Experimental ultrasonic communications through tissues at Mbps data rates

Abstract

Remote monitoring of patients using wireless capabilities increasingly plays an important role in healthcare and the communication with implanted medical devices (IMDs). In the case of IMDs, transmission is characterized by low peak power and low duty cycle to reduce the potential for adverse bio-effects. Currently, most IMDs use radio-frequency (RF) electromagnetic waves to communicate through the body. The corresponding maximum bandwidth allowed is 300 kHz as regulated by the FCC, which inherently limits the communication rates of these devices (limiting to maximum data rates of 50 kb/s). Beyond bandwidth restrictions, the main limitation for using RF electromagnetic waves in the body is loss of signal because of attenuation in the body. For these reasons the output power of RF devices is limited to 25 µW, which also limits data rates to 50 kb/s. In this study, an ultrasonic communication channel is explored for IMD communications as an alternative to EM transmission. We demonstrate the ability to communicate through tissues using the ultrasonic communications channel while maintaining data rates necessary for streaming of high definition video. In our experiments we used a matched pair of 5-MHz f/3 single-element transducers operating in a pitch-catch configuration in a tank of degassed water. Between the transducers was placed either a slab of pork tenderloin or a slab of beef liver (each more than 2 cm thick). Quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) signals were coded for transmission into the transmitting transducer using an arbitrary waveform generator. Signals were transmitted and captured by sending 10 snapshots (packets) of 50,000 samples (10,000 training / 40,000 decision-directed) using a fractionally-spaced (2 samples per symbol) decision feedback equalizer with up to 40 taps in the feed forward section, and 40 taps in the feedback section. Signals were successfully decoded through the samples using QPSK, 16QAM and 64QAM codes with bit error rates of (BER) < 1e-4. A maximum data rate of 30 Mbps using the 64QAM was achieved through both the pork tenderloin and the beef livers. These experiments successfully demonstrate that communications through the ultrasonic channel can provide data rates capable of streaming high definition video.