Cross-Medium Communication Combining Acoustic Wave and Millimeter Wave: Theoretical Channel Model and Experiments

Communication across the water–air interface is an open research problem. A common solution is to place a relay on the water–air interface, which transmits received underwater acoustic signals to shore via electromagnetic waves. However, this solution has limitations, such as surface relays easily floating away or underwater relays becoming resource-intensive due to frequent surfacing, which make water–air communications unstable and costly. In this article, we investigate a recently proposed cross-medium communication technique, translational acoustic radio frequency (TARF) communications, in which underwater nodes can directly communicate with airborne nodes via acoustic waves and millimeter waves. For the first time, we introduce a closed-form, end-to-end channel and present its experimental validation. The high-frequency TARF communication is achievable with a frequency of up to 4.4 kHz. In the experiment deployment, underwater acoustic signals traveled as pressure waves, which produced water surface waves when they impinged on the water–air boundary. Airborne radar was employed to measure and decode these water surface waves. Experimental results and theoretical analysis demonstrated that 1) the cross-medium communication channel is frequency selective and time invariant; 2) cross-medium communications with both single-carrier and multicarrier signals are feasible; and 3) high-frequency signals from 3.6 to 4.4 kHz can be detected, which was considered difficult previously. This study provides a theoretical and experimental basis for achieving cross-medium communications between deep-sea moving objects, such as autonomous underwater vehicles and airborne nodes.