# Acoustic Backscatter Communication and Power Transfer for Batterless Wireless Sensors
## 5 Discussion  
The investigation of WPT through an elongated metal bar showed that more than 4 mW AC power could be transferred over a distance of 3 m, supporting a wide range of sensing applications: [PERSON] et al. (2018) report SHM measurements using lamb waves at sub-milliwatt power. Many other sensors require even less power, e.g., low-resolution image recording ([PERSON] et al., 2018) can be achieved at several hundred microwatts, acceleration measurements at tens of microwatts, strain gauge, temperature, and humidity measurements consume even less. Furthermore, a duty cycle can support even higher-power applications requiring extended computations.  
In theory, the signal power of guided lamb waves is attenuated only linearly with distance, enabling much longer distances. Practical measurements on the large structure support the ability to transfer power over large distances, i.e., the maximum observed voltage at the receiver after 5 m was still about 75% of the observed voltage over 1 m. The presented harvester prototype required at least a signal power of 200 uW to fully charge the capacitor, which can potentially be reduced to about 40 uW when cap voltage is reduced to 3 V, as discussed in Section 4.1.2. The results relate well to harvesters presented in RF harvesting, e.g., the harvester presented in ([PERSON] et al., 2018) showed dramatic drops in efficiency when the energy source only provides several tens of uW. The total efficiency of WPT was 8% over 1 m, which exceeds the observations from ([PERSON] et al., 2018) (0.1% at 1 m). However, the efficiency is likely strongly related to the structure, the transducer's geometry, and the bounding method. Therefore, a direct comparison of these results is not informative.  
Tailoring the energy transfer method to a specific structure could greatly increase the overall transfer efficiency of less than 3% over 3 m distance drastically. For example, [PERSON] et al. (2018) used a finite element model simulation of the metal waveguide to find its resonance frequencies and chose transducers with matching resonance and determined their optimal placement position to achieve the highest transfer efficiency. Other groups used rectangular transducers to generate directed waves and increase transmission efficiency ([PERSON] et al., 2018). However, such optimizations are hard to achieve as real structures often have complex geometry and unknown boundary conditions, prohibiting previous determination of optimal locations and frequencies. We, therefore, aimed to investigate a _plug-and-play_ solution that works without prior knowledge about the channel. However, dynamic beamforming using a piezo array may be a promising optimization.  
The presented backscatter tag in this study required less than 300 uW during backscattering, of which only 20 uW were required for modulation, and the MCU required the rest. [PERSON] reports a power consumption of 100 to 1000 uW during active uplink communication, dependent on the data rate. While ASIC implementation is potentially much more energy efficient, we showed that a working system is achievable with commercially available off-the-shelf components. For research prototypes and small series production, this is desirable as ASIC production is very costly and time-consuming. The backscatter communication decoding with DPLLs proved robust against the CFO and STE induced by low-power oscillators. Furthermore, the signal strength over 3 m was more than sufficient due to canceling of the self-interference. The WPT will likely be the limiting factor for longer distances in large structures. However, backscatter communication can potentially be used over much longer distances in battery-powered sensor nodes to reduce the power requirement of communication and increase battery life.