As another example, the D2D discovery transmission probability may be based on a fixed probability or an adaptive probability. (1) When it is based on the fixed probability, whether to transmit a D2D discovery signal on a discovery resource may be determined based on a random function including a probability value P. (2) When it is based on the adaptive probability, this case may be similar to the case based on the fixed probability but the probability value P may adaptively vary. For example, when D2D discovery signal transmission is not executed in a previous period, the probability value P may increase by k, and when D2D discovery signal transmission is executed, the probability value P may increase by m. Alternatively, the probability value P may increase gradually, and may decrease by a predetermined value when a predetermined condition is satisfied.

In an exemplary embodiment, it is assumed that a D2D synchronization Signal (D2DSS) and a Physical D2D Synchronization Channel (PD2DCH) are located in a resource that is configured by a network or a resource determined in advance, so as to effectively support D2D discovery or D2D communication. Therefore, when a D2DSS or a PD2DSCH is located in a resource for D2D discovery or D2D communication (SA/data), a D2D signal and a Wide Area Network (WAN) signal may be multiplexed in the corresponding resource. Alternatively, a D2D synchronization signal and a D2D synchronization channel may be multiplexed with other signals, independently. The WAN refers to a network that configures a wide coverage in a cellular network and provides mobile UEs with voice/data traffic, and the WAN includes WCDMA, LTE, WiMAX, and the like. Radio access networks are generally referred to as a WAN.