I received the following comment from Bo Thide', one of the authors of the paper where Fabrizio Tamburini and collaborators explain their novel method to multiply the transmission of information via EM waves (see here). I think his points are of interest to many so I decided to elect his comment to a independent posting here.
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
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