2X sidelink resource pool based on the zone in which the UE is located. The UE further performs sensing for (re) selection of sidelink resources. Based on sensing results, the UE (re)selects some specific sidelink resources and reserves multiple sidelink resources. The UE can perform up to two parallel and independent resource reservation processes. It is also allowed to perform a single resource selection for its V2X sidelink transmission [6]. A UE does not establish and maintain a logical connection to receiving UEs before one-to- many sidelink communication. Higher layer protocols establish and maintain a logical con- nection for one-to-one sidelink communication, including ProSe UE-to-network relay opera- tion. The control plane functions for establishing, maintaining, and releasing the logical connections for unicast sidelink communication are shown in Fig. 7.12. Sidelink discovery is defined as the procedure used by the UE supporting sidelink discovery to find other UE(s) in its proximity, using LTE direct radio signals via PC5 (see Fig. 7.13). Sidelink discovery is supported regardless of whether the UE is within network coverage or out-of-coverage. The service was originally limited to ProSe-enabled public safety UEs to perform sidelink discovery when they were out-of-network coverage where the allowed operating frequency was preconfigured in the UE, and is used even when UE is out-of- network coverage in that frequency. The preconfigured frequency is the same frequency as PC5 Signaling PC5 Signaling protocol protocol PC5-C Signaling protocol Figure 7.12 Control-plane protocol stack for sidelink broadcast and unicast transmissions [6]. Vehicle-to-Everything (V2X) Communications 811 ProSe protocol ProSe protocol PC5-D Figure 7.13 PC5 interface for sidelink discovery [6]. the public safety ProSe carrier [6]. The AS protocol stack for sidelink discovery consists of only MAC and PHY. The AS layer interfaces with upper layer (ProSe protocol) where the MAC sublayer receives the discovery message from the upper 