# Software-Defined Small Cell-Linked Vehicular Networks: Architecture and Evaluation
## 1 Introduction  
The deployment of vehicular applications based on the 5G-vehicle-to-everything (5G-V2X) is the hottest approach within recent years [1]. 5G-V2X is an efficient technology that manages comprehensively solutions for road safety and traffic efficiency by allowing vehicle-to-vehicle (V2V) communication, vehicle-to-pedestrians (V2P), vehicle-to-vulnerable road users/motorcycles via smartphones, road infrastructure (V2I), and mobile network operators (V2N, P2N, I2N) to provide full coverage and continuity of vehicular services [2]. 5G-V2X performs greatly, as the vehicle is out of the road side units (RSUs) coverage but the placement of the RSUs scales the quality of reception of safety messages exchanged between vehicles [3]. In addition, V2X applications use RSUs as relay nodes for V2V, V2P, and V2I [4]. The V2X architecture integrated with vehicular ad hoc network (VANET)-based 5G and RSUs enables a multi-protocol access communication. 5G represents the next major phase of V2X communication. This is due to the fact that such a system will connect everything seamlessly and ubiquitously. Aside from addressing massive internet of things (IoT) deployment, 5G is also expected to meet the requirements of previously unavailable V2X applications that are based on ultra-reliable and low-latencycommunications [5]. To accomplish this, the 5G research community [6] discussed new technologies, including, among others, the densification of the network with small cells (SCs) [7]. The use of SCs improves spectral efficiency. The LTE-A with SCs densification is positioned as a potential candidate technology for V2I communications. However, frequent handoffs and the increased signaling overhead of LTE-A small cells issues were addressed. For instance, Ahmed et al. [8] proposed a novel selection scheme-based mobile gateway with one-hop clustering to efficiently relay traffic from neighboring vehicles toward the serving SC. Exploring spectrum and energy efficiency is a great challenge in 5G vehicular small-cell networking (V-SCN) due to the fast vehicle mobility and varying communications environment. Refs. [9; 10] designed the non-orthogonal multiple access technology to address the interference management and handover in 5G V-SCNs. Software-defined network (SDN) is another potential technology that has been available for decades and needs to be reconsidered for the design of new concepts suitable for 5G use cases. Most visions [11; 12] agree that 5G-V2X networks will be extremely densely populated by a heterogeneous combination of cells from different generations of 3G, 4G, and 5G. As illustrated in Figure 1, the 5G V2X communication technologies enabled an advanced ITS road system (safety message and weather service solution), which includes a suite of sophisticated facilities and services designed for public convenience and road traffic safety. In the 5G vehicular network, the RSUs and 5G V-SCNs must provide adequate information about the network status and geolocation services. The access networks with a diverse set of fields related to V2V, V2N, and V2I networking uses are deployable in smart cities or even for internet-of-vehicles (IoV) applications Moreover, selecting the appropriate access point (AP) or small cell base station (BS) from the accessible heterogeneous wireless access network that meets the minimum necessary requirements for quality of service (QoS) is a great challenge. In this paper, we propose the use of the SDN concept, where the controller plane performed a decision-making procedure that took into account a variety of situational factors, as well as the network performance. To maintain a reliable connection and QoS, we opted for the topology discovery method to accurately localize the small cell-linked RSU (co-located) infrastructure. The deployment of the SDN for 5G V-SCNs can play an essential role in providing V2X services to the general public. New radio access and advanced technologies, such as SDN and small cells in 5G network, pave the way to the new next generation of vehicular networks.