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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.
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
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.
Lessig: ...Coase's arguments reflected the state of the art at the time. Property was the best way to allocate spectrum in 1959. But it's the wrong answer today. Not because property does no good -- in fact, it does a great deal of good. This should not be taken to imply that administrative allocations are inevitably worse -- a market has costs, and if those costs exceed the value, then markets result in misallocation. Coase's insight -- most prescient -- is that spectrum is not in its nature rivalrous. It's not a thing at all. Colors, sounds correspond to frequency.
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.
Fact: All radio, visible light, cosmic rays, infrared, x-rays all the good Electromagnetic radiation exhibits interference. Do a search for the double slit experiment if you don't believe light acts the same way. If you let light through a pinhole as he suggests, and then sent it through two more pinholes so there were essentially two sources of coherent light spaced apart, you'd get interference. And, this still happens if you're only letting a photon through at a time. Basic quantum theory people....
The radio spectrum isn't a finite resource. How much can you increase frequency? You can infinitely increase it. What is limited is usable frequency. Usable frequency is limited not just by technology, but also by the physics of the environment. I have always said that trying to implement 802.11b like what has been done with cellular tech cannot be done because of it's frequency. 802.11b uses 2.4 GHz band of frequency. The physics of the problem makes 2.4 GHz not suited for long haul. 2.4 GHz can go through buildings but can only go around 50 feet. You could extend that by using a beam or a better omnidirectional antenna, but your definitely not going to go miles in most current instalations. Now HF frequencies can go thousands of miles with current equipment. I am sure BOTH RF frequency bands can and do go thousands of miles and maybe even light years, but current technology limits that. If the signal is so low in strength that current recievers can't detect it, then it's not useful. It's finite. Theoretically, if you can develop a reciver that can recieve the very very low strength signal, then you could....possibly say that a RF wave can be infinite.....but conditions have to be perfect. No walls and a total vacuum. On the other hand, interference that we currently have comes from going for that extra buck. If one were to build proper recievers and transmitters, they would be very expensive, but they would not be susceptible to interference. Cheap devices absolutly breed interference.
Gorkman
Um, if two people are flashing a huge green light at me from different directions, I sure can tell the difference and know where the pulses come from - dunno about you...
The directional anntena has what it is called a main lobe whitch is usually measured in degerees and it is greater than 0, therefore two radio signals using the same frequency and resising in the same lobe will certainly interfere.
http://ebgp.net/ccc/
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.
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.
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.
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
Try building some linear amplifiers that work for ALL frequencies..... You did? Next stop oscillators that work on ALL frequencies. Then an antenna system that works well on ALL frequencies.....
You are still left with a limited piece of the spectrum and in this piece you are still going to run out of space (either in the frequency domain or in the code word domain). Shannon's law still applies for the signal/noise ration.
Jeroen
Secure messaging: http://quickmsg.vreeken.net/
Imagine you have a transmitter and receiver that send signals using exactly one frequency and no others. That is, the signal is a perfect sine wave of a particular frequency. How much information can you send on this frequency?
The answer is none; you can't change the signal at all, so you can't send information. Once you start changing the signal, (i.e. change the amplitude) you are actually adding in more frequencies - this is Fourier 101.
To send information, you have to use a band of frequencies. The wider the band, the more information per channel, but the fewer channels. So there is a limited amount of information that can be sent.
All is Number -Pythagoras.
What Reed doesn't talk about is that interference is a receiving
problem, not a transmission problem. You also have to remember that
radio broadcasting predates the internet by almost 100 years. His main
focus seems to be to get needed spectrum for the expansion of the
internet into the wireless world. In the early days, the only way to
prevent interference was to separate the spectrum into pieces and assign
each user a specific piece. Up until the 1970's, there was no frequency
sharing between active users. This begin to change in the late 1970's
with the introduction of spread spectrum techniques. This is the
bandwagon that Reed seems to be jumping onto. However, there are
theoretical limits on how many users can share the same piece of
spectrum even using spread spectrum techniques - thus you still need a
spectrum policeman to decide who gets what.
"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."
This sounds like the argument the phone company used to argue against allowing the Internet. Yes, computers cost a lot more than dumb phones, but people are willing to pay more for something that does more and especially for something that allows them to do more.
Phones haven't gone away, but allowing the internet has added greatly to our lives over the past ten years.
You are 100% correct, I'm pretty sick and tired of these monkeys with no grasp of technology going on about light affecting radios. It's not the first time I see such nonsense.
It was probably the light traffic controller, a PLC probably, going through some sort of timing loop, waiting for the moment to change the light. Since the code is probably different when changing from red to green, than for green to yellow, the noise spectrum of the processor is different.
This sort of frequency hopping happens all the time in, for example, GSM and Bluetooth. It doesn't make the interference go away.
The "few errors" you refer to are still interference. With a sensible frequency hopping pattern, the interference will spread out around users and be evenly spread in time, hopefully to the point where error correcting codes can catch it and compensate. But add more users and the error rate will pile up until your network falls apart, just as with non-hopping.
This effect is called "interference diversity" and is well studied in the literature.
Additionally, your throwaway line about "ask anyone who's signal you can see to choose a different color or time division on that particular color" would be enormously, insanely complex to implement. The amount of traffic necessary to keep this sort of scheme working would dwarf the useful traffic the network would handle; plus, this whilst it would improve things for a single user, it would likely make the next user over worse. It would not lead to a better network overall.
[Disclaimer: frequency hopping is my PhD thesis topic]
You win again, gravity!
Yeah.... Further, in general, the wider you are from the carrier, the better the quality. CW (Morse code) sends nothing but a 1-bit binary pulse train, not even the tone (that's recreated in the receiver) but As Seen in the Movies(tm) on ID4 a CW signal is so narrowband and bounces off the ionosphere that you can talk around the world under the right conditions. Single-Sideband-supressed-carrier punches a voice thru where FM fears to tread- again, narrowband (not as narrow as CW) because they supress the carrier- don't even bother transmitting it. They just send the band of frequencies deviating above (or below) the imaginary carrier. But it ain't KROC-FM. Then there's AM, the gold standard for voice broadcast until the 60's or so. Both sidebands and the carrier, bigger RF footprint, but it doesn't sound like donald duck if you mistune slightly, like SSB. Commercial FM the way its used in US broadcast, is the bandwidth pig. Oddly, FM stereo (twice the information, right?) doesn't have a bigger footprint than the original FM broadcast spec. It's encoded with a 19khz pilot signal. But FM broadcast is limited in frequency (remember that 19khz pilot tone?) somewhat more (limited) than most modern stereo components.
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
You can't filter noise. Noise is, by definition, not a band-limited signal. Furthermore, due to the nature of its generation, your filter will add noise to its output signal.
basically what your saying is this:
noise = rand();
rx_sig= desired_sig+noise;
therefore:
desired_sig= rx_sig-rand();
how does that work? You can't know in advance what the noise spectra is.