The fundamental question answered by this research program is what is the minimum possible energy that a communication node must expend to transfer information wirelessly? The more you understand about conventional wireless communications, the more the answer will surprise you.
Load modulation of antennas, for example, is one of the lowest-powered methods for transferring information between points in space, since an RF reader supplies the power for communications. In this scheme, an RF tag backscatters radio signals back towards the RF reader. Much of the leading (and bleeding) edge physical layer innovations in backscatter radio have come from Georgia Tech Propagation Group in the last 15 years.
This research program started with design and characterization of object-resistant antennas, when Joshua Griffin developed his radio assay for characterizing RFID antenna degradations when tags were placed on various materials. This has led to all sorts of interesting antenna designs and measurements: characterizing antennas that approach metal ground planes, designing antennas that resist the corona shielding of high-voltage power lines, and antennas that can be placed onto building facades to track people moving inside.
|Photographs of a flexible, silver-deposited antenna (left) and a testing apparatus for measuring the object-degradations of impedance and pattern on an RFID antenna.|
The group has produced numerous papers on how to optimize the coding scheme for backscattered signals, from unorthodox long-ranged coding schemes, to high-throughput techniques that surpass today’s state-of-the-art RFID protocols. One interesting side branch of research is the use of perfect pulses for scattering signals on ambient radio signals. Perfect pulses — invented by Propagation Group researchers — are binary waveforms that maximally evacuate DC spectral content, producing the deepest possible spectral null when constructing an information signal.
|Reflection of Modulated Radio (ReMoRa) scatters ambient RF signals with additional information using low-powered, load-modulated antennas (above). Perfect pulses can be used to construct ReMoRa waveforms that optimally evacuate spectrum about the center of transmitted signals (below).|
Some of the most revolutionary work has been performed in the area of retrodirective structures for backscatter modulation. Starting with Albert Lu and Greg Koo’s work on retrodirective-phase modulation, our group has invented numerous low-power structures that retrodirectively reflect radio waves while adding information to them. This work promises to enable low-powered wireless sensors and a new-generation of RFID applications.
|A retrodirective RF tag reflects and modulates incident radio waves back in the direction of transmission, much like a corner-reflector in optics. The rat-race structure on the right can provide retrodirective behavior under certain load modulation conditions.|
One of the most exciting areas of research is the use of quantum tunnel reflectors (QTRs) — low-power, active diodes that amplify and reflect RF power incident upon an antenna. In 2016, Francesco Amato demonstrated a QTR at 5.8 GHz that was able to transmit a detectable waveform 1.2 kilometers across the city of Atlanta, consuming only 23 microWatts in the process.
|Map of midtown Atlanta with several QTR links tested using a 5.8 GHz low-powered backscatter link.|
Active areas of backscatter radio research in the group include:
- retro-directive structures for microwave and mm-wave backscatter
- optimal RF scattering and RF harvesting antenna structures
- channel coding for long-distance/high throughput backscatter
- ambient signal scavenging
- quantum tunnel reflectors (QTRs)
- backscatter receiver design
- object-resistant and environment-resistant antenna design