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EETE DEC 2014

Smart wall re-focuses scattered RF signal By Julien happich Ever experienced bad radio communication, struggling to find the right spot at home, making the most of a faint signal to place a phone call? Through the use of a passive array of tunable microwave mirrors, researchers from Langevin Institute found a way to focus a scattered and seemingly chaotic RF signal towards the receiver (for example your cell phone). In a paper titled “Shaping complex microwave fields in reverberating media with binary tunable metasurfaces”, professors Mathias Fink and Geoffroy Lerosey from the Ecole Supérieure de Physique et de Chimie Industrielles (ESPCI) at ParisTech disclose an experiment that boosts signal reception ten folds. The researchers rely on what they describe as electronically tunable metasurfaces, in effect, spatial microwave modulators implemented as rectangular copper reflectors etched from a PCB substrate, each combined with a parasitic strip controlled through a pin diode – see figure 1. The parasitic strips are nearfield coupled to the reflectors, forming a hybridized resonator that can perform binary phase modulation (reflecting the waves either positively or negatively) depending on whether the pin-diode Fig. 1: the resonant unit cell (middle) and its two operating states (left-right). Source Scientific Reports) . allows or not full strip resonance (when biased with a voltage, 0V or 5V). For their experiment, they used an array of 102 such controllable electromagnetic reflectors, each measuring 31x45mm and spaced by 6cm, half a wavelength at the working frequency f0 of 2.47GHz for commercial WiFi. Each element of the reconfigurable metasurface elements was individually controlled through one of two single-board microcontrollers (Arduino Mega 2560) each with 54 digital output ports. In total, the whole array used in the experiment covered an area of 0.4m2 and was sufficient to boost signal in a scattering furnished office room of roughly 3×3×4m3. “The experiment was a proof-ofconcept and now we are looking at spinning out a startup to bring commercial products to market”, told us Lerosey in a phone interview. “The beauty of this metasurface is that it is completely passive in that sense that it doesn’t generate any additional microwaves. It just reflects and re-shapes the existing reverberated waves in a room to maximize the From right to left, Mathias Fink, Geoffroy Lerosey and collaborators inspecting the metasurface panels crafted from etched PCB. (copyright Benjamin Boccas). transmission between a remote emitter and a receiver antenna”. “Smart materials are a hot topic nowadays, with improvements on thermal or acoustic performance, or even designed for energy-harvesting such as photovoltaic windows. These tunable metasurfaces could create a new category of electromagnetically smart materials” continued Lerosey. In practical applications, the metasurface could be applied to office furniture, construction panels or even carpets or ceiling tiles, either to increase communication performance on a same power budget, reducing the number of radio cells, or to decrease the overall radiated RF power and the users’ exposure to electromagnetic fields. It could also find use in planes or trains, with smart walls able to tune themselves to refocus the RF signal to multiple users. “People are increasingly concerned about electromagnetic pollution and our solution goes in the right direction”, added Lerosey who declined to comment on a tentative name for the startup. “The resonators can be sized to match any target frequency, and in fact, with WiFi moving to 5GHz soon, we’ll be able to use smaller reflectors and pack more of them on the same surface for increased focus and gain”. The CNRS (Centre national de la recherche scientifique) has secured various international patents and both researchers are confident that their findings will attract serious investors. “We are currently talking to a number of players, including construction material providers and RF infrastructure builders, but our solution could easily be deployed by the consumers themselves, so we are open to all partnerships” explained Lerosey who will play a consultant role together with Fink while remaining in academia. Although the lab experiment used a sturdy hand-crafted prototype based on a commercial PCB substrate, printed electronics would be the best candidate to make large conformable metasurfaces, acknowledged Lerosey. So far tuning optimization was carried out manually, by switching “on” and “off” the individual reflectors serially, checking the intensity feedback at the receiving antenna to maximize RF reception. But the researchers are working on light algorithms that could automate the procedure in real-time for a moving receiver. By increasing the number of reflectors, larger “RF focus zones” could be created too. The reverse manipulation, tuning the reflectors to minimize or cancel out as much electromagnetic field in a given volume, could be used to create RF-free zones. This could find applications either in lab experiments, or in consumers’ homes (near your pillow) to reduce electromagnetic pollution locally. 24 Electronic Engineering Times Europe December 2014 www.electronics-eetimes.com


EETE DEC 2014
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