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“Twisted” Radio Beams Data at 32 Gigabits per Second

by IEEE SpectrumSeptember 22, 2014

Image: Alan Willner/USC Viterbi

Multiple channels of data orbiting orthogonally around a single radio wave

A team led by engineers at the University of Southern California has demonstrated a new way to send information via radio waves. By twisting several polarized beams carrying information together into a spiraled beam, the team was able to send up to 32 gigabits per second across 2.5 meters of open air.

The high data rate, reported in the journal
Nature Communications
, was made possible through a technique known as orbital angular momentum (OAM) multiplexing, says USC electrical engineering professor Alan Willner, who partnered with researchers from the University of Glasgow and Tel Aviv University on the experiment.

A  property of electromagnetic waves first identified in the 1990s, OAM can be harnessed to let multiple channels of information ride along a single frequency. “I could have a wave that twists slowly, and one that twists a little faster, and those waves are now orthogonal to one another,” Willner says. “If you put them together and send them spatially co-located through the same medium, you have doubled your capacity.”

Willner and others have previously demonstrated the twisting technique with beams of light reaching data transmission speeds of 2.56 terabits per second in 2012 and 1.6 terabits per second over optical fiber in 2013. Demonstrating the technique using another band of the electromagnetic spectrum, though, helps to drive home its viability across a variety of waves.

Putting these principles to work in radio waves, Willner and his team used four antennas to send polarization-multiplexed beams, resulting in a total of eight channels of data. Those beams of data were sent through specially shaped “spiral phase plates,” Teflon plates that don’t absorb the beams, but do change their shape, twisting them slightly. The twisted waves are then gathered by a multiplexer and sent through a single transmitter aperture. Since each wave has a slightly different orbital angular momentum, they can travel along a shared axis without interfering with one another, similar to how several subway riders can steady themselves around a single pole by holding it at different points.

The combined beam, which takes on a helical shape like, travels through another aperture at the receiver, after which it is split back into four beams by a demultiplexer. The four beams then pass through another set of spiral phase plates. These plates are inverted versions of the first set, which undo the initial twisting and prepare the waves to deliver their data payload.

This isn’t the first time radio waves have been used to demonstrate the potential of OAM. Italian and Swedish researchers in 2012 used the same principles to send a pair of radio waves sharing a single frequency between two islands in Venice. At the time, some communications engineers criticized that work, suggesting it was not significantly different than traditional multiple-input, multiple-output (MIMO) techniques. At the time, Lund University radio systems professor Ove Edfors told IEEE Spectrum that the technique was “traditional MIMO, but with a more esoteric antenna.”

Willner says that this latest study demonstrates that there are clear differences between MIMO and orbital angular momentum multiplexing. MIMO sends different streams of data from different antennas broadcasting on the same frequency and decodes the inevitable crosstalk on the receiving end using digital signal processing. Orbital angular momentum multiplexing sends multiple channels of information along a single beam without any interference between them. That means once the phase plate at the receiver unwinds the helical beam into its component channels, they don’t have to undergo further cleanup.

Edfors, however, remains unconvinced that OAM is anything but a different flavor of MIMO. “In both cases multiple input (MI) signals are used to create the beam at the transmitter side and at the receiver side the beam is split in multiple output (MO) signals…hence MIMO,” says Odfers. While he doesn’t question the results of the latest Nature Communications paper, Odfers also remained incredulous that radio-based OAM could be made practical for long range communications. “To allow efficient OAM-based communication, near-field conditions must be maintained, which at low frequencies forces…antennas to rapidly grow to impractical sizes as the distance increases.” Odfers says. 

Fabrizio Tamburini, one of the scientists behind the 2012 Venice OAM experiment, though, sees a lot of promise in the latest work, saying that “very good ideas can come from it.” Tamburini, who is now working on ways to refine OAM for use in telecommunications and other industries, also sees a fairly clear line between MIMO and OAM. He welcomes the views of his critics, though, saying via email that the debate around OAM “helped us to understand and better focus our studies. Competition, controversies, and exchanges of ideas are necessary for research.”

As work on OAM goes forward, Willner wants to find ways to integrate it with other technologies, including MIMO, to build a system with better transmission rates that can work over longer distances. “Spatial multiplexing can take many forms, and this is not MIMO versus orbital angular momentum,” says Willner. “They come at things in a different way, they use different techniques, and they can complement one another.”

If OAM pans out the technology could see adoption in places where high-speed, line-of-sight wireless connections are in demand, such as for wireless backhaul in cellular networks. OAM could be a good fit for transmitting data among “a dense network of small base stations without…the stringing of fiber to connect them to the core network,” says Willner. He and USC colleague Andy Molisch, also see the potential for OAM in data centers. “With better equipment, [transmission rates] could go much higher,” Willner says. “A radio backhaul like that could be a huge pipe for data centers or building to building connections.”

Orbital angular momentum techniques might also see use in other fields, such as microscopy, Willner says. “There are potential applications outside of communications. We’re going to continue learning how to tailor and manipulate the structure of waves in ways we’ve never thought of before.”

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