25th June 2012
American and Israeli researchers have used twisted vortex beams to transmit data at 2.5 terabits per second. As far as we can discern, this is the fastest wireless network ever created — by some margin. This technique is likely to be used in the next few years to vastly increase the throughput of both wireless and fiber-optic networks.
These twisted signals use orbital angular momentum (OAM) to cram much more data into a single stream. In current state-of-the-art transmission protocols (WiFi, LTE, COFDM), we only modulate the spin angular momentum (SAM) of radio waves, not the OAM. If you picture the Earth, SAM is our planet spinning on its axis, while OAM is our movement around the Sun. Basically, the breakthrough here is that researchers have created a wireless network protocol that uses both OAM and SAM.
In this case, Alan Willner and fellow researchers from the University of Southern California, NASA’s Jet Propulsion Laboratory, and Tel Aviv University, twisted together eight ~300Gbps visible light data streams using OAM. Each of the eight beams has a different level of OAM twist. The beams are bundled into two groups of four, which are passed through different polarization filters. One bundle of four is transmitted as a thin stream, like a screw thread, while the other four are transmitted around the outside, like a sheathe. The beam is then transmitted over open space (just one meter in this case), and untwisted and processed by the receiving end. 2.5 terabits per second is equivalent to 320 gigabytes per second, or around seven full Blu-ray movies per second.
This huge achievement comes just a few months after Bo Thide finally proved that OAM is actually possible. In Thide’s case, his team transmitted an OAM radio signal over 442 meters (1450ft).
According to Thide, OAM should allow us to twist together an “infinite number” of conventional transmission protocols without using any more spectrum. In theory, we should be able to take 10 (or 100 or 1000 or…) WiFi or LTE signals and twist them into a single beam, increasing throughput by 10 (or 100 or 1000 or…) times. For fiber networks, where we still have a lot of spare capacity, this isn’t all that exciting — but for wireless networks, where we’ve virtually run out of useful spectrum, twisted radio waves could provide an instant, future-proof solution. For the networking nerds, Willner’s OAM link has a spectral efficiency of 95.7 bits per hertz; LTE maxes out at 16.32 bits/Hz; 802.11n is 2.4 bits/Hz. Digital TV (DVB-T) is just 0.55 bits/Hz.
The next task for Willner’s team will be to increase the OAM network’s paltry one-meter transmission distance to something a little more usable. “For situations that require high capacity… over relatively short distances of less than 1km, this approach could be appealing. Of course, there are also opportunities for long-distance satellite-to-satellite communications in space, where turbulence is not an issue,” Willner tells the BBC. In reality, the main limiting factor is that we simply don’t have the hardware or software to manipulate OAM. The future of wireless networking is very bright indeed, however.