  This paper proposes an ultrasonic backscatter communication (UsBC) system for
passive implantable medical devices (IMDs) that can operate without batteries,
enabling versatile revolutionary applications for future healthcare. The
proposed UsBC system consists of a reader and a tag. The reader sends
interrogation pulses to the tag. The tag backscatters the pulses based on the
piezoelectric effect of a piezo transducer. We present several basic modulation
schemes for UsBC by impedance matching of the piezo transducer. To mitigate the
interference of other scatters in the human body, the tag transmits information
bits by codeword mapping, and the reader performs codeword matching before
energy detection in the reader. We further derive the theoretical bit-error
rate (BER) expression. Monte Carlo simulations verify the theoretical analysis
and show that passive UsBC can achieve low BER and low complexity, which is
desirable for size- and energy-constrained IMDs.
