 resources should be allocated among the PLMNs, but introduces a coordi- nation mechanism (through the V2X control function) between PLMNs to avoid packet col- lisions [17]. The frame timing synchronization is an important consideration. Transmission of the ith sidelink radio frame from the UE starts at (NTA,SL + NTA offset)T seconds before the start of the corresponding timing reference frame at the UE (see Fig. 7.22). The UE is not required to receive sidelink or downlink transmissions earlier than 624T's following the end of a side- link transmission. The parameter NTA,SL differs between channels and signals where NTA,SL = NTA for PSSCH in sidelink transmission mode 1 and 0 otherwise [3]. 7.4.1 Sidelink Physical Resources and Resource Pool In LTE-based sidelink communication, physical layer transmissions are organized in the form of radio frames with duration Tframe = 10 ms, each consisting of 20 slots of duration Tslot = 0.5 ms. A sidelink subframe consists of two consecutive slots. A physical channel or signal is transmitted in a slot and is described by a resource grid of NSLNRB SC subcarriers symbol SC-FDMA symbols. The sidelink bandwidth is equal to the uplink bandwidth NSL = NUL RB if the cell-selection criterion is satisfied for a serving cell having the same uplink carrier frequency as the sidelink; otherwise, a preconfigured value is used. The side- link cyclic prefix is configured independently for type 1 discovery, type 2B discovery, side- link transmission mode 1, sidelink transmission mode 2, control signaling, and PSBCH as well as synchronization signals. The configuration is done per resource pool for discovery, sidelink transmission mode 2, and control signaling. The PSBCH and synchronization sig- nals always use the same cyclic prefix value. Normal cyclic prefix is only supported for PSSCH, PSCCH, PSBCH, and synchronization signals for a sidelink configured with trans- mission mode 3 or 4 [3]. Each resource element in the resource grid is uniquely defined by the index pair 0, , NSLNRB - 1; l =