March 02, 2012

Encoding many channels on the same frequency through radio vorticity: first experimental test

Science Daily - A group of Italian and Swedish researchers appears to have solved the problem of radio congestion by cleverly twisting radio waves into the shape of fusilli pasta, allowing a potentially infinite number of channels to be broadcast and received.

To demonstrate this, the researchers transmitted two twisted radio waves, in the 2.4 GHz band, over a distance of 442 metres from a lighthouse on San Georgio Island to a satellite dish on a balcony of Palazzo Ducale on the mainland of Venice, where it was able to pick up the two separate channels.

"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," said Tamburini.

This appears to be using a form of three dimensional modulation (a new kind of polarization) to get more bandwidth.

New Journal of Physics - Encoding many channels on the same frequency through radio vorticity: first experimental test

Diagram of the monophonic audio recordings of the twisted/untwisted beams. The output of the two transmitters was adjusted to ensure the same maximum input voltage of 2 V when both channels were present, and 1 VCC max for each individual channel. The first minimum is found at about 1 cm of antenna shift for the ℓ = 1 mode (continuous line). Here the ℓ = 0 channel (marked with the symbol 'o') has a maximum and the associated audio tone is clearly audible. The same was found for the ℓ = 0 mode around the 9 cm antenna position. The inner boundaries of the two minima regions are separated in distance by half the radio wavelength. Between these positions there was a forest of minima of the ℓ = 1 mode, a phenomenon due to the sampling of the field from a finite-sized antenna. Beyond the minimum located at 9 cm, two additional alternating signal minima due to the cross-talk of the two Yagi–Uda antennae were found.

We have shown experimentally, in a real-world setting, 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. This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. This paves the way for innovative techniques in radio science and entirely new paradigms in radio communication protocols that might offer a solution to the problem of radio-band congestion.

Already with this setup, one can obtain four physically distinct channels on the same frequency by additionally introducing the use of polarization (SAM), which is independent of OAM. A further five-fold multiplicative factor from implementing multiplexing would yield a total of 20 channels on the same frequency. The utilization of multiport techniques (e.g. MIMO) could increase the capacity further.

Our experimental findings that EM OAM can be used for increasing radio transmission capacity without increasing bandwidth is likely to open up new perspectives on wireless communications and radio-based science. History tells us that Marconi invented the wireless telegraph and from that the communication world spread its branches in all directions. All current radio communication services are based on various forms of phase, frequency and/or amplitude modulation of the EM radiation in the form of EM linear momentum (i.e. integrated Poynting vector or energy flux). In order that many different broadcasting stations are able to transmit simultaneously without overlapping their radio signals, Marconi suggested that the total available spectrum of radio frequencies be divided into many non-overlapping frequency subbands. Now, the wide use of wireless communication has unavoidably led to the saturation of all available frequency bands, even after the adoption of artificial techniques that increase band capacity. We have experimentally shown that by using helicoidal parabolic antennae, the use of OAM states might dramatically increase the capacity of any frequency band, allowing the use of dense coding techniques in each of these new vortex radio channels. This might represent a concrete proposal for a possible solution to the band saturation problem.

Moreover, our experimental findings demonstrate that the spatial phase signature was preserved even in the far-field region and for incoherent non-monochromatic wave beams. These results open up new perspectives not only for wireless communication but also for physics and astronomy, including the possible detection of Kerr black holes in the test general relativity

In addition to increasing the quantity of information being passed around our planet, this new discovery could also help lend an insight into objects far out in our galaxy. Black holes, for example, are constantly rotating and as waves pass them, they are forced to twist in line with the black hole.

According to Tamburini, analysing the incoming waves from the supermassive black hole at the centre of the Milky Way, Sagittarius A, could help astronomers obtain crucial information about the rotation of this "million-solar mass monster."

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