'Twisted' Waves Could Boost Capacity of Wireless Spectrum

New submitter Ogi_UnixNut writes "In Venice, Italy, physicists have shown 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. In principle this allows the implementation of an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. It's potentially a boon for congested spectrum problems, although at the moment I suspect it would only work for directional links."

Comments

Multipath

[rullywowr]

What about the issue of multipath, where one wave inverses the phase because its reflection arrives at the antenna slightly delayed from the original direct LOS (line of sight) signal?
I work with wireless microphones and deal with spectrum issues on a daily basis. With the shrinking spectrum, this would be extremely good news if it actually was feasible and practical in the real world. As it stands right now, two transmitters operating on the same frequency is simply a recipe for disaster.
oh yeah, first!

Re:Multipath

[Forty Two Tenfold]

Leeloo Dallas multipath!

Re:Multipath

[YoopDaDum]

That's completely different. GPS uses CDMA, which is a way to multiplex several users on the same channel. Here it's a way to create additional independent channels. The former is sharing one channel capacity, the later is adding channels and capacity. If you want to compare this to an existing technology, it's closer in spirit to MIMO with spatial multiplexing.

But as the grand-parent remarked, and if I understand correctly, this shouldn't be robust to multipath (i.e. all the reflections that adds up at the receiver). And all practical use cases you care about as an end user must support multipath (OFDMA used in WiMAX and LTE main strength is its robustness to multipath) as they must operate in non line of sight (NLOS) conditions. So that would limit the application to line of sight (LOS) systems like microwave trunking. Possibly still useful, but not for you and me.

And by the way, although you're correct that wireless microphones are basic tech, satellites links are by no mean state of the art. Satellite is LOS, the challenge is very low signal level but the channel is easy. The state of the art is in terrestrial broadband (mostly LTE and its evolutions now) with mobility and multipath to handle with a constrained (size and power) receiver in a smartphone.

Re:Multipath

[tibit]

Hah, CDMA should be plenty robust to multipath, you can use more than one adaptive correlator per channel, and each correlator gives you the relative multipath phase as a diagnostic output, too. Ideally you'd want more than one antenna to make the adaptive scheme more robust, but it'll work with just one. What's more, you can always record the high-bandwidth datastream from the digital radio I/Q inputs for offline data recovery: whatever processing you do online is limited by the maximum latency allowed in your decoder, offline has no such problem. To integrate "offline" with other recording equipment, you can simply have two outputs: a realtime output that goes to the program mixing console, and a more delayed offline output that goes to the multitrack "source" recorder for a studio mix (where you can easily shift things around, time-wise).

Admittedly satellite links have stable channel properties, but the error correction codes that they use are as close to optimal for given datarate-to-bandwidth as is feasible, and that's not very common in non-cellular consumer point-to-point gear. I agree that terrestrial channels are more challenging when it comes to varying channel properties.

A wireless mike is a very specific application. It shouldn't ever need a receiver, so the only way to deal with potentially strong narrowband interference is to use CDMA and as wide of a transmit bandwidth as possible -- using frequency division (one channel per mike) is not robust enough, usually. All of the "brains" need to be in the base station, the transmitter circuitry can be hardwired in a relatively simple FPGA that takes input from an audio codec, a couple jumper settings (node ID / code selection) and pushes it via a DAC to the filter/upconverter/final amp. All the encoding etc. is done completely digitally and can be probably modeled in a few pages of Verilog.

Shannon-Hartley still in effect.

[neyla]

This might help, but it doesn't expel Shannon-Hartley. They don't get "inifinite channels" in finite bandwith. Not unless each channel has infinitely low capacity, anyway.

Re:Shannon-Hartley still in effect.

[Idbar]

I think AT&T is fine with that. ;-)

Re:Shannon-Hartley still in effect.

[YoopDaDum]

CDMA is different than this. CDMA is just a multiplexing mechanism, splitting a single channel capacity between users. Here the scheme adds new independent channels.

