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In wireless systems, radio waves do not propagate simply from
transmit antenna to receive antenna, but bounce and scatter wildly
off objects in the environment. This scattering is known as
"multipath", as it results in multiple copies ("images") of the
transmitted signal arriving at the receiver via different scattered
paths. In conventional wireless systems, multipath represents an
impairment to accurate transmission, because the images arrive at
the receiver at slightly different times and can thus interfere
destructively, canceling each other out. For this reason, multipath is
traditionally viewed as a serious impairment. Using the BLAST
approach however, it is possible to exploit multipath -- that is, to
use the scattering characteristics of the propagation environment to
enhance, rather than degrade, transmission accuracy by treating the
multiplicity of scattering paths as separate parallel subchannels.
BLAST accomplishes this by splitting a single user's data stream into
multiple substreams and using an array of transmitter antennas to
simultaneously launch the parallel substreams. All the substreams
are transmitted in the same frequency band, so spectrum is used
very efficiently. Since the user's data is being sent in parallel over
multiple antennas, the effective transmission rate is increased in
roughly in proportion to the number of transmitter antennas used.
Rohit Khare -- UC Irvine -- 4K Associates -- +1-(626) 806-7574 http://www.ics.uci.edu/~rohit -- http://xent.ics.uci.edu/~FoRK
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[in the news--bell labs innovations] [Lucent Technologies]
Bell Labs Scientists Shatter Limit on Fixed Wireless[in the news] Transmission [projects] -----------------------------------------------------------[people]
MURRAY HILL, N.J. (Sept. 9, 1998) -- Lucent Technologies [resources] announced scientists at Bell Labs, the company's research an[faqs] development arm, have developed a breakthrough technology that may potentially boost the capacity of certain wireless link[index] by 10 to 20 times. (Technical information about the techn[employment] is available on a separate page.) [fellowships] This innovation, known as BLAST, may allow so-called "fixed[search] wireless technology to rival the capabilities of today's wired networks, while also providing faster and more cost-effective deployment. One potential application would be for busine[Bell Labs return] where wires no longer would be necessary to transmit data between desktop computers, notebook computers and hand-held devices. Another possible use would be providing phone service to remote and rural areas, where wireless networks would connect homes and businesses to copper-wired public telephone service providers.
"Technologies that provide a 10-fold improvement in wireless capacity come along once a decade," said Bell Labs President Dan Stanzione. "This is a very significant scientific development with long-term potential impact on our wireless business."
The BLAST technology is not well suited for mobile wireless applications, such as hand-held and car-based cellular phones, because multiple antennas -- both transmitting and receiving -- are needed. In addition, tracking signal changes in mobile applications would increase the computational complexity.
The inspiration for BLAST (Bell Labs Layered Space-Time) can be traced to a challenge from Rich Gitlin, chief technical officer and Data Networking Technology Vice President in Lucent's Data Networking Systems business unit. Gitlin asked the researchers to take a fresh look at a 50-year-old mathematical theory developed at Bell Labs, which is the theoretical foundation of today's high-speed communications systems. The end result was cramming roughly 10 to 20 times more information on the same frequency band by developing new signal processing techniques.
"The breakthrough results prove the feasibility of a technology which leapfrogs what we assumed about the limitations of radio communications," said Jim Brewington, president of Lucent's Wireless Networks Group. "While there is still a great deal of applied research required before we apply this discovery, we are very excited about its potential implications for our future wireless systems."
The BLAST technology essentially exploits a concept that other researchers believed was impossible. The prevailing view was that each wireless transmission needed to occupy a separate frequency, similar to the way in which FM radio stations within a geographical area are allocated separate frequencies. Otherwise, the interference is too overwhelming for quality communications.
The BLAST researchers, however, theorized it is possible to have several transmissions occupying the same frequency band. Each transmission uses its own transmitting antenna. Then, on the receiving end, multiple antennas again are used, along with innovative signal processing, to separate the mutually interfering transmissions from each other. Thus, the capacity of a given frequency band increases proportionally to the number of antennas.
The BLAST prototype, built to test this theory, uses an array of eight transmit and 12 receive antennas. During its first weeks of operation, it achieved unprecedented wireless capacities of at least 10 times the capacity of today's fixed wireless loop systems, which are used to provide phone service in rural and remote areas.
"This new technology represents an opportunity for future wireless systems of extraordinary communications efficiency," said Bell Labs researcher Reinaldo Valenzuela, who headed the BLAST research team. "This experiment, which was designed to illustrate the basic principle, represents only a first step of using the new technology to achieve higher capacities."
The advanced signal-processing techniques used in BLAST were first developed by researcher Gerard Foschini from a novel interpretation of the fundamental capacity formulas of Claude L. Shannon's Information Theory, first published in 1948. While Shannon's theory dealt with point-to-point communications, the theory used in BLAST relies on "volume-to-volume" communications, which effectively gives Information Theory a third, or spatial, dimension, besides frequency and time. This added dimension, said Foschini, is important because "when and where noise and interference turn out to be severe, each bit (of data) is well prepared to weather such impairments."
Remarkably, the initial BLAST experiment designed by researchers Glenn Golden and Peter Wolniansky did not use the technology of error correction coding to correct signal errors, nor did the transmitter have prior knowledge of which signal components would propagate easily and which would be severely impaired.
Also, BLAST research by Michael Gans includes determining the optimal placement and number of transmitting and receiving antennas. If, for instance, the distance between antennas on each end were further reduced, the number of potential applications, such as mobile communications, might increase. In addition, researchers are trying to boost capacity even further and exploring how to enhance BLAST for all wireless formats.
Technical background information about BLAST is available. More information about the BLAST research project is available at <http://www.bell-labs.com/projects/blast>. For technical information on the BLAST architecture, see Gerard J. Foschini, Layered Space-Time Architecture for Wireless Communication in a Fading Environment when Using Multiple Antennas, Bell Labs Technical Journal, Volume 1, Number 2 Autumn 1996, pp 41-59. For more information on Claude Shannon's Information Theory, see <http://www.lucent.com/informationtheory>.
* BLAST Technical Background
This information is based on a press release written by Steve Eisenberg and Dick Muldoon of Bell Labs Media Relations.
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