# Development of Radio-Frequency Sensor Wake-Up with Unmanned Aerial Vehicles as an Aerial Gateway
## 6 Experimentation and Results
### Energy Consumption Analysis and Verification  
Energy consumption was one of the primary concerns of this paper. Our main study focused on energy consumption in the sensor nodes that were potentially deployed in the hard-accessible field. Specifically, we classified each sensor module hardware into four subunits represented by the primary device: the MCU, XBee module, wake-up receiver, and the sensors. In Table 3, the typical current consumption with a 3.3 V supply voltage for this hardware is listed. We did the comparative experiment \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Coding Pattern** & **Time between t\({}_{1}\) and t\({}_{2}\)** & **Time between t\({}_{2}\) and t\({}_{3}\)** & **Total Time Delay** \\ \hline
16-bit, Single pattern & 12 & 49 & 61 \\
32-bit, Single pattern & 18 & 49 & 67 \\
16-bit, Double pattern & 18 & 49 & 67 \\
32-bit, Double pattern & 31 & 48 & 79 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average time delay (in terms of milliseconds) within the wake-up procedure. Figure 9: One image captured in the high-speed video. In the center is the wake-up transmitter, on the left is the wake-up receiver, and to the right is the microcontroller unit (MCU). This image is the frame when the receiver decodes the signal and finds that it matches, thus, the LED on the receiver board is lit up. on one default (duty-cycle) energy-saving mode and two wake-up mechanisms, as explained earlier, and recorded the power consumption:  
1. Solution 1--the simple duty-cycle method (default in the Libelium sensor network). In this method, no out-band wake-up method was used. We used the XBee as our data communication as well as the wake-up channel. The XBee module on the sensor node stayed in listening mode when no UAV was nearby. Using this solution, the XBee and MCU on the sensor board had to always be turned on. 2. Solution 2--infrared wake-up method implemented in this paper. The infrared receiver was used as the wake-up channel. Since the IR receiver was connected to the MCU GPIO, it required the MCU to always stay on, while the XBee module could be turned off. 3. Solution 3--RF wake-up method proposed and implemented in this paper. The RF was used as the wake-up channel. When the receiver was connected to the MCU's interrupt pin the MCU stayed in sleep mode. The MCU only required 55 uA of current while in sleep mode. We assumed the same schedule for the different solutions, in which T\({}_{\text{i}}\), T\({}_{\text{s}}\), and T\({}_{\text{tran}}\) represented the duration of the idling state, sensing state, and the transmitting state of the sensor node, respectively. Considering 24 h as a working period, if every 4 h the node sensed the data and transmitted to the gateway, the values of T\({}_{\text{s}}\) and T\({}_{\text{tran}}\) in 24 h were usually less than 10 min, and the remaining time belonged to T\({}_{\text{i}}\). This means that the significant part of the power consumption was spent within the T\({}_{\text{i}}\) period when accumulated over time. Figure 10 illustrates the electric current consumption, considering the aforementioned typical durations. The power consumption for one sensing cycle can be calculated by the following formula:  
\[E_{cycle}= \left[T_{i}\times I_{i}+T_{s}\times I_{s}+\,T_{tran}\times I_{tran }\right]\,V_{dc} \tag{1}\]  
where I\({}_{\text{i, s, tran}}\) is the current variable at the state of idling, sensing, and data transmitting as defined in Table 3, respectively. The resulting \(E_{\text{cycle}}\) defines the energy consumption in a designated cycle proportional to the constant battery DC voltage. Using this formula and the data from Table 3, we can qualitatively state that Solution 3 needed much less energy for the idle state. By algebraic calculation based on Table 3 and the assumed typical idling (4 h), sensing duration (1 min), and transmitting duration (1 min), Solution 3 required only 1.6% of the energy compared to Solution 1, and 6% of energy compared with Solution 2.