A first device senses scheduling assignment (SA) and received power of a second device, and a total received energy of each subband. The first device determines a reference value of received power of the second device and a reference value of total received energy, based on a sensing result of the second device. The first device selects a resource, based on the reference value of received power and reference value of total received energy, the first device transmits data. By adopting the method of the present disclosure, decoding performance of SA is improved, and accuracy for measuring received power of the SA is also enhanced. Subsequently, on the basis of SA and received power, performances for selecting/re-selecting channel resources are improved.