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'Twisted' Waves Could Boost Capacity of Wireless Spectrum

Posted by Soulskill on from the make-them-squirm dept.

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."

33 of 147 comments (clear)

  1. Multipath by rullywowr · · Score: 5, Insightful

    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!

    1. Re:Multipath by Forty+Two+Tenfold · · Score: 4, Funny

      Leeloo Dallas multipath!

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      Upward mobility is a slippery slope - the higher you climb the more you show your ass.
    2. Re:Multipath by tibit · · Score: 2

      GPS transmitters on the space segment all share the same channels and it's not a recipe for a disaster. Your GPS receiver works just fine listening to all those satellites all jabbering on the same channels. You engineer the whole system for it. Let's put it this way: wireless microphones are not anywhere near state-of-the-art in digital data transmission techniques. Extraterrestrial links are where the state of the art is at, and mostly has been, too, for a good while.

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      A successful API design takes a mixture of software design and pedagogy.
    3. Re:Multipath by Sponge+Bath · · Score: 2

      Milla Jovovich with a lisp... very sexy.

    4. Re:Multipath by YoopDaDum · · Score: 4, Insightful

      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.

    5. Re:Multipath by tibit · · Score: 3, Informative

      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.

      --
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  2. Not really new by scsirob · · Score: 2, Interesting

    This has been used for ages by HAM radio operators. Horizontal and vertical polarization antenna's can be used independently, or even together to create circular polarization. See: http://www.astronwireless.com/topic-archives-antennas-polarization.asp and http://en.wikipedia.org/wiki/Antenna_(radio)

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    1. Re:Not really new by virgil_disgr4ce · · Score: 4, Insightful

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

      --
      Limina.Log
  3. Shannon-Hartley still in effect. by neyla · · Score: 4, Insightful

    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.

    1. Re:Shannon-Hartley still in effect. by Idbar · · Score: 3, Funny

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

    2. Re:Shannon-Hartley still in effect. by tibit · · Score: 2

      Shannon's theory applies to an abstract concept of a channel. It says nothing about how you map such an abstract channel to a physical realization of it. So, you cannot make a leap from an abstract channel and abstract bandwidth to a physical realization using some means of transmission without saying how those concepts map to underlying physical reality. Do that first, otherwise your statement makes no sense.

      --
      A successful API design takes a mixture of software design and pedagogy.
    3. Re:Shannon-Hartley still in effect. by MadKeithV · · Score: 2

      42.

    4. Re:Shannon-Hartley still in effect. by YoopDaDum · · Score: 4, Interesting

      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.

    5. Re:Shannon-Hartley still in effect. by JoshRosenbaum · · Score: 2

      You get 56k out of it because of compression. Actual physical bandwidth is limited to about 34kbps of actual data transfer.

      I'm no expert, but I did go to Wikipedia and it appears to indicate that your statement is false and 56k is indeed the base speed. http://en.wikipedia.org/wiki/56_kbit/s

      However, with upload speed you appear to be more correct. (33.6 for V.90 and 48 for V.92)
      http://en.wikipedia.org/wiki/56_kbit/s_modem

      Modem compression (v.44 for example) can provide much faster rates than 56k. For highly compressible text, Wikipedia suggests topping out at 3:1 (~150kbit/s) rates.
      http://en.wikipedia.org/wiki/V.44#Error_control_and_data_compression

      If you feel my understanding on these Wikipedia articles is off, I'm definitely interested in hearing more.

  4. Terrestrial Microwave Links. by neBelcnU · · Score: 2

    "It's potentially a boon for congested spectrum problems, although at the moment I suspect it would only work for directional links."

    Wouldn't that mean a huge boon for telcos and state gov'ts that still use terrestrial microwave links? Could a state network take advantage of this, and sell off the unused portion? Speaking for IL and MN, both have microwave line-of-sight to all their toll booths, truck depots and weigh stations.

    There are inevitably issues to this, but if this first appears in LoS, wouldn't these networks (telco+local gov't) be able to use it?

  5. Think of it as a phased array by Anonymous Coward · · Score: 2, Informative

    I think that what's happening is no different from what you could achieve with a 802.11/n MIMO system. Think of their twisted antenna as a ring of patch antennas.

    Essentially, the trade-off they are making is that they broaden the beam by warping their antenna, so they have a lower-gain antenna with a wider beam. Consequently, you need more power in each of the two orbital angular momentum states to transmit the data, consequently Shannon-Hartley is preserved.

    Another way of looking at it is that their dish makes a broader beam because it is twisted. If you wanted to keep the beam width (and thus the gain) of the antenna the same as an unmodified dish, you'd need a bigger dish. Alternatively, instead of a bigger dish, you could use two unmodified dishes sending two separate beams.

    So, I don't think they have accomplished anything except that they've (a) produced a nifty new antenna design that might occasionally be useful but isn't a great advance, and (b) shown some interesting math. And, they've also managed to confuse themselves and let themselves believe that they did something wonderful.

  6. Re:not really by NicknameAvailable · · Score: 2

    Probably sounds insane to cite, but Rodin coils put out em fields with orbital angular momentum - I've measured it personally from them.

  7. Re:not really by jo_ham · · Score: 3, Informative

    Photons do.

    Photons are part of the EM spectrum.

  8. BULLSH!T by AB3A · · Score: 3, Interesting

    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.

    --
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    1. Re:BULLSH!T by Angst+Badger · · Score: 5, Interesting

      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.

