Scattered signals boost capacity
By Kimberly Patch, Technology Research NewsHelp may be on the way for those despairing about the sheer density of cellphone towers popping up all over the planet.
A team of researchers from Bell Labs has figured out how to triple the theoretical capacity of wireless communications channels by making use of signals that take a roundabout route to an antenna, bouncing off objects like buildings along the way.
This scheme could increase the capacity of existing satellite networks and cellphone towers considerably, said Michael Andrews, a physicist at Lucent Technologies' Bell Labs. "Our result is saying that there's a factor of three there that people haven't considered [in terms of] the amount of information measured in bits per second that you can fit in a given bit of spectrum, " he said.
In a practical sense, people who design radio communications systems could use this to gain more bandwidth, support more users, or to use less spectrum to transfer the same amount of information, Andrews said.
Conventional radio and cellphone telecommunications transmit and receive information on one channel for each frequency of radiowave using a single antenna. Other types of communications, particularly satellite communications, make use of two polarization states -- vertical and horizontal -- in the radio waves to transmit two channels per frequency.
A signal is received when photons that make up a radiowave hit the receiving antenna. These photons are vibrating, or oscillating, and it is these oscillations that carry information by conveying force to, or pushing, the electrons in the antenna.
The two polarization states make use of left, right and up, down oscillations of the electrons to convey information. Because photons exist in three dimensional space, they are capable of oscillating in the third dimension -- front, back -- but this oscillation cannot be used in wireless communications because of a law of physics: the electric field can't vibrate in the same direction that the wave is moving.
The researchers have figured out how to use the third dimension in a different way -- by using the signals that bounce off objects in the environment and thus hit the receiving antenna at different angles from the photons that did not bounce.
The way this type of radio transmission works is analogous sending out light beams in free space versus sending out light beams in a hall of mirrors, he said.
In free space, where there are no objects for beams to bounce off "you are only going to see some fraction of all those waves being sent out from the transmitter because some of them are just going in directions you can't see," Andrews said.
In a hall of mirrors, on the other hand "you get multiple vantage points on the transmitter... you're seeing stuff that would never have reached you previously,” he said.
These multiple vantage points allow the researchers to use the third dimension to transmit information. Each wave that bounces off an object in the environment hits the antenna at its own angle, adding a third dimension to the two provided by the wave's polarization states. "All these two-dimensional planes are inclined relative to one another. And so you're really getting access to all three dimensions from these mismatched two-dimensional planes," said Andrews.
The electric and magnetic fields that make up a radio signal can both, independently, be used this way, providing a total of six channels. "The electric field is a three-dimensional object, and so is the magnetic field. What those fields tell you is how much force there would be and what direction that force would point in... on an electron in the metal of the receiving antenna," said Andrews.
The researchers have successfully sent signals on the three electric channels with a three-antenna prototype, according to Andrews. In theory, the three magnetic dimensions would also work same way, providing the other three channels, he said.
With the addition of signal processing to separate the six signals and an array of six antennas that could send the six different types of signals at the same spot, the theoretical capacity of a given frequency of wireless spectrum would triple, Andrews said.
Signal processing is needed to untangle the separate signals because they don't arrive at the receiving antenna in orderly way. "You could never control how those waves bounce around and also all of those waves are adding and subtracting to each other [when they meet] in space. Rather than just keeping track of one channel you need to keep track of six channels and how they interfere with each other," he said.
It's a good idea, and the antenna and signal processing work required to use the extra channels is doable, said David Rutledge, an engineering professor and chair of the engineering department at the California Institute of Technology. Figuring out how to untangle the six signals is "within the range of their [signal] processing ability to do reasonably cheaply," he said. Making an array of six antennas to extract the six different channels from the same spot in space "would take some work," he added.
It's a significant development that could both increase electromagnetic spectrum capacity and advance the general understanding of radio signals, according to Daniel Stancil professor of electrical and computer engineering at Carnegie Mellon University. "It gives new insight... as to what is possible in an environment that is rich with scattered and reflected signals," he said.
Because six antennas are needed to receive the six signals and higher frequencies require smaller antennas, handheld devices could more easily use this technology for the personal communications systems (PCS) frequencies, which are around 1.9 gigahertz, than the cellular frequencies, which are around 850 megahertz, Stancil added.
The engineering solutions required to use the extra channels could probably be solved within a couple of years, Rutledge said.
Andrews' research colleagues were Partha P. Mitra of Bell Labs and Robert deCarvalho of Bell Labs and Harvard University. They published the research in the January 18, 2001 issue of Nature. The research was funded by Lucent.
Timeline: 2 years
Funding: Corporate
TRN Categories: Wireless Communications
Story Type: News
Related Elements: Technical paper, "Tripling the Capacity of Wireless Communications Using Electromagnetic Polarization," Nature, January 18, 2001.
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January 24, 2001
Page One
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Scattered signals boost capacity
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