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.

Comments

Wha?

[sg3000]

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.

Re:Wha?

[philg]

Well, Salon's oversimplifying (surprise!). He's sorta right, in that radio force-carriers don't interfere with each other's movements through space (or whatever's analogous for freaky massless stuff). That isn't how we define "interference" as we understand it, though, as your "green object in a green room" analogy makes clear.

Interference as we know it is the inability to derive meaning from information about the local radio environment. That's what happens when two people broadcast on the same frequency -- your receiver can't figure out which information to care about, because all it knows is "stuff on this frequency is important information" and we keep more than one person from broadcasting on more than one frequency by convention.

Where he seems to be going is treating the endpoints of radio communication more like endpoints in a network. Something analogous to modulation of a carrier frequency (in terms of complexity) is voltage modulation of wires in CAT5 cable. But network interfaces lay the notion of connections between two endpoints over something a good deal more abstract than that. They abstract the modulations into a binary stream, decode the binary into discrete data structures, interrogate the data structures to get meta-information about the data, demux the data (or defrag the packet, or reassemble the stream) based on the meta-information, and so on.

What he seems to be proposing is that radio receivers and transmitters do the same thing that network interfaces and protocol stacks do -- make the actual dance of bits considerably more complicated (to allow for things like error-correction when traditional "interference" is a problem, and to add more meta-information), then apply layered abstractions on top of it to get at the actual data.

Spread-spectrum communication does this already -- two SS messages can be sent to two SS receivers in the same range of frequency, because the two transmitters won't usually be broadcasting on the same frequency, and redundancy can be built into the transmission protocol so that when collisions occur, information isn't lost.

The article overpromises -- if I understand, this mode of communication is no better or worse than what we enjoy by using the OSI model to structure network communications. Even if the information space is "theoretically infinite" (which I doubt), we have to get increasingly more creative in how we utilize the space. In the networking world, however, we can talk at gigabit speeds over the same physical media that only supported 10mbps 10 years ago. We accept that wireless networking can find ways to squeeze increased "bandwidth" out of what is, in reality, a fixed width of spectrum allocated by the FCC.

What Reed seems to be agitating for is that the FCC and others get out of the way entirely, architecting a basic framework for the exchange of information and letting the transmitters/receivers figure out the rest of the details -- essentially the same thing he advocated for the Internet. I don't think it's a crackpot idea at all, though the style of the article masks that pretty well.

Re:shrinking the required spectrum....

[stevew]

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.

The Stanford Spectrum Conference...

[Remik]

...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

Re:The article is crap

[W32.Klez.A]

" This guy looks like a CS grad or (oh horrors) a digital designer. I would really love to see his credentials."

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.

Re:complete bunk

[coult]

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.

He's correct, on a technicality....

[James McTavish]

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.

Someone hand this guy a physics book, stat!

[Hal-9001]

I can't even begin to discuss all the things that are wrong with Reed's theories as described in the article, but I'll address some howlers.

"Photons, whether they are light photons, radio photons, or gamma-ray photons, simply do not interfere with one another," he explains. "They pass through one another."

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.

Reed uses the example of a pinhole camera, or camera obscura: If a room is sealed against light except for one pinhole, an image of the outside will be projected against the opposite wall. "If photons interfered with one another as they squeezed through that tiny hole, we wouldn't get a clear image on that back wall," Reed says.

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.

If you whine that it's completely counterintuitive that a wave could squeeze through a pinhole and "reorganize" itself on the other side, Reed nods happily and then piles on: "If photons can pass through one another, then they aren't actually occupying space at all, since the definition of 'occupying' is 'displacing.' So, yes, it's counterintuitive. It's quantum mechanics."

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.

Re:Someone hand this guy a physics book, stat!

[Hal-9001]

Wow, I provoked a lot more responses than I expected! I will try to organize my responses to the most common comments or objections.

• The number one objection seems to be to the soap bubble example. The soap bubble probably isn't the clearest example of interference phenomenon to explain, but it is an example from everyday experience, which is why I chose it.
  
