Method for triggering a sidelink buffer status reporting in a D2D communication system and device therefor

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

• a Radio Frequency (RF) module including a transceiver configured to transmit and receive information;
• and
• a processor operably coupled with the RF module and configured to:
• configure one or more Proximity based Services (ProSe) destinations for SideLink (SL) communication,
• wherein each ProSe destination is related to one or more Logical Channel Groups (LCGs), and each LCG includes one or more sidelink (SL) Logical Channels (LCHs);
• control the RF module to receive first SL data at an upper layer of a Medium Access Control (MAC) entity;
• and
• in case that the first SL data is related with a ProSe Destination that already has second SL data available for transmission, trigger a SL buffer status reporting process by the MAC entity when the first SL data belongs to a SL LCH which has a higher priority than any SL LCH of any LCG related to the second SL data within the ProSe Destination.