## 1 Introduction  
An Intelligent Transport System (ITS) is a sophisticated concept grouping a plethora of advanced services to make a more intelligent usage of current and future transport networks, thus improving on current procedures regarding persons and goods mobility, traffic management, and the flow of information towards users of transport networks. A key part of an ITS are Vehicle-to-Everything (V2X) communication technologies, which represent a set of standards that give way to services and applications aimed at supporting the goal of an ITS of improving road safety, reducing vehicle fuel consumption, reducing travel time, etc., also in a sustainable way. An intelligent transportation system not only contemplates intelligent transportation solutions, but also \"green\" transportation solutions. So, it has been shown that through communications between vehicles or between vehicles and network infrastructures it is possible to automate certain maneuvers that vehiclesmust perform on the roads, which leads to efficient driving that reduces fuel consumption and greenhouse gas emissions [1]. In addition, in the case of electric vehicles, proper management of vehicular traffic on the roads leads to savings in energy consumption, which extends the life of the batteries and, therefore, reduces pollution due to their manufacturing process [2]. The idea of establishing new vehicular communications systems to improve transportation and road safety is not new. In 1998, the US Congress studied spectrum requirements for the operation of intelligent transportation systems, including the spectrum for the dedicated short-range vehicle-to-wayside wireless standard, known as Dedicated Short Range Communication (DSRC) [3]. Thus, the first standards were based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, also known as Wi-Fi. Complementarily, services based on vehicular communications are also supported by Third Generation Partnership Project (3 GPP) radio access technologies, i.e., Long Term Evolution (LTE) and Fifth Generation (5G) New Radio (NR) [4; 5]. Such technologies are known as Cellular-based V2X (C-V2X) and, unlike IEEE technologies, they enable new use cases and applications that were not contemplated in previous releases. The V2X communications umbrella term includes Vehicle-to-Vehicle (V2V) (communications between vehicles), Vehicle-to-Pedestrian (V2P), Vehicle-to-Infrastructure (V2I) and Vehicle-to-Network (V2N) [6]. Through V2V, two or more vehicles can exchange packets for different V2X services, even without cellular network coverage. The V2I communication mode allows a vehicle to communicate with road safety transmission nodes installed along the roads. In addition, V2I communications can extend the communication range between two or more vehicles. Through V2P, vehicles can establish communications with V2X devices carried by a pedestrian. This makes it possible to send alert messages and warnings to avoid an accident. Using V2N vehicles can establish communications via the cellular network. This allows, for example, a human operator to remotely control a vehicle whose communications pass through the cellular network. A large number of use cases, applications and services have been defined for V2X [4; 5]. Each use case demands certain potential requirements that the network must guarantee in order for that use case to be implemented efficiently. These potential requirements are related to: high transmission rates, low latencies, high reliability, large coverage ranges, etc. To meet these potential requirements, it is necessary to take advantage of certain communication modes and intertwworking mechanisms to improve the robustness and reliability of communications. That there will be a progressive adoption of 5G from Fourth Generation (4G) and coexistence of both technologies is already considered in the specifications. There are architectures that allow combining the use of several access technologies in a heterogeneous environment.