Slashdot Mirror
A Wireless Revolution From The Garage
Saige writes "There's an interesting article in this month's Discover magazine about a lone inventor who's managed to develop a new method of wireless transmission that he was told was impossible - using "time-coded ultra-wideband electromagnetic pulses" - something that apparently can offer vast amounts of spectrum, allowing ways around the billion-dollar bidding going on now, with the bonus of better transmission through walls, higher speed/capacity, and higher resolution."
It isn't VMSK. VMSK was the digital equivilant of that tired old april fool's joke of reducing FM deviation until the signal had essentially no measurable bandwidth. Shannon's Limit shows why it's not workable.
This stuff is real, but the hype and spin is nothing short of amazing. Folks, get an electrical engineering degree and you'll learn that it is hardly revolutionary.
Look up "Costas Loop" on your favorite search engine and you'll see how he stays synchronized. No, you don't need an atomic frequency standard. And by the way, this design as been around since at least the late 1950's.
I first learned about pulse phase modulation from an ARRL publication in 1978 while studying for an Extra Class license. The idea of spreading it further was simply a matter of changing the timing in a pseudo random fashion. BFD.
In any case, UWB is not much of an issue UNLESS you give it to the general public to take everywhere. Before you know it, Radio Astrononomy gets hosed. Next, a few ham radio operators complain. Then a few incidents in airliners regarding Portable Electronic Devices start making GPS go nuts, and before you know it, police radios stop working within a few hundred feet of Starbucks when they start offering wirless internet portals...
That's right, you can be compliant with 47 CFR 15 and still mess with licensed radio services. It's called the near/far problem with spread spectrum. Look it up.
If we could somehow all turn off all of our narrow-band radio gear and magically turn on our computer enhanced digital communicators, we'd all probably love it. But that's not practical. We will have narrowband communications for a very long time to come.
UWB devices will have to avoid trashing GPS frequencies (it's a spread spectrum system too) without introducing filters that introduce excessive group delay distortion. They'll also have to figure out how to get good dynamic range and sensitivity with a very wide band front end that lets in some remarkably powerful signals.
Most computer professionals know so little about the transmission media they use for their LANs and WANs that it's comical. They would love to dispense with FCC licensing but they have no concept of what they're getting in to. Folks, unlicensed operation means you must ceace and desist when a licensed operation asks you to. It also means you must accept any interference you may receive. And people want to build a wireless infrastructure on THAT?
We need the FCC. We need narrowband channels. Just because they're not doing their job poorly doesn't mean it doesn't need doing.
erhaps I don't fully understand this invention, but to me it sure looks like snake oil.
You don't understand the technology.
Check out the Ultra WideBand Working Group, time domain (mentioned in the article) and dozens of other sites. The pulses are what makes this thing work and the spectral splattering is exactly intended. They're trying to get FCC approval since it doesn't cause (enough) harmful interference to take down communications with existing equipment.
It's way cool stuff. Check the UWB link provided; they want to use this for positioning, through-wall "radar" and communications. It has very serious potential. If I can find the EDN issue which gave an in-depth study of this I'll post back.
Now we're all gonna have to dip into our pockets so that SETI can add search capabilities for *this*. Man, I wish the aliens would just go rap on G. Dubya's noggin and say "Howdy" instead making us waste all this energy on screensavers that probably are gonna result in jack squat.
Aliens on the brain... And my ass hurts. Hmm.
- Rev.
- A full duplex 1.3 GHz system with an average output power of 250 microWatts, and a variable data rate of either 39 kbps or 156 kbps. The radio has been tested to beyond 16 kilometers (10 miles).
- A full duplex 1.7 GHz walkie-talkie with an average output power of 2 milliWatts, a data rate of 32 kbps and a range of 900 meters. The unit was also capable of measuring the distance between radios with an accuracy of 3 cm (0.1 ft).
- A simplex 2.0 GHz data link with an effective average output power of 50 microWatts, a data rate of 5 Mbps at bit error rate (BER) of 0 with no forward error correction (FEC) and a range of 10 meters (32 ft) through two walls inside an office building.
Also, their pulses are not remotely square, but gausian monocycles of the form: V=t*e^(-t^2). In addition to low power, they claim that they can do really dense channelization by using different clock sequences -- very similar to the way FHSS works, only by varying the time base, rather than frequency hopping.Another plausable advantage is that since they don't use continious waves, multi-pathing isn't a big problem. The wave packets from the two paths are completely distinguishable, and therefore do not interfere. However, this makes each path look like a seperate transmitter on a different channel. So you sacrifice total bandwidth (by reducing the number of channels availbable) in exchange for reducing fade-out from point of destructive interference.
