# Development of Radio-Frequency Sensor Wake-Up with Unmanned Aerial Vehicles as an Aerial Gateway
## 6 Experimentation and Results
### RF Wake-Up Distance  
RF wake-up, however, did not require a direct LOS. The receiver, AS3933, had three wake-up channels, each connecting to an antenna. In our test, the three antennas were set up in a perpendicular fashion. Any antenna that received the correct pattern may trigger the wake-up. To test the wake-up range, we used a 125 KHz antenna connected to the UAV's MCU and external power supply. When the supply voltage for the transmitter was set to 9 V, the maximum range was around 7 m in an indoor environment. A similar result in the outdoor test using the same receiver was found in [71], which resulted in a 5 m range using a 12 V supply for the transmitter. One question related to the wake-up distance was whether the power consumption would increase as the distance became greater. To get more detailed data on the wake-up distance and its energy consumption, a ground test was implemented where the position of the same transmitter was fixed, and the AS3933 receiver was moved away from it. We recorded the received signal strength indicator (RSSI) of the receiver at different distances, and the data is shown in Figure 7. The RSSI was measured in digital values witha 5-bit accuracy. The relation to the input voltage provided by the AS3933 manufacturer shows an approximately a linear relation [78]. Combining our testing result in Figure 7 and this information, they gave us a general idea that how far the receiver could work and what the voltage was at the AS3933 input.  
Following the RSSI test, we used a 6.5 precision meter, Fluke 8845A, to measure the power consumption under these distances. In this setup, we programmed the receiver as a standard listening mode with three active channels and used the resistor-capacitor (RC) oscillator as the clock generator. From the receiver's manual [79], it showed the typical current was 6.1 uA, which was also confirmed by our measurement (Figure 8). Then, we measured the current of the receiver while it was decoding the signal from the transmitter. The results are shown in Table 1. It was found that as the distance increased, the current of the receiver increased. This was mainly because as the distance became further, the RSSI value lowered as discussed, and the receiver's variable gain amplifier increased the gain; therefore, the signal could be decoded properly. We also observed that, interestingly, within a 2.5 m range the receiver's working duration remained around 200 ms. Once the range passed 3 m, the working duration increased to 460 ms, which indicated that more power was consumed.  
Figure 8: Current measurement of the AS3933 in a standard listening mode with three active channels and the RC-oscillator as the clock generator.  
Figure 7: Received signal strength indicator (RSSI) of the receiver at different distances from the transmitter.  
As the voltage for the receiver was 3 V, the results above indicated that the lowest possible energy consumption was 4.632 uJ (at 0.5 m), while the highest was 13.053 uJ. Considering that most of the time the receiver was in a listening mode with the maximum current of 6.1 uA (with 3 channels on), and further assuming that the listening time was 30 min before a wake-up signal arrived, the energy consumed during the listening states was 32.94 mJ. Compared between 32.94 mJ and 13.053 uJ, the increased energy due to a larger distance can be ignored.