RF-Mediator: Tuning Medium Interfaces with Flexible Metasurfaces

Emerging wireless IoT applications increasingly venture beyond over-the-air communication, such as deep-tissue networking for medical sensors, air-water communication for oceanography, and soil sensing for agriculture. These applications face the fundamental challenge of significant reflection and power loss at medium interfaces. We present RF-Mediator, a programmable metasurface placed near a medium interface to mask the presence of a physical boundary. Our hardware design comprises a single layer of varactor-based surface elements with specific metallic patterns and wiring. With the biasing voltage tuned element-wise, the surface dynamically mediates between the adjacent media to minimize unwanted reflection and boost transmission through the medium interface. A multi-stage control algorithm efficiently determines the surface configuration to handle all dynamic adaptation needs for medium impedance matching and beamforming jointly. We implement a lightweight and flexible metasurface prototype and experiment with diverse cross-medium setups. Extensive evaluation shows that RF-Mediator provides a median power gain of 8 dB for air-tissue links and up to 30 dB for cross-medium backscatter links.