Traditionally, active implants are powered by batteries, which, when discharged, have to be either replaced or recharged by an inductive power link. In the recent years, ultrasonic power links are also being investigated, as they promise more available power for deeply implanted miniaturized devices. Such devices often need to transfer information back, for example sensor data or some status information of the implant. For ultrasonically powered implants, this data transfer is usually achieved

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... ith On- Off Keying (OOK) based on load or backscatter modulation, or active driving of a secondary transducer. In this paper, we propose to superimpose OOK with subcarriers, effectively leveraging Frequency-Shift Keying, which increases the robustness of the link against interference from other nearby scatterers and fading, due to patient movements, for example. This approach also allows for simultaneous powering and communication, and inherently provides the possibility of frequency domain multiplexing to address individual nodes in implant networks. The proposed modulation scheme can be implemented in miniaturized application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and microcontrollers, with little additional complexity. We have validated this communication scheme in a water tank setup during continuous ultrasound powering. In these experiments we achieved symbol rates of up to 100 kBd (limited in this implementation by the selected microcontroller), were able to detect the modulated signal when placing the implant transducer about 4 cm away from the focal axis and showed that this modulation scheme is more robust than OOK. This approach could provide a more stable uplink communication, particularly for miniaturized implanted devices that are located deep inside the body and need continuous ultrasonic powering.