  The colors from a soap bubble are due to light interfering with light. Light is partially reflected from each surface of the soap film, and the reflected beams do interfere with each other and result in the colors that you see. That's about all the detail I want to go into describing it, but if I still don't believe me, it's probably described better and in more detail in either Hecht Optics, Born & Wolf Principles of Optics, or Lipson, Lipson & Tannhauser Optical Physics (in any of those books, look for the section on "multiple-beam interference").
  
  It is true that when two beams of light cross paths in vacuum, if you were to observe them after they cross, you could not tell that they crossed. However, in the region in which they cross, they can interfere with each other. Again, any of the references I mentioned above will probably explain this much better than I can.
• The second most common objection was to my description of diffraction in a pinhole camera. On this count, I will admit that I was playing it very fast and loose when I said that diffraction was due to photons interfering with each other, but OTOH Reed used the phrase "photons interfering" to describe a phenomenon that in optics and electromagnetics is normally described as diffraction. Reed explicitly denies the existence of diffraction in the pinhole camera. On this count, he is dead wrong because you can conduct an experiment, observing the images of pinhole cameras with smaller and smaller pinholes, and eventually making the pinhole smaller makes the picture worse! (not just dimmer, but blurrier as well) This is because of diffraction, but again, you'd be much better off reading about it from a book than listening to me try to explain it.
• The other interesting objection introduced the notions of Laplace (or Fourier) transforms, and the spectra that arise from these mathematical operations. This is a different spectrum than the physical spectrum associated with light or radio waves. However, even the abstract world of signals and systems or communications theory, you can arrange for two signals to interfere with each other, and even to interfere in such a way that it is impossible to recover the original signals without a prioriknowledge. For example, if you multiply sin x with cos x, you get out a sine wave at twice the signal frequency of either of the original waves. If you received this signal without a priori knowledge, there would be no way to tell if if was meant to be one signal or two signals. Admittedly, this is a very simple and contrived example, but this can still occur with more complicated signals.
  
  Even worse, once you physically manifest this signal by modulating it onto an electromagnetic carrier wave (like radio does), this communications spectrum is now superimposed on the physical spectrum of the electromagnetic wave. Now the signal is subject to the physical phenomenon of interference, which can further corrupt the signal if you don't allocate communications channels in the electromagnetic spectrum properly. And I think it's the allocation of commmunications channels which is what the article is trying to be about. However, that doesn't change the fact that Reed is dead wrong in the way he describes or interprets many of his physical examples, probably because he has a lot of background in computer science but not as much in physics.
  
  Furthermore, Reed is wrong if he thinks that ultrawideband (UWB) or frequency hopping will increase the Shannon limit within a given range of the electromagnetic spectrum. Ultrawideband will interfere with other electromagnetic signals. It requires a lot of electromagnetic bandwidth, hence the name. :-p This increases the likelihood that it will overlap with other channels, which means that it probably would be a less efficient way to allocate spectrum than FM radio, for example. This may not be an issue for the other channels if the signal-to-noise ratio is high enough to compensate, but it does not mean that the interference phenomenon does not exist or does not take place. The advantage of ultrawideband is that it has a wide bandwidth, which enables faster data transfer rates, but it wouldn't be any faster than multiplexing the same data across enough FM channels to have an equivalent bandwidth (coding and SNR ratios and all other things being equal). The problem is that allocating a ton of bandwidth to a single UWB channel means that instead of several somewhat underutilized channels occupying some range of the spectrum, you might end up with one highly underutilized channel filling that entire range of the spectrum.
  
  Frequency hopping can improve the efficiency of the spectrum allocation by moving communications channels to unused regions of the spectrum, but it does not create communication capacity where there is none. Furthermore, those channels have to be allocated in advance to prevent them from with other signals.

Reed is probably right that the electromagnetic spectrum is inefficiently utilized. But the many of the physical examples or explanations of physical phenomena that he presents are dead wrong, which was the point that I was trying to make in my original post.

What I found astounding...

[gilroy]

was Dr. Reed's willingness to wave away two hundred years of well-established physics. Waves of the same frequency crossing the same point in space do interfere. How do I know? Because the very definition of interference is the effect they have.

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 ... what do all those "dumb" radios make of the frequency-hopping signal as it passes through their current band? In any event, I found his tone to be wildly optimistic (if one is generous) and far too disingenuous in throwing out a well-defined technical term.