Similar to diffusing reverberant acoustic wave spectroscopy (DRAWS), the techniques relies on the room having walls that reflect radio waves – reflecting sufficiently to fill the room with a complex field of interference patterns.
An object introduced or moved within the room changes the complex interaction of reflections within the volume, even if they are, for example, around a corner in the room and not in direct sight of any antennas involved.
Earlier this year the team showed that they could detect movements like a fan blade turning or a person breathing – work that is described in ‘Dynamic metasurface aperture as smart around-the-corner motion detector‘ in Nature Scientific Reports.
“We do not only managed to detect cyclic motion, but any sort of motion, and that moreover we could then additionally infer details such as whether it was cyclic or not,” researcher Philipp del Hougne told Electronics Weekly. “This allows one to distinguish aperiodic motion like a human moving through a room from periodic one like a rotating fan or breathing human.”
Since that earlier work, training has been added – the demonstration system was taught the pattern of radio waves scattered by a triangular block placed in 23 different positions on a floor (photo right).
“That calibration is enough not only to distinguish between the learned 23 scenarios, but to also distinguish the positions of three identical blocks placed in any one of 1,771 possible configurations,” said Duke University.
“While DRAWS links the wave field’s autocorrelation to the quantity and scattering cross section of the scatterers, here we borrow the assumption that under the stated conditions, the interplay of multiple scattering off the objects and multiple reflections off the cavity walls can be unravelled,” said the team, from Duke University and Institut Langevin, in the paper ‘Precise localization of multiple noncooperative objects in a disordered cavity by wave front shaping‘, published in Physical Review Letters (complete paper freely available).
Complex maths are involved – specifically the Green’s function between two antennas arbitrarily placed inside the cavity or room – the ‘Precise localization’ paper has details.
The technique requires changing fields.
“It doesn’t even matter what those particular wave shapes are,” said Duke engineer Professor David Smith. “As long as they’re diverse, the detector will pick up enough different patterns to determine if something is there and where it is.”
Options for creating changing wavefronts include a single frequency signal time-switched between multiple antennas around the room, or multiple frequencies bouncing between a pair of antennas.
Instead, in the research described in the ‘Precise localization…’ paper, there were two simple antennas, one transmit and one receive, plus a digitally-adjustable meta-material surface on the back wall of the room (photo right).
Digitally-controlled PIN diodes on the metasurface control the phase of the wave reflected off each of its elements element, turning what would be a static field into a dynamic field.
“There are other technologies that could achieve similar wave front shaping capabilities, but they are much more expensive both in cost and energy usage,” said Dule researcher Dr Mohammadreza Imani.
Another way to use such a surface to create a varying field is to bounce transmit energy off it before the energy radiates into the room – something like a phased array, but fed with an external wavefront and reflecting a complex variable field instead of beams.
This scheme is used in the ‘Dynamic metasurface…’ paper, and in this case the metasurface (pink, pictured at top) is tuned using varactor diodes.
According to del Hougne, applications for the technology could include security (intruder detection and tracking), smart homes (heating and lighting control), health care (monitoring the elderly or infants) and gaming.
Greenerwave is a start-up intended to exploit the technology.