## 2 Related Work  
As a new mode of traffic organization, signal-free intersections have attracted a lot of attention with various control approaches being proposed, which can be divided into two groups: distributed control and centralized control. The distributed control architecture regards vehicles as autonomous agents. Vehicles can pass the intersection through the strategic interaction via Vehicle-to-Vehicle (V2V) technology, which has the advantage of low infrastructure cost and low communication and computing requirements. Thus, an agent-based approach is commonly adopted to address the collaborative passing problem at a two-way two-lane intersection. [PERSON] et al. [11] proposed a multi-agent system and scheduled the departure times rather than just obeying the First-In-First-Out (FIFO) or the First-Arrive-First-Past (FAFP) rule. [PERSON] et al. [17] proposed a task-area partition framework to decompose the problem into three main tasks, including vehicle state observation, arriving time optimization, and trajectory tracking control problems, and then used a distributed algorithm to accomplish these tasks without global coordination. [PERSON] et al. [13] modeled the sequential movements of vehicles as Multi-Agent Markov Decision Processes (MAMDPs) and tried to alleviate the curse of dimensionality problem in multi-agent learning.  
In the centralized control architecture, sensing and computing resources can be shared via the Vehicle-to-Infrastructure (V2I) technology, and global traffic improvement can be obtained. There is at least one task in the system conducted by the central controller, and V2I is used to capture the real-time information of traffic flow [18]. Some researchers [19; 20] proposed reservation-based Autonomous Intersection Management (AIM) methods. In them, vehicles need to send messages to an intersection manager and reserve a sequence of spaces and time to cross the intersection. Then, the intersection manager will decide whether to approve or reject the requested reservation according to the intersection control policy. First-arrive-first-past is the most common policy; it takes the arrival time of each vehicle as a unique priority [15]. Therefore, the vehicle arriving earlier will have a higher priority to pass the intersection area. [PERSON] et al. [21] proposed a Discrete-time occupancies trajectory-based Intersection traffic Coordination Algorithm (DICA) and clarified the interaction between a CAV and an intersection control agent.  
Some other studies in this field focused on the optimization of traffic sequences. Their objective function was usually related to safety performance [14], traffic efficiency [22], fuel consumption and pollutant emissions [23; 24; 25], ride comfort [26], fairness [27], and other multiple performance measures [28]. A platoon mechanism has also been investigated to find the impact on integrated traffic performance [29; 30]. Additionally, many researches tried to avoid conflicting vehicles entering the intersection at the same time to guarantee traffic safety, while some researches tried to allow vehicles with conflicting routes to enter intersections simultaneously so as to pursue higher traffic efficiency. To achieve this objective, [PERSON] et al. [31] divided the intersection into grid cells or points, where only one vehicle is allowed to pass the cells or points at each time step to avoid collisions. Thus, other vehicles can exist within the same intersection cannot be within the same cell at the same time.