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 10 -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.Advances in miniaturized, wireless, and deeply implantable medical devices (IMDs) can enable coordinated closed-loop diagnostics and treatments for applications like neuromodulation and drug delivery. 1 As their capabilities become more complex and numerous, robust and low-power communication becomes increasingly important. Research into wireless networks in the body have largely focused on radio frequency (RF) communications. 2 However, a major challenge for the propagation of electromagnetic (EM) waves in the body is power absorption in tissue. The absorbed power is dissipated as heat leading to both health concerns and significant path loss. 3 More recently, there has been increased interest in using ultrasonic waves for intra-body communication and power transfer. 2,4 At low MHz frequencies, reduced scattering due to relative homogeneity in tissue density and compressibility as well as low attenuation of about 1 dB·cm -1 ·MHz -1 in soft tissue allow ultrasonic waves to safely propagate much farther in tissue than EM waves. 1 The orders of magnitude slower propagation velocity of acoustic waves in the body (~1500 m/s) also results in millimeter wavelengths around a MHz, allowing for simpler circuits, beamforming capabilities, and smaller transducers. 1 On the other hand, the lower operating frequency and fundamentally smaller bandwidth drastically limit the achievable data rate and available modulation schemes. Required data rates can vary considerably depending on the application. For example, glucose monitoring may need kbps speeds while imaging/video may require Mbps speeds. 2 Recent studies have proposed and demonstrated different protocols with varying data rates for intra-body ultrasou...

Waves, modes, communications, and optics: a tutorial

“…In communications, we continually want larger amounts of useful bandwidth. This need is strong for wireless radio-frequency transmission [13], for optical signals in fibers [14 -17] or free space [17 -20], and even for acoustic information transmission [21][22][23]. Recent progress in novel optical ways to separate different [16] and even arbitrary modes [24 -29], including automatic methods [24 -29], gives additional motivation to consider the use of different modes (or "spatial degrees of freedom") in communications.…”

“…We presume that a bounded linear operator 21 A maps from vectors in space 1 H to vectors in space 2 H -for example, mapping an vector η in 1 H to some vector σ in 2 H…”

“…(Note we use the same notation for these two technically different things -the operator and its matrix representation -but the context will resolve the confusion: if we are using Dirac notation, then we are using the matrix representation of the operator.) Then the sum 21 , j k j k t a r * ∑ can be interpreted as the vector-matrix-vector product 21 ,…”

“…For the moment, we will be explicit about what spaces the vector are in by using "1" and "2" subscripts on the vectors. Specifically, we write a bilinear expansion [104] 21…”

“…Note that a linear operator like 21 A from one Hilbert space to another can be written in such an "outer-product" form as in Eq. (110) on any desired basis sets for each Hilbert space.…”