# V2X Communication over Cellular Networks: Capabilities and Challenges
## 2 V2X Use Cases  
V2X UCs involve various scenarios with different application requirements. Consequently, they are typically characterized by diverse network performance requirements in terms of latency, reliability, and throughput. In UC definitions, E2E latency is the time needed to transfer a given piece of information from a source to a destination, measured at the application layer [18]. Reliability is defined as the proper reception of a message within specific time constraints [19]. The experienced throughput is the bit rate required for the delivery of the service [8]. 5 GAA has defined a methodology for describing V2X UCs and respective service level requirements. All UCs have been grouped according to whether they serve user safety, assist with driving, enhance vehicle operation, provide convenience, improve the traffic efficiency, or enable autonomous driving [20]. Furthermore, UCs are classified as \"initial/day-1\" or \"advanced\" UCs, partly based on the support level in various releases of 3 GPP's Cellular-V2X (C-V2X) specifications, and partly based on 5 GAA's analysis of whether they will be used from the first phase of C-V2X deployments or in later phases. Regarding the classification of the mode of communication, Vehicle to Vehicle (V2V) refers to direct communication between on-board units (OBUs) of different vehicles, using the C-V2X (PC5) interface, and in bands that are designed to support Intelligent Transportation Systems (ITS) services (e.g., the globally harmonized 5.9 GHz band). It should be noted that V2V communications using the C-V2X (PC5) interface (also called sidelink) can--in principle--also be accommodated in frequency bands that are designated for use by mobile communication networks. Vehicle to Infrastructure (V2I) refers to communication between the OBUs of vehicles and roadside units (RSUs) or locally relevant application servers. Vehicle to Network (V2N) refers to communication between the OBUs of vehicles and the base stations (BS) of mobile communication networks, using the C-V2X (Uu) interface. Because day-1 UCs have non-demanding requirements, it is assumed that these UCs will be supported by LTE networks. As mentioned earlier, in our analysis, we have concentrated on the most advanced UCs. 3 GPP has defined its own UCs for the identification of functional and non-functional requirements, as well as respective evaluations for various scenarios. The families of UCs considered by 3 GPP are as follows: (a) Non-safety V2X services (e.g., connected vehicles, mobile hot spots, dynamic digital map updates, etc. ); (b) safety-related V2X services (e.g., autonomous driving, car platooning, etc. ); and (c) V2X services in multiple 3 GPP Radio Access Technologies (RATs) (LTE, 5G New Radio, or 5G-NR) and network environments. These UCs are described in detail in [2,6]. The UCs proposed by the two organizations present some expected similarities and some interesting differences, so we have attempted to harmonize the terminology. Therefore Table 1, presents a mapping of UCs based on the desired service outcome, the underlying functionalities, and the descriptions provided by the two organizations. As mentioned above, the UCs examined and compared in this paper are those that fall under the overall category of \"advanced, safety and autonomous driving\" UCs, as these demonstrate the most stringent network requirements for Key Performance Indicators (KPIs). Table 1 also presents the UC titles set by 5 GAA and 3 GPP and provides a short description of the expected services per UC. As these two organizations have selected different names for several UCs, we have provided a common nomenclature that is used in the following sections, where information about the delay and the spectrum analysis is provided.