PMUT-BASED HIGH DATA RATE ULTRASONIC WIRELESS COMMUNICATION LINK FOR INTRA-BODY NETWORKS

This paper reports on the first demonstration of a high data rate (0.6 Mbit/s) ultrasonic wireless communication link implemented through Aluminum Nitride (AlN) Piezoelectric Micro Machined Ultrasonic Transducers (PMUTs). Real-time video streaming is demonstrated through a phantom mimicking human tissue, thus proving the feasibility of PMUT-based implantable Body Area Networks (BANs). Two 20 × 20 PMUT arrays were used as transceiver elements and an Orthogonal-Frequency-Division-Multiplexing (OFDM) modulation scheme was implemented resulting in a wideband digital transmission link with a data rate of up to 0.6 Mbit/s at a 5 cm distance between transmitter and receiver. Channel estimation and Bit-Error-Rate (BER) versus Signal-to-Noise-Ratio (SNR) curves were obtained for both the PMUT link and an implementation based on custom-made and miniaturized ultrasonic bulk Lead Zirconate Titanate (PZT) transducers for comparison. The PMUT implementation, characterized by a wider bandwidth than the PZT transducers, showed two orders of magnitude lower BER for the same SNR as compared to its PZT counterpart, while occupying an approximately 100 times smaller volume (∼3mm×3mm×0.5mm PMUT array vs. 9.5mm diameter, 6mm thick PZT transducer). INTRODUCTION The use of PMUTs in applications such as ultrasound imaging [1], fingerprint scanning [2] and three-dimensional gesture recognition [3] has been widely investigated. Furthermore, several studies focusing on the optimization of specific device performance metrics (such as output pressure or bandwidth) have been reported [4]. However, the application of the PMUT technology for communication purposes remains largely unexplored. As an example, a high data rate intra-body communication network has potential applications in high resolution wireless endoscopy [5], imaging or biosignal telemetry, continuous ambulatory monitoring of chronic patients, and early disease detection. The great potential of ultrasonic wireless communication for intra-body networks has been recently demonstrated [6]. Even though a great degree of miniaturization and power consumption optimization was achieved, the implementation is based on Lead Zirconate Titanate (PZT) ultrasonic transducers that remain relatively bulky and are a not fully biocompatible. In fact, the presence of lead in the composition can be toxic for human tissues, particularly in long-term use. In this article, an ultrasonic transceiver based on miniaturized arrays of AlN PMUTs is demonstrated for the first time, attempting to address these fundamental challenges currently hindering the full deployment of implantable ultrasonic wireless communication devices. AlN is a much more inert material for the intra-body chemistry, which would allow for implants immune to degradation and toxic effects in the long term operation required. Another dramatically enabling feature is that, as the PMUT transducers are built on silicon technology, a single chip implementation of the transducer array and Application-Specific Integrated Circuits (ASICs) can be envisioned, allowing for drastic miniaturization and power consumption reduction. Therefore, the achievement of a PMUT-based intra-body communication link perfectly fits the development of an implantable, integrated system offering communication, energy harvesting and sensing capabilities [6]. The use of arrays of PMUTs in an ultrasonic transceiver is also attractive because it enables the implementation of functionalities such as focusing and beamforming that are not attainable with the conventional single-element transducers. Figure 1: System-wide overview of the application of the PMUT array within an ultrasonic intra-body transceiver.