Résumé: © 2019 IEEE. Ambient backscatter communications have emerged as a promising technology for a sustainable development of the Internet of Things (IoT). In such system, a radio frequency (RF) tag can transmit data to a receiver without battery and without generating any new RF wave, just by backscattering the incident RF wave originated by an ambient RF source. The simplest tag is a dipole that is either in an absorbing mode or in reflecting mode to send "0" or "1", and thus leads to a modulation order of 2. Previous solutions to reach higher modulation order, so as to achieve higher data rate, are based on antenna arrays. However, such solutions are not suitable for connected objects, due to their sizes. In this paper, for the first time, we propose a new tag that uses a compact antenna with reconfigurable radiation patterns to provide a high modulation order. We present experimental bit error rate measurements obtained with an experimental test-bed and two tag antennas, both providing 4 states and thus being able to convey 2 bits per symbol. These measurements show that the performance improves with increasing number of receive antennas, and lead to the conclusion that a tag antenna with low cross-correlations between reconfigurable radiation patterns is suitable for backscattering applications in IoT.

Résumé: © 2019 IEEE. Locally resonant metamaterial crystals are artificial materials built from small spatially local resonant inclusions periodically arranged at subwavelength scale. Unlike conventional continuous metamaterials, for which spatial dispersion originates mostly (but not exclusively) from the nonlocality of their inclusions, they exhibit large spatially nonlocal effects that emerge solely at the array level because of the periodic structuration of simple spatially local scatterers, often allowing for an intrinsically subwavelength granularity. Herein, we demonstrate the unique relevance of metamaterial crystals to induce nonreciprocal electromagnetic propagation at deep subwavelength scales. This is obtained by combining the breaking of time-reversal symmetry, using an externally biased magnetic material, with appropriate spatial-dispersion engineering, via subwavelength structural modification of the metamaterial crystal. Interestingly, the material unit cell can be scaled down without affecting this functionality, leading to the exciting possibility of largely enhanced wave-matter interaction at deep subwavelength scales. Altogether, our proposal provides an interesting route for transposing the rich physics of nonreciprocal systems down to the subwavelength scale.