Water-To-Air backscatter communications
Over 70% of the Earth’s surface is covered by water, while the majority of the underwater areas are unexplored yet. Owing to growing underwater activities, e.g., undersea oil & gas exploration, and Internet of underwater things, reliable water-to-air wireless communications are urgently needed by the industry, military, and scientific communities. As it is difficult to use a single type of signal, e.g., acoustic, radio frequency (RF), or optical, for communication across a water-air interface, most existing water-to-air communications rely on sonobuoys to relay acoustic signals from underwater transmitters to RF receivers on/above the water surface. The global sonobuoy market is predicted to reach $595.9 million at a 6.62% CAGR over the period 2018-2023. Since sonobuoys might not be recovered from remote, hostile areas of the ocean, battery-powered sonobuoys (which have battery packs full of toxic metals) are posing a great threat to the marine environment. Although energy-harvesting technologies (e.g., photovoltaic, wave action, or seawater based semi-fuel cell) have been developed for sonobuoys to charge onboard energy storage systems, the challenge of miniaturizing and incorporating these technologies into the sonobuoy volume constraint remains a dominant issue. 
This project will develop novel water-to-air backscatter communication (WABCom) technology to enable the design of lightweight, battery-free sonobuoys by leveraging RF backscatter communications, where the RF energy harvested by the sonobuoy from an unmanned aerial vehicle’s (UAV) transmission is used to support the circuit operation at the sonobuoy, and the sonobuoy forwards the received underwater signal to the UAV via an RF backscatter link (i.e., by modulating the signal on the RF waveform reflected back to the UAV). Such miniature battery-free sonobuoys can be flexibly deployed (e.g., air-dropped by UAVs) where and when needed and are disposable with minimum impact on the marine environment.