As the mega-money auctions for broadcast bandwidth demonstrate, there are a finite number of frequencies and they are almost all allocated. Put another way, the world is running out of broadcast frequencies. That also amounts to a challenge for the world’s physicists and radio engineers – How to get more signal (information) onto existing bandwidths? When so motivated, it’s good to note how creative human minds can be: As proposed by a team of scientists in New Journal of Physics [1 March 2012, Open Access, Encoding many channels on the same frequency through radio vorticity: first experimental test] the idea is to twist radio waves so that multiple streams of information can be carried on a single bandwidth. This potentially important idea has a multi-disciplinary origin involving astrophysics – from observing the twisted light coming from certain black holes.
A key aspect to the story is that the idea for twisting radio waves comes from a study involving Bo Thidé (Swedish Institute of Space Physics, Uppsala, Sweden) who is an astrophysicist. Last year he and a group of scientists published a study (Nature Physics [13 February 2011, paywalled, Twisting of light around rotating black holes]) of the way light escapes from around Kerr black holes (a specific type). They noticed that the spin of the black hole imparts an orbital angular momentum to light (makes it spiral) and that this twisted light can be captured and measured. It occurred to Thidé that this concept, orbital angular momentum, could also be applied to radio waves of various kinds.
The basic concept is usually explained like this: The Earth spins on its axis; this is spin angular momentum (rotational spin). The Earth also orbits the Sun at the same time; this is orbital angular momentum (orbital spin). The particles of light, photons, can have both kinds of spin. The angular momentum spin is the effect we know as polarization, where for example, light can be split into two phases for 3-D glasses. In a similar way, orbital angular spin can be produced at different amounts so that more than one stream of information can be carried on the same wavelength.
Thidé teamed up with a group of Italian scientists to put this concept, which looked very good in mathematical models, into practice. In grand Italian fashion, a public demonstration was performed in St. Mark’s Square of Venice in 2011. The team used a modified antenna, splitting it into two sections with the edges a specific space apart. As a signal is transmitted from the antenna, the wave from one side is ‘ahead’ of the other side, producing a broadcast beam that if it were visible would look like a corkscrew. Using this corkscrew design, they nested two audio signals together at the same frequency in the 2.4 GHz band, the same as used by Wi-fi. They repeated the experiment with two TV signals. The results were received and converted to audio/visual results on the walls of the Palazzo Ducale. This was fitting showmanship for an idea that, if it can be made practical, could make it possible to put far more signals into a single bandwidth.
Discussions have already begun with industry to find ways of commercializing the technique. It will take several years to develop appropriate applications and equipment, and there is no guarantee that the idea can actually be made sufficiently reliable and cost effective (always a big concern). Still, this is one potential solution for the crowded bandwidth problem that works by really screwing things around.