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Abstract-Biomedical systems of implanted miniaturized sensors and actuators interconnected into an intra-body area network could enable revolutionary healthcare and clinical applications. Given the well-understood limitations of radio frequency (RF) propagation in the human body, in our previous work we investigated the use of ultrasonic waves as an alternative physical carrier of information [1], and proposed Ultrasonic WideBand (UsWB), an ultrasonic multipath-resilient integrated physical and medium access control (MAC) layer protocol [2]. In this paper, we discuss the design and implementation of a software-defined testbed architecture for ultrasonic intra-body area networks, and propose the first experimental demonstration of the feasibility of ultrasonic communications in biological tissues. We first discuss in detail our FPGA-based prototype implementation of UsWB. We then demonstrate how the prototype can flexibly trade performance off for power consumption, and achieve, for bit error rates (BER) no higher than 10 −6 , either (i) high-data rate transmissions up to 700 kbit/s at a transmit power of -14 dBm (≈ 40 µW), or (ii) low-data rate and lower-power transmissions down to -21dBm (≈ 8µW) at 70kbit/s. Finally, we show how the UsWB MAC protocol allows multiple transmitter-receiver pairs to coexist and dynamically adapt the transmission rate according to channel and interference conditions to maximize throughput while satisfying predefined reliability constraints.

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This article describes the design of a custom software-defined modem with adaptive physical layer for underwater acoustic (UWA) communications. The modem consists of a commercial software-defined radio (SDR) interfaced with a wideband acoustic transducer through amplifying circuitry. With this custom-built platform, we focus on the unique physical layer challenges of the underwater acoustic channel to demonstrate the benefits of real-time adaptation in such rapidly varying environments. We first focus on an Orthogonal-Frequency-Division-Multiplexing (OFDM) transmission scheme. In particular, for the forward link, we consider and implement a high-data rate Zero-Padded OFDM (ZP-OFDM) physical layer with a superimposed convolutional error-correction coding scheme. ZP-OFDM offers high reconfigurability in terms of number of OFDM subcarriers, modulation type (e.g., BPSK, QPSK), and error-correction coding rate. Real-time adaptation at the transmitter is achieved through a robust feedback link based on a binary chirp spread-spectrum modulation (B-CSS). We demonstrate that joint real-time adaptation of system parameters such as modulation constellation and channel coding rate leads to significant data rate increase under preset bit-error-rate (BER) constraints. Moreover, in the same context, we present for the first time a seamless switch of our SDR transmitter between different signaling technologies such as OFDM and direct-sequence spread-spectrum (DS-SS).

Abstract-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 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.