ommunication, the following radio-layer enhancements are considered to improve sidelink communication performance in distributed resource allocation mode: Group radio-layer feedback When transmitter node sends a sidelink transmission/message to a group of UEs, the UE that has not successfully received physical sidelink shared channel (PSSCH) sends a NACK on physical sidelink control channel (PSCCH) resource reserved by the transmitter for acknowledgment. Group radio-layer (re-)transmission - If the UEs in a group detect a NACK from at least one of the group members, the UEs can retransmit successfully the received packet on a resource that can be either reserved by the UE which is failed to receive or by the original source of transmis- sion. Note that for groupcast communication, the same principles of sensing and resource selection as well as resource reservation can be reused. The group radio- layer feedback and (re-)transmissions can be developed using the same sidelink channel access and resource selection mechanisms. The latency of V2X systems is categorized into transmission time interval (TTI) dependent and TTI-independent, depending on whether the latency is proportional to or independent of the transmission time interval of the radio air-interface [15]. Each data transmission, that is, control signaling, scheduling, HARQ (re-)transmission, and SO on, consumes at least one TTI. The V2X services can be categorized into three groups: (1) safety-related services, (2) non-safety-related services, and (3) automated driving-related services. The safety-related services handle real-time safety messages, such as warning messages (e.g., abrupt brake warning message) to reduce the risk of car accidents. In this type of services, timeliness and reliability are the key requirements. On the other hand, non-safety-related services are intended to optimize the traffic flow on the road SO that travel time is reduced. Therefore, Vehicle-to-Everything (V2X) Communications Table 7.1: Performance requirements of different V