Technique puts more data into airwaves As telecommunications designers add

electronic mail and Internet access to cel- lular phones, they find it’s like connecting a fire hose to a straw. Each phone user’s tiny slice of the airwaves is too narrow to handle the full deluge of available data.

Now, scientists at Lucent Technologies’ Bell Labs in Murray Hill, N.J., have found a way to boost the datacarrying capacity of each radio frequency as much as six- fold. They outline their scheme in the Jan. 18 NAW.

Besides feeding more data into each cell-phone link, the development could ex- pand telecommunications in other ways, says Lucent’s Michael R. Andrews. For in- stance, wireless companies might pack more customers onto each radio channel.

To test the new approach, Andrews, Partha P. Mitra, and Robert decarvalho recorded three independent electric sig- nals encoding the red, green, and blue hues in a Joan Miro painting. Ordinarily, each signal would require a separate f r e quency. But in their new scheme, the re- searchers transmitted those signals across a cafeteria at Bell Labs simultaneously on one frequency. A receiver adapted for the new technique reproduced the Miro im- age from those signals.

Alfred 0. Hero I11 of the University of Michigan in Ann Arbor comments that the “intriguing and interesting” result

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stokes an already hot technology known as smart antennas. It provides a compact way to further increase capacity, he adds.

Smart-antenna pioneers have recently discovered that using 10 or so antennas at each transmission and receiving sta- tion can greatly increase the number of independent channels per radio frequen- cy (SN: 7/15/00, p. 38). But the scheme works only in environments with many objects, such as buildings, that scatter radio waves. That way, each signal ends up taking several independent paths.

Although the signals arrive in a scram- bled condition, sophisticated software un- tangles them by accounting for the effects of the different paths. So far, the technique has been unsuitable for handheld devices

This antenna sends or receives multichannel data on a single frequency.

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because it requires spacings between the multiple antennas that are wider than the devices themselves.

Andrews’ team has now recognized that the scattering of radio waves not on- ly multiplies the number of pathways they can take from transmitter to receiv- er. It also adds an extra information-car- rying dimension to those waves.

Scientists have known since the 19th century that electromagnetic waves con- sist of electric and magnetic fields orient- ed, or polarized, along distinct directions in space. Therefore, polarization is a fea- ture of every transmitted signal. Andrews says, however, that textbook explana- tions of electromagnetic waves preclude polarization along the same direction a wave travels, leaving only the updown and left-right dimensions.

Not so, he and his Lucent colleagues have now found. When waves are scattered along multiple pathways, fields do polarize along the direction of travel. So, there are three polarization dimensions. The addi- tion of the third permits the polarization of the electric and magnetic fields to vary in- dependently. That means six separate pcr larization signals can be transmitted simul- taneously on the same frequency.

What’s more, exploiting these six new- ly recognized channels of a radio signal requires only one antenna, albeit a bristly one, at each end of the link. That simplic- ity may make the discovery applicable to handheld cell phones. -P Weiss

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