An intra-body ultrasonic signal can be converted into a first electrical signal, a local oscillator signal can be generated in an implantable system. The first electrical signal and the local oscillator signal can be mixed in an implantable system, such as to generate a demodulated signal, processed, such as using a filter. The filtered, demodulated signal can be further processed, such as implantably determining a peak amplitude of the first portion of the demodulated signal received from the filter over a time interval, implantably generating a dynamic tracking threshold that starts at an amplitude proportional the first portion of the demodulated signal and exponentially decays over a time interval, and determining a noise floor in the absence of a received intra-body ultrasonic signal and implantably comparing the peak amplitude and the tracking threshold and generate the digital output based on the difference.