A method for performing wireless communication by a first device and a device for supporting same are provided. The method may include receiving a plurality of reference signals from a second device through a physical sidelink control channel (PSCCH) or a physical sidelink shared channel (PSSCH), herein, the plurality of reference signals are transmitted through a plurality of antenna ports, determining a reference signal received power (RSRP) value using the plurality of reference signals based on indexes for the plurality of antenna ports, and transmitting the RSRP value to the second device.