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Boffins biff over ‘twisted radio’

Is infinite capacity possible?

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Six months after an Italian/Swedish group set the comms world alight with a wireless technology of theoretically infinite capacity, debate over his work is becoming increasingly bitter.

To recap the original story: today’s wireless technologies use a handful of venerable modulation schemes to carry information (amplitude, frequency and phase). To pack more data on a carrier, you can increase its baseband frequency, use a wider wireless channel, or find some way to multiplex the wireless channels.

Frequency division multiplexing gets extra capacity by allowing stations to transmit simultaneously over multiple carriers; and spatial multiplexing (using multiple antennas – MIMO) does so by using the same carrier over different paths.

The “spin modulation” group – led by Fabrizio Tamburini of the University of Padua and Bo Thidé of the Swedish Institute of Space Physics and first demonstrated this year in Venice – proposed using a previously-unexploited characteristic of radio waves, their orbital angular momentum, or “spin”.

The theory, then, is this: if Bob and Alice both want to communicate on the same frequency, they can do so by applying different spin values to their signal, so long as the receiver can detect that spin. In the original test, that spin was imparted by a modified dish antenna.

Multiple-signal demonstration antenna

Did the spinning antenna create a MIMO channel?

Since there’s no limit to the number of spin states that can exist, this was heralded all over the world as “infinite capacity wireless”. There were, however, sceptics at the time, and more recently, that’s become what the BBC calls “’he says, she says’ with references”.

One issue is that the demonstration earlier this year only multiplexed two signals. According to this paper (lead author Michele Tamagnone of EPSL), the antenna configuration for the “spin modulation” test could be seen as creating a MIMO case – sending Bob and Alice’s signals over different paths created by the shape of the dish.

It’s important to note that Tamagnone’s group do not dispute whether multiplexing was observed in the “Venice experiment”, only that OAM isn’t needed to explain the results.

Tamburini and Thide disagree, here, reiterating their belief that OAM was observed in their experiments both at Venice and at Padua.

The other dissenting paper, (here) from Lazlo Kish and Robert Nevels of Texas A&M University’s department of electrical and computer engineering, doesn’t dispute whether the original experiments demonstrated OAM – but vigorously disputes the “infinite capacity” notion. These two authors argue that if a free-space system multiplexed more than two signals, OAM would violate the Second Law of Thermodynamics.

A final note from El Reg, so as those leaving comments don’t get drawn down a rabbit-burrow: multiple spins have been encoded onto optical waves and transmitted down fibres. The current – and presumably ongoing – furore is about whether the phenomenon can be used for free space radio transmissions. ®

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