Method for transmitting a MAC PDU on SL-DCH in a D2D communication system and device therefor

A User Equipment (UE) operating in a wireless communication system, the UE comprising:

• a transmitter and a receiver;
• and
• a processor, operatively coupled to the transmitter and the receiver, wherein the processor is configured to:
• trigger a new transmission of a Medium Access Control (MAC) Protocol Data Unit (PDU) of a sidelink process for a sidelink Hybrid ARQ (HARQ) entity, wherein the sidelink HARQ entity is configured with a first counter to count a number of transmissions within a transmission period and a second counter to count a number of transmission periods, and
• control the transmitter to transmit the MAC PDU in a HARQ buffer of the sidelink process until a value of the first counter reaches the first maximum value and a value of the second counter reaches the second maximum value,
• wherein the value of the second counter is incremented by 1, each time the MAC PDU is transmitted as a new transmission during a first transmission opportunity of each transmission period,
• wherein a value of the first counter is counted only within a last transmission period when the value of the second counter reaches the second maximum value, and
• wherein when the value of the first counter reaches the first maximum value and the value of the second counter reaches the second maximum value, the UE flushes the HARQ buffer.