  The use of miniaturized biomedical devices implanted in the human body and
wirelessly internetworked is promising a significant leap forward in medical
treatment of many pervasive diseases. Recognizing the well-understood
limitations of traditional radio-frequency wireless communications in
interconnecting devices within the human body, in this paper we propose for the
first time to develop network protocols for implantable devices based on
ultrasonic transmissions. We start off by assessing the feasibility of using
ultrasonic propagation in human body tissues and by deriving an accurate
channel model for ultrasonic intra-body communications. Then, we propose a new
ultrasonic transmission and multiple access technique, which we refer to as
Ultrasonic WideBand (UsWB). UsWB is based on the idea of transmitting
information bits spread over very short pulses following a time-hopping
pattern. The short impulse duration results in limited reflection and
scattering effects, and its low duty cycle reduces the thermal and mechanical
effects, which are detrimental for human health. We then develop a multiple
access technique with distributed control to enable efficient simultaneous
access by interfering devices based on minimal and localized information
exchange and on measurements at the receiver only. Finally, we demonstrate the
performance of UsWB through a multi-scale simulator that models the proposed
communication system at the acoustic wave level, at the physical (bit) level,
and at the network (packet) level.
