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."

Some facts

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 [uwb.org]
Aetherwire [aetherwire.com] - makers of an ultrawideband gizmo called the locator which is both exciting and very frightening.

privacy?

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

several problems: a technical analysis

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