Towards infinite-capacity wireless networks, with twisted vortex radio waves
by Sebastian Anthony / September 18, 2014
Researchers at the University of Southern California, building on its previous work on infinity-capacity twisted laser vortex networks, has now adapted its technology to work with boring ol’ radio waves. The previous laser-based technique was only workable over short distances, with minimal atmospheric interference. Twisted radio waves are more rugged and can be reliably transmitted over much larger distances, potentially allowing for wireless networks that can carry much more data than your existing WiFi router — perhaps into the hundreds- or thousands-of-gigabits-per second range. Two and a half years ago, we wrote about a Swedish researcher who — after many years of work — finally proved that you could transmit radio waves in three dimensions, rather than two. Every wireless network that you’ve ever used — from WiFi to 3G to satellite TV — uses radio waves that oscillate (go up and down) in just two dimensions. Bo Thide found that, by simply twisting the antenna, you could impart some kind of corkscrew action to the radio waves so that they also travel width-ways, in a third dimension. In theory, hundreds — or perhaps thousands or millions — of wireless connections could share the same carrier frequency if they all had a slightly different level of twist. In technical terms, these twisted radio waves have orbital angular momentum (OAM). Currently, all radio-based network technologies only use spin angular momentum (SAM). SAM is comparable to the Earth spinning on its axis; OAM is comparable to the Earth’s orbit around the Sun.
A couple of years ago, USC’s Alan Willner used OAM to twist a bunch of lasers together, creating one of the fastest wireless networks ever — but only over a distance of 1 meter. Now, Willner and friends have done much the same thing, but with 28GHz radio waves. Using a “spiral phase plate” — basically a satellite dish with a slice taken out of it, and then twisted slightly (pictured below) — the USC researchers used OAM to squeeze four 8Gbps radio links over the same frequency, for a total link speed of 32Gbps. The range in this case was 2.5 meters — better than the laser-based approach, but still a long way short of commercial applicability. 32Gbps isn’t a world record for radio-based networks — a group in Germany has that honor at the moment, with 100Gbps — but USC’s method has the advantage in that it’s fairly simple, and theoretically could be deployed with just a few new radios and antennas. In theory, it should be fairly easy for USC to keep adding more and more 8Gbps streams to the vortex until some crazy capacities are realized. For now, the USC researchers are targeting wireless backhaul — the high-speed links that connect cell towers and rural broadband back to the core network. Currently these links use microwaves and are only capable of a few gigabits per second — a problem when you want to roll out 150Mbps LTE to millions of people. Assuming the “spiral phase plate” can be miniaturized, though — and I see no reason it can’t be — these infinite-capacity wireless links could also be used at home or in the office by some kind of futuristic 802.11 WiFi standard, too.
Research paper: doi:10.1038/ncomms5876 – “High-capacity millimetre-wave communications with orbital angular momentum multiplexing”
ORBITAL ANGULAR MOMENTUM (OAM)
Infinite-capacity wireless vortex beams carry 2.5 terabits per second
by Sebastian Anthony / June 25, 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.
DIY RADIO VORTEX
Vortex radio waves could boost wireless capacity “infinitely
by Sebastian Anthony / March 2, 2012
Thide’s approach is rather simple. Basically, electromagnetic waves can have both spin angular and orbital angular momentum (OAM). If you picture the Earth-Sun system, spin momentum is the Earth rotating on its axis (producing the day-night cycle), and orbital momentum is the Earth rotating around the sun (producing the seasons). In standard wireless communications — radio, TV, WiFi — we only modulate the spin angular momentum of waves. For years, Thide had theorized that orbital angular momentum could also be added to wireless signals, effectively creating a spiral signal that looks like fusilli pasta; or, in the words of Thide, a “radio vortex.” Now, in an experiment in Venice, Thide and his Italian colleagues have transmitted two signals at the same time, on the same frequency, over a distance of 442 meters (1450ft). Pictured above is the antenna that the team used. No, your eyes don’t deceive you: To create these radio vortices, all you have to do is make a cut in a standard parabolic reflector and twist it slightly. If you imagine a corkscrew of radio signals being continuously transmitted from the outside edge of the antenna, that’s effectively what’s occurring. On the receiving end, there are two “normal” TV antennae (Yagi-Uda) set apart by the same angle as the break in the transmitter. These antennae “decode” the vortex, and voila: Two radio signals transmitted over the same frequency.
It is hard to put into words just how significant Thide’s discovery could be. If the vortex preserves other aspects of wireless communications, such as multiplexing, then in the short term we could be looking at a wireless spectrum that can carry 10 or 20 times as much data. In the long term, as our understanding of orbital angular momentum grows, our wireless spectrum could effectively be infinite. To be honest, this is such a huge twist for wireless communications that the full repercussions are not yet known. With radio and TV, and now cellular networks, wireless spectrum is one of humanity’s most valued resources. It is because airwaves are so clogged that companies like Verizon or Vodafone pay billions of dollars for just a few megahertz. If Thide’s discovery pans out, not only would wireless spectrum lose most of its value, but the trouble and strife surrounding LightSquared, international roaming, LTE rollout, white space wireless, and digital TV simply cease to be.