# Acoustic Backscatter Communication and Power Transfer for Batterless Wireless Sensors
## 1 Introduction  
Recently, the Internet of Things (IoT) has made tremendous progress in creating a smarter and more efficient infrastructure, driven by the emergence of sensor nodes that are small, cheap, and easy to deploy [1]. These sensor networks rely on low-power communication techniques, such as LoRa, enabling runtimes of several years on a battery or even batteryless sensors driven by energy harvesting [2; 3]. However, conventional wireless communication is inapplicable in many scenarios due to electromagnetic shielding. Applications include structural health monitoring, where sensor nodes are embedded within a structure [4; 5; 6], underwater communication [7; 8], fallback systems in nuclear power plants [9], and generally sensors within enclosed metal containers, e.g., pipelines, ship hulls, or cargo containers [10]. In contrast to electromagnetic waves, acoustic waves experience little attenuation in these scenarios and allow efficient transmission over longer distances. Powering these sensors is an additional challenge, as it is often desirable to avoid cable-bound solutions because of high deployment costs in large structures or compromised structural integrity, e.g., in pressurized containers or underwater applications. At the same time, regular battery replacements must be avoided as well because sensors may be in hard-to-reach places. Therefore, several research groups have investigated the transfer of energy and data using acoustic waves through metal, concrete, and water. We sketch a through-metal sensor network for a potential application, pipeline monitoring, in Figure 1.  
In through-metal acoustic communication and power transfer, we distinguish between two types of channels: the Sandwich-Plated Channel (SWP), where the two transducers are placed coaxially on opposing sides of a metal boundary, and the Guided-Wave Channel (GWC), where the transducers are placed at different locations on a plate-like metal structure [11]. In the former case, distances are usually only a few centimeters, and bulk waves with relatively high bandwidths may be used. The SWP has been modeled analytically and numerically, and models are well suited to predict power transfer efficiency [12]. However, in the case of GWC, the transducers may be several meters apart. Guided waves, also called lamb waves, travel with little attenuation over long distances through these structures. Lamb waves are best excited at lower frequencies, significantly reducing the channel's bandwidth. Several research groups have investigated power transfer, communication, or combined power transfer and communication using lamb waves. However, all of them employed active communication schemes. In many applications, passive backscatter communication is advantageous because active communication requires oscillators, amplifiers, and analog circuitry for signal generation, consuming significant amounts of energy and increasing the size and cost of the sensor node. Consequently, communication modules often require orders of magnitude more power than the sensors employed on the nodes [13]. Therefore, backscatter communication has the potential to reduce overall power consumption significantly. In other domains, acoustic backscatter communication has been investigated in combination with wireless power transfer, i.e., for underwater [7; 8] and through-concrete applications [4]. In our previous work, we have shown that such approaches can also be transferred to lamb-wave-based through-metal communication [11; 14]. However, to our knowledge, no fully energy-autonomous implementation has been demonstrated that combines acoustic power transfer and passive backscatter communication through metals. Furthermore, most of the existing studies in the field use expensive and bulky high-power lab equipment, i.e., signal generators and oscilloscopes, to establish a proof of concept.