I have been following the recent results about practical use of Orbital Angular Momentum (OAM) for electromagnetic communication (radio, light, gamma rays), mostly reported in AAAS’s top journal “Science”, but also in various physics journals. What’s interesting to me is that there appears to be some set of “radio experts” who find this distressing and are close to treating it like “Cold Fusion”. I wrote the following little mini-essay to clarify the issues as I see them. At the end, I’ve added some deeper, more technical references.
The BBC picked up the surprising controversy here: ‘Twisted light’ data-boosting idea sparks heated debate — An idea to vastly increase the carrying capacity of radio and light waves has been called into question.
My thoughts
I personally side with the physicists on this one, long term. (at the very
end of this note I explain the non-obvious reason why I say “long term”)
What’s going on here? Well, most radio engineering is done using the
“scalar” properties of electromagnetic propagation: the magnitude of the
electrical and magnetic field intensity at a point. That’s what standard
antennas measure, and is processed by historical analog radios and most
current digital radios.
But electromagnetic waves (or photons) are a little more (lot more?) complex
than the voltage variation at the source antenna over time, and the
consequent voltage variations at the destination antenna. Maybe a lot more
complex.
First of all, the vector fields that make up the propagation medium are more
complex. Waves don’t propagate as rays (blame simplistic science
curricula), but rather like the waves on the surface of the ocean, through a
complex process of transferring energy from one bucket to another and back.
In the ocean, that is accomplished by an interchange between the potential
energy stored in the gravitational potential energy of the water (vertical),
and the pressure field of the water. In radio waves, it is the interchange
of energy between the electric field and the magnetic field, which are
perpendicular to each other at every point, and follow Maxwell’s equations
(which actually can be written as a single simple differential equation
using Geometric Algebra).
Waves don’t look like sine waves – EM waves can have any 3D shape (pulses,
triangles, etc.). But we typically in radio systems measure only
“narrowband” and spatially local properties by comparing the waves to a
“sine wave” of a particular frequency oriented in a particular direction,
ignoring the rest.
This is a property of the receiver (including its antenna) as a measurement
system. It throws away most of the information in the wavefront, or gets
confused by it.
Let me repeat that: our radios today are designed to measure sinusoidal
scalar field variation. They don’t (and can’t) measure anything else! In
other words, they are blind to other significant properties of the EM vector
field dynamics.
Polarization is a simple property of some waves in a 3D time-varying vector
field. In particular, a polarized wave stores its entire scalar electric
field energy in a particular direction, and its entire scalar magnetic field
energy in a perpendicular direction. Both of these fields are perpendicular
to the local path by which energy is transferred (that direction is the
Poynting Vector).
Todays’ antennas include antennas that narrow their measurements to measure
just the electric or magnetic field in a specific direction. This means
that polarized antennas can “ignore” signals polarized perpendicular to that
direction, allowing two communications to proceed at once without being
confused.
Again, I emphasize that we receive what our antennas and radios are designed
to measure about the propagating waves in the vector EM field. Have we
exhausted what can be measured (or shaped by the transmitter)?
The answer is no – far from it. And here’s the simple reason why. The
simple law of physics that governs the field (Maxwell’s equations or the
quantum version of them) have many more solutions than the ones I’ve
mentioned above.
To get an idea about this, think about the ocean again. Is the state of
the ocean waves across the ocean and at any beach merely the sum of a set of
ripples of the sort that arise from dropping pebbles periodically at points
throughout the earth? Well, the answer is no. The reason is that
wavefronts are not constrained to be sinusoidal, and they are not required
to be aligned in “parallel” or “concentric circle” patterns.
So how do we construct and measure these other waveforms? Well, that’s the
trick that the physicists are playing with.
There’s one more variable here, and that’s the quantum rules. Not the
“particle/wave duality”, just the discovery by Einstein and others that
*all* EM phenomena are “quantized”. That doesn’t violate Maxwell’s
Equations, but it is another constraint on the behavior of the EM field and
its waves.
When the energy is quantized, there are a few new phenomena (such as all the
phenomena that make semiconductors and superconductors work – e.g. cooper
pairs, …).
But the basic thing is that radio waves are quantized, which doesn’t mean
they are composed of tiny “pellets” of energy, since photon quanta have
spatial extent.
But there are additional phenomena in quantum mechanical models of
photon/waves.
One of these is OAM. OAM is a property of an individual photon, an EM wave
“quantum”. The photon has an additional property.
But that property *can’t be measured* by our ordinary radio receivers and
antennas. To them, all photons look the same, because all that is measured
is the magnitude of a quanta’s electric field in a particular direction
perpendicular to its propagation.
So here’s the bummer for radios: “legacy radios” that cannot see OAM of
photons will get confused when they see a mixture of photons that include
many different OAM states. (note that this is a problem with polarization as
well – most radios cannot separate distinctly polarized photon/waves mixed
together).
To separate the independent “channels” one needs a “smart receiver design”
that includes a front-end that can measure the OAM state of each
photon/wave, plus a computational system that can exploit this separation.
If we can do that, and the recent results indicate we can, then we can
design new radio systems that have much higher capacities.
MIMO systems are related in that they provide more complex ways to structure
waves in the EM field, but they do not have the capability of measuring OAM.
They do, however, allow the generation of waveforms that carry more
information than the information that can be carried over a single
antenna-single antenna path of the traditional sort. They do this by
measuring the phase differences created by the many paths between pairs of
antennas at source and destination. Phase differences are scalar field
magnitude based. They don’t measure the vector field properties of the EM
field, nor do they measure the quantized properties that arise from quantum
theories.
There is one problem with all of these, however: to get the advantage of
MIMO or OAM requires rethinking two key “regulatory assumptions” that come
to us from Marconi’s day.
The regulatory assumption is that the primary mode of information
transmission is “narrowband” that the primary solution to sharing is
“exclusive/clear channel” ownership of frequencies. Implicit in that is
that there is no legal recognition of the distinguishability of wavefronts
based on their 4 dimensional shape.
In other words, the FCC and other bodies regulate only by considering the
scalar magnitude and instantaneous frequency of transmitters. They have
forgotten that *by requiring much more sophisticated receivers* we can get
*far more capacity* out of radio systems.
Instead, the companies claim that their frequencies are “fully utilized” and
they have a “spectrum crunch”. Why do they do that? Well the answer is
this: all frequencies are allocated to somebody already, so new competitors
cannot start a competitive business!
What happens when a new, potentially, non-interfering mode of communications
is invented? Well, the incumbents then claim that the new mode will
“interfere” with their legacy radio systems (or someone else’s). THis of
course will cause planes to fall out of the sky, and now GPS will fail.
It’s amplified by claims of “spectrum shortages” that add credibility to the
idea, and panic the regulators and Congress.
And then it all becomes a mess.
So, that’s why I say OAM will be a benefit “in the long run”. It could be
great in the “short run” too. But I doubt that.
References
Potential for multiplexing more signals in the same bandwidth:
https://www.sciencemag.org/news/2011/07/high-bandwidth-interplanetary-connection
OAM for radio and light demonstrated:
A Different Angle on Light Communications
http://iopscience.iop.org/1367-2630/14/3/033001/
Chips for producing OAM modulated light:
http://spectrum.ieee.org/semiconductors/optoelectronics/chip-makes-twisted-light-for-communications