 cal safety services when network coverage is unavailable or if the vehicle does not have an active cellular subscription. These parameters can also be securely updated by the vehicle manufacturers or the mobile operators. For V2I applications, the infrastructure that includes an RSU can be implemented in an eNB or a stationary UE. Fig. 7.10 shows two RSU implementation options and the func- tional entities in each case. 7.3.2 Sidelink and Radio Access Protocols Sidelink communication is a mode of operation whereby UEs can communicate with each other directly over the PC5 interface. This operation mode is supported when the UE is served by an eNB/gNB or outside of the network coverage. The use of sidelink communica- tion was originally limited to those UEs that were authorized for public safety operation; however, the application of sidelink communication was further extended to V2X services. In order to perform synchronization for out-of-coverage devices, the originating device may act as a synchronization source by transmitting sidelink broadcast control channel (SBCCH) and synchronization signals. The SBCCH carries the most essential system information needed to receive other sidelink channels and signals. In LTE V2X, the SBCCH is transmitted along with a synchronization signal with a fixed periodicity of 40 ms [6]. 808 Chapter 7 When the UE is in network coverage, the content of SBCCH are derived from the para- meters signaled by the serving eNB. When the UE is out-of-network coverage and if it selects another UE as a synchronization reference node, then the system information is obtained from SBCCH transmitted by the reference node; otherwise, the UE uses preconfi- gured parameters. The system information block type 18 (SIB18) provides the resource information for the synchronization signal and SBCCH transmission. There are two precon- figured subframes every 40 ms for out-of-coverage operation. The UE receives the synchro- nization signal and the SBCCH in one subframe and transmits the synchronization signa