ecent years to deploy V2X communications using IEEE 802.11p. However, IEEE 802.11p uses a carrier sense multiple access scheme with collision avoidance which may not be able to guarantee stringent reliability levels and network scal- ability as the traffic increases. As an alternative, 3GPP LTE Rel-14 included support for V2X communications. The LTE-based V2X physical layer improves the link budget relative to IEEE 802.11p. In addition, it can improve the reliability, under certain conditions, by adding redundant transmission per packet. The C-V2X standard includes two radio inter- faces. The LTE-based Uu radio interface supports vehicle-to-infrastructure communications, while the PC5 interface supports V2V communications based on LTE-based sidelink com- munications. LTE sidelink (or D2D communication) was originally introduced in Rel-12 for public safety, and included two modes of operation: mode 1 and mode 2 (see Fig. 7.14). Both modes were designed with the objective of prolonging the battery life of mobile devices at the cost of increased latency. Connected vehicles require highly reliable and low- latency V2X communications; therefore, modes 1 and 2 are not suitable for vehicular appli- cations. 3GPP Rel-14 introduced two new communication modes (modes 3 and 4) specifi- cally designed for V2V communications (see Fig. 7.15). In mode 3, the cellular network selects and manages the radio resources used by vehicles for their direct V2V communica- tions. In mode 4, vehicles autonomously select the radio resources for their direct V2V communications. In mode 4, the UE (the vehicle) can operate without cellular coverage; therefore, this is considered the baseline V2V mode since safety applications cannot depend DCI Format 5 on (e)PDCCH with CRC Scrambled with SL-RNTI ((III) ) EPC with preSe function SIB18 Provides reception resource pools V2X UE1 V2X UE2 SCI Format 0 on PSCCH Data on PSSCH according to Information in SCI Format 0 scrambled with group destination ID Figure 7.14 Scheduling transmission resources for 