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  9. Re:Huh? Is this polarization? by jo_ham · · Score: 2

    If you do something totally crazy and actually RTFA you'll note that they address this very question.

    It is distinct from polarisation, which the FA talks about considerably, including an analogy for the layman.

  10. *Orbital* angular momentum by Urban+Garlic · · Score: 3, Insightful

    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.

    --
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  11. Re:So what are they orbiting then? by Anonymous Coward · · Score: 3, Informative

    Around the propagation direction of the beam. Read this:

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

  12. Re:So what are they orbiting then? by Baloroth · · Score: 3, Informative

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

    --
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  13. Re:So what are they orbiting then? by tibit · · Score: 4, Informative

    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.

    --
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  14. Signal Power by pavon · · Score: 2

    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.

    The other limiting factor in Shannon-Hartley is signal power. Transmitting with infinite power does allow you to have infinite channel capacity, and transmitting over an infinite number of channels each with finite power over does just that. That said, I am sure that practical limitations in hardware design will place a limit on how close the orbital angular momentum spacing can be and still be able to discriminate the channels.

  15. NOTHING NEW - PROVEN by OveE · · Score: 3, Informative

    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.

  16. Spectrum not overcrowded, mismanged by colsandurz45 · · Score: 4, Interesting

    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.

  17. A Fire Upon the Deep - Vernor Vinge by Guppy · · Score: 2

    "...There are simple tricks that are almost never noticed till a very high technology is attained. For instance, quantum torsion antennas can be built from silver and cobalt steel arrays, if the geometry is correct. Unfortunately, finding the proper geometry involves lots of theory and the ability to solve some large partial differential equations. There are many Slow Zoners who never discover the principle."

  18. The same as MIMO by Jott42 · · Score: 3, Informative

    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

    1. Re:The same as MIMO by goodmanj · · Score: 2

      Yes. It's easy to get lost in the weeds when talking about rotational systems, especially when light is involved. Here's an analogue for what I think is going on:

      Suppose I had a rectangular grid of directional transmitting antennas, and a rectangular grid of directional receivers. If I point one antenna array at the other, I can send data between each pair. With enough antennas and receivers, I can send arbitrarily large amounts of data using a fixed bandwidth. But there are problems: if I don't have really good directional antennas (which must be large), signals from one Tx-Rx pair will bleed onto nearby channels. If I mis-align the antennas, or have stray reflections, same problem. And eventually, I can't afford that many antennas.

      This "twisted wave" thing is exactly the same concept, wrapped round in a spiral. It too will require large, expensive antennas with many components to distinguish each beam pattern. It too will have potential crosstalk problems if the antennas aren't large enough. It too will have to deal with crosstalk when the antennas are misaligned, or signals are reflected en route.

  19. A lay perspective by MajroMax · · Score: 2
    I am a scientist, but not an E&M specialist. Take this with a grain of salt.

    I've read through the New Journal of Physics article. The ``radio vorticity'' means that the phase of the signal goes through a 180 flip across the beam centre, and the zero-point of this phase shift rotates as you move along the beam. The receiving antennas in the experiment were a pair of yagis, used to create a radio interferometer. The math and experimental results behind this appaer sound, but there are a few limitations:

    • This is a highly directional effect. Not only would multipath interference destroy the crap out of this signal, but they also needed pairs of antennas on opposite sides of the beam centre to discriminate between mode-0 and mode-1 rotations. Directionally-wide beams will have more interference, and building the interferometer will be more difficult with less than a 180 separation.
    • The transmitting antenna was very specialized. The transmitter itself not so much, but the antenna was a parabolic antenna ``mechanically modified'' -- they sliced through the top of it to turn the atenna into one loop of a parabolic spiral. If you have access to the article online, take a look at the picture, it's kind of neat.
    • ``In principle an infinite number of channels'' my ass. They're building an interferometer, so they need at least one antenna per mode they wish to discriminate between, and when they used antenna-separation to do the phase filtering for them they saw some significant interference form secondary lobes for intervals where the match wasn't perfect. This was okay for the two-channel experiment (mode 0 and 1), but the receiving antenna design would really start messing with higher channels, where those secondary lobes start seriously interfering themselves.
    • As written, the receiving antenna design is highly sensitive. The phase cancellation used required some pretty precise antenna positioning, since they needed a displacement of one half-wavelength in the beam direction for proper interference (to discriminate the mode 1 angular momentum). Trying this in a production environment is going to be pretty tricky -- perhaps they could get somewhere with electronic phase delay.

    So for controlled channels -- perhaps even microwave links -- I'm optimistic about engineers being able to build something useful out of this. But the basic math isn't going to generalize to omnidirectional links, and it certainly isn't going to deal well with strong multipath interference. Simply being able to discriminate between modes requires straddling the beam centre, so this absolutely isn't going to work for general consumption.

    Also, I don't think that practical antenna design will ever allow more than three or four channels of angular momentum outside of a lab setting. Even that may potentially be a huge win for fixed microwave links, though.

    --
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  20. Re:Multiple transmitters on same freq is a reality by rullywowr · · Score: 2

    http://en.wikipedia.org/wiki/Single-frequency_network

    AFAIK however its only used for digital transmission where you can do a lot of signal processing. I don't think it would work well with analog - look what happens on AM at night.

    However SFNs are used with the DAB digital radio system in europe.

    All transmissions are inherently analog in nature. The phrase "digital" only refers to the processing before and after (companding etc). AM refers to a modulation scheme. FM also refers to a modulation scheme. FM is more reliable and has better consistency than AM, however both are analog technologies.