Technique puts more data into airwaves

As telecommunications designers add electronic mail and Internet access to cellular 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.

This antenna sends or receives multichannel data on a single frequency. Andrews et al./Nature

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

Besides feeding more data into each cell-phone link, the development could expand telecommunications in other ways, says Lucent’s Michael R. Andrews. For instance, 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 signals encoding the red, green, and blue hues in a Joan Miro painting. Ordinarily, each signal would require a separate frequency. But in their new scheme, the researchers transmitted those signals across a cafeteria at Bell Labs simultaneously on one frequency. A receiver adapted for the new technique reproduced the Miro image from those signals.

Alfred O. Hero III of the University of Michigan in Ann Arbor comments that the “intriguing and interesting” result 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 station can greatly increase the number of independent channels per radio frequency (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 scrambled condition, sophisticated software untangles them by accounting for the effects of the different paths. So far, the technique has been unsuitable for handheld devices 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 only multiplies the number of pathways they can take from transmitter to receiver. It also adds an extra information-carrying dimension to those waves.

Scientists have known since the 19th century that electromagnetic waves consist of electric and magnetic fields oriented, or polarized, along distinct directions in space. Therefore, polarization is a feature of every transmitted signal. Andrews says, however, that textbook explanations of electromagnetic waves preclude polarization along the same direction a wave travels, leaving only the up-down and left-right dizmensions.

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 addition of the third permits the polarization of the electric and magnetic fields to vary independently. That means six separate polarization signals can be transmitted simultaneously on the same frequency.

What’s more, exploiting these six newly recognized channels of a radio signal requires only one antenna, albeit a bristly one, at each end of the link. That simplicity may make the discovery applicable to handheld cell phones.

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