# Leveraging Energy Harvesting and Wake-Up Receivers for Long-Term Wireless Sensor Networks
## 6 Experimental Results
### Energy Microbenchmarks  
To evaluate the energy efficiency of a MAC protocol, it is important to measure the energy consumption of the transmission and the reception of a single packet. Therefore, the energy traces of both operations were measured for the three evaluated protocols, by capturing the voltage drop across a 10.2 \(\Omega\) resistor in series with a 3.5-V power supply using an Agilent Technologies MSO-X-3024A oscilloscope. In addition to allowing detailed analysis of the energy consumption, these microbenchmarks were used to set the \(\tau_{i}\) values introduced in Section 3.2 and to compute the \(H\) and \(\xi_{\infty}\) values related to the EUC metric. The results of the measurements are exposed by Figure 10, in which \(P_{\text{C}}\) is the power consumption of the node. Figure 10a shows that sending a data packet using the proposed SNW-MAC protocol achieves the lowest power consumption compared with the other protocols, as it requires only the sending of the data frame (B). The reception of the WuB does not appear in this figure, as the power consumption of the WuRx when decoding a WuB is 284 \(\mu\)W, which is too low to be visible on the scale of Figure 10a. Moreover, the energy cost of sending a packet is constant if the data payload length is fixed. Regarding the sink in Figure 10b, the two stages of a packet reception, sending the WuB (A) then receiving the data frame (B), can be seen in this figure. As sending the WuB is done at a lower bitrate and higher transmission power than for non-WuB frames, polling a node is energetically expensive for the sink. This result motivates the piggybacking of the wake-up interval of each node in data packets, allowing the sink to poll them only at the right time (see Section 4). Figure 9: False wake-ups of the PIC over a 24-h period in an indoor environment. Figure 10c,d shows respectively the energy cost of a packet transmission and reception using PW-MAC. Sending a packet with this protocol requires the receiving of a beacon (A) and an ACK (C) frame, making the energy cost of sending a packet higher than with SNW-MAC. Moreover, the sender wakes up a short time before the sink transmits a beacon to prevent prediction errors. This time interval varies at each transmission, leading to a non-constant energy cost per packet transmission. The prediction error becomes significant due to the clock drift, and when it exceeds a fixed threshold, an update of the prediction state is triggered, leading to even higher energy consumption. Regarding the sink, receiving a data packet does not require the sending of a WuB and is thus less energetically expensive than with SNW-MAC. Nonetheless, SNW-MAC does not require the transmission of an ACK frame, which partially counterbalances the energy overhead incurred by the WuB transmission when compared to PW-MAC. Figure 10: Microbenchmarks of the MAC protocols. (A)–(C) respectively correspond to the transmission/ reception of a beacon/WuB, data frame and ACK frame. (**a**) SNW-MAC: transmission of a data packet. (**b**) SNW-MAC: reception of a data packet. (**c**) PW-MAC: transmission of a data packet. (**d**) PW-MAC: reception of a data packet. (**e**) UPMA-X-MAC: transmission of a data packet. (**f**) UPMA-X-MAC: reception of a data packet. Figure 10e shows the energy cost of sending a packet for a node using UPMA-X-MAC. In this case, the packet was successfully received by the sink at the sixth attempt. For each attempt, the two stages, sending the data packet (B) and listening for an ACK (C), can be seen. As shown in Figure 10f, the sink woke up during the fifth attempt (B) and thus did not receive the complete data packet. It stayed awake to receive it at the next attempt (B) and sent an ACK (C). The cost of sending one packet with UPMA-X-MAC greatly varies for different transmissions because of the randomness of the sink wake-up time relative to the node transmission starting time. On average, a node has to wait half of the wake-up interval of the sink before a data packet is successfully received.