# Acoustic Backscatter Communication and Power Transfer for Batterless Wireless Sensors
## 2 State of the Art  
### Ultrasonic Energy Transfer  
Multiple studies have shown the transmission of power through acoustic waves in GWCs. We summarize the results in Table 1. Unfortunately, these studies cannot be directly compared against each other because they used vastly different setups, i.e., transducers, frequencies, impedance matching circuits, and structures, yielding output powers ranging from less than 100 uW to 470 mW. The following paragraphs explain the variables in which existing studies vary.  
Piezoelectric transducers come in many different shapes. The most commonly used are disk-shaped and generate a circular wave, propagating in all directions uniformly. In contrast, rectangular transducers may generate directed waves, increasing efficiency at the cost of generality [15]. Some research groups have used transducer arrays to enable beam formation and thus dynamically concentrate the emitted energy in the direction of a specific receiver [16].  
Impedance matching at the transmitter and receiver is critical to achieving high efficiency, e.g., [PERSON] [17] reports a 6.75-times increase in output power when using an ideal complex impedance matching circuit at the receiver's transducer. Unfortunately, in GWCs, the input and output impedances of the transducers are coupled to the structure's mechanical impedance and vary strongly--even switching between inductive and capacitive behavior--when the carrier frequency changes by only a few hundred hertz. As a result, the optimal matching impedance is not known prior to transducer deployment. Therefore, optimal complex impedance matching requires the design of a dedicated impedance matching circuit after deployment, complicating the setup of such systems, especially in hard-to-access places. Alternatively, the receiver's Power Management Unit (PMU) can perform Maximum Power Point Tracking (MPPT), where the circuit automatically matches the resistance of the transducer but not the reactance. Furthermore, in some studies, the carrier frequency is automatically tuned to maximize received power, thereby avoiding strong impedance mismatches and improving transmitted power by more than 15 dB over a fixed carrier frequency [5].  
When reporting transmission efficiency, it is essential to mention the scope of the measurement. While some studies report the DC output power the harvester provides after rectification and voltage regulation, other studies measure the AC power available at the receiving transducer. Rectification and voltage regulation can be very inefficient and strongly influence the overall Wireless Power Transfer (WPT) system's efficiency. Another difference considers the presence of a storage element. Although the harvester's output can be used directly to power the sensor node, transferring the harvested energy into a battery or supercapacitor for later use is also possible. Using a storage element may enable harvesting from sources that provide very little power. The harvester can collect energy and only activate the sensor node once sufficient energy has been harvested. Interestingly, all the reported studies only tested short channels of less than 1 m in length. However, ultrasonic lamb waves can propagate long distances with little loss, as their power is attenuated only linearly with increasing distance. Therefore, we expect much longer distances than what are currently reported.