# MCST Scheme for UAV Systems over LoRa Networks
## 5 MCST Scheme over LoRa Network  
This section describes the design of the MCST scheme for optimizing the transmission performance in the LoRa network. We consider that the node in the LoRa network can perform concurrent transmission. We design the MCST scheme over the LoRa network  
Figure 4: An example of MCST scheme for a BRF transmission. (MCST/LoRa scheme) via the drone-to-drone communication for the UAV system by integrating and modifying the three classes of LoRaWAN MAC, especially class A and class B, to enable the DCF mechanism in the MAC layer of the OSI model. Figure 5 shows an example frame exchange sequence for a BRF transmission through the MCST scheme in which the \(SR\) and \(RD\) transmissions perform the sequential transmission in the time fraction of \((1-\tau)\) for optimum transmission capacity as case 1 and case 2, respectively. As illustrated in the figure, the data transmission can be successfully completed with the four-way handshaking mechanism. Since we neglect the time synchronization problem in this paper, the order of operating sequential transmission and concurrent transmission does not matter in the MCST scheme for theoretical study. However, in the implementation of the real network scenarios, the MCST scheme performing the sequential transmission in the prior time fraction could be beneficial to time synchronization. Figure 6 shows the request-to-send/clear-to-send (RTS/CTS) message handshaking of the MCST/LoRa scheme over the LoRa network. As the figure represents, if the node wants to begin the transmission, it will first send the LoRaWAN request-to-send (LRTS) control frame set, which is a combination of request-to-send (RTS) control frame uplink transmission and LoRa answer (ANS) frame transmission from the gateway. Different from [17], this paper considers that the LoRaWAN-enabled end device in the network is in the acknowledged configuration; on the other hand, there is an answer from/to the gateway for the uplink/downlink transmission. Therefore, to follow the LoRaWAN standard of bi-directional transmission, the gateway will send the ANS frame in the receive window to indicate the reception of the uplink/downlink transmission [22]. We assume that the ANS frame transmission is a function of transmit power, transmission channel, and transmission rate of the uplink/downlink transmission and has a similar transmission  
Figure 5: The frame exchanging sequence for successful BRF transmission through MCST over LoRa network (MCST/LoRa scheme). time to the uplink/downlink transmission. If the receiving node is ready to accept the transmission, it will send the LoRaWAN clear-to-send (LCTS) control frame set, which is a combination of clear-to-send (CTS) control frame and ANS frame transmissions. Before processing the DATA transmission, the relay node sends the LoRaWAN set-to-send (LSTS) control frame set, which is a combination of set-to-send (STS) and ANS frame transmissions, to pass the MCST information, i.e., time fraction and transmission control for instructing the sequential transmission. After the DATA transmission is complete, the destination and relay nodes inform the transmission completion with the LoRaWAN acknowledgment (LACK) control frame set, which is a combination of acknowledgement control frame and ANS frame transmissions. For theoretical study purposes, this paper considers zero receive delay and only the first receive window with a simple ANS frame downlink/uplink transmission following each uplink/downlink transmission for both class A and class B LoRaWAN MAC. As mentioned in [22], the acknowledgment control frame transmission consumes the devices' energy.