# Leveraging Energy Harvesting and Wake-Up Receivers for Long-Term Wireless Sensor Networks
## 5 Experimental Setup
### Node Architecture  
Multiple EH-WSN platforms have been proposed by academia and industry over the last decade. In this work, we consider a single-path architecture version of the Multiple Energy Source Converter (MESC) architecture proposed in [45]. In the single-path architecture, there is only one energy storage device, and all the harvested energy is used to charge the storage device, which directly powers the node through a DC-DC converter. Figure 5 shows the block architecture of MESC that can be used with a variety of energy harvesters (e.g., photovoltaic cells, thermoelectric generators and wind turbines) using the appropriate energy adapter to normalize the output energy. Supercapacitors were chosen as storage devices as they are more durable and offer a higher power density than batteries [46]. In this work, the PowWow platform [47], based on the MESC architecture and equipped with a Texas Instruments CC1120 radio chip, is used as the testbed. The energy storage device is a 0.9 F supercapacitor with a maximum voltage of 5.0 V, and the minimum voltage required to power the node is 2.8 V. PowWow embeds a voltage measurement chip, the INA3221 from Texas Instruments, which allows measurement of the supercapacitor voltage, denoted by \(V_{C}\), with a precision of 0.1 mV. The residual energy \(e_{R}\) can thus easily be computed as follows:  
\[e_{R}=\frac{1}{2}{CV_{C}}^{2}, \tag{26}\]  
where \(C\) is the supercapacitor capacitance. When SNW-MAC is evaluated, the ULP WuRx is added to the node, and the so-obtained mote is shown in Figure 6. The EM introduced in Section 3 was implemented on PowWow, in addition to SNW-MAC introduced in Section 4 and two othersstate-of-the-art MAC protocols presented in Section 5.3. The parameters used for experimentations are shown in Table 1.  
As the supercapacitor supplies the node via a DC-DC converter as shown in Figure 5 and the efficiency of the DC-DC converter varies with the input voltage, the power consumed by the node depends on the charge of the supercapacitor. Therefore, the power consumption of each of the \(N_{S}\) states (introduced in Section 3.2) was measured for different input voltages of the DC-DC converter ranging from 2.8 V-5.0 V, and piecewise linear interpolation was used to get the \(P_{i}\) values as functions of the supercapacitor voltage. Figure 7 shows the so-obtained measures and the corresponding interpolated functions for two states of the node. As we can see, piecewise linear interpolation permits an accurate modeling of the node power consumption.  
\begin{table}
\begin{tabular}{c l c} \hline \hline  & **Parameters** & **Values** \\ \hline \multirow{3}{*}{MAC} & Sink wake-up interval & \multirow{3}{*}{250 ms} \\  & (UPMA-X-MAC and PW-MAC) & \\  & Maximum number of retransmissions & 2 \\ \hline \multirow{3}{*}{PHY} & WuB bitrate & \multirow{3}{*}{1 kbps} \\  & Data/ACK/beacon bitrate & \multirow{3}{*}{20 kbps} \\  & WuB transmission power & \multirow{3}{*}{12.5 dBm} \\  & Data/ACK/beacon transmission power & \(-6\) dBm \\ \hline \multirow{3}{*}{EBC} & \(M_{\mathrm{C}}\) & \multirow{3}{*}{0.01} \\  & \(K_{\mathrm{C}}\) & \multirow{3}{*}{2.0} \\  & \(M_{\mathrm{D}}\) & \multirow{3}{*}{0.5} \\  & \(K_{\mathrm{D}}\) & \multirow{3}{*}{2.0} \\  & \(\Delta\epsilon_{B}\) & \multirow{3}{*}{5 mJ} \\  & \(E_{R}^{fail}\) & \multirow{3}{*}{3.528 J} \\  & \(E_{ENI}^{down}\) & \multirow{3}{*}{12.40 J} \\  & \(E_{ENI}^{up}\) & \multirow{3}{*}{12.45 J} \\  & \(E_{R}^{max}\) & \multirow{3}{*}{12.50 J} \\  & \(E_{B}^{min}\) & \multirow{3}{*}{0.04 J} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters used for the experiments.  
Figure 5: Hardware architecture of a WSN node using the Multiple Energy Source Converter (MESC) architecture and a WuRx. UPMA, Unified Radio Power Management Architecture.