Wideband communication with batteryless sensors embedded in metal structures

Structural health monitoring and predictive maintenance promise to significantly reduce costs, increase availability, and improve safety of civil infrastructure and industrial facilities. Wireless and batteryless sensors fuel these applications with the required data. As metal structures often shield sensor nodes from electromagnetic waves, acoustic power and data transfer are promising alternatives. However, the metal channel suffers from severe multipath propagation, limiting data rates to typically less than 200 bits-1. We investigate wideband pulses as alternative to commonly used continuous wave modulation schemes to increase robustness. In a simulation study, we first compare wideband modulation with narrowband modulation schemes and assess their robustness against noise and clock deviations. We then construct a wirelessly powered tag prototype to validate the simulation results in real-world metal channels. Furthermore, we propose a reader-based synchronization scheme to mitigate clock mismatch, which is inevitable with ultra-low-power tags. The results show that wideband pulse-position modulation is the most favorable modulation scheme, increasing the median data rates in many scenarios by 132% over continuous-wave schemes. Additionally, wideband pulses are advantageous for power transfer in the highly frequency-selective channel when optimal carrier frequencies are yet unknown. However, simultaneous power transfer and communication on the same transducer are shown to interfere with each other, complicating the receiver design.