Ultrasonic data transmission through a rebar-reinforced concrete acoustic channel

Abstract

For cases in which digital data must be transferred through a solid object while maintaining its structural integrity, commonplace methods using radio waves or wires running through the object may be impractical. Materials like metals and concrete may block radio waves to an extent that achievable data rates are insufficient, and drilling into an object to pass wires through may result in leakage of fluids or compromised integrity of the material. In these cases, ultrasonic signals may be used to transfer data, requiring only that transducers be attached to the surface of the object. This document presents the results of using an orthogonal frequency-division multiplexed (OFDM) system using bit-loaded quadrature amplitude modulated (QAM) subcarriers to send data through a 70 cm thick rebar-reinforced concrete pillar using ultrasound. Data rates nearing 90 kbits/s are achieved with a bit error rate below 10⁻⁵, with approximately 2 mW of transmit power. Transient variability of the acoustic channel through the pillar is found to limit the bit error rate for a single bit-loading configuration, requiring the channel to be measured frequently or the bit-loading configuration to be adapted to the changing channel.