are designed to support public safety application requirements, but unlike C-ITS applications, reliability and low latency are not required for such applications. Therefore the Rel-14 introduced modes 3 and 4 are most relevant for LTE C-V2X applica- tions. Both modes 3 and 4 support direct vehicular communications but differ on how resources are allocated. In mode 3, vehicles are within the coverage of cellular network, resources are selected, allocated, and reserved by the base station (eNodeB). Mode 4 was Chapter 11 Differentiation by use case Mode 3 Mode 4 FIGURE 11-5 LTE C-V2X modes. designed to work when vehicles are not under the coverage of cellular network. Resources in mode 4 are autonomously selected by vehicles. As it is considered as an ad hoc system, it is challenging to avoid collisions when selecting resources. Distributed scheduling algorithms and Distributed Congestion Control (DCC) algorithms are developed for this purpose. Mode 4 is required for V2X since C-ITS safety-related applications must be established even outside network coverage. C-V2X is a synchronous communication method, which means that the system needs to get time aligned, either through Global Positioning System connectivity or by synchronizing to other vehicles/infrastructure (Fig. 11-5). NR C-V2X (PC5) 3GPP's 5G standards have been finalized in Release 15 (Phase 1, September 2018) and Release 16 (Phase 2, Q4 2019). It is to be expected (given the deployment timeline as dis- cussed below) that a 5G version of C-V2X will be more widely deployed than a 4G version given that the 4G version has limited legacy. NR C-V2X is based on similar design principles as LTE C-V2X but supports flexible numerology and forward error correction using Polar and Low Density Parallel Codes (LDPC) as defined in the 5G standard. Standards summary The following table summarizes the different wireless protocol stacks: 802.11p LTE C-V2X 5G C-V2X Synchronization Asynchronous Synchronous Synchronous Multiplexing TDM + FDM TDM + FDM Channel coding Convolutio