'Twisted waves' Technology Demonstrated in Venice
March 9, 2012
A technology I first wrote about a year ago in Vorticity Transmission Could Increase Spectrum Efficiency was demonstrated over a 442 meter path from a lighthouse on San Georgio Island to a satellite dish on a balcony of Palazzo Ducale on the mainland of Venice. The Institute of Physics published details of the demonstration, including video and audio, mathematical analysis and illustrations showing the use of the orbital angular momentum (OAM) quality associated with the "helicoidal phase profile of the EM beam in the direction orthogonal to the propagation axis" to increase the capacity of communications channels.
The article Encoding many channels on the same frequency through radio vorticity: first experimental test notes "[OAM] offers, in addition to the conventional translational linear momentum and polarization (SAM) rotational degrees of freedom, which spans only a two-dimensional (2D) state space, additional rotational degrees of freedom that are distinctly different from SAM. Without increasing the frequency bandwidth, the OAM states can be used as a new, very large set of communications channels that are mutually orthogonal to each other in the OAM state space."
The experiment used two identical FM transmitters operating at 2.414 GHz to feed two antennas. A 26 dBi gain commercial off-axis parabolic antenna was modified to generate the vortex beam. A picture of the antenna is shown in an appendix at the end of the article. In it you can see the antenna has cut from the rim to the center and the surface at the cut is offset, making the antenna part of a spiral, or "vortex reflector". A 16.5 dBi Yagi-Uda antenna was used for transmitting beams without OAM. Two Yagi antennas with precision offset were used for receiving the signals. The 2.414 GHz signals were FM modulated using both audio and video with two different tones and two different images.
The experiment proved it was possible to separate the two signals transmitted on the same frequency but in different OAM states. Audio recordings showing the discrimination are available on the Supplementary Data tab of the New Journal of Physics article Webpage.
The researchers responsible for this breakthrough in radio communications are Fabrizio Tamburini, Elettra Mari, Anna Sponselli, Bo Thidé, Antonio Bianchini and Filippo Romanato.
Tamburini described the waves this way:
"In a three-dimensional perspective, this phase twist looks like a fusillli-pasta-shaped beam. Each of these twisted beams can be independently generated, propagated and detected even in the very same frequency band, behaving as independent communication channels." He added, "Within reasonable economic boundaries, one can think about using five orbital angular momentum states, from –5 (counter-clockwise) up to 5 (clockwise), including untwisted waves. In this instance, we can have 11 channels in one frequency band. It is possible to use multiplexing, like in digital TV, on each of these to implement even more channels on the same states, which means one could obtain 55 channels in the same frequency band."
If all this sounds too complicated, a video explains how radio vorticity works and includes video of the first experiment showing reception of the two signals.
The obvious question is how soon before we see devices using this technology?
The use of MIMO (multiple-input / multiple-output) technology, was still being developed in the lab when I described it in my Jan. 1, 2001 article Exotic Modulation – Beyond 8-VSB. The term MIMO was not in common use 11 years ago. I called the technology "modulating space." Fast-forward 10 years and Wi-FI routers using this technology were on the shelves at Best Buy and Wal-Mart, and wireless carriers had begun large scale rollout of the technology as part of LTE. It's possible the looming "spectrum crisis" FCC Chairman Genachowski constantly warns of won't be an issue 10 years from now (the deadline Congress has set for completing auctioning UHF broadcast TV spectrum), although as with the rollout of LTE and the DTV transition, some additional spectrum will be needed during the move from today's technology to systems using OAM states.
I still have a few questions about the technology, though.
From what I've been able to determine, it isn't clear what happens to vorticity waves when they bounce off irregular surfaces. The experiment was line-of-site with carefully positioned directional antennas. Can the isolation be maintained over a non-line-of-site path? The other problem is the transmission and reception of specific OAM states. As shown in the article and video presentation described earlier, the antennas used would not work in portable or even handheld devices. However, if the power of digital signal processing (DSP) increases, will it be possible to send signals with the correct time and phase to the different antennas to create a specific OAM state and also decode it? How much more difficult is it to process a vorticity wave signal from multiple antennas than a MIMO signal from a Wi-Fi router or LTE cell phone tower? Is it possible to receive or transmit OAM signals with omnidirectional antennas?
It's difficult to understand how the size of the antenna or antenna array needed to send and receive vorticity waves will to allow this technology to be used for handheld or portable devices in the UHF TV band without substantially more work. It's difficult to get sufficient isolation between small antennas for good MIMO reception at UHF. I expect the first applications will be for longer fixed links at GHz frequencies to feed more conventional systems for the "last mile" to the consumer.