'Twisted' Waves Could Boost Capacity of Wireless Spectrum

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."

Multipath

[rullywowr]

What about the issue of multipath, where one wave inverses the phase because its reflection arrives at the antenna slightly delayed from the original direct LOS (line of sight) signal?
I work with wireless microphones and deal with spectrum issues on a daily basis. With the shrinking spectrum, this would be extremely good news if it actually was feasible and practical in the real world. As it stands right now, two transmitters operating on the same frequency is simply a recipe for disaster.
oh yeah, first!

Shannon-Hartley still in effect.

[neyla]

This might help, but it doesn't expel Shannon-Hartley. They don't get "inifinite channels" in finite bandwith. Not unless each channel has infinitely low capacity, anyway.

Re:Not really new

[virgil_disgr4ce]

Try reading the article. The innovation is to use orbital angular momentum, NOT spin angular momentum (polarization).

*Orbital* angular momentum

[Urban Garlic]

I RT first part of the FA (no, not actually new here...), and an important point is that the paper is talking about *orbital* angular momentum of the light beam. The circular polarization states correspond to *spin* angular momentum of the photons, orbital angular momentum is a different thing with its own phase space.

Infinite channels still seems unlikely, it has to be true that detectors for orbitally-tuned light beams won't be perfect, and will detect "nearby" orbitally-tuned beams as well, and it's likely that some parts of the space of orbital angular momentum will be more difficult to generate than others, so I remain skeptical of the claim.

But, the mechanism is not a trivial one. I note with some surprise that TFS actually correctly notes that it's orbital angular momentum they're talking about.

Re:Multipath

[YoopDaDum]

That's completely different. GPS uses CDMA, which is a way to multiplex several users on the same channel. Here it's a way to create additional independent channels. The former is sharing one channel capacity, the later is adding channels and capacity. If you want to compare this to an existing technology, it's closer in spirit to MIMO with spatial multiplexing.

But as the grand-parent remarked, and if I understand correctly, this shouldn't be robust to multipath (i.e. all the reflections that adds up at the receiver). And all practical use cases you care about as an end user must support multipath (OFDMA used in WiMAX and LTE main strength is its robustness to multipath) as they must operate in non line of sight (NLOS) conditions. So that would limit the application to line of sight (LOS) systems like microwave trunking. Possibly still useful, but not for you and me.

And by the way, although you're correct that wireless microphones are basic tech, satellites links are by no mean state of the art. Satellite is LOS, the challenge is very low signal level but the channel is easy. The state of the art is in terrestrial broadband (mostly LTE and its evolutions now) with mobility and multipath to handle with a constrained (size and power) receiver in a smartphone.