The flexible Rotman lens-based rectifying antenna (rectenna) system developed at Georgia Tech is the first that is capable of harvesting energy in the 28GHz band that can be used for 5G.
The Rotman lens is key for beamforming networks and is frequently used in radar surveillance systems to see targets in multiple directions without physically moving the antenna system. However, to harvest enough power to supply low-power devices at long ranges, large aperture antennas are required.
The problem with large antennas is they have a narrowing field of view. This limitation prevents their operation if the antenna is widely dispersed from a 5G base station.
“We’ve solved the problem of only being able to look from one direction with a system that has a wide angle of coverage,” said senior researcher Aline Eid (above, centre) in the ATHENA lab, established in Georgia Tech’s School of Electrical and Computer Engineering.
The 5G energy harvesting rectenna design could open up new passive, long-range, mm-wave RF power for wearable and ubiquitous IoT applications. The researchers used inhouse additive manufacturing to print the palm-sized mm-wave harvesters on a multitude of everyday flexible and rigid substrates.
“With this innovation, we can have a large antenna, which works at higher frequencies and can receive power from any direction. It's direction-agnostic, which makes it a lot more practical,” said Jimmy Hester, senior lab advisor and the CTO and co-founder of Atheraxon, a Georgia Tech spinoff developing 5G radio-frequency identification (RFID) technology. With the Georgia Tech design, all the electromagnetic energy collected by the antenna arrays from one direction is combined and fed into a single rectifier, which maximizes its efficiency.
“People have attempted to do energy harvesting at high frequencies like 24 or 35GHz before,” said Eid. Such antennas only worked if they had line of sight to the 5G base station; there was no way to increase their angle of coverage until now.
Operating just like an optical lens, the Rotman lens provides six fields of view simultaneously in a pattern shaped like a spider. Tuning the shape of the lens results in a structure with one angle of curvature on the beam-port side and another on the antenna side. This enables the structure to map a set of selected radiation directions to an associated set of beam-ports.
The lens is then used as an intermediate component between the receiving antennas and the rectifiers for 5G energy harvesting. This approach achieved a 21-fold increase in harvested power compared to a reference system with an identical angular coverage.
“The fact is 5G is going to be everywhere, especially in urban areas. You can replace millions, or tens of millions, of batteries of wireless sensors, especially for smart city and smart agricultural applications,” said Emmanouil (Manos) Tentzeris, Ken Byers Professor in Flexible Electronics in the School of Electrical and Computer Engineering.
“I've been working on energy harvesting conventionally for at least six years, and for most of this time it didn't seem like there was a key to make energy harvesting work in the real world, because of FCC limits on power emission and focalization,” said Hester. “With the advent of 5G networks, this could actually work and we’ve demonstrated it. That’s extremely exciting — we could get rid of batteries.”
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