'Twisted' Waves Could Boost Capacity of Wireless Spectrum 147
New submitter Ogi_UnixNut writes "In Venice, Italy, physicists have shown 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. In principle this allows the implementation of an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. It's potentially a boon for congested spectrum problems, although at the moment I suspect it would only work for directional links."
Think of it as a phased array (Score:2, Informative)
I think that what's happening is no different from what you could achieve with a 802.11/n MIMO system. Think of their twisted antenna as a ring of patch antennas.
Essentially, the trade-off they are making is that they broaden the beam by warping their antenna, so they have a lower-gain antenna with a wider beam. Consequently, you need more power in each of the two orbital angular momentum states to transmit the data, consequently Shannon-Hartley is preserved.
Another way of looking at it is that their dish makes a broader beam because it is twisted. If you wanted to keep the beam width (and thus the gain) of the antenna the same as an unmodified dish, you'd need a bigger dish. Alternatively, instead of a bigger dish, you could use two unmodified dishes sending two separate beams.
So, I don't think they have accomplished anything except that they've (a) produced a nifty new antenna design that might occasionally be useful but isn't a great advance, and (b) shown some interesting math. And, they've also managed to confuse themselves and let themselves believe that they did something wonderful.
Re:not really (Score:4, Informative)
Photons do.
Photons are part of the EM spectrum.
Re:So what are they orbiting then? (Score:3, Informative)
Around the propagation direction of the beam. Read this:
https://en.wikipedia.org/wiki/Light_orbital_angular_momentum
Re:So what are they orbiting then? (Score:4, Informative)
Re:So what are they orbiting then? (Score:5, Informative)
The notion of "what are they orbiting" is nonsensical here -- we're talking about quantum objects. It's like saying that electrons "orbit" the nucleus: in the description of their motion, the concept of a classical "path" doesn't quite apply either, and classical mechanics can't describe what an electron does when bound to the nucleus! Now, Maxwell's theory is "classical" in a way, but it describes AFAIK an aggregate (macroscopic) behavior of inherently non-classical, quantum objects, the photons. To get the behavior at the quantum level right, you need quantum electrodynamics (QED).
It is well known from Maxwell's theory that electromagnetic radiation carries both energy and momentum. The momentum may have both linear and angular contributions; angular momentum has a spin part associated with polarization and an orbital part associated with spatial distribution
- from "Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes" by Allen et al. In the same paper, you can read that you can measure those properties of light using fairly simple opto-mechanical instruments:
A suspended lambda/2 birefringent plate undergoes torque in transforming right-handed into left-handed circularly polarized light. Suspended cylindrical lenses undergo torque in transforming a Laguerre-Gaussian mode of orbital angular momentum -l*hbar per photon, into one with +I*hbar per photon.
NOTHING NEW - PROVEN (Score:3, Informative)
The same as MIMO (Score:4, Informative)
Re:Multipath (Score:4, Informative)
Hah, CDMA should be plenty robust to multipath, you can use more than one adaptive correlator per channel, and each correlator gives you the relative multipath phase as a diagnostic output, too. Ideally you'd want more than one antenna to make the adaptive scheme more robust, but it'll work with just one. What's more, you can always record the high-bandwidth datastream from the digital radio I/Q inputs for offline data recovery: whatever processing you do online is limited by the maximum latency allowed in your decoder, offline has no such problem. To integrate "offline" with other recording equipment, you can simply have two outputs: a realtime output that goes to the program mixing console, and a more delayed offline output that goes to the multitrack "source" recorder for a studio mix (where you can easily shift things around, time-wise).
Admittedly satellite links have stable channel properties, but the error correction codes that they use are as close to optimal for given datarate-to-bandwidth as is feasible, and that's not very common in non-cellular consumer point-to-point gear. I agree that terrestrial channels are more challenging when it comes to varying channel properties.
A wireless mike is a very specific application. It shouldn't ever need a receiver, so the only way to deal with potentially strong narrowband interference is to use CDMA and as wide of a transmit bandwidth as possible -- using frequency division (one channel per mike) is not robust enough, usually. All of the "brains" need to be in the base station, the transmitter circuitry can be hardwired in a relatively simple FPGA that takes input from an audio codec, a couple jumper settings (node ID / code selection) and pushes it via a DAC to the filter/upconverter/final amp. All the encoding etc. is done completely digitally and can be probably modeled in a few pages of Verilog.