1. A method for broadcasting a device-to-device (D2D) signal by a user equipment (UE) positioned outside a coverage of a base station (BS), the method comprising:transmitting a first message including a preamble sequence to a first D2D UE in a cluster, wherein an index of the preamble sequence is determined based on a resource used for transmission of the first message;
receiving a second message from the first D2D UE in the cluster as a response to the first message, wherein the second message contains scheduling information to broadcast a third message;
broadcasting the third message in the D2D signal to a plurality of D2D UEs in the cluster based on the scheduling information;
receiving a plurality of ACK/NACKs (acknowledgement/negative-acknowledgements) for the third message from the plurality of D2D UEs in the cluster; and
re-broadcasting the third message in a D2D signal to the plurality of D2D UEs in the cluster if a proportion of NACK among the plurality of ACK/NACKs is equal to or larger than a certain value.