Higher-order modulation for acoustic backscatter communication in metals

Backscatter communication enables miniature, batteryless, and low-cost wireless sensors. Since electromagnetic waves are strongly attenuated in several scenarios, backscatter communication in metals via acoustic waves can leverage various applications, e. g., in structural health monitoring. When backscattering, the Tag has little control over the modulation it performs on the carrier wave. Therefore, existing approaches commonly employ differential binary modulation schemes, limiting the achievable data rates. To overcome this limitation, we derive a channel model that accurately describes the modulation in an acoustic backscatter channel---as, e. g., found in steel beams---and leverage it to achieve higher-order load modulation. We present an open-source Reader and Tag pair prototype based on COTS components that we have developed for communication and on-the-fly channel characterization. We explore the influence of various parameters on communication performance on different channels. Moreover, (i) we are the first to demonstrate that acoustic backscatter is feasible in guided-wave channels, covering up to 3 meters, and that (ii) our modulation scheme achieves up to 211% higher data rates than binary modulation schemes, and (iii) provides reliable communication through channel coding.