  In this paper, we present BatNet, a data transmission mechanism using
ultrasound signals over the built-in speakers and microphones of smartphones.
Using phase shift keying with an 8-point constellation and frequencies between
20--24kHz, it can transmit data at over 600bit/s up to 6m. The target
application is a censorship-resistant mesh network. We also evaluated it for
Covid contact tracing but concluded that in this application ultrasonic
communications do not appear to offer enough advantage over Bluetooth Low
Energy to be worth further development.
