he standard does not specify a resource management algorithm for mode 3. Each operator can implement its own algorithm that should fall under one of two categories: (1) Dynamic scheduling where the vehicles request subchannels from the eNB for each packet transmission, which increases the signaling overhead, and delays the packet transmission until vehicles are noti- fied of their assigned subchannels; and (2) SPS-based scheduling where the base station reserves subchannels for the periodic transmissions of a vehicle. However, in contrast to mode 4, the eNB decides how long the reservation should be maintained and it can activate, deactivate, or modify reservation of subchannels for a vehicle. The vehicle must inform the eNB of the size, priority, and transmission frequency of its packets SO that the eNB can semi-persistently reserve the appropriate subchannels. This information must be provided to the eNB at the start of a transmission, or when any of the traffic characteristics (size, prior- ity, and frequency) change [17]. Vehicles operating in mode 3 can be supported by different cellular operators. To enable their direct communication, 3GPP has defined an inter-PLMN architecture that can support vehicles subscribed to different PLMNs to transmit over different carriers. In this case, vehi- cles must be able to simultaneously receive the transmissions of vehicles supported by other PLMNs on multiple carriers. Therefore, each PLMN broadcasts in the sidelink V2X config- urable parameters the necessary information SO that the subscribed vehicles can receive the packets transmitted by other vehicles supported by different PLMNs. In an alternative sce- nario, the vehicles may be supported by different PLMNs sharing the same carrier, where each PLMN is assigned a fraction of the resources of the carrier. The standard does not Vehicle-to-Everything (V2X) Communications 821 Timing reference radio frame i Sidelink radio frame i (NTA, SL+ NTA offset) Ts seconds Figure 7.22 Sidelink timing alignment [3]. specify how the