PSBCH. These signals have a periodicity of 40 ms. Assuming the device cannot detect an eNB, due to possibly being out-of-coverage, the device starts looking for SLSS from other devices and performs signal quality measure- ments (S-RSRP) on those synchronization signals. The SLSS comprise a primary sidelink synchronization signal (PSSS) and a secondary sidelink synchronization signal (SSSS). The PSSS and SSSS are both transmitted in adjacent time slots in the same subframe (see Fig. 7.26). The combination of both signals defines a sidelink ID (SID), similar to the physi- cal cell ID transmitted in the downlink. The SIDs are split into two sets. The SIDs in the range of {0, 1, , 167} are reserved for in-coverage, whereas the SIDs {168, 169 are used when the device is out-of-coverage. The subframes to be used as radio resources to transmit SLSS and PBSCH are configured by higher layers and no PSDCH, PSCCH, or PSSCH transmissions are allowed in these subframes. The resource mapping is slightly dif- ferent for normal and extended cyclic prefix. Fig. 7.27 shows the mapping for normal cyclic prefix. In the frequency domain, the inner six RBs are reserved for SLSS and PSBCH trans- mission. More specifically, a physical layer sidelink synchronization identity is represented {0, 1, ., 335}, divided into two sets consisting of identities {0, 1, , 167} and {168, 169, ., 335}. The PSSS is transmitted in two adjacent SC-FDMA symbols in the same subframe. Each of the two sequences di(0), di(61), is used for the PSSS the two SC-FDMA symbols with root index 26 and u=37, otherwise. The sequence d(n) used for the primary synchronization signal is derived from a frequency Vehicle-to-Everything (V2X) Communications 831 Subframe 1 ms DMRS Location for Rel-12/13 PSCCH/PSSCH DMRS Location for Rel-12/13 PSBCH DMRS Location for V2V PSCCH/PSSCH DMRS Location for V2V PSBCH Figure 7.27 LTE V2X sidelink frame structure. domain Zadoff-Chu sequence according todu(n)exp-jun(n+1)/63])n=0,1,...,30 and du(n) = exp[-jru(n + 1)(n + ,61, where U denot