A method for performing wireless communication by a first device is proposed in an embodiment. The method may comprise the steps of: selecting a synchronization source on the basis of a sidelink synchronization priority; acquiring synchronization on the basis of the synchronization source; transmitting a sidelink-synchronization signal block (S-SSB) to a second device on the basis of the acquired synchronization; transmitting, to the second device, information related to a pattern of a physical sidelink shared channel (PSSCH) demodulation reference signal (DMRS) for decoding a PSSCH through sidelink control information (SCI) on a physical sidelink control channel (PSCCH); mapping the PSSCH DMRS onto a time resource related to the PSSCH on the basis of the information related to the pattern of the SSCH DMRS and an interval of a time resource scheduled for transmission of the PSSCH related to the PSCCH; and transmitting the PSSCH DMRS to the second device through the PSSCH.