Would Orbital Angular Momentum (OAM) multiplexing work over HF bands?

Question

OAM (Orbital Angular Momentum) Multiplexing is a form of multiplexing capable of high data speeds on limited bandwidth using the physical properties of EM waves.

An experiment in 2012 transmitted two data signals simultaneously on the same frequency over a distance of 442 meters on the 5ghz band.

Would the properties of OAM multiplexing work on the same level if done on the HF band (say for example 20 meters) with ionospheric propagation? If this was possible, could the signal coherently be transmitted and received?

Answer 1

This is a physicist level experiment, and its benefit over established methods disputed, and even Wikipedia says that it's performance benefit over MIMO are nonexistent to negligible.

So, no. Not in any realistic way.

Also, orbital angular momentum is a result of the mode of the electromagnetic wave and its propagation. Hence, if you want to use it, you'd need precise control of the mode.

You can (maybe) do that (at high effort) for light (which is also an electromagnetic wave) in a glass fibre (which is a waveguide). Cross-mode talk is a thing in the finely controlled environment of a fiber, still.

You can certainly not do that reliably for HF in propagation over a medium as inconsistent as the air, with boundaries varying in material and shape as the earth and ionosphere.

So, no. Totally different realm in which this highly immature thing is happening.

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I find it kind of entertaining that someone would mention this moonshot technology in the context of HF communications in amateur radio applications.

I find it hard to find any area of hobbyist technology usage where the difference between the actual state of the art and the way the technology is used is as large as in HF communications done by amateur radio enthusiasts. Seriously, there's 60 to 90 years between what science and commercial technology can do, and what the average ham operator on HF does. Maybe we should catch up these decades, before you start following methods that try to improve upon what can be done so far.

That starts with retiring all analog modes, and all transceivers that are basically AM or SSB devices in favor of proper devices that can deal with both sides of a spectrum. Then, application of modern modulation, channel sensing and synchronization methods. Going for modern channel codes. Classical MIMO. Proper network and routing protocol design. Lobbying against counter-productive rate-limiting and legacy-mode-preserving regulation. Requiring a minimum effective spectral efficiency, especially for higher-powered transmitters.

Things like FT8 are a start. They tackle maybe half of these points.

Answer 2

Orbital angular momentum is a near-field property of the electromagnetic field. To detect the orbital angular momentum one has to use a receive antenna that is big enough to detect the different phase and/or amplitude at different points in space. This can be used in optical communication where a light beam can use different OAM modes to transmit multiple messages on the same frequency between two antennas at the same time. It is a special case of MIMO.

To illustrate: Think of two large antenna arrays pointing towards each other at a modest distance. One antenna can then form a lobe that is small enough to illuminate only the upper left corner of the receiving antenna. It is then possible to simultaneously form three more lobes that illuminate the other three quadrants at the same time on the same frequency and thus transmit four channels on the same frequency. Instead one could combine those four channels to form a field with orbital angular momentum at the receive side. Imagine four feed-points each one exciting the entire tx array to point to one of the four quadrants. Feed them all with the same power but with different phases.

1. The same phase in all four quadrants.
2. 0 degrees in the upper two and 180 degrees in the lower two
3. 0 degrees in the left side pair and 180 degrees in the right side pair.
4. 0 degrees in the upper left and the lower right, 180 degrees in the other two.

Arrange the rx array as four independent antennas. Each one would have a twice as wide lobe and thus see the entire tx antenna. Combine the four rx antenna cables according to the above pattern to get four independent channels on the same frequency. As I see it, using OAM would be nearly equivalent to MIMO in which case one would have four independent channels by using the four channels of the tx side independently to point to the four parts of the rx antenna. Possibly the combination to make OAM waves would make the system more robust to variations in the transmission path, in particular if the near field is extended by being confined in an optical fibre.

The near field can be very far from the transmit location. Here is an example "Measurement of the spin of the M87 black hole from its observed twisted light": https://arxiv.org/pdf/1904.07923.pdf They write "because the radio source has been spatially resolved at 1.3 mm wavelength by the ∼10000km EHT radio-interferometric baseline."

It seems to me OAM might become useful at very high frequencies for short range communication to 5G base stations where extremely high data rates are needed and optical fibres might be unpractical: https://www.nec.com/en/press/201812/global_20181219_02.html I do not think OAM would be useful on HF with ionospheric propagation. The antennas would have to be big enough to form lobes that are narrower than the size of the antenna on the other side.

Certainly an antenna system like this one will easily detect OAM on HF: http://www.lofar.org/about-lofar/general-information/introduction.html