Tripling the capacity of wireless communications using electromagnetic polarization

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Abstract
Wireless communications are a fundamental part of modern information infrastructure. But wireless bandwidth is costly1, prompting a close examination of the data channels available using electromagnetic waves. Classically, radio communications have relied on one channel per frequency, although it is well understood that the two polarization states of planar waves2 allow two distinct information channels; techniques such as ‘polarization diversity’ already take advantage of this3. Recent work4,5,6,7 has shown that environments with scattering, such as urban areas or indoors, also possess independent spatial channels that can be used to enhance capacity greatly. In either case, the relevant signal processing techniques come under the heading of ‘multiple-input/multiple-output’ communications, because multiple antennae are required to access the polarization or spatial channels. Here we show that, in a scattering environment, an extra factor of three in channel capacity can be obtained, relative to the conventional limit using dual-polarized radio signals. The extra capacity arises because there are six distinguishable electric and magnetic states of polarization at a given point, rather than two as is usually assumed.