Method for changing a link connection in a communication system and device therefor

1. A method for changing a connection by a user equipment (UE) connected with a relay UE through a device-to-device (D2D) link in a wireless communication system, the method comprising:

receiving information on a threshold value through a broadcast signaling;

measuring a quality of a channel from a cell;

establishing a radio resource control (RRC) connection with the cell and starting a timer, when the measured quality is higher than the threshold value;

transmitting to the relay UE an indication which indicates an application server of the relay UE to stop data transmission; and

releasing the D2D link with the relay UE until the timer expires.

2. The method of claim 1 , wherein if data is being received through the D2D link, the D2D link is released after the data reception is completed.

3. The method of claim 1 , wherein the D2D link is released when the RRC connection establishment is completed.

4. The method of claim 3 , wherein the information on the threshold value comprises validity information,

wherein the validity information includes at least one of time information indicating a time condition in which the threshold value is applied effectively and area information indicating an area condition in which the threshold value is applied effectively, and

wherein the timer is set with a value in the time information.

5. The method of claim 1 , wherein the releasing the D2D link comprises: transmitting assistance information on the relay UE to the cell; receiving a message for releasing the D2D link from the cell in response to the assistance information; and releasing the D2D link based on the message.

6. The method of claim 5 , wherein the releasing the D2D link further comprises:

transmitting a D2D link release message to the connected relay UE; and

receiving a response message in response to the D2D link release message.

7. The method of claim 1 , wherein the threshold value is related to a condition for a PC5 interface.

8. The UE of claim 7 , wherein the threshold value is related to a condition for a PC5 interface.

9. A user equipment (UE) connected with a relay UE through a device-to-device (D2D) link in a wireless communication system, the UE comprising:

a transceiver configured to transmit and receive signals; and

a processor configured to process the signals,

wherein the processor is further configured to:

control the transceiver to receive information on a threshold value through a broadcast signaling,

measure a quality of a channel from a cell,

establish a radio resource control (RRC) connection with the cell and start a timer, when the measured quality is higher than the threshold value,

transmit to the relay UE an indication which indicates an application server to stop data transmission, and

release the D2D link with the relay UE until the timer expires.

10. The UE of claim 9 , wherein if data is being received through the D2D link, the D2D link is released after the data reception is completed.

11. The UE of claim 9 , wherein the processor releasing the D2D link is further configured to release the D2D link when the RRC connection establishment is completed.

12. The UE of claim 11 , wherein the information on the threshold value comprises validity information,

wherein the validity information includes at least one of time information indicating a time condition in which the threshold value is applied effectively and area information indicating an area condition in which the threshold value is applied effectively, and

wherein the timer is set with a value in the time information.

13. The UE of claim 9 , wherein the processor is further configured to control the transceiver to transmit assistance information on the relay UE to the cell, control the transceiver to receive a message for releasing the D2D link from the cell in response to the assistance information, and release the D2D link based on the message.

14. The UE of claim 13 , wherein the processor is further configured to control the transceiver to transmit a D2D link release message to the connected relay UE, and control the transceiver to receive a response message in response to the D2D link release message.