By Edwin Cartlidge of Nature magazine
The research on which this story is based has now been published in the New Journal of Physics. Nature's coverage, published on 22 February, 2011 and presented again below, is based on the version of the paper submitted to ArXiv.
The bandwidth available to mobile phones, digital television and other communication technologies could be expanded enormously by exploiting the twistedness as well as wavelength of radio waves. That is the claim being made by a group of scientists in Italy and Sweden, who have shown how a radio beam can be twisted, and the resulting vortex detected with distant antennas.
The simplest kind of electromagnetic beam has a plane wavefront, which means that the peaks or troughs of the beam can be connected by an imaginary plane at right angles to the beam's direction of travel. But if a beam is twisted, then the wavefront rotates around the beam's direction of propagation in a spiral, creating a vortex and leaving the beam with zero intensity at its centre.
Physicists have been able to create twisted beams of visible light for about 20 years, having initially noticed that such beams were being produced inside some laser cavities. These twisted beams of light are useful in nanotechnology, as optical 'tweezers' or 'spanners' to manipulate tiny particles. To date, however, no-one has attempted to do the same thing at the radio wavelengths used in telecommunication.
Spiral waves
Now, a group led by Bo Thidé of the Swedish Institute of Space Physics in Uppsala and Fabrizio Tamburini of the University of Padua, Italy, has succeeded in twisting the waves emitted by the type of antenna used by standard wireless routers to transmit data over long distances. The team did this by reflecting the waves off an eight-stepped, spiral-staircase-like structure positioned a couple of metres from the antenna, the axis of which lined up with the beam. The idea was that different sections of the wavefront would bounce off different steps, introducing a delay between the reflection of neighbouring sections and so causing the wavefront to become twisted and take on the shape of the reflector.
To prove that they really had twisted the beam, Thidé and his colleagues measured the beam's intensity with a pair of antennas seven metres away. They found that the combined intensity from the two antennas varied as they kept one fixed and moved the other around in a plane at right angles to the beam. This, they point out, is what would be expected if different portions of the wavefront traverse the plane at different times. The signals registered by the two antennas would be in-phase (that is, two peaks or two troughs), out of phase or somewhere in between depending on their relative orientation. The intensity pattern more or less matched that predicted by a computer simulation of the propagating twisted beam.
Thidé, Tamburini and others recently showed how this detection scheme, carried out using radio telescopes, could identify the tell-tale twisted radiation from spinning black holes (see 'How to spot a spinning black hole'). But Tamburini thinks it could also have "revolutionary" implications for radio communications. He envisages that just as waves of different frequencies can propagate together without interference -- thereby multiplying the number of signals that can be sent between an emitter and a receiver -- so too could bandwidth be expanded by simultaneously transmitting waves with the same frequency but different degrees of twistedness.
The next important milestone will be the demonstration of twisted radio transmission in noisy, real-world conditions -- so far the experiments have been done in an electromagnetically and acoustically insulated room at the University of Uppsala. The researchers hope to start testing a partially spiralled satellite dish within the next few days, then to use a similar device to transmit a twisted radio beam several hundred metres across the lagoon in Venice three months from now.
Tamburini thinks that the bandwidth available to mobile phones and laptop computers could be increased by a factor of nine almost immediately, and at relatively little extra cost, by carefully positioning four antennas inside the devices. He estimates that this technology could enter the market within the next two to five years. Technological improvements could make even more bandwith available.
Taco Visser, an electrical engineer at Delft University of Technology in the Netherlands, thinks that twisted radio beams would "certainly increase capacity" in telecommunication channels. But he cautions that atmospheric turbulence, which causes fluctuations in the amplitude and phase of a signal, would probably limit the extent to which beams could be twisted and therefore restrict the number of available channels. He also says it is not clear how portable devices such as mobile phones could emit such twisted beams, because each channel would require its own spiral reflector.