In any case, anyone interested should check out this whitepaper more info. I doubt this is a scalable as they claim, but they do have some interesting ideas, and the single-chip positioning and radar sounds cool, too.
Not to be a naysayer, but this really isn't that remarkable. In particular, it basically has exactly the same constraints as "normal" frequency domain signals. In particular, it has to take up the same sized region of the spectrum as conventional broadcasting, and all of your electronic components have to work at the same high frequency to resolve data.
If you want to resolve picosecond pulses, you need electronics that can pass frequencies up to 1 THz.
That isn't to say there aren't applications, and I am going to read their whitepapers to see how they get such phenomenally low power usage, even when constrained by the inverse square law, but it isn't a revolutionary technology that is going to eliminate the bandwidth problem.
Short-range transmitters with closely spaced receivers connected by fibers could solve the bandwidth problem, but A) would require massive investment in infrastructure and B) be no better or worse than Bluetooth type proposals using more conventional radio technology.
Ultrawideband is not exactly new, I worked on a UWB radar almost 10 years ago. It has promise, but it also has problems that you don't see with traditional carrier based methods- since you are talking about picosecond resolution, communication with a reciever that is moving with respect to your transmitter is hard- in a picosecond, light travels about .3 mm. Virtually any motion can cause your reciever to start looking at the wrong part of the data- since the data is all time-coded.
Many have said that you can't detect UWB, but usually, they are looking at it with a traditional carrier based spectrum analyzer- you can't see it, but it is there. It just looks like additional noise. which looks very similar to spread spectum- which is exactly what UWB is, but you're spreading the spectrum very, very wide- an impulse in the time domain is uniform (all frequencies, equal amplitude) in the frequency domain.
Time domain has been around, and pretty big in the UWB arena for a long time, but most of what they have is vapor- big promises, but they fail to deliver. There are other players in the field, Aetherwire is one of the other biggies of UWB. There has been a huge amount of money poured into UWB, but dreadfully little usable technology.
The FCC and other government institutions are always going to have a hand in anything that scatters signal at RF.
My bet is on optical line-of-sight being developed by companies like Terabeam. OK, right now it's expensive technology only practical for corporate customers. So were computers 30 years ago.
The advantage of LOS technology is that there is no shared spectrum, and therefore no legitimate need to regulate. The only people who might want to regulate it are neighborhood associations, for aesthetic reasons. However, most of them have accepted satellite dishes so a well designed optical LOS transceiver should pass also.
Regardless of who or how, the problem will be solved. One day, people will pay only the cost of the transceiver, and they will get free internet, phone, you name it. I told this to my Dad, who owns stock in the phone company and it made him a bit nervous. I told him not to sell yet, but in 20 to 30 years you might not want to be holding stock in any of the companies that sell bandwidth.
For all intensive purposes, "whom" is no longer a word. That begs the question, "who cares"?
The bottom line is that there is a fundamental limit to how much data can pass through a given amount of radio spectrum in a given period of time. Let's say you have 100 MHz of bandwidth available and 100 signal sources each generating enough data to fill 1 MHz of bandwidth. Now, we can divy up the spectrum in several different ways.
The obvious one is to place each source on a different frequency and since each one is 1 MHz wide we can just fit them all in. This is called frequency division multiplexing and its how FM radio works.
Next, we could get each signal to buffer 1 second of data and then transmit it at 100 times normal speed for 1/100th of a second. Increasing the data rate increases the bandwidth so each signal is now 100 MHz wide and uses the entire available spectrum, but if we get them to transmit one after the other we get 100 bursts each 1/100th second long and once again the channel is full. This is called time division multi access (TDMA) and it is how your GSM cellphone works.
Lastly, we can take each signal and mix it with a very high speed pseudo random data stream. The signal gets much wider, but the energy per unit bandwidth goes down. At the other end the received signal is mixed with the same random stream and the original signal re-appears. The maths is complex, but it turns out that if I spread each signal out to fill the 100 MHz then I can just about get them all in without them interfering with each other too much. This is called code division multiple access (CDMA) and is how GPS and 3rd generation cellphones work.
Now for the important bit. As you can see, we have 3 different ways to get 100 signals into out 100 MHz of bandwidth. We can choose any one of them, but we can't choose all three. If our 100MHz is already full of TDMA signals and we put 100 CDMA signals in the same spectrum then they will interfere.
