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SCIENCE AND TECHNOLOGY
How to look through walls
BESIDES its use in communications (see article), ultra wideband (UWB)
pulse radio might have a future as a radar that can see through
walls, and do so in great detail. It should, its manufacturers hope,
be able to distinguish a cat from a cat burglar, or detect barely
breathing bodies under several metres of rubble after an earthquake.
More mundanely, do-it-yourself enthusiasts will be able to use it to
check for power cables and pipes beneath the plaster before they
start drilling.
UWB radar works like normal radar in so far as it depends on sending
out radio signals and listening for the reflection. But unlike
ordinary radar, which takes the form of continuous waves, UWB signals
are short pulses of energy.
As a means of radio communication, UWB works because the chips in the
receiver are able to time the pulses they are hearing to within a few
thousand-billionths of a second. Even at the speed of radio (ie, the
speed of light), a pulse will travel only a few millimetres in that
time. Since, in the case of radar, the receiver is also the
transmitter, it knows exactly when a pulse was sent. By measuring how
long that pulse takes to return, it can place the distance to the
point of reflection to within that level of accuracy-enough to tell
whether an aircraft's wing-flaps are up or down. Four million pulses
a second are sent out to provide a near-perfect picture of what the
target looks like.
Conventional radar relies on high-frequency (and therefore short
wavelength) radio waves to achieve high resolution. Long waves would
produce fuzzy images. But when the resolution depends on
pulse-length, wavelength does not matter. So UWB radar can employ
significantly longer wavelengths, and these can penetrate a wide
range of materials, such as brick and stone, which are denied to
their shortwave cousins.The result is "RadarVision", which, like the
communication
technology, is manufactured by Time Domain. Though still
experimental, it is being tested by several police forces around
America. They are using it to look inside closed rooms that might be
harbouring suspects, before the guys with the sledgehammers batter
the door down. If it works, television cop-shows will never be the
same again.
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Bandwidth from thin air
Two new ways of transmitting data by wireless exploit unconventional
approaches to create valuable additional capacity
THEY may be invisible, yet chunks of radio spectrum are fought over
just as much as parcels of land. Governments raise billions by
auctioning parts of the spectrum to mobile-phone companies and radio
and television stations. Other frequencies are reserved for
air-traffic control or the sending of distress signals. The most
desirable addresses on the spectrum, like apartments in the trendiest
parts of town, are in short supply-hence the high prices paid for
them. To make the most of limited "bandwidth", as it is known,
engineers have devised elaborate schemes to allow several devices
(such as mobile telephones) to share a single frequency by taking
turns to transmit.
Two emerging technologies now promise to propel such trickery into
new realms, by throwing conventional ideas about radio transmission
out of the window. The first involves multiple simultaneous
transmissions on the same frequency. The second, by contrast,
transmits on a huge range of frequencies at once. Outlandish though
it sounds, the effect in both cases is to create hitherto unforeseen
reserves of valuable bandwidth, practically out of thin air.
Don't all talk at once. Actually, do
Turn the dial (or press a button) on a radio, and you determine which
station's signal is played through the speaker. Now imagine that
several radio stations are transmitting on exactly the same
frequency, so that their signals interfere with one another. Is it
possible to build a new kind of radio, capable of separating the
signals, so that just one of them can be heard clearly? The
conventional answer is no. Once radio signals have been mixed
together, trying to separate them is like trying to unscramble an
egg. In 1996, however, Gerard Foschini of Bell Labs (the research arm
of Lucent Technologies, based in Murray Hill, New Jersey) suggested
that multiple transmissions on a single frequency could be separated
after all-by using more than one receiving antenna and clever signal
processing. The result was a technology called Bell Labs Layered
Space-Time, or BLAST.
The prototype system, which is now being tested, transmits via an
array of 12 antennae, all of which broadcast a different signal, but
on exactly the same frequency. At the receiving end are 16 antennae,
also spaced out, each of which receives a slightly different mixture
of the 12 broadcast signals-which have bounced and scattered off
objects along the way. Computer analysis of the differences between
the signals from the receiving antennae, helped by the fact that
those receiving antennae outnumber the transmitting ones, enables the
12 original signals to be pieced together.
