# Acoustic Backscatter Communication and Power Transfer for Batterless Wireless Sensors
## 2 State of the Art
### Through-Metal Communication  
Through-metal acoustic communication was thoroughly investigated in several different scenarios. We distinguish them by two criteria: channel type and signal generation. Many studies investigate SWP, where transducers are aligned coaxially on opposite sides of a metal barrier. Bulk waves are used in this channel type, and distances are usually small (in the range of a few centimeters). In contrast, lamb waves are used in GWCs that travel along a thin metal structure, e.g., a steel beam, a pipeline, or a ship hull. Such waves can travel over long distances with little attenuation. However, lamb waves are generated efficiently only in low-frequency ranges, limiting communication bandwidth severely. At the same time, existing studies can be distinguished by active and passive communication. Although passive communication has been studied extensively within SWP [21], all previous work on GWC used active communication schemes, which require active circuitry for waveform generation and amplification at the tag node. Instead, passive communication requires essentially only a switch to modulate the transducer's load impedance, thereby changing its reflection coefficient. This enables smaller, cheaper, and more energy-efficient sensor nodes. In this work, we focus on passive GWC communication but give an overview of existing active approaches for comparison in Table 2.  
A challenge in GWC is strong multipath propagation leading to inter-symbol interference (ISI). Different methods have been developed to mitigate the effects of ISI. Some use the correlation with the channel impulse response (IR), where the receiver measures the IR first and then correlates every received pulse with the recorded IR. Other methods use the so-called time-reversal technique [24], where the impulse response is measured \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Study** & **Distance m** & **Frequency kHz** & **Output Power mW** & **Efficiency** & **Impedance Match** & **Scope 1** & **Storage** \\ \hline [PERSON], 2022 [17] & 0.40 & 150 & 1.54 & 0.217\% & Ideal & DC & Yes \\ [PERSON], 2022 [5] & \textless{}0.30 & 400 & 0.08 & - & MPPT & DC & Yes \\ Sun, 2021 [18] & 0.40 & 150 & 3.18 & - & Ideal & AC & No \\ [PERSON], 2020 [15] & 0.24 & 25 & 470.00 & 56\% & Ideal & AC & No \\ [PERSON], 2020 [16] & 0.50 & 250 & 0.18 & - & MPPT & DC & Yes \\ [PERSON], 2017 [19] & 1.00 & 48 & 18.00 & 0.1\% & Resistive & AC & No \\ Kural, 2014 [20] & 0.54 & 220 & 17.00 & 30\% & N/R & AC & No \\
**This work** & 3.00 & 225 & 4.00 & 2.8\% & MPPT & DC & Yes \\ \hline \hline \end{tabular}  
* \({}^{1}\) DC: Efficiency was measured, including rectification and voltage regulation.  
\end{table} 
Table 1: Summary of existing studies on lamb-wave-based WPT through metal channels. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Study** & **Spec.** & **Distance m** & **Frequency kHz** & **Modu** & **Data Rate** & **ISI Mitigation 1** & **Impl. Level** \\ \hline Tang, 2022 [5] & Plate (CFRP) & \textless{}0.3 & 400 & OOK & 15 & None & CMOS IC \\ Bahouth, 2022 [22] & Plate (Alum.) & 0.08 & 200 & BPSK & 350 & CCIR & Lab Equipment \\ Sun, 2021 [18] & Plate (Alum.) & 0.4 & 500 & OOK & 100 & CCIR \& MD & Lab Equipment \\ Heifetz, 2021 [23] & Pipe & 1.8 & 2000 & ASK & 2 & None & Lab Equipment \\ Jin, 2013 [24] & Pipe & 1.5 & 250 & PPM & 10 & TR & Lab Equipment \\ \hline
**This work** & Beam (Steel) & 3 & 225 & PSK & 2 & Equalizer & MCU + AF 2 \\ \hline \hline \end{tabular}  
* \({}^{1}\) CCIR: cross-correlation with impulse response, MD: mechanical damping, TR: time reversal; 2 AF: Custom Analog Frontend.  
\end{table} 
Table 2: Summary of existing studies on lamb-wave-based active communication studies compared to the presented backscatter communication study. and known to the transmitter, which uses the inverse of the IR as the transmit pulse.