## 1 Introduction  
Over the past decade, the transport demand as well as transport volume per capita in Europe has steadily risen [1]. Apart from the positive effects such as economic growth and better access to education and healthcare, this phenomenon brings along many challenges including safety, road congestion and negative environmental impact. To help to address these challenges, the transport sector increasingly relies on advanced technology, e.g., vehicular communication, to streamline its operations. Vehicular communications have experienced a rapid increase of interest in the last years. It is believed that establishment of communication links between vehicles and the rest of the traffic actors, known as the vehicle-to-everything (V2X) communication, will dramatically improve the road traffic safety and efficiency. This belief represents a main driving force behind the increased interest in the vehicular communications experienced in the last years. This paradigm is entitled the Cooperative Intelligent Transportation Systems (C-ITS) in the literature and its benefits are naturally very important. Therefore, this research area has been considered as a strategic research area in the European Union and most developed countries. It should be noted here that when it comes to the V2X communication, it covers different kinds of communication between vehicles or vehicles and infrastructure or even pedestrians, i.e., Vehicle-to-Vehicle (V2V), Vehicle-to-Infrastructure (V2I), Vehicle-to-Network (V2N) and Vehicle-to-Pedestrian (V2P). V2V communication refers to the exchange of communication messages between vehicles, usually through a self-organized ad hoc communication network, i.e., a network without central management. The messages are exchanged directly between vehicles equipped with V2X-capable Onboard Units (OBUs). In the case of V2I communication, the messages are transmitted between a vehicle and a Roadside Unit (RSU), which is an electronic device capable of receiving and processing communication messages generated by vehicles and generating and transmitting compatible messages to vehicles and other digital devices composing the transport infrastructure. V2I communication can be facilitated either by ad hoc or a centrally managed communication network. V2P communication refers to the exchange of communication messages between vehicles and V2X-enabled wearable or handheld devices carried by pedestrians, cyclists, etc. The communication is, in this case, usually facilitated by a centrally managed infrastructural network, such as LTE- or 5G-based cellular network. Finally, V2N communication is exclusive to cellular networks and facilitates the communication between vehicles and the remaining parts of the Intelligent Transport System (ITS), i.e., the V2X management system and also the V2X application server, using the existing cellular infrastructure. Regarding V2X communication technologies, two key technologies are currently deployed. First of them is Dedicated Short-Range Communications (DSRC), which is also referred as the IEEE 802.11p. The second technology is Cellular-V2X (C-V2X), which is portrayed in the corresponding specifications of The 3 rd Generation Partnership Project (3 GPP) Release 14. This represents the so-called 4G-based C-V2X. Moreover, there is an extension of the C-V2X concept towards 5G cellular networks, entitled 5G-based C-V2X, described in the corresponding specifications of the 3 GPP Release 16. The main reason to develop the DSRC technology was to allow a deployment of collision prevention applications [2]. It is perceived as reliable, rather easy to implement and cheap, almost patent-free and, most importantly, immediately available. However, an application range of the C-V2X is wider in the areas of navigation, traffic data, entertainment, and, most notably, driving automation.