## 2 Related Works  
In recent years, VANETs have gained popularity for various applications and services in the civilian domains [13]. Compared with the mobile ad-hoc networks (MANETs), the high mobility and wide range cause significant LQoS changes in VANETs [14]. Therefore, in order to realize efficient cooperation and information exchange among multiple network nodes, it is critical to design an efficient relay selection protocol for VANETs. Many papers have proposed relay selection schemes in VANETs [15; 16; 17; 18]. Specifically, based on the VANET-Cellular network architecture, authors in [15] proposed a safety message broadcast relay selection method. This method by jointly considered the cellular signal strength, the link stability, the channel quality, and the geographical location to select the best broadcast relay node. Mathematical analysis and simulations showed that the proposed relay selection method can improve the VANET resource utilization. In addition, [PERSON] et al. [16] proposed a cross-layer relay selection scheme, which can reduce the end-to-end transmission delay. Moreover, [PERSON] et al. [17] proposed a data forwarding scheme based on trajectory, which was specially designed for roadside reports in sparse VANETs. Furthermore, [PERSON] et al. [18] proposed an analytical framework for the path delay in bidirectional vehicular traffic. They used this analytical framework to select the relay node with the lowest expected delay. However, the above works [15; 16; 17; 18] are based on fixed infrastructure road side units (RSUs) to relay selection. In the large space scale, the obstacle, the complex terrain, the severe weather,and other factors may lead to LQoS degradation or even unavailability. In particular, in some extreme environments, there is a lack of fixed infrastructure RSUs to assist vehicle communication. Thus, it is difficult to meet the demand solely relying on the ground-based VANETs. To address the problem discussed above, the Consultative Committee for Space Data Systems (CCSDS) [19] proposed that ground nodes can communicate with space networks. However, the number of access network nodes in this method is limited, which cannot meet the demand for multi-vehicle simultaneous access in UAV-assisted VANETs. Recently, research works have also emerged that use UAVs in VANETs [20; 21; 22; 23; 24; 25; 26; 27; 28]. Specifically, [PERSON] et al. [20] studied the opportunistic utilization of UAV-base stations (BSs) in future VANETs and proposed the UAV stations that can be used in cellular wireless VANETs. [PERSON] et al. [21] proposed a relay selection method in space information VANET environment, which can effectively reduce the network overhead while satisfying security constraints. [PERSON] et al. [22] proposed an aerial-ground cooperative vehicular networking architecture, in which UAVs can act as the relays when network partitions happen in the ground-based VANETs. [PERSON] et al. [23] proposed a software defined air-to-ground (A2G) network architecture for supporting diverse vehicular services in a seamless, efficient, and economical manner. [PERSON] et al. [24] proposed an intersection UAV-assisted VANET relay selection protocol, which was beneficial in developing more intelligent connected network nodes. [PERSON] et al. [25] studied a wireless system consisting of distributed vehicles and UAVs and proposed the cyclical multiple access method, which can improve the network throughput. [PERSON] et al. [26] calculated the probability distribution of the end-to-end information propagation delay of a message in UAV-assisted VANETs with the SCF method. [PERSON] et al. [27] proposed the task of using one or more UAVs to relay messages between two distant ground nodes by the SCF method. [PERSON] et al. [28] devised an UAV-assisted VANET system, which utilized UAVs to boost vehicle-to-vehicle data message transmission under instructions conducted by their distributed vehicle location prediction algorithm.