# Software-Defined Small Cell-Linked Vehicular Networks: Architecture and Evaluation
## 2 Related Works  
We highlight relevant proposals that have already been proposed in the literature in order to develop new reliable frameworks for prospective 5G-VANETs. The first methods take advantage of the benefits of the SDN paradigm to propose a more adaptable vehicular communication architecture. In [14], a hybrid SDN approach is proposed, in which network control is shared between the RSU controller (RSUC) and local SDN controllers installed in a subset of RSUs. The goal is for the new architecture to be more reconfigurable due to vehicle mobility. The new adaptable architecture solves the issue of connectivity loss between RSUC and the data plane. M.A. Salahuddin et al. [15] took another step by integrating the SDN into the RSU cloud. Their proposed architecture consists of traditional and specialized RSUs that dynamically instantiate, replicate, and/or migrate services via software-defined networking. The micro datacenter RSUs, such as RSU clouds, are reconfigured to meet more service demands. They consider a heuristic algorithm to reduce the cost of the reconfiguration overhead.  
Having the same vision, the authors of [16] were intrigued by the increasing demand for computational resources in the next vehicular networks. They proposed scaling up the computing infrastructure by considering vehicles as potential candidates. Vehicle owners are offered a number of attractive incentives in exchange for adding their vehicles' computing resources to the fog computing (FC) service provider. SDN and FC provide great computing frameworks for vehicular networks. Since SDN divides the software plane from the data plane, FC alleviates the pressure of the core network. Yaomin et al. [17] proposed the SDFC-VeNET architecture. They presented mobility management and resource allocation using the SDFC-VeNET. Even though these approaches have made significant advances in the design of future 5G vehicular networks, we consider 5G-SCN linked to RSUs as a potential candidate for the new generation of 5G and beyond vehicular networks. Furthermore, proposals that considered FC as small cell-linked RSU infrastructures failed to address how these SCs linked to RSUs will be efficiently controlled and supervised. Toaddress this gap, our proposed architecture employs a distributed SDN approach to manage small cell-linked RSU infrastructures and address the issue of heterogeneous wireless access networks in 5G-V2X networks.  
The requirements of various vehicular applications impose different wireless communications in vehicular networks. The bandwidth and coverage range of cellular networks for vehicular applications are costly. The work in [18] proposed the SDVN communication using heterogeneous wireless interfaces for network selection, which utilizes the concept of controller layering and link duration. In addition, Ref. [19] addressed the issue of loss of connectivity with the said controller entity by proposing a hierarchical SDN-based vehicular architecture with the goal of improving performance in the event of a loss of connection with the central SDN controller.  
The majority of the works are built on an RSU and fog computing architecture, which may have scalability issues in network selection for vehicular applications with varying QoS requirements. Furthermore, most of these works do not deal with multiple RSUs linked to SCs to keep communication alive when no communication infrastructure is managing the computing resources of deployed RSUs. The following subsection discusses the motivation of small cells-linked RSU paradigm for V2X services.