of IEEE 802.11a, leveraging a radio technology that is over two dec- ades old and was originally developed for wireless Ethernet cable replacement and not for high-speed mobile applications. IEEE 802.11p performance results indicate its limited range and unreliable large-scale field performance, derived from susceptibility to conges- tion and lack of minimum performance guarantees, which would limit its usefulness. Most of the present-day cars are equipped with several active sensors, including camera, radar, and LIDAR, which compel the V2X wireless sensor to provide longer range and reliabil- ity, especially in non-line-of-sight (NLoS) scenarios where other vehicles and buildings obstruct the vehicle's vision systems. In the meantime, 3GPP has continued to evolve LTE device-to-device (D2D) technology, specified as part of Rel-12, and to optimize it for automo- tive applications in Rel-14, also referred to as C-V2X communications, over proximate radio interface (PC5) or sidelink and/or the LTE Uu interface. The C-V2X incorporates LTE's mobility support and further extends the baseline standard for automotive applications, while learning from IEEE 802.11p issues and shortcomings. The C-V2X includes both direct com- munications and network-based communications. Fig. 7.2 shows various scenarios for sidelink and direct link communications where UEs are located in-coverage and out-of-coverage of a cell. The future transportation technologies will include connected and intelligent vehicles, which can cooperate with each other and a transportation infrastructure that can provide safer and more convenient commute/travel experience. A wide range of use cases that require longer range or higher throughput can be supported with LTE-based/NR-based V2X communica- tions. C-V2X is the technology developed in 3GPP and is designed to operate in the follow- ing two modes: 792 Chapter 7 Direct communication Direct discovery Synchronization ((<000)) Control LTE/NE-Uu LTE/NE-Uu LTE-Uu/NR-Uu Radio interface downlink broadcast (MBSFN/S