Lots of Clutter? Doesn't Matter

Is your office a jumble of filing cabinets, furniture, and piles of old magazines? That's okay--at least as far as the hottest technology in wireless communication is concerned. Researchers have shown that a new technique dubbed "array-to-array" or "multipath" communication will work in the messy surroundings of everyday life. The finding may speed the replacement of today's wired computer networks with wireless systems.

An enduring problem of wireless communication is interference, the jumble of signals that occurs when two radio stations or cell phone transmitters broadcast on the same frequency at the same time. Engineers have always gotten around this by spacing stations far apart. But in 1998, engineers at Bell Laboratories in Crawford Hill, New Jersey, showed that the capacity of wireless networks could actually be boosted by using an array of transmitters and receivers close together, all tuned to the same frequency. "That's the opposite of what you would do in a conventional system," says Reinaldo Valenzuela, whose team developed the concept. "Normally, interference is something you avoid at all costs." But in this case, interference between signals allows the waves to travel to their destination by paths other than a straight line, the researchers found; computers can use the so-called phase changes this causes to resolve the signals from different transmitters in the array.

The find "was a paradigm shift," says wireless systems engineer Mary Ann Ingram of Georgia Institute of Technology in Atlanta. "Now, multipath is your friend." But the underlying theory was still somewhat primitive: It assumed that the space between the arrays of transmitters and receivers was empty--not a very realistic assumption in, say, a busy office. Aris Moustakas, a physicist with Bell Laboratories in Murray Hill, New Jersey, realized he could use the equations describing how electrons diffuse through impure metals to calculate how multipath radio waves bounce off objects such as office furniture and filing cabinets. In the current Science, Moustakas shows that the boost in information capacity given by an array is still present, and can still be accurately calculated, with furniture and other objects that scatter radio waves lying around.

Ingram expects Moustakas's theory to save her "time and money" in the development of new arrays. Although they would probably be too large for cell phones, the arrays might well fit on the back of a laptop computer, she says. And if they do, adds Valenzuela, all-wireless computer networks would become feasible.

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