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Reflections
RevMike writes "The New York Times (reg required) is reporting that Bell Labs/Lucent has developed a method to multiply the bandwidth of cellular networks by using multipath. Robert Lucky developed the system called Blast. He claims that it should multiply the data rates in existing spectrum 300% to 400%. One prototype took a network from 2.5Mb/s to 19.2 Mb/s. Interestingly, the system works better in cluttered environments." And on a related note, Kimberley Burchett writes "The latest Physics News Update mentions that skyscrapers could actually help wireless communication. 'The more scatterers between the transmitters and receivers, the more channels that are available. For the time being, the communication technique is limited to ultrasonic communication - the electronics necessary for exploiting scatterers with wide-band time-reversal antennas at cell phone frequencies simply don't yet exist.'"
wide-band time-reversal antennas ... simply don't exist yet.
And in related news, still no flying cars.
So far, this new millennium sucks.
/syle
Bouncing Signals Push the Limits of Bandwidth
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."
OK, I'm a physical chemist, and this junk drives me up the wall.
All these journalists assume that the wattage is what matters, and that being 1 cm from a 4 watt transmitter could be unhealthy. However, the wattage is irrelevant - Einstein showed this with the photoelectric effect. Basically, it is the frequency of the radiation that is dangerous, not the wattage, because one photon interacts with one electron - so the number of photons is irrelevant. For example, which would you rather be near - a 100W light bulb, or a 1W gamma ray emitter? Thought so...;)
To give you a baseline, cancer due to radiation occurs because photons of succicient energy actually break chemical bonds in your DNA. This requires low-UV or better to accomplish. That's why sunburn gives you cancer. Now, compare this to radio waves, which are far, FAR to low in energy to accomplish anything of the sort. In fact, radio waves are even to low to excite the vibrational or rotational states of a molecule (which is how a microwave oven works), so there is no risk of "cooking" your brain, either.
Ultimately, when pressed, these cell-phone-cancer freaks point at two cases where some habitual cell-phone user got cancer on the right side of his brain. Ergo, it was the cell phone. These "doctors" (and I use the term loosely) have never proposed any sort of mechanism or ANYTHING to explain how it could occur. Because it can't.
Bottom line, you have a better chance of getting cancer from your own body heat (you emit infrared radiation) than you do from a cell phone.
-Looking for a job as a materials chemist or multivariat