# Adaptive Virtual RSU Scheduling for Scalable Coverage under Bidirectional Vehicle Traffic Flow
## 2 Related Work  
The content allocation for distributed RSUs in a large scale has been greatly explored in recent years [13; 14; 15; 16]. Generally, these works can be divided into two types, including the RSU allocation and cache replication. RSU allocation refers to the position selection according to the vehicle distribution. For example, [PERSON] et al. [13] proposed a cost-effective strategy for RSU placement in VANETs with the formulation of vehicle population distribution and the vehicle speed. In [14], a connectivity-oriented maximum coverage RSU deployment strategy was proposed to maximize vehicle-to-infrastructure (V2I) communication performance in urban areas. In [15], an RSU placement strategy was proposed to improve the location impacts on the efficiency of vehicular network with minimal cost provisioning. Furthermore, [PERSON] et al. [16] presented an analytical RSU placement framework, which takes into account the randomness of vehicle data traffic and the statistical variation of the disrupted communication channel. However, none of these works have taken into account the bidirectional traffic flow, which is prevalent in modern urban areas. Most previous works use the vehicle density instead of the traffic flow, and can be considered as static scenarios in which the traffic condition is independent in different time slots. Even though some previous works analyze the single-direction traffic flow, they assume a simplified scenario in which the vehicles will pass by the regions in a sequence, neglecting the impact of traffic flow in the opposite direction. Recently, some works have presented the bidirectional traffic model to analyze the efficiency of vehicle-to-vehicle (V2V) communications in VANET. For example, [PERSON] et al. studied the message delivery delay in VANETs with the impact factors of message delivery distance and density of vehicles based on the bidirectional traffic model in [17]. According to the driving directions and relative positions of vehicles, [PERSON] et al. designed a time slot division policy to assign the vehicle clusters with specific slots in [18]. In [19], an inter-vehicle ad hoc routing metric based on the density and velocities of vehicles was proposed, and the clusters in opposite directions were used as a bridge to reduce the message propagation delay. [PERSON] et al. also analyzed the data delivery delay on a bidirectional road segment with carry-and-forward vehicle communications [20]. Nevertheless, these works mentioned above only investigate the influence of bidirectional traffic on the V2V communications with the assumption that the RSUs are allocated at the ends of a road segment. None of previous research has explored the RSU allocation under the bidirectional traffic flows, which make the problem more complicated. Intuitively, the RSUs should be allocated in the regions with highest vehicle density for a larger coverage. However, it is not the case for the bidirectional traffic flow scenario, in which the region with high traffic density may experience transmission bottleneck in either direction. Therefore, we are motivated to present a comprehensive model for the bidirectional traffic flow and analyze the solutions for the RSU allocation problem. As to the cache replication, it refers to replica allocation in the fixed RSUs with limited capacity according to various request demands. For example, in Ref. [21], a novel cache invalidation algorithm was proposed to take advantage of the underlying location management scheme to reduce the number of broadcast operations and corresponding query delay. [PERSON] et al. [22] proposed an optimal, a sub-optimal, and a greedy strategy for RSU content replication to minimize the average request delay. However, none of these works have taken into account both the RSU allocation and request popularity during the content replication. In addition, [PERSON] et al.