Friday, April 13, 2012

Twisted Radio Waves

For over 30 years we have been know that we can "twist" light. This was first detected inside some optical  cavities. It's called a "screw dislocation or phase singularity". (I will refrain from using that phrase ever again.) It was first documented in a paper by John F. Nye and Michael V. Berry, in 1974. If you think you can imagine a "dislocations in wave train" please try to read their work here. I'll admit most of it's over my head.  But the general idea is that light can be twisted around its axis of travel, like the thread of a screw. Interestingly, through this twisting, the light waves at the axis itself cancel each other out and leave a null. It actually reminds me of the way twisted pair reduce noise.
So even if you don't fully understand all that,  you do already know that light and radio waves are both just different forms of electromagnetic energy. So what is possible with one segment of the spectrum is usually applicable to another. Astrophysicist Fabrizio Tamburini made that leap. In March of this year he published a paper Encoding many channels on the same frequency through radio vorticity: first experimental test. I'll quote the abstract here:
"We have shown experimentally, in a real-world setting, that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels. This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. This paves the way for innovative techniques in radio science and entirely new paradigms in radio communication protocols that might offer a solution to the problem of radio-band congestion."
You can read it all here. Being Italian, he goes on to falsely credit Marconi in the introductory paragraphs. Let's skip all that and get to the exciting stuff. In a test within the 2.4 GHz WiFi band, they broadcast on 15 and 27 MHz. This is difficult to visualize as most people visualize radio waves like ripples on the surface of water. This is not inaccurate, (were you under the water yourself) but is poorly expressed without the math.

The unfamiliar variable here is the orbital angular momentum (OAM) [More here.] It expresses a fundamental physical quantity each with a set of of eigenstates. This is important because linear momentum and polarization are two-dimensional concepts. OAM by comparison is orthogonal, and can be used to describe three-dimensional vectors. These concepts can be used to mathematically describe and thus quantify the "twist" of the wave. If that can be done consistently it can be used to add additional layers of multiplexing to radio waves and embed them with even more data.  If Mr. Tamburini is correct this could be done not only in broadcast but also in wired networks. His tests succeeded at a distance of 442 meters (1,414 feet) that is from lighthouse of San Giorgio Island to the balcony of Palazzo Ducale in Venice (Italy). While open ocean is an absolutely ideal test ground in terms of propagation, the point of reception is in a major city with all the local sources of interference you might expect. It appears that this is an effective way to increase transmission capacity without increasing bandwidth. It may have some limitations but so does FM.
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