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Wireless "Pulse" Technology
mustard writes "
This is an article in USA Today about a technology that
uses energy pulses to transmit data.
It's fast as the speed of light, cell phones could be as
small as a wristwatch, and you could have only 1 tower
every 100 miles. It uses new chip technology from IBM,
and as an example, they cite that it could support over
2,000 cellphones per block, as opposed to coventional
cellular today which is about 400 per block. But it's not
limited to that, it can be used for cheap personal radar
as well. Well worth a read, fascinating stuff. In a
related story, the inventor of the patent is in a dispute
with a government funded lab who, according to congress,
stole the idea."
Anyone else distrubed by the thought of "hand-held radar that police can use to see inside a room before bursting in"?
Here in New York City we'd be happy to have something that would ensure that the police can see a person clearly outside of a building prior to pumping them full of bullets.
Outside that bit of current events, I imagine that something like hurricane fonts will happen - things that people can use to generate interference. At least I hope such things are common. After all, there are so many things that interfere with my AM reception here at home - from the microwave to my monitor - that I expect that future technologies will evolve associated technologies that provide static as a side-effect.
It just may take some time.
-Peter
== Just my opinion(s)
Er, that was the meaning of my smiley immediately after Walsh Function ... "no, it's not pizza, it's tomato and cheese on a flour and water base." In other words, maybe the "new invention" is one of the types of SS the world has known for many years but described in a way that makes it sound different.
I'll be most interested to see whether this thing makes it to market. We shouldn't prejudge it on the basis of just an article or two: really new inventions are rare, but they *do* appear occasionally.
"The question of whether machines can think is no more interesting than [] whether submarines can swim" - Dijkstra
It's different. SS/CDMA encodes information in the phase of fixed-rate transitions of a fixed-frequency phase-locked carrier. As far as I can make out, the article is describing a type of Time Hopping SS in which information-coded pseudo noise modulates the time-domain position of a pulse, so none of the RF bits of SS/CDMA are present at all.
"The question of whether machines can think is no more interesting than [] whether submarines can swim" - Dijkstra
The fourier transform and the concept of "spectrum" is so deeply ingrained in so many engineers that they have lost touch with the time domain, essentially the "real world". Many things are a lot simpler in the frequency domain, like AM and FM. But for many probelms the frequency spectrum is the wrong tool. I see so many people who don't understand even simple transistor circuits because they try to think in the frequency domain.
So stop talking about spectrum - it is the wrong tool for understanding this technology. Maybe the power spectral density is useful to figure out if you're polluting radio or TV signals, but that's it.
And don't talk about bit synch either. While that's important in conventional communication systems, it isn't here. With a correlation detector, you just sit waiting around until the output of your correlator jumps up - that's the detection of the pulse - then you read your picosecond stopwatch. No need to expect the signal, just detect it. So simple!
If you're unfamiliar with correlation, and want to hear some math, here goes. The correlation of two functions (say f(t) and g(t)) is the integral S(f(t)*g(t))dt. Essentially multiuply the two signals together and then integrate. For two signals that don't look at all alike, the correlation is small. But when the two signals are very close, the integral is much much larger.
So imagine that you want to detect a pulse with very high resolution. At every instant in time you use a little integrator in some electronics to integrate your incoming signal with the expected signal (whatever shape the pulse has). When there's no pulse, the output of your correlator is very small. When the pulse comes along and lines up with the pulse in your integrator, your correlation gets really big relly fast and then small again as the pulse passes.
Try the math yourself: do the integral S(f(t)*f(t-d))dt where f(t) is the e^-3x pulse and notice that there is a huge peak at d=0.
Since the autocorrelation of that pulse (the pseudo gaussian e^-3x) they are using is very very high, you can detect this pulse with very high precision using correlation techniques. Sub wavelength resolution even.
(For all you communications engineers out there, this is the legendary matched filter technique. Except the typical use of the matched filter samples its output at the middle of the bit interval, when its output is supposed to be biggest(for a one) or smallest (for a zero). Here you do the opposite: when the matched filter output is maximum, that's the middle of the bit!)
So all you need is a correlation detector, a really accurate timer, and a pseudorandom noise generator to whiten up your spectrum and allow multiple channels. And if you do some dsp, your timer allows you to turn reflections off of objects into a pretty good radar image. (Except it's more like typical sonar sounding than typical radar).
If Fullerton's correlator is as good as he says, this stuff is very much for real. Believe it!
The Companies web site can be found at http://www.time-domain.com
Privately held.... for how long?
It's not that this tech doesn't use radio waves, it just doesn't rely on the radio waves themselves as data.
Confusing statement, I guess.
