# Research Progress on Synergistic Technologies of Agricultural Multi-Robots
## 3 Research Progress on Synergistic Technologies of Agricultural Multi-Robots
### Communication
Since explicit communication and implicit communication have their advantages [120], explicit communication is used for the integrated control of robots in the upper layer, and implicit communication is used for integrated control of robots in the bottom level. Explicit communication means that the robot communicates directly or indirectly with other robots via wireless networks. For example, robot 1 sends a message to all robots in the communication range in broadcast communication, that is, without specifying a particular robot, robot 2, which does not need the message, receives the message. In implicit communication, the intermediary for inter-robot communication is often the surrounding environment. For example, the UAV can be informed about the farmland in advance and build a model of the farmland environment, and the ground robots operate on the ground based on this farmland model [122]. The combination of both communication modes can be used to develop their advantages, improve the flexibility to confront the various dynamic and unknown environments, and complete many complex tasks in agricultural production. For the implicit communication of multiple robots, you can refer to Section 3.2 environment perceptions, here we focus on robot explicit communication techniques. In literature [25], two aerial drones were equipped with GPS, visible and near-infrared spectral cameras, which took pictures of the farmland at a set series of ordered waypoints and uploaded them to the backend, which sends the processed information of weeds in the farmland to the ground robot. The ground robots were equipped with RTK-GPS, RGB camera, and LIDAR. RTK-GPS provided accurate heading for the ground robot, RGB camera detects weeds and crop rows, and LIDAR detects obstacles on the vehicle trajectory. While the ground robots were safely walking along their respective set paths, weeding operations start if the weeds detected by the cameras were the same as the weed information in the farmland. In this multi-robot system, the aerial drones and ground robots did not communicate directly but completed the cooperative operation through the interaction of environmental information. Figure 17: Communication modes of multiple robots. * Wireless communication net of multiple robots As shown in Figure 18, the agricultural multi-robots need to adjust their pose in real-time. Therefore, the data exchange of communication among multiple robots is mainly based on wireless communication technology in agricultural production. This technology mainly involves a wireless local area network (WLAN) and a wireless personal area network (WPAN), such as WI-FI, Bluetooth, ZigBee, and IRDA (infrared data association). Among them, WI-FI technology has been developed most rapidly in agricultural multi-robots. * The wireless communication protocol of multiple robots The wireless communication protocols are primarily used based on wireless communication standards and the unlicensed band. Taking the WLAN as an example, the IEEE 802.11 series standards and the 2.4 GHz or 5 GHz bands are used in this communication. The IEEE 802.15 series of transmission technology protocols are selected in WPAN. Combined with the above communication technology, the research progress of agricultural multi-robots in communication in the past 10 years is summarized, as shown in Table 7. \begin{table}
\begin{tabular}{c c c c} \hline
**Communication Technology** & **WI-FI** & **Bluetooth** & **ZigBee** \\ \hline Transmission distance [m] & 10–300 & 10–100 & 10–75 \\ \hline Theoretical transmission speed [bps] & 54–300 M & 100 k & 10 k \\ \hline Wireless communication network & Wireless local area network (WLAN) & Wireless personal area network (WPAN) & WPAN \\ \hline Working frequency [Hz] & 2.4–5.4 G & 2.4 G & 2.4 G (global); 868 M (Europe); 915 M (United States); \\ \hline \end{tabular}
\end{table} 
Table 7: Comparison of wireless communication technologies of agricultural multi-robots in the past 10 years.