Exploiting this result, it should become possible to transmit far
more data than before over a wireless channel of a particular size.
For convenience, the researchers used a channel "width" of 30kHz, the
size of the channel used by analogue mobile phones. Normally, a
data-hungry process such as accessing a web page over such a link is
painfully slow. But using BLAST, transmission speeds of up to 1m bits
per second have been achieved. By increasing the number of antennae
at each end, it should become possible to squeeze even more capacity
out of a fixed-size channel, albeit at the cost of far greater
computational effort.
The technology is not, however, intended for mobile use. The multiple
transmitting and receiving antennae, and the powerful
signal-processing hardware involved, will be difficult to fit inside
portable devices. In any case, too much moving around causes the
mixture of signals received by each of the antennae to vary in ways
that even the most sophisticated computer cannot cope with. Instead,
according to Reinaldo Valenzuela, who is in charge of the research,
BLAST is more suitable for use in fixed wireless applications, such
as providing high-speed Internet access to homes, schools and
offices, or establishing telephone networks in isolated areas without
laying cables.
If transmitting several signals on the same frequency sounds odd,
what about transmitting on many frequencies simultaneously? That is
the principle behind another novel form of wireless-communications
technology known as ultra-wideband (UWB). This is being developed by
a small company called Time Domain, which is based in Huntsville,
Alabama. The technology is the brainchild of Larry Fullerton, an
engineer who has spent the past 23 years working on the idea.
Whereas conventional transmitters (and BLAST transmitters) operate at
a particular frequency, just as a single key on a piano produces a
particular note, a UWB transmitter emits a pulse of radiation that
consists of lots of frequencies at once, akin to the cacophony that
ensues when all the keys on a piano are pressed at the same time. The
pulse is very short-just half a nanosecond (billionth of a
second)-and is transmitted at extremely low power. Because it is a
mixture of so many frequencies, such a pulse passes unnoticed by
conventional receivers, which are listening for one particular
frequency.
But to a UWB receiver, listening on a wide range of frequencies at
once, it registers as a distinct pulse. Information is sent by
transmitting a stream of pulses-apparently at random (to fool
conventional receivers), but actually at carefully chosen intervals
of between 50 and 150 nanoseconds, in a pattern known to both
transmitter and receiver. By varying the exact timing of each pulse
to within a tenth of a nanosecond, slightly early and slightly late
pulses can be used to encode the zeroes and ones of digital
information. The resulting system can transmit data at 10m bits per
second, without any interference with conventional transmissions.
Or so Mr Fullerton and his backers at Time Domain contend. So far,
however, America's Federal Communications Commission (FCC) has not
approved the technology for anything more than experimental use. But
there are signs that UWB could, after a long gestation, soon emerge
into the marketplace. At a conference in September to rally support
for it, Susan Ness, an FCC commissioner, spoke in support of the
technology and said regulations permitting it to be used would be
announced next year.
Several firms are lining up to make products based on UWB technology.
Time Domain, which owns the relevant patents, plans to supply these
firms with its chip, called PulsON, to do the hard work of generating
and detecting UWB pulses. And as well as communications, UWB also has
an intriguing potential use in radar (see article).
Neither BLAST nor UWB quite create something out of nothing. Both
technologies cunningly conjure up extra bandwidth at the cost of
increased computational complexity. Over the past few years, however,
the cost of computing power has plummeted, and demand for bandwidth
has soared. Trading one for the other could prove to be a very good
deal.
LINKS
See Lucent Technology's website for more information about BLAST
technology. Ultra-wide band technology is described on Time Domain's
site. Start with the Ultra Wideband Working Group for more
information on the subject.
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Overview
The Ultra Wideband Working Group (UWBWG) has been founded in response
to interest voiced by the UWB Community at the UWB Communications
Workshop on May 25-27, 1998, as well as a result of the FCC's NOI on
UWB. Membership is open to interested parties, and you can become a
member just by subscribing. Members will be actively involved in
setting industry standards, and voting on issues related to
technology deployment.
1999 International UWB Conference
The Ultra Wideband Working Group recently hosted the 1999
International UWB Conference. The event was held on September 28 -
30, 1999, in Washington, DC at the Crowne Plaza/ Sphinx Club. For
more Information visit our Conference Site.
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