By the way, Bo Thide' is a Swedish professor at the Uppsala department of Physics and Astronomy. For his CV see here.
From Bo Thide':
A few points about MIMO vs OAM and OAM vs SAM (wave polarisation):
1. Every EM field from any arbitrary radio or TV antenna, laser, light bulb, star or whatever object that emits radiowaves, lightwaves etc carry angular momentum density. Precisely as they all carry energy and linear momentum density (also known as Poynting vector). This follows directly from Maxwell's equations.
2. Wave polarisation (left-hand/right-hand, or horizontal/vertical) is the spin part of the electromagnetic angular momentum. It is therefore called SAM (spin angular momentum) and is entirely different from the OAM which is the orbital part of the angular momentum. It is precisely as with the motion of the earth. It spins on its own axis at a rate of approximately 1 rev per 24 h. This is the earhts SAM. It also orbits around the sun at a rate of about 1 rev per 365 days. This is the earth's OAM. The SAM (24 h) and the OAM (365 days) have nothing to do with each other. The same is true for photons/electromagnetic fields.
3. MIMO is based on the use of multiple antennas. It is *necessary* for MIMO to be able to increase the spectral capacity to use multiple antennas at both the transmitting and receiving end. OAM can increase the spectral capacity using only a single antenna at the transmitting and receiving ends. The fact that it is *possible* or *sufficient* to use multiple antennas to generate and detect OAM (at least approximately) does not mean that it is *necessary*.
4. MIMO requires massive, time- and energy consuming digital postprocessing, OAM requires almost no digital postprocessing.
Short summary of angular momentum radio:
Transferring information wirelessy with electromagnetic (EM) fields amounts to encoding information onto physical observables carried by these fields, radiating them into the surrounding space, and detecting them remotely by an appropriate sensor connected to an information-decoding receiver. Each observable is second order in the fields and fulfills a conservation law. Of all available observables, only the linear momentum is fully exploited in present-day radio. A fundamental physical limitation of this observable, which represents the translational degrees of freedom of the fields and of the charges (typically an oscillating electric current along a linear antenna), is that it is single-mode. This means that a linear-momentum radio communication link comprising one transmitting and one receiving antenna, known as a single-input-single-output (SISO) link, can provide only one transmission channel per
frequency (and polarization). In contrast, angular momentum, which represents the rotational degrees of freedom, is multi-mode, allowing an angular-momentum SISO link to accommodate an arbitrary number of independent transmission channels on one and the same frequency (and polarization).
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