Title: Exploiting Spatial Degrees of Freedom for High Data Rate Ultrasound Communication with Implantable Devices,
Abstract: We propose and demonstrate an ultrasonic communication link using spatial
degrees of freedom to increase data rates for deeply implantable medical
devices. Low attenuation and millimeter wavelengths make ultrasound an ideal
communication medium for miniaturized low-power implants. While small spectral
bandwidth has drastically limited achievable data rates in conventional
ultrasonic implants, large spatial bandwidth can be exploited by using multiple
transducers in a multiple-input/multiple-output system to provide spatial
multiplexing gain without additional power, larger bandwidth, or complicated
packaging. We experimentally verify the communication link in mineral oil with
a transmitter and receiver 5 cm apart, each housing two custom-designed
mm-sized piezoelectric transducers operating at the same frequency. Two streams
of data modulated with quadrature phase-shift keying at 125 kbps are
simultaneously transmitted and received on both channels, effectively doubling
the data rate to 250 kbps with a measured bit error rate below 1e-4. We also
evaluate the performance and robustness of the channel separation network by
testing the communication link after introducing position offsets. These
results demonstrate the potential of spatial multiplexing to enable more
complex implant applications requiring higher data rates.