuming you use low-side injection, your receiver's LO would need to generate a frequency of 135.8200MHz (this, by the way, is why scanning receivers are not permitted in commercial aircraft. 135.8200 is in the aircraft comm band), which is merely 146.5200MHz minus 10.700MHz.
Anyway... What I'm driving at is this; Think of a mountain top transmitter site that's got a ton of broadcast, public safety, amateur, and other kinds of transmitters on top of it, many of which are producing hundreds, if not thousands, of watts worth of RF.
There's going to be signal mixing. Lots of it. That means tons of the very "interference" that Reed doesn't seem to think exists.
The techniques mentioned in the article, BTW, including software-defined radios, are nothing new. They've been around for decades, and ham radio folk are already experimenting with them. For one example of a purely software-controlled radio, take a look at this radio kit from TAPR.
73 de KC7GR
Bruce Lane, KC7GR,
Blue Feather Technologies