C-PTM) eNB/RSU LTE-Uu/NR-Uu Radio In-coverage Partial coverage Out-of-coverage interface uplink unicast Sidelink (PC5) Autonomous LTE/NR PC5 Radio communications interface sidelink broadcast ((<000)) gNB/eNB Autonomous discovery Network-assisted discovery Network-assisted V2X UE communication Figure 7.2 Various V2X scenarios 12-14]. Device-to-device: This mode is sidelink communication between two or more devices without network involvement. Device-to-network: This mode uses the traditional cellular links to enable cloud services as part of the end-to-end solution by means of network slicing architecture for vertical industries. Some key performance metrics for low-latency local communications that are addressed by LTE-based V2X as part of Rel-14 include support of vehicle speeds up to 160 km/h and rel- ative speeds of 280 km/h; extended range to provide the driver(s) with sufficient response time (e.g., 4 seconds); message sizes for periodic broadcast messages between two vehicles/ UEs with payloads of 50-300 bytes and for event-triggered messages up to 1200 bytes; and maximum latency of message transfer of 100 ms between two UEs and 1000 ms for mes- sages sent via a network server [27]. In subsequent releases, more stringent requirements for latency, range, speed, reliability, location accuracy, and message payloads have been specified in order to support more advanced use cases. For example, NR-based V2X sys- tems are expected to provide end-to-end latency between vehicles of less than 5 ms and guarantee more than 99% reliable packet delivery within a short-to-medium range (80-200 m). RSUs are the new network nodes that are part of the LTE-based V2X communications sys- tem. These entities can be co-located with eNBs or operate in standalone mode. The V2I communication mode allows RSUs to monitor traffic-related conditions, such as traffic sig- nals and tolls and subsequently notify the surrounding vehicles. The evolution path for LTE-based V2X is going to exploit the NR features and to provide ubiquitous cover