Optical boffins tweak antennae with photons so MIMO can make WiFi serve more masters
And also to make 5G sing, and cary radar so it can spot missiles at Mach 4
Phased-array antennas, a technology crucial to modern Wi-Fi systems that use beam-forming to improve throughput, has a speed limit in how quickly beams can be manipulated.
Beams are formed by adjusting a microwave signal's timing at different antennas in a multiple-in, multiple-out (MIMO) system so that they reinforce in some directions and cancel in other directions – but it takes milliseconds to adjust the delay.
Australian photonics researchers have decided to cheat a little: photonic systems can be tuned a lot quicker, so they've created a chip-level delay line that operates in the optical domain, rather than at microwave frequencies.
Adding a fixed delay to a microwave signal is easy – just run it through an appropriate length of cable, and let the propagation delay take care of it.
To create an adjustable delay, systems typically send the signal around a ring. Heating and cooling the ring changes its geometry, but that takes milliseconds, and that's not fast enough for emerging wireless applications.
From 5G to electronic warfare
Ben Eggleton, director of Australia's Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), explained to The Register that high-speed wireless communications (such as millimeter-wave 5G), radar, and electronic warfare all demand fast antenna reconfigurability.
That's what CUDOS announced yesterday: a demonstration of an on-chip delay line that can be reconfigured at gigahertz frequencies – by modulating the microwave signal onto an optical signal, and operating in the photonic domain rather than on the radio waves.
“When it comes to manipulating and processing microwave signals … as you go beyond 10 GHz, it becomes inherently more difficult,” he said, so “rather than manipulate the microwave signal directly, we take it in through the coax, and modulate it onto a photonic carrier.”
In an optical signal, something like a delay line is very simple both to implement and to reconfigure, using interference and the power of the optical signal.
What that means, in the world of wireless communications, is that the phased beam-forming used in multiple-in-multiple-out (MIMO) WiFi systems can serve more users.
Since the CUDOS chip can reconfigure antennas at gigahertz rates (Eggleton said it might be able to reach 100 GHz), the same approach can be applied to emerging millimeter-wave systems.
And there's a more deadly-serious application in military systems – for example, in radar.
“Configurability has to be on a timescale that allows [radars] to track something like an incoming missile arriving at Mach 4”, he explained.
Fast-configure phased antennas would also be useful in evading an adversary's jammer (another application of CUDOS's microwave photonics work, a notch filter, was demonstrated to the US military recently).
Eggleton added that much of the technology needed for CUDOS's reconfigurable delay lines is common: “we can leverage all the infrastructure developed for telecom – 1.5 micron technology, fibre optics, and all the processing techniques we've developed for that.”
Since the photonic system doesn't need to heat the chip, it's more energy-efficient, which is also important if wireless systems are trying to support high throughput for large number of users.
As Eggleton pointed out, that's exactly what's wanted in a 5G world where devices outnumber humans, with the associated proliferation of small-cell base stations.
The lead author on the paper is Yang Liu, a CUDOS and Sydney University School of Physics PhD candidate, and he worked at the Australian Institute for Nanoscale Science and Technology. ®