# Formation Control of Dual Auto Guided Vehicles Based on Compensation Method in 5G Networks
## 1 Introduction  
As the new generation communication technology, 5G networks have gained vital development and attention in recent years. International Telecommunication Union (ITU) has defined three typical scenarios for 5G: enhanced Mobile Broadband (eMBB), massive Machine Type Communications (mMTC), and ultra-Reliable Low Latency Communication (uRLLC). The 3 rd Generation Partnership Project (3 GPP) has developed specifications for 5G New Radio and finished the 5G standards Release 15 (5G R15) in June 2018. As the first full set standard of 5G, 5G R15 has been commercialized and available for individuals and businesses. Nowadays, academia and industry are widely recognized that 5G has significant potential in industry application and is the key wireless technology in Industry 4.0 [1]. Many companies have applied 5G R15 to their intelligent manufacturing systems, for example, the aircraft surface inspection with 5G and 8K video of Commercial Aircraft Corporation of China, the 5G and smart manufacturing of SANY heavy industry [2] and multi-sensor platforms monitor system of Fraunhofer Institute for Production and Technology [3]. The capabilities of 5G R15 mainly meet the demand of eMBB scenarios, so the above applications are concentrated on intelligent management and data transmission. There are few real-time applications due to the uRLLC capability of 5G R15. To promote the application of 5G in real-time control, frozen R16 enhance the uRLLC capability of5G and subsequent R17 R18 will continue to strengthen support for uRLLC scenarios [4]. Meanwhile, academia and industry companies are actively exploring the technologies for industry application [5; 6; 7]. Many 5G research projects have also been implemented like 5G Smart in Europe Union [8], Koordinierte Industriekommunikation (KoI) project in Germany [9] and Industrial 5G in Siemens [10]. Many researchers focus on control and management of movable devices in 5G networks [11; 12; 13] and the Auto Guided Vehicle (AGV) has become one of the most concerned objects for 5G application in real-time control [14]. AGV is one of the main transport equipments in factories and it has the characteristics of long-distance motion, changeable trajectory, and flexible grouping, which could take full advantage of 5G wireless communication. Moreover, collaborative transport is becoming an important application of AGV in industry. It will take additional cost to design the dedicated AGV to meet the demand of transferring materials in different weight and size. Multi-AGVs collaborative transport is a way to decrease cost and increase AGV utilization. Stability of formation is the key factor to ensure the effectiveness of collaborative transport. Many researchers pay attention to formation control and proposed a variety of research methods [15; 16; 17]: leader-follower method, behavior-based method, virtual structure method etc. However, introducing 5G into multi-AGV collaborative transport will bring some challenges: there is non-negligible delay in 5G transmission process compared to the control cycle; there is also data out of order and dropout due to the multi-band characteristics of 5G. These make the control system in 5G networks a typical networked control system (NCS), with a loss of stability and timing compared to wired system. Therefore, it is necessary to redesign the control method according to the characteristics of 5G networks. There are some challenges when applying 5G to industrial control:  
1. Out-of-order and packet dropout. Due to the data multichannel transmission mechanism, data packets from the same node sometimes reach the receiving end out of order. The setting sequence number in packet and data buffer in receiving end could set data in order but may mean that there is no new data for control system. The transmission process could cause data dropout because of the signal attenuation and resource conflict. The network system usually uses retransmission to ensure the correctness of data packet. However, retransmission will increase the transmission delay.