However, Tamburini says that he has devised a scheme in which individual spiral reflectors are not needed. He has shown in a simulation that this scheme works and is now looking to build a prototype system and patent it, adding that the most complicated aspect is how to send and recieve twisted beams from a device when it is moving about.
Visser says that the work could also have other useful applications. For example, he says, radio waves could be used to make a scaled-up version of an optical spanner. So rather than manipulating objects just a few millionths of a metre across, twisted radio waves could be used to manoeuvre objects several millimetres long. Conceivably, he says, this could allow small toxic or radioactive objects to be handled remotely.
This article is reproduced with permission from the magazine Nature. The article was first published on March 2, 2012.
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15 Comments
Add CommentVery interesting! The link to the arxiv preprint archive research report is contained in the Nature article: http://arxiv.org/pdf/1101.6015v1.pdf
Reply | Report Abuse | Link to thisWhile the paper mentions some experimental applications of the technique using lasers, it would seem that multiplexing optical signals in fiber channels would be a natural application for increasing fiber bandwidth as well. This is not mentioned - is it not already being done? If not, why not?
The research report abstract states:
"Since the OAM [orbital angular momentum] state space is infinite, our findings provide new tools for achieving high efficiency in radio communications and radar technology."
While this is correct in theory, practical limitations in the ability to reliably transmit and detect OAM states practically constrains potential capacity improvements.
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?"
Reply | Report Abuse | Link to thishttp://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=2062936&fileOId=2339120
Excellent reference, thanks! I also suspect that similar methods have been used for years to multiplex fiber optic signals, as your reference shows for radio communications.
Reply | Report Abuse | Link to thisIt seems that sometimes the theoretical infinities run afoul of physical engineering realities...
Perhaps OAM can still be used as suggested to produce optical spanners - you know, to tighten down those nano-nuts!
Anders:
Reply | Report Abuse | Link to thisYes, I've looked at your paper: It makes sense to me that the OAM concept boils down to a variety of MIMO. It seems to me that it might suffer from line-of-sight limitations; and that could additionally be a headache for moving terminals.
Nonetheless, it is an interesting way to look at the EM spectrum. I admit that it never occurred to me to look at different AM states of the EM field as one looks at different frequency states.
jtdwyer:
Reply | Report Abuse | Link to thisThe fiber-optic equivalent of looking at the different vorticity (OAM) states of the radio-wave field would be looking at the different modes of the EM field in the fiber-optic transmission line. Unfortunately, different modes propagate at different speeds, and are also attenuated at different rates. So that means that bits sent out at one time would arrive down the line at quite different times, and at different intensities; this would be a problem for using all these channels at the same time.
And that is assuming that there would be a way to separate out the different modes from each other. As far as I know (but I'm not necessarily up-to-date on fiber-optic technology), no one has a technique for reliably distinguishing energy received from different FO modes; so that all that would happen is that the energy from the different modes gets smeared together and just blurs the nice clean bit into a blob.
This is why, traditionally, people use single-mode fiber for long-distance transmission; multi-mode is cheaper (because your detector doesn't have to be as good) but has a much worse bandwidth-distance product.
Thanks - I haven't really looked into fiber-optic communications for a few decades, but a quick reference indicates that:
Reply | Report Abuse | Link to this"Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as 160 channels to support a combined bit rate in the range of terabits per second."
Please see:
http://en.wikipedia.org/wiki/Fiber-optic_communication#Wavelength-division_multiplexing
http://en.wikipedia.org/wiki/Wavelength-division_multiplexing
Neil J. King: It is definitely interesting to view RF communications in general, and especially MIMO, from this point of view.
Reply | Report Abuse | Link to thisjtdwyer: It is used/proposed to be used in optical communications. It is possible to detect different OAM-modes by special gitters.
jtdwyer:
Reply | Report Abuse | Link to thisWDM is a method of creating separate channels by means of distinct frequencies (essentially, different colors). The OAM approach creates separate channels by the shape of the field, at a given frequency: Roughly, dependence of the field that goes like exp(jx), exp(2jx), exp(3jx) ... where x is the angle around the direction of propagation. The fiber-optic analog is the different modes of the FO cable, at a fixed frequency.