And CDMA has been removed from DOCSIS 3.0. It had been added in DOCSIS 2.0, then people eventually realized it was a dumb idea over cable, and then removed it. The company that had pushed it went bankrupt, but not before its share peaked and some people made a lot of money selling at the right time...
What you mention (channel bonding) is also called carrier aggregation in HSPA and LTE (LTE advanced, not the current one). It's just adding the capacity of different physical channels and treating them as one logical pipe. Very similar to Ethernet bonding, although it's more complex when you get to the details. But it has nothing to do with CDMA.

CDMA is the most hyped multiplexing technology. It's been hyped to death, so much some people think it's some form of magic. But it's not, and it's our past now. CDMA key point was that it was the first mechanism that enabled deploying cellular over a single frequency, which maximized at the time cellular capacity. This was very useful in cellular system, but it's a non issue in cable (there's no cell, duh). So CDMA over cable is a marketing/hype driven monstrosity that should never have happened (CDMA may by useful for a contention channel though). And even in cellular there are better schemes which have become practical now. All 4G system are based on OFDMA for example, with just the contention channel using some form of code multiplexing to be more robust to collisions.
Even HSPA, which is still CDMA based, went back to something closer to TDM in spirit than CDMA: there are still codes, but they're usually allocated to a single user over a short duration, and multiplexing is mostly TDM. Instead of having multiple user at the same time using different codes, which is the essence of CDMA. The HSPA way to send with more density over a shorter period of time instead of spreading the signal is more power efficient.

Re:Not really new

[virgil_disgr4ce]

Try reading the article. The innovation is to use orbital angular momentum, NOT spin angular momentum (polarization).

Re:not really

[jo_ham]

Photons do.

Photons are part of the EM spectrum.

BULLSH!T

[AB3A]

Any time someone starts talking of infinite channel capacity, you know they're going to be full of crap. Shannon's limit is a Mathematical principle. There is no such thing as "infinite" bandwidth/channel capacity.

What they're actually discussing is the spatial equivalent of spread spectrum. In other words, they have their own custom reflector with its own unique shape that can be reversed so that a coherent signal with minimal inter-symbol interference would be present. It is not a bad idea, except that you would need a line of sight path with very little exposure to the first Fresnel zones. Reflections would be a bitch to deal with.

Also note this method reduced point source noise, but it doesn't eliminate it. Likewise, a spread spectrum signal is still detectable as increased noise in a narrow-band radio.

Re:BULLSH!T

[Angst Badger]

Shannon's limit is a Mathematical principle.

Unfortunately, most people have next to no understanding of mathematics beyond some rote memorization from school. This is just another example of people confusing analog signals with magic. To be fair, the actual researchers involved probably understand this quite well, but the scientifically uneducated class from which science and technology journalists are drawn is another matter.

The non-mathematical version, for those interested, is that yes, analog signals are continuous and so can occupy an infinite number of states. The reason you can't get infinite bandwidth out of that is because both the transmitters and receivers have limited precision, and because there is always noise, which is another manifestation of the Second Law. For example, there are an infinite number of real numbers between 0 and 1. If you could actually use all of that space, you could encode any amount of information in an arbitrarily short signal. (Well, there's a limit to that, too, for which see Georg Cantor.) In practice, you can't use all of that space, because your instruments might distinguish quite well between 0.001 and 0.002, but they can't reliably tell the difference between 0.001 and 0.0005. On top of that, there is noise, which is also a big topic, but you can think of it as a random fluctuation in the signal. If the ambient noise varies between 0.0 and 0.0005 in the same example, you can't even reliably tell the difference between 0.001 and 0.002.

What the parent is getting at is that laws of physics, being derived from observations of nature with limited precision, might occasionally be overturned by better observations. Fundamental mathematical principles, on the other hand, are much more reliable. There might be a difference between rest mass and inertial mass that we could exploit for thrustless propulsion. It's extremely unlikely, but it can't be ruled out. But there is zero possibility that 2 + 2 will ever equal anything other than four. Shannon's limit and, for that matter, the Nyquist sampling theorem are a little more complex than a simple integer sum, but the actual math for both would fit on an index card with plenty of room to spare to blather on about "infinite" analog signals. We use digital signals most of the time these days because it makes the hardware easier to design, but neither digital nor analog can be used to make an end run around the Second Law.

