# Development of Radio-Frequency Sensor Wake-Up with Unmanned Aerial Vehicles as an Aerial Gateway
## 4 Proposed Energy Efficient Sensing Network
### General Active Out-Band Wake-Up Mechanism  
As reviewed earlier, an active, out-band wake-up mechanism was chosen for the proposed AG-WSN. To understand this mechanism, Figure 3 summarizes the state and action diagram for the wake-up mechanism and the sensor-node operation. A description is given as follows:  
1. When the sensor node is deployed in the field it is pre-programmed with a duty-cycle sensing schedule. It starts off in sleep mode, which we call the initial state. Only the wake-up receiver is listening in our example, which is either an infrared or RF wake-up receiver.
2. If a wake-up signal is received by the sensor node, it will check whether the signal matches the pre-stored pattern. If not, the node ignores the signal and changes back to the initial state.
3. If the wake-up signal matches the stored pattern the node will wake up, start the XBee module, and turn off the wake-up receiver. Then, the XBee begins scanning the gateway on the UAV.
4. If the XBee module fails to find the gateway in a couple of tries, the node shuts down the XBee module and returns to the initial state.
5. If the XBee module successfully connects to the gateway, it starts communication with the UAV as programmed (e.g., sensing data or updated schedules).
6. After the communication ends, the node returns to the initial state again.  
It is noted that in this design the sensing cycle is preprogrammed and may not match the wake-up cycle when the UAV is approaching. This is because the sensing cycle happens more often than the UAV data collection cycle. In a practical application such as in an SHM or PA sensing system, these sensors may be pre-programmed to wake up serval times a day to sense the ground-truth data, while the UAV only approaches these individual sensor nodes once a day, or even less frequently in  
Figure 3: State and action diagram of the ground sensor nodes and the unmanned aerial vehicle (UAV).  
the case of flying on demand. In this case, the sensor nodes can store the data locally and send all the data to the gateway once the UAV wakes up the nodes. Although this would possibly force the sensor nodes to send massive amounts of data in practice, we find the data sizes vary significantly in different applications. One example is the environmental monitoring project, as shown in Figure 4. In this garden monitoring project, the sensor nodes included the modalities of soil temperature and moisture, atmosphere pressure, humidity, and temperature. The results showed that each sensing cycle produced less than 500 bytes of data to transmit with the sensing cycle setup at every hour, and the total data collected from one sensor node was less than 12 K bytes. In another test the vibration sensors were attached, which required the sensor to run for 1 min at 20 Hz for each cycle. The data produced in each cycle was about 5 K bytes, which led to a total of 120 K bytes of data for a single day if the sensing cycle was set to every hour. That said, the Xbee module adopted in this work claims to have a maximum data rate of 30 K bytes/s, which is fast enough for the UAV to collect all the data from the sensor nodes.