Bouncing Signals Push the Limits of Bandwidth

January 16, 2003
By IAN AUSTEN

IT is a phenomenon well known to people who drive through
urban high-rise canyons. Just as you stop at a traffic
light, the car radio loses its signal. Once the light turns
green, the car only has to creep forward a few feet to
restore the radio reception.

Those dead spots, which can also cut off cellphone calls
and mobile computer communications, are often caused when
signals bounce wildly off the surrounding buildings. This
scattering creates pockets in which two reflections of the
same signal collide and cancel each other out.

Avoiding the undesirable effects of multipath, as this
scattering effect is formally known, has long been a
preoccupation of people who design wireless communications
systems. Now, however, a system developed by Bell Labs
actually embraces radio reflections not only to improve
reception but also to boost the speed of wireless networks.
Prototypes of the system, called Blast, can send data over
third-generation, or 3G, cellphone networks at rates about
eight times those of 3G.

"Normally multipath is the source of confusion, it's the
enemy," said Robert W. Lucky, who recently retired as vice
president for applied research at Telcordia Technologies
and is familiar with the Bell Labs work. "Here you put the
confusion back together Humpty Dumpty style. It's like
getting something for nothing."

Gerard J. Foschini, a 40-year veteran of Bell Labs, came up
with the theory behind Blast about a decade ago while
working on a long-term project to find the limits of a wide
variety of technologies. As part of that project, he
reviewed the work of Claude Shannon, the Bell Labs
mathematician who published a paper in 1948 that
established the field of modern information theory. Dr.
Shannon's work still provides the basis for much
information theory, including the notion of system capacity
limits.

"He found the ultimate limits," Dr. Foschini said. "But he
was basically dealing with one transmitter and one
receiver. It was obvious to us that we could deal with many
transmitting antennas and many receiving antennas for the
same transmission."

So Dr. Foschini began developing mathematical models to see
whether sending data through arrays of antennas would
expand network capacities.

Antenna arrays have long been used in radar systems. But
Dr. Foschini said that radar arrays are used to focus radio
beams, whereas he wanted to scatter them. He hoped to
discover whether wireless capacity could be boosted by
dividing up data in space as well as time. Rather than
point-to-point communications, his plan was to create
volume-to-volume exchanges.

He had found through mathematical research that the concept
would not work if the transmitter had only a single
antenna. "If you send the same signal from one antenna many
times all radiating in the same band, you come out
statistically right where you started," Dr. Foschini said.

Instead, he developed a system that divided data into
multiple streams that were then transmitted on the same
frequency by several antennas. At the receiving end, the
different streams of data were picked up by other antenna
arrays.

Normally more than one transmission on a single radio
frequency produces nothing but electronic noise. But Blast
can make sense out of the noise because of the physical
separation of the antennas sending the messages. Processing
software reassembles the scattered data streams into their
original form.

When Dr. Foschini tested the plan mathematically, the
results were surprising. "We found the capacities were
enormous - far, far in excess of what people were thinking
of,'' he said. "If you put more and more antennas at the
transmitting end, the capacity kept increasing. We were
coming out with such ridiculously large capacities that at
first, we didn't believe it."

Prototype systems proved that the experiments were correct.
Each additional antenna added another element of space and
because of that, additional capacity.

Just as surprising was the finding that the reflections
that plague current wireless systems actually expanded the
capacity of Dr. Foshini's system by effectively introducing
more points in space. "Heavy scattering, which I always
thought was a bad thing, is with this a good thing," he
said. In fact, he anticipates that Blast-based wireless
systems will work more effectively in Manhattan rather than
"somewhere where it's flat as far as the eye can see."

Bell Labs has made prototype chips that would allow Blast
to operate at speeds of 19.2 megabits per second over a 3G
wireless network. Currently the highest speed those
networks can offer is 2.5 megabits per second. Ran Yan,
vice president for wireless research at Bell Labs, said
that the prototype chips were intended for use in a
cellphone or wireless hand-held computer.

Dr. Foschini declined to estimate the ultimate transmission
speeds that could be achieved with Blast. One restraint on
speed is the intense data processing it requires. With
current technology, higher speeds would demand chips that
are too large and too power-hungry for hand-held devices.

Dr. Yan said that the first systems offered by Lucent
Technologies, the lab's parent company, would probably use
just four transmitting antennas. Because wireless data
systems operate with high frequencies and the transmitting
antennas must be separated by only half a wavelength, he
said, it will not be hard to squeeze more antennas into
even the most compact mobile phones or palmtop organizers.

Because of economic problems, the wireless industry has
been slow to adopt even 3G networks in the United States.
So Blast is unlikely to become available soon. But unlike
3G, Blast does not require the construction of new
networks. It only needs relatively inexpensive equipment,
like new base stations, to be installed on current systems.
 

"It's a minimal upgrade," Dr. Yan said. "But it will allow
service providers to get 300 to 400 percent increases in
data rates in first deployments, and much higher quality."

While Lucent is already making network base stations for
wireless service providers that can be converted to use
Blast, Dr. Lucky anticipates that those companies will wait
for the military to pioneer use of the system. He said
there were concerns that the complexity of Blast might
create unforeseen problems when used by large numbers of
people on congested networks.

Assuming that problems do not develop there, however, Dr.
Lucky said, the system could completely alter all systems
that depend on radio waves. "I had this idea that spectrum
was all used up,'' he said. "Now, with new technologies
like Blast, maybe spectrum is infinite."
 

Copyright 2002 The New York Times Company