## 3 Model Formulation and Scheme Design
### Communication Strategy  
#### 3.4.1 LoRa Adaptive Data Rate Strategy  
To reduce the data transmitting time between the UAV and the cluster head nodes, a LoRa 2.4 GHz adaptive data rate strategy with a dual channel is designed. The UAV is equipped with two LoRa 2.4 GHz transceivers--SX1280, and the ground cluster head node with one. On the UAV, one SX1280 is used for Channel 1 to establish a connection and transmit a command between the UAV and the cluster head node, the other is used for Channel 2 to transmit sensor data. SX1280 can be configured to different Data Rates (DR) as shown in Table 2 Each DR value corresponds to a specific combination of Modulation mode, Spreadind Factor (SF), and Bandwidth (BW). In LoRa mode, a lower DR can have a longer transmission distance, and increase the success rate, as the receiver sensitivity will be better. The FLRC modulation with 1040 Kbit/s rate is also supported for applications requiring higher data rates and operating over substantially shorter ranges. In our system, the DCMDR (Dual Channel Multiple Data Rate) scheme is presented, as shown in Figure 4. The Channel 1 constantly uses DR0 with the farthest communication distance, and the Channel 2 uses the variable DR according to the wireless condition. The ground cluster head nodes use the (low power listening) LPL-like method to check the Channel 1 using DR0, to reduce the energy consumption. In the course of the flight, the UAV continually sends wake-up packets by DR0 in Channel 1 with the target cluster head node ID. When the ground cluster head node is awakened and receives the wake-up packet, if the node ID is not equal to its own, it returns to LPL status. Otherwise, it replies with an ACK packet with the RSSI, SNR, and the data size information to the UAV. The UAV chooses the proper DR for data transmitting, according to the distance to the cluster head node and the ACK packet information piggybacked. When it negotiates the DR for data transmitting with the cluster head node, it switches to the new DR in Channel 2 for data receiving. The cluster head also switches to the same DR and starts to transmit the data packet to the UAV. When the UAV receives one data packet, it returns a data ACK packet. If the cluster head node does not receive the ACK within the preset timeout period and times, it switches to DR0, and negotiates the new and slower DR for data transmitting with UAV. The above process is repeated until the data packet is successfully transmitted. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Data Rate (DR)** & **DR6** & **DR5** & **DR4** & **DR3** & **DR2** & **DR1** & **DR0** \\ \hline Bandwidth (kHz) & 1200 & 300 & 812 & 812 & 812 & 406 & 203 \\ Mode & FLRC & FLRC & SF7 & SF9 & SF11 & SF12 & SF12 \\ Physical Bit Rate (kb/s) & 1040 & 260 & 44.41 & 14.27 & 4.36 & 1.19 & 0.595 \\ Receiver Sensitivity (dBm) & \(-\)100 & \(-\)106 & \(-\)112 & \(-\)117 & \(-\)123 & \(-\)128 & \(-\)130 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Spread spectrum factor and data rates. Figure 4: DCMDR scheme. The moving status of the UAV is determined based on the completion of receiving data. Thus, the UAV adaptively adjusts \(DR\in\{6,5,\ldots,0\}\) according to the distance, RSSI, and SNR to reduce data transmission time, the DR should be as high as possible, while ensuring the packet loss ratio. According to (13), the threshold condition of DR6 is determined first, then the condition of DR5 is determined, and so on. \[DR=\left\{ \begin{array}{l}6\;if\;SNR_{th6}<\overline{SNR}\;and\;RSSI_{th6}< \overline{RSSI}\;and\;Dist_{th6}<\overline{Dist}\\ 5\;else\;if\;SNR_{th6}<\overline{SNR}\;and\;RSSI_{th5}<\overline{RSSI}\;and\; Dist_{th5}<\overline{Dist}\\ n\;else\;if\;SNR_{thm}<\overline{SNR}\;and\;RSSI_{thm}<\overline{RSSI}\;and\; Dist_{thm}<\overline{Dist}\\ 0\;else\end{array} \right.