[LINK] An amazing breakthrough in wireless communication, or a pipe dream?
Encoding many channels on the same frequency through radio vorticity: first experimental test
“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.”
“Moreover, our experimental findings demonstrate that the spatial phase signature was preserved even in the far-field region and for incoherent non-monochromatic wave beams. These results open up new perspectives not only for wireless communication but also for physics and astronomy, including the possible detection of Kerr black holes in the test general relativity”
This looks too good to be true, but I cannot see any obvious issues, and they have an experimental confirmation.
If this pans out, it would be a black swan in the making for many of the wireless spectrum allocation/licensing authorities and companies.
Channels distinguished by vorticity experience crosstalk in the presence of plane scatterers like buildings, the ground, the sky… It seems pretty well restricted to what they used—a parabolic mirror, aiming it along a direct line of sight to the receiver.
This could potentially be very useful for communications in space, where we can’t spatially resolve a large number of faraway receivers. You can address by vorticity instead of frequency, and each receiver gets a full spectrum of bandwidth.
I could be off on this, but in short, it looks legit, if specialized.
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This doesn’t technically increase the number of channels available. What it does make possible is distinguishing these channels without having to have lots of antennas cooperating on sending every signal. One antenna, one channel.
Italian nationalism in the sciences fascinates me.
And frightens me a little—how can I give this result any credence now that I have read this?
Yeah. The only way it could be worse was if the researchers started making anime references. Like yelling ‘this signal is the signal that will pierce the heavens’ or something.
Oh, wait. ^^
Neither; just a subset of existing MIMO technology.. And MIMO is already part of standards like 802.11n.
Yes. The orbital angular momenta spans the same space as the linear momenta, so it cannot add anything in principle to MIMO and similar. (Practical issues can of course in some cases make the one or the other basis more effective under various circumstances.)
… nm
Not correct! Linear momentum and angular momentum are entirely different things. This was shown by Euler already 1776. Any good mechanics book will show why. Read the books by Clifford Truesdell to get all the details right.
Not correct. MIMO is essentially a radio transmitter/antenna combination trick; angular momentum is a fundamental property of all fields and matter. Angular momentum comes in two distinct forms, spin angular momentum (like the Earth spinning around its own axis one revolution per day) and orbital angular momentum (like the Earth orbiting the Sun one revolution per year). All vector fields, including the EM field, has this property,
Well, then, rather than tell me I’m wrong in a forum where that will have no effect, go submit a paper to the IEEE refuting the paper they published by Ove Edfors and Anders J Johansson, which I linked.
How does that work? Photons have spin 1. There’s only three possible angular momenta a photon can have.
Also, what kind of equipment would be necessary to use that? I doubt it would be worth while.
First, two and not three, the spin projection 0 disappears for massless fields. Second, those are basis states, any normalized linear combination is also allowed. Third, they are talking about orbital angular momentum, not spin (I am unsure about the details, though).
They show their helix-shaped satellite dish, doesn’t seem overly high-tech.
Not correct! The Hilbert space for spin angular momentum has dimension 2 (-1, +1; left, right; up, down). The Hilbert space for orbital angular momentum has a denumerably infinite dimension (..., −3, 2, −1, 0, 1, 2,3 , …).
Not sure what you are arguing with so emphatically. DanielLC’s statement “Photons have spin 1.” implies spin, not orbital angular momentum, and the Hilbert space for photon spin is indeed 2D, as both you and I said.
Oh, this is a good idea. Yeah, totally possible. The only issue is that now you need two antennas to receive the signal, and you need to have electronics good enough to get at the phase information. The backwards-incompatibility might be a problem, and I don’t know whether the electronics requirements are already easy to satisfy or not.
Not too hard on the receiver end. If you know the phase relationship, you can do it analog—I’ve even seen cheap rabbit-ears antennas for TVs have a phase-changing dials to effectively reorient the array.
If this works, could it also be applied to fiber optics?
No. Fiber optics experience enough crosstalk between modes that this would fail in short order. It’s best to use single-mode fibers, and pack them close to each other.
I don’t quite grok rotational effects in EM waves, perhaps someone can enlighten me: doesn’t circular/elliptical polarization allow the same kind of effect? (I.e., a theoretically infinite number of overlapping but separable signals.) Or is circular polarization fixed to a single rotation per wavelength?
No. Circular polarization allows exactly two channels per frequency, just like linear polarization.
Thank you. For the record, circular polarization is tied to the wavelength (I hadn’t noticed that before for some reason). Also, if I understand it right, you can superpose many (more than two) different elliptically or linearly polarized signals with different axes, but you can’t separate them.