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 (pictured right).
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. For more technical info, I suggest you read this story: Infinite-capacity wireless vortex beams carry 2.5 terabits per second.
UCS’s OAM wireless network setup. You can see the four different beams, each with a different OAM (level of twist).
Back to the story. 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”
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