Cross-Medium Communication Via Passive Relay

Subsea Internet of Things (IoT) networks have rapidly developed but still suffer the inefficient underwater-toair communication. Due to wireless signals exhibiting different properties in different media, it is difficult to use any single modality of signal for cross-medium communication. In this paper, we propose Exocoetus, a novel passive relay based water-to-air communication system. By taking advantage of the out-of-specification characteristics of the RF switch, Exocoetus can trigger acoustic-to-RF conversion even if the input voltage is below the standard threshold and use a clamp circuit to further maximize the efficiency of the acoustic-to-RF signal conversion. We design a dual-capacitor circuit-based pulse position modulation method to amplify the acoustic signals emitted by powerconstrained underwater nodes, ensuring reliable communication over greater distances. We implement a prototype of Exocoetus and evaluate its performance in the real environment. The results show that Exocoetus can achieve a communication distance of 6 meters above water and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1.25}$</tex> meters underwater.