Twisted Signals Could Increase Spectrum Capacity ‘Infinitely’

A team of researchers say that they have devised a method which could in theory infinitely boost the amount of information that can be carried on radio waves.

The technique uses “spiral” wave signals with a different orbital angular momentum. More than one of these signals can be sent independently on one wavelength, according to a demonstration in Venice, by Professor Bo Thide of the Swedish Institute of Space Physics and a team of Italian colleagues.

Infinite expansion

The researchers were able to send two signals on the same frequency 442 metres from San Giorgio Island to the Palazzo Ducale in St Mark’s square, lighting up a display message of “signal received” when the experiment worked.

The key to the method lies in the distinction between the spin angular momentum and orbital angular momentum of electromagnetic waves. Previously wireless signals have only used spin angular momentum, which is related to its circular polarisation.

Orbital angular momentum is independent of the polarisation, and allows a spiral signal, which can be distinguished from other signals with a different orbital angular momentum. This allows a number of data streams to fit in the frequency.

To send the signal, the team split a satellite dish and separated the two edges that resulted to create a ‘corkscrew’ effect.

“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,” wrote the researchers in the New Journal of Physics. “This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarisation, multiport or dense coding techniques.”

Fears of overcrowding

It is hoped that the technology could be used for Wi-Fi, radio, television and even mobile phones and could be a solution to an increasingly crowded spectrum.

“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,” said the researchers.

The technology could be particularly useful in the UK, where there is little room left for competing signals and no way of increasing bandwidth. Mobile networks in particular are feeling the strain as smartphone users increasingly demand greater amounts of bandwidth.

The advent of 4G technology is expected to alleviate some of this overcrowding, but the perpetually delayed auction for spectrum has resulted in Ofcom warning that such networks may not be available in the UK for another four years. Concerns of a lack of capacity have led Ofcom to borrow spectrum from the Ministry of Defence and other agencies for this summer’s Olympic Games.

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