By now, you’d expect that communications engineers would have explored every trick in their century-old effort to cram more data into a limited number of frequencies.
But researchers in Italy and Sweden have shown that there is still uncharted territory. A little-explored quantum property, they claim, has the potential to boost the number of channels available in a single-frequency microwave link, perhaps as much as elevenfold.
Last year the researchers simultaneously transmitted two radio beams at exactly the same frequency between two of Venice’s islands, a distance of 442 meters. The signals were received and decoded as clearly as if they’d been sent at two different frequencies. Ordinarily, coding schemes require either different frequencies to distinguish the signals or the division of a channel into time slots.
The new trick hinges on using a quantum state of photons called orbital angular momentum. A photon can carry angular momentum just as a rotating body does and can even transfer the momentum to small particles, causing them to rotate. The orbital angular momentum of photons has been intensively explored in the optical region of the electromagnetic spectrum. But its study in the radio-frequency region is quite new.
In theory, a photon can occupy any one of an infinite number of these quantum states, each associated with an integer value. These quantum states impart the radio beam with a distribution of phases as it travels through space that gives the beam the shape of fusilli pasta (a helix).
The researchers started with two off-the-shelf transmitters and receivers designed to operate at the Wi-Fi frequency of 2.414 gigahertz. “We chose this frequency because equipment that can easily be controlled is available,” says Fabrizio Tamburini, an astrophysicist at the University of Padova, in Italy. The setup also included two Yagi-Uda antennas (similar to old television aerials) for reception and a radio dish and a Yagi-Uda antenna for transmission, all commercial models. But the team modified the dish antenna by bending it into a somewhat helical shape.
The shape of the dish was important, because when an ordinary wave of radiation from the antenna horn struck it, what reflected off was a wave whose phase was shifted into the shape of a continuous helix, corresponding to an orbital angular momentum of 1. At the same time, a beam of exactly the same frequency was emitted with a Yagi-Uda antenna, which imparted no phase twist, corresponding to an orbital angular momentum of 0.
At the receiver, the team could easily separate the twisted radiation from the nontwisted beam by measuring the phase.
Tamburini stresses that the experiment was just a proof of principle and that real-world systems for data transmission will use phased-array antennas for transmission and reception instead of bent dishes. These consist of arrays of small antennas, each fed with a signal at a shifted phase to create the helical wave.
Tamburini and his colleagues plan to perform experiments with more transmission channels and to develop smart phased-array antennas that can generate radio waves with several orbital angular momentum states simultaneously. Tamburini says that satellite companies are interested in the technology and that he and his colleagues plan to start a spin-off company in collaboration with the university.
Convincing communication companies that twisted radio waves can add capacity where there was none before will require the simultaneous demonstration of three or more channels at the same frequency, each having different angular momentum, argues Michael Steer, an RF and microwave expert at North Carolina State University. “That would have been convincing; now the [researchers] are really asking us to trust them.”
Indeed, some radio communication specialists are skeptical, saying that the technique will add no capacity. Ove Edfors and Anders J. Johansson, both at Lund University in Sweden, argued in the February 2012 issue of IEEE Transactions on Antennas and Propagation that at its heart, radio transmission using orbital angular momentum is no different from the multiple-input multiple-output (MIMO) communication technology in use today. MIMO, which involves transmitting and receiving on several antennas, increases data throughput and range without increasing power or using more bandwidth. Though the Lund group submitted their paper before the demonstration in Venice, Edfors says, “I still argue that this is traditional MIMO, but with a more esoteric antenna.”
Stefano Maci, an IEEE Fellow and antenna expert at the University of Siena, agrees. “I have some doubt about the practical feasibility of actual systems based on radio vorticity. One should compare this system to a MIMO system,” he says.
This story was corrected on 13 July 2016. The experiment took place in 2011 not 2012.