What the researchers in TFA claim to have figured out is another way to use part of the signal outside of the frequency domain to stuff data into. It's a really ingenious approach that might be quite useful if it pans out in actual practice, but it's not magic, and it's not infinite.

*Orbital* angular momentum

[Urban Garlic]

I RT first part of the FA (no, not actually new here...), and an important point is that the paper is talking about *orbital* angular momentum of the light beam. The circular polarization states correspond to *spin* angular momentum of the photons, orbital angular momentum is a different thing with its own phase space.

Infinite channels still seems unlikely, it has to be true that detectors for orbitally-tuned light beams won't be perfect, and will detect "nearby" orbitally-tuned beams as well, and it's likely that some parts of the space of orbital angular momentum will be more difficult to generate than others, so I remain skeptical of the claim.

But, the mechanism is not a trivial one. I note with some surprise that TFS actually correctly notes that it's orbital angular momentum they're talking about.

Re:So what are they orbiting then?

[Anonymous Coward]

Around the propagation direction of the beam. Read this:

https://en.wikipedia.org/wiki/Light_orbital_angular_momentum

Re:So what are they orbiting then?

[Baloroth]

See Wikipedia for details. It isn't polarization, but I can't exactly explain how it isn't.

Re:So what are they orbiting then?

[tibit]

The notion of "what are they orbiting" is nonsensical here -- we're talking about quantum objects. It's like saying that electrons "orbit" the nucleus: in the description of their motion, the concept of a classical "path" doesn't quite apply either, and classical mechanics can't describe what an electron does when bound to the nucleus! Now, Maxwell's theory is "classical" in a way, but it describes AFAIK an aggregate (macroscopic) behavior of inherently non-classical, quantum objects, the photons. To get the behavior at the quantum level right, you need quantum electrodynamics (QED).

It is well known from Maxwell's theory that electromagnetic radiation carries both energy and momentum. The momentum may have both linear and angular contributions; angular momentum has a spin part associated with polarization and an orbital part associated with spatial distribution

- from "Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes" by Allen et al. In the same paper, you can read that you can measure those properties of light using fairly simple opto-mechanical instruments:

A suspended lambda/2 birefringent plate undergoes torque in transforming right-handed into left-handed circularly polarized light. Suspended cylindrical lenses undergo torque in transforming a Laguerre-Gaussian mode of orbital angular momentum -l*hbar per photon, into one with +I*hbar per photon.

NOTHING NEW - PROVEN

[OveE]

The claims made by Thidé et al. about finding an entirely new mechanism that can improve wireless communication, as reported by BBC in "'Twisted' waves could boost capacity of wi-fi and TV" (http://www.bbc.co.uk/news/science-environment-17221490), have been proven incorrect in the following peer reviewed journal paper: "O. Edfors, A. J. Johansson: Is orbital angular momentum (OAM) based radio communication an unexploited area? IEEE Transactions on Antennas and Propagation, Vol. 60, No. 2, pp. 1126-1131, 2012." Existing and well known techniques produce the same 'twisted' radio waves. These 'twisted' waves bring nothing conceptually new in the area of wireless communications and cannot boost capacity further. The claims have been appearing repeatedly in media over the last few years, while consensus among experts in the area of wireless communications is that they are incorrect.

Spectrum not overcrowded, mismanged

[colsandurz45]

I did my MS thesis on wideband spectrum sensing (just about everything under 2.2 GHz). Turns out the spectrum isn't actually overcrowded, it's underutilized, especially over 500 MHz. Look at some papers by the Shared Spectrum Company www.sharedspectrum.com/. This is common misperception and it's the result of FCC policies (that they're working on changing). The underlying problem is that institutions that have spectrum allocated for them now actually need it, just not most of time.

The same as MIMO

[Jott42]

This is actually a subset of MIMO, which is already widely used in WiFi and other wireless networks. Thus it will, regrettably, not give access to any additional bandwidth. The details on the equivalence is in a paper from IEEE Transactions on Antennas and Propagation, titled "Is orbital angular momentum (OAM) based radio communication an unexploited area?" http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=2062936&fileOId=2339120