Of course, you can ALSO & separately separate channels by frequency; but that's already done in all types of electromagnetic transmission; there's nothing new there.
Thanks for explaining.
Reply | Report Abuse | Link to thisNot sure how any of this differs from circular polarization and spin modulation - both of which have been known for some time.
Reply | Report Abuse | Link to thisAs shown in most textbooks on electrodynamics, an arbitrary time-varying charge and current distribution will radiate both linear momentum (Poynting vector) and angular momentum. These two components of the radiation will have different properties, characteristics and even direction of propagation but today only the linear momentum is used in conventional radio; at much higher frequencies both linear and angular momentum is now beginning to be used.
Reply | Report Abuse | Link to thisTo be more specific and "engineering oriented": Take a piece of conducting wire, curl it up in random shape, connect it to a transmitter and, voilà, this system will emit both linear momentum and angular momentum. Depending on the shape and configuration of the conductor the proportion between the two momenta will vary within wide margins and it is quite possible that the linear momentum is almost undetectable while the angular momentum is quite strong. This is fundamentally different from MIMO which is a technique for optimising the radiation in various ways. So far this technique has only been applied to the linear momentum component of the radiation as used in current radio science and communication, but may equally well be applied to the angular momentum to optimise that component in various ways. This has not been done yet.
To summarise: Linear momentum and angular momentum are physical observables that are described by the fundamental laws of physics. The are constants of motion (conserved quantities). MIMO, and multiport techniques in general, are (very clever) engineering tricks to enhance the usefulness of - to date - linear momentum and - in the future - angular momentum in radio communications.
Polarisation is spin angular momentum and spans a state space of dimensionality. It is related to, but distinctively, different from, orbital angular momentum whose state space is infinite.
Reply | Report Abuse | Link to thisPolarization is the manifestation of the fundamental physical quantity spin angular momentum (rotation of the field/photon around its own centre) which can take on two values. We use the related but distinctively different orbital angular momentum (rotation around an external axis) which can take on an infinitely large number of values.
Reply | Report Abuse | Link to thisAs shown in most textbooks on electrodynamics, an arbitrary time-varying charge and current distribution will radiate both linear momentum (Poynting vector) and angular momentum. These two components of the radiation will have different properties, characteristics and even direction of propagation. Take a conducting wire of random shape, connect it to a transmitter and the system will emit both linear momentum and angular momentum - simultaneously. Depending on the shape and configuration of the conductor the proportion between the two momenta will vary within wide margins and it is quite possible that the linear momentum is almost undetectable while the angular momentum is quite strong. MIMO is a technique for optimising the radiation in various ways. So far this technique has only been applied to the linear momentum component of the radiation as used in current radio science and communication, but might equally well be applied to the angular momentum to optimise that component in various ways. This has not been done yet.
Reply | Report Abuse | Link to thisA year ago we published a paper that showed that radiation from a region with Kerr metric, as that around a rotating black hole, contains orbital angular momentum. This emission process is so far removed from MIMO antenna engineering as one can get.
To summarise: Linear momentum and angular momentum are physical observables that are described by the fundamental laws of physics and are produced naturally in nature. They are both conserved quantities (constants of motion). MIMO or multiport techniques are (very clever) engineering transmitter and antenna tricks developed to enhance the usefulness of, hitherto, linear momentum and, in the future, angular momentum in radio communications.
Oh Great information that's why for sending images, audio and text in the form of signals, radio is most popularly used actually the credits for being early pioneers in the section of radio are given to Nikola Tesla and Guglielmo Marconi.Great work, By the way I heard that The waves with the wavelength that travel around an average 10 meters are absorbed by the atmosphere. If not that then the radio waves bounce back and forth between the ionosphere and the ground which makes the radio ideal to transmit over the horizon.Is this true? and what is a scientific reason behind it?
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