1. A method for a base station, the method comprising:transmitting, to a pair of device-to-device (D2D) devices, spectrum information for indicating a plurality of frequency spectrums, wherein the D2D devices are unlicensed for the plurality of frequency spectrums;
receiving, from the pair of D2D devices, load information on a load level of a frequency spectrum among the plurality of frequency spectrums detected by the pair of D2D devices, wherein:
the frequency spectrum among the plurality of frequency spectrums is allocated to another licensed user device of a cellular communication, and
the load level indicates how frequently the frequency spectrum is occupied by the other licensed user device of the cellular communication;
determining a mode between a cellular mode and a D2D mode based on the frequency spectrum being occupied by the other licensed user device of the cellular communication more frequently than a predetermined threshold according to the load information; and
transmitting, to the pair of D2D devices, instruction information indicating the mode.