Anyhow, as an example, digital cell phones pollute the radio spectrum, because (quote your favorite signal analysis source, since I'm not an expert) sending a fast sharp clear pulse (dirac deltas!) can be described as an infinite series of signals in differing frequencies (Fourier series, each term describing a different radio frequency)
Did I get that description right?
Anyway, this pulse technique, rather than using frequency hopping to distribute data across many different frequencies and allowing multiple devices to coexist at once, uses many frequencies at once, relying on a time domain discriminator to differentiate multiple devices. I think. I am unsure how they can do this, and perhaps someone else can supplement my data, spotty as it is.
The very use of many frequencies is necessary for ultrafast digital communication, or inversely, the decision to use digital communication forces the use of entire swathes of radio frequencies. Both are the same statement, I think.
It may not interfere with traditional radio frequency devices, but I think they would appear as noise and such to today's digital wireless devices, such as cell phones.
I also imagine this tech doesn't work very well across large distances, say a state or country without conversion to an alternative communications method, though within a city, what with its extremely dense packing of people and devices, it may be perfectly useable and possible. I say it may not work across long distances because each frequency would be attenuated differently by the atmosphere, would reflect differently on the layers of the atmosphere, and may be detected at different times as the originally sharp pulse gets smeared into a fuzzier packet of data.
Still, should make wireless lans a distinctly enticing possibility
AS
-AS
*Pikachu*
I'm not sure if it can be detected...
IE, it isn't a directed pulse as seen in a radar gun, so if you don't know the signal coding, I don't see how you can detect it.
Likewise to interfere with it.
If you wanted to just overwhelm them with static, you'd also probably interfere with other legitimate devices, such as your own cell phone, or the cell phone in the car next to you.
Heck, it would also mean that the police could tell how fast you were going if they knew your cellphone coding, without necessarily being able to decode and listen in on your conversation!
I'm pretty sure you can't block a radar gun functioning on this principle except to absorb all the radar with a stealth coating.
AS
-AS
*Pikachu*
Hey! Weren't those telegraphs you're speaking of, that used pulses to send signals, spark gap arc telegraphs? They were incredibly dangerous, incredibly power hungry, incredibly spectrum polluting, and didn't transmit using antennas at all, if I recall correctly....
Cool. I think =)
AS
-AS
*Pikachu*
Here are couple of related patent numbers I found, doing a quick search at IBM's Patent site; search for "Time Domain" and "Fullerton; Larry W." as the Inventors & Companies, look at referenced patents:
There are a good 40 patents referencing the older, USP4641317, originally filed on Dec 3, 1984.
...I wonder what a prior art search would turn up.?. As the saying goes, a rose by any other name, is still a rose.
Anyhow, just a little FYI. Enjoy!
--The more you know, the less you know.
Upside Today: Was Fullerton ever at Lawrence Livermore, or did they just develop it independently?
Petroff: He made a presentation to an audience that included almost a dozen Livermore people. Within several days they started working on a similar kind of thing, trying to come up with this technology. Livermore's credibility has been tremendously undermined [because of this] and it is becoming less of an issue. But for a long time, there were many people who knew about the Fullerton technology, but they were concerned about investing because Livermore might come in and sue them.
Interview with the CEO of Time Domain from Upside.
Anyone else distrubed by the thought of "hand-held radar that police can use to see inside a room before bursting in"?
The signal emitted from antenna depends on the derivative (speed of change) of the signal that you drive the antenna with. The usual sine wave has about the smallest derivative - and thus minimum possible effectiveness at dissipating radio waves.
A pulse wave - which has near vertical slopes is _much_ more effective. (example - a toy car with a dc motor using brushes makes enough noise to interruptions when you watch tv. it doesn't use that much power)
A drawback to this is that only sine waves allow you to control which part of the spectrum your transmission controls. Incidentally the first "wireless telegraph" also used pulses - and thus in a given area you could only have one wireless telegraph working at a given time.
So how can one get around this limitation ? One way is to transmit two singals, not one. Imagine that both you and you partner have sources that produce identical noise. Since noise also has big slopes it's transmission is very efficient. You transmit your signal as pulses of that noise. Your partner receives all radio he/she can handle and correlates the result with the noise source he/she has. The output signal should be your pulses. And as pointed out above you don't really need to have real noise - even pseudo-random pulses will suffice.
However, whatever method you choose there is a question of how finely you can tune the receiver. In the case of correlation you won't be able to filter out all stray signals.
Thus I think the bulk of the patents aren't on the method of transmission. Rather, they should be on how tune more finely to the selected bandspace.
Ultrawideband is a form of spread spectrum. The major difference between it and traditional forms of spread spectrum is that it is spread over a band which is wide relative to the center frequency (>25% of center frequency)
For example, Qualcomm's CDMA is spread over 1.25MHz around a center frequency in the 800MHz band while a typical UWB system covers over 1GHz starting at around 500MHz.
