# Development of Radio-Frequency Sensor Wake-Up with Unmanned Aerial Vehicles as an Aerial Gateway
## 6 Experimentation and Results
### RF Wake-Up Delay  
Since it took time for the MCU to wake up from sleeping, we expected that this may cause a delay in the data gathering for the WSN. Other wake-up receiver designs, like the one reported in [80], claimed to have a latency of 214 ms, while in [70], the author achieved around 0.9 ms. We wanted to compare our setup with other wake-up designs to make sure the latency value was in an acceptable range. To evaluate this and measure the time it took for the signal to reach the MCU after sending a signal from the transmitter, we set up a novel photogrammetric test environment. In the RF wake-up setup we had three different components: the wake-up transmitter, the wake-up receiver, and the MCU. They are introduced in Figure 6a The measurement operates as follows:  
1. When the transmitter sends out the wake-up signal, the LED on the transmitter will flash. This moment is defined as t1. 2. When the receiver receives the signal and decodes it if the signal matches the pre-stored key, then the LED on the receiver will flash. This moment is t2. 3. When the receiver sends out the wake-up trigger to the MCU interrupt pin, then the MCU wakes up and the LED on MCU board will flash. This moment is t3. We used a high-speed camera (Sony RX100 V), which can record 1000 frames per second, to record the above sequence in a video format. We can obtain the time delay by calculating the video frames between each LED light up, as each frame is equal to 1 ms. One picture frame of the video is shown in Figure 9.  
After the test, we found that the receiver had some failed wake-up instances due to environmental RF noise when simple coding (short pattern) was used. The solution to this would be to increase the wake-up single pattern from 16-bit Manchester coding to 32-bit, or double the wake-up single pattern length. Four coding conditions were considered and tested, and the time intervals between t1 and t2 and between t2 and t3 were recorded. Combining all these coding solutions, the time delay values averaged over multiple tests are reported in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Distance (m)** & **Average Current (\(\mu\)A)** & **Working Duration (ms)** \\ \hline
0.5 & 8.0 & 193 \\
1 & 8.0 & 198 \\
2 & 8.2 & 198 \\
2.5 & 8.1 & 198 \\
3 & 9.4 & 459 \\
4 & 9.4 & 462 \\
7 & 9.5 & 458 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The relation of distance between receiver and transmitter, the current of the receiver, and the working duration of the receiver. The above results reveal that with different coding setups for the transmitter and receiver, first, the time delay between t\({}_{1}\) and t\({}_{2}\) increased when longer-bit Manchester codes or double patterns were transmitted. This was because both transmitting and decoding phases took longer if the coding was more complex. Second, the delay times between t\({}_{2}\) and t\({}_{3}\), as shown in Table 2, stayed comparatively the same. This was because t\({}_{2}\) was the time when the wake-up receiver sent out the signal through wiring to the MCU, when all decoding procedures were already completed. Thus, the delay between t\({}_{2}\) and t\({}_{3}\) only represented the wake-up time of the MCU and would not be affected when different coding patterns were used. Regardless, since the total delay caused by using this RF wake-up mechanism was less than 80 milliseconds (from sending out the wake-up signal by the transmitter to the MCU's wake-up), we safely concluded that this wireless wake-up mechanism did not affect the data communication between the gateway and the sensor nodes in the field for most sensing applications, specifically when the sensors sense data packets at one time and transmit at a different time, then stay idle with much longer durations. The study in [81] claimed to reduce the time between t\({}_{1}\) and t\({}_{2}\) roughly from 13 ms to about 2 ms using 16-bit and a single pattern with a similar setup in this paper. We thought this could potentially further reduce the latency introduced by this out-band wake-up mechanism.