## 2 Literature Review
In [PERSON] and [PERSON]'s studies, a runway resource scheduling model was developed for multi-runway airports to optimize the scheduling of runway resources in terms of runway sequencing and takeoff and landing times to improve runway operational safety and efficiency [27; 28]. According to [PERSON], a two-stage multi-objective optimization model was constructed to integrate the joint scheduling of airfield area resources by considering the scheduling requirements of airfield area resources, and a comprehensive allocation of spatial and temporal resources of the whole airfield area was carried out [29]. The above-mentioned papers have carried out some preliminary studies in the field of airport surface operation optimization and model construction. As can be seen from the above, most current research works establish the model of airport surface operation based on specific operation procedures or rules, adopt scientific assessment of the surface operation capability, and optimize the scheduling of the surface spatial-temporal resources to improve the safety and efficiency of the arrival and departure aircraft, but they neglect to optimize the design of the \"airport surface operation procedures\" from the root level. From the perspective of key resources, the surface operation procedure involves three subsystems: runway, taxiway, and apron. As the runway operation mode and taxiway operation rules have been gradually researched on a large scale and widely applied, the apron operation procedures have not been studied systematically and thoroughly. Especially in the context of civil aviation industry-wide transfer of apron control, the problem of apron operation procedure optimization becomes more urgent. In the apron area of complex airports, the dense geographical distribution of parking stands, the limited space for aircraft operation, the frequent interaction of arrival and departure activities, as well as the existence of certain blind areas for control operation often lead to frequent problems in the apron such as aircraft cut-off, vehicle-aircraft cut-off, arrival and departure conflicts, taxiing avoidance, etc. Therefore, it is particularly important and urgent to use scientific methods to optimize the design of apron operation procedures and to scientifically evaluate the implementation performance of the designed operation procedures. In fact, scientific and reasonable apron operation procedures can essentially improve the safe, efficient, and smooth operation of arrival and departure aircraft in the apron area, while unreasonable apron operation procedures largely restrict the operational efficiency, fuel saving, and emission reduction level of the apron area and even the whole airport surface system. Given this, this paper puts forward the optimization design and performance evaluation method of a U-shaped area operation procedure for arrival and departure aircraft operation activities in the complex aprons. First, based on the actual operation data of the airport and the configuration of the apron, we innovatively designed a diversified operation procedure for the apron U-shaped area and propose differentiated operation rules and traffic models under different operation procedures. Then, from the perspectives of safety, capacity, efficiency, and environmental protection, we construct a multi-dimensional performance evaluation index system of the apron U-shaped area and propose the specific measurement methods and comprehensive evaluation methods of various indices. Finally, to comprehensively verify the optimization of U-shaped area operation procedures, a simulation model of U-shaped area operation is integrated and developed. In addition, the advantages and shortcomings of various performance indices under different types of U-shaped area operation procedures are compared and analyzed in the final part.