Now, if I have a bandwidth which isn't being fully utilised (suppose for example I have only 10 frequency multilexed signals in my 100 MHz) then I can slip in a few CDMA signals and all will be well, but only a few. If I bung 50 in then the FM signals will start to suffer interference.
What Mr Fullerton has done is basically produced a modified version of CDMA which is spread over a very wide band. The radio spectrum is still not used very efficiently, so there is enough room over that huge bandwidth for quite a few of his transmitters to be fitted in. Provided that their average power is low and there not too close to your receiver. But you don't get something for nothing. He's not using new spectrum, just old spectrum in a different way. We could achieve exactly the same effect by taking conventional CDMA and just making the random bit stream much faster.
Finally, there are costs to his approach which is related to the huge bandwidth and that is making an effecient wideband antenna. As someone who designs antennas for a living I can tell you that making a wideband antenna which is efficient means sacrificing gain or size or both.
http://www.aetherwire.com/
I've been watching this subject for a couple of years now... The first link I found was at Lawrence Livermore National Laboratory's MIR site, where they have a very similar setup.
Here are a couple of applications:
Cheap radar. These things, since they are spread spectrum, don't interfere with each other and are ideal for watching reflected signals (since you are the sender and know the chirping pattern you used to transmit with)
Complicated imaging. They had some pictures of a larynx -- instead of doing speech recogintion on sound waves, they were doing it by watching the actual parts of the body that move to form the sounds!
Even more complicated imaging. They had a 3-d radar system to detect reinforcement rods in concrete. Pretty neat
The above site also has the FCC rules regulating the transmission (since these are single pulses, not repetitive waves, the FCC isn't sure what to do with them) and LLNL's response. One curious thing about ultra-wide radar is that the frequency response of the antennas themselves are the limiting factor on what frequencies actually get transmitted -- so aftermarket antennas might not be so easy to use since they are a vital part of the circuit.
HIV Crosses Species Barrier... into Muppets
The early adopter in me thinks this is wonderful, and can't wait to buy it. Where's the global-access web-surfing PDAs?
The pragmatist in me realizes we won't see this for many years. How long have we waited for Bluetooth?
The Democrat in me thinks this could bring widespread, afordable telecommunications to the masses, and should be encouraged. What's the problem, FCC, why won't you approve this?
The free-market capitalist in me says this could disrupt the established players in the business and should be watched closely. Who should I buy? Who should I short?
The Republican in me says this would disrupt the established players in many businesses and must be stopped. Why isn't the FCC working to ban this subversive activity?
The cynic in me notes who's in charge now. Guess who will win?
If all this should have a reason, we would be the last to know.
There's no free lunch with spread spectrum signals. They do not magically provide more information carrying capacity, lower probability of intercept or defense against intentional jamming.
All modulation techniques use a set of basis vectors to transmit information. The traditional non-spread sprectrum methods use the frequency, amplitude or phase of a sinusoid as a basis. Spread spectrum methods use "weirder" bases, such as pseudo-random codes. The only real differences between the SS methods and the traditional ones are that the SS bases are highly unlikely to look like "common" interfering signals (carriers, impulse noise, etc.) and that they tend to have a wider bandwidth as well.
"Ultra wideband" is just another spread spectrum technique that uses the time position of pulses as its basis set.
There is only so much bandwidth out there and the amount of data you can send through it is bounded by the Shannon limit which is a function of bandwidth and noise floor. Any signal you transmit, no matter what modulation technique it uses, will interfere with other signals, at the very least by raising the noise floor, and thus lowering the Shannon limit for that section of the spectrum. It may be possible to demonstrate that a few ultra-wideband signals don't interfere with exisitng transmissions, but that certainly won't be the case for a large number of ultra-wideband signals.
The one advantage of ultra-widband is that it has very good time resolution because of the short pulses, and since the speed of light is constant, this equates to very good spatial resolution. Lawrence Livermore Labs was demonstrating something called "micropower impulse radar" (MIR) based on these techniques a number of years ago. This was the gadget that lets you "see" through walls. The range was very short, but it did work. This looks like the best application of ultra-wideband.
I hope the FCC isn't fooled by snake-oil claims of non-interference and unlimited bandwidth. They should not approve ultra-wideband for normal communications use.
God I love people like this. They're told it can't be done, and they obsess until they develop a solution. These are usually the greatest inventions, IMAO, because they shake up the world, and hopefully make people think a bit farther outside of the box.
If god had intended you to be naked, you would have been born that way.