Conventional spread spectrum systems use frequency hopping or direct sequence to spread the signal. UWB uses a simple and often forgotten form of spread spectrum called time hopping where short pulses are transmitted at pseudo-random intervals. The reason this modulation is used is simply because FH and DS cannot be practically implemented over such a wide bandwidth.
It's not new. It has been used in jamming resistant radars for at least two decades. What's new is an implementation on a single chip which is potentially cheaper than even conventional carrier-based RF technology at large quantities.
The primary advantage of ultrawideband is its insensitivity to fading. Narrowband transmissions can experience significant attenuation of the signal due to signals travelling through different reflection paths canceling out each other. A wideband signal is virtually immune to this and therefore requires about 20db less power usually taken as a safety margin against fading.
Ultrawideband systems can communicate over significant distances using a lower power spectral density than the electromagnetic noise generated by a typical computer.
The primary limitation to using ultrawideband systems is the wording of part 15 of FCC rules - apparently while your computer is allowed to pollute the spectrum for no good reason it is not allowed to transmit the same power INTENTIONALLY.
The FCC has issued a NOI (Notice Of Inquiry) seeking comments on possible change to these rules. Opposing comments come from the usual suspects: mostly users of the restricted bands such as government agencies.
Links:
Ultrawideband working group
Aetherwire - makers of an ultrawideband gizmo called the locator which is both exciting and very frightening.
Stop worrying about the risks of nuclear power and start worrying about the risks of not using nuclear power.
I don't see how this technology allows for half of the speculation described in the article.
How can the same tech that allows directional distance pinpointing of a handheld cellular watch also be undetectable and untraceable in a marine communications device?
I would imagine the directionality and distance is a direct product of data smearing, that differnt frequencies and such of the same data pulse would travel at different velocities, so a single pulse train, under observation, can be analyzed to figure out how far it traveled, and the relative direction if an array of 3 receivers were used to determine which gets distorted most and least to triangulate a direction
AS
-AS
*Pikachu*
A lot of the claims made in the article are misleading or overblown. The idea of using very short pulses for data transmission is not new, and as someone has already pointed out this is merely a special case of spread spectrum encoding.
First: An extremely short pulse approximates a delta function, which has infinite frequency content; "DC to daylight." This is still a form of RF transmission, it just happens that you are dumping energy into a very wide range of frequencies.
Second: Transmissions using this technique _do_ interfere with other RF transmissions. In fact, they interfere with _all_ other transmissions, but that interference is spread over the entire spectrum so it does not interfere with any one frequency very strongly (this raises FCC regulatory questions). In addition, a time-domain spread spectrum encoding makes the likelihood of interfering with another pulsed time-domain spread-spectrum transmission very small, if a good spreading algorithm is chosen.
Third: This is not a new idea (we were looking at this a few months ago for a data transmission application) and there is a reason why this hasn't been widely implemented: timing. In order to receive a pulsed time-domain spread-spectrum signal, you must synchronize your receiver's spread-spectrum decoder to the transmitter's encoder. The shorter the pulses, the more exact the timing and the more difficult this synchronization becomes.
Here is an analogy:
Imagine transmitting a signal by encoding it as a time-varying sequence of baseballs being fed to a pitching machine. The receiver catches the balls, decodes the sequence and reconstructs the signal.
If the transmitter is the only one pitching, the task of decoding is easy.
The problem is, the transmitter is not the only one feeding the pitching machine -- the noise in the environment is also feeding balls in. The best way to encode the signal to avoid any particular noise source (and to avoid interfering with anyone else) is to make the encoding look as random as possible, which is what spread-spectrum encoding is all about.
The resulting stream of baseballs looks random, since it is a combination of a spread-spectrum signal and random interference. In order to decode the signal, you want to catch only the balls that represent the signal.
In order to do this, you install a shutter in front of the receiver -- the spread spectrum decoder -- which will only let the "signal" balls through. This requires the decoder driving the shutter to be exactly synchronized with the encoder.
As the pulses become narrower, the "balls" are coming faster and timing the shutter must become more exact to exclude non-signal balls. If a non-signal ball passes through the shutter (or a signal ball is missed), the error will break the syncronization between the tranmitter and receiver. Narrow pulses also make it more difficult to lock the receiver's decoder to the transmitter's encoder in the first place. Once the pulses become short enough, maintinaing synchronization becomes almost impossible without an additional, non-spread communication channel. If an additional, non-spread chanel is used, then you are back to the problems of ordinary RF transmission.
There is great potential in this technology, but the technical chalenges (and regulatory hurdles) are large.
Rich
parallax