Method for transmitting D2D signal and apparatus for same

1. A method of performing a HARQ (hybrid automatic repeat request) procedure, which is performed by a first D2D (device-to-device) UE in a wireless communication system, comprising the steps of:

receiving data from a second D2D UE;

transmitting a HARQ response to the second D2D UE in response to the data; and

if the HARQ response corresponds to NACK (negative acknowledgement),

receiving a retransmission indicator from the second D2D UE at a first timing, and

receiving a retransmission signal from a base station in response to the data at a second timing,

wherein the second timing is determined based on the first timing.

2. The method of claim 1 , further comprising:

receiving a PDCCH (physical downlink control channel) scheduling the data,

wherein a CRC (cyclic redundancy check) of the PDCCH scheduling the data is masked with a group-RNTI (radio network temporary identifier), and

wherein the group-RNTI is determined based on a group ID (identifier) of a group to which the first D2D UE and the second D2D UE belong thereto.

3. The method of claim 1 ,

wherein, if the data includes a plurality of continuously received data, the HARQ response is transmitted in a subframe satisfying (10*SFN+SF) mod N=0 only,

wherein the SFN corresponds to a radio frame index,

wherein the SF corresponds to a subframe index in a radio frame,

wherein the mod indicates a modulo calculation, and

wherein the N corresponds to an integer equal to or greater than 2.

4. The method of claim 1 , wherein if the HARQ response corresponds to ACK, transmission of the HARQ response signal is omitted at a timing assigned to transmit the HARQ response.

5. A first D2D (device-to-device) UE configured to perform a HARQ (hybrid automatic repeat request) procedure in a wireless communication system, comprising:

an RF (radio frequency) unit; and

a processor, the processor configured to

receive data from a second D2D UE, transmit a HARQ response to the second D2D UE in response to the data, and

if the HARQ response corresponds to NACK (negative acknowledgement),

receive a retransmission indicator from the second D2D UE at a first timing, and

receive a retransmission signal from a base station in response to the data at a second timing,

wherein the second timing is determined based on the first timing.

6. The first D2D UE of claim 5 ,

wherein the processor is further configured to receive a PDCCH (physical downlink control channel) scheduling the data, and

wherein a CRC (cyclic redundancy check) of the PDCCH scheduling the data is masked with a group-RNTI (radio network temporary identifier), and

wherein the group-RNTI is determined based on a group ID (identifier) of a group to which the first D2D UE and the second D2D UE belong thereto.

7. The first D2D UE of claim 5 ,

wherein, if the data includes a plurality of continuously received data, the HARQ response is transmitted in a subframe satisfying (10*SFN+SF) mod N=0 only,

wherein the SFN corresponds to a radio frame index,

wherein the SF corresponds to a subframe index in a radio frame,

wherein the mod indicates a modulo calculation, and

wherein the N corresponds to an integer equal to or greater than 2.

8. The first D2D UE of claim 5 , wherein if the HARQ response corresponds to ACK, transmission of the HARQ response signal is omitted at a timing assigned to transmit the HARQ response.