l and the SBCCH in another subframe, if it assumes the role of the synchronization node. The UE performs sidelink communication in subframes defined over the duration of side- link control period. The sidelink control period is the time interval during which the resources are allocated in a cell for sidelink control information (SCI) and data transmis- sion. Within the sidelink control period, the UE sends SCI followed by sidelink data. SCI indicates a physical layer identifier and resource allocation parameters; for example, modu- lation and coding scheme (MCS), location of the resource(s) over the duration of sidelink control period, timing alignment, etc. The UE performs transmission and reception over LTE-Uu and PC5 (without sidelink discovery gap) where Uu transmission/reception is the highest priority, followed by PC5 sidelink communication transmission/reception and then by PC5 sidelink discovery announcement/monitoring which is considered the lowest prior- ity. The UE further performs transmission/reception over Uu and PC5 (with sidelink discov- ery gap) starting with Uu transmission/reception for RACH, followed by PC5 sidelink discovery announcement during a sidelink discovery gap for transmission; non-RACH Uu transmission; PC5 sidelink discovery monitoring during a sidelink discovery gap for recep- tion; non-RACH Uu reception; and PC5 sidelink communication transmission/reception [6]. The UE radio protocol structure for sidelink communication consists of user-plane and control-plane protocols. Fig. 7.11 shows the protocol stack for the user plane (the access PC5-U Figure 7.11 User-plane protocol stack for sidelink communication [6]. Vehicle-to-Everything (V2X) Communications 809 stratum protocol stack over PC5 interface), where packet data convergence protocol (PDCP), radio link control (RLC), and medium access control (MAC) sublayers, which are terminated at the other UE, perform similar functions defined for LTE protocols with some exceptions. The user plane corresponding to sidelink communication doe