Method and apparatus for changing, by terminal, priority in MCPTT

1. A method for changing, by a user equipment (UE), a priority in a wireless communication system, the method comprising:

changing the priority of a group for sidelink communication from a first priority to a second priority, wherein the UE belongs to the group;

after changing the priority of the group for sidelink communication from the first priority to the second priority, selecting a first transmission resource pool related to the first priority;

transmitting information regarding the second priority to at least one UE in the group, using the selected first transmission resource pool related to the first priority;

after the information regarding the second priority is transmitted, selecting a second transmission resource pool related to the second priority; and

transmitting data to the at least one UE in the group, using the selected second transmission resource pool related to the second priority.

2. The method of claim 1 , wherein the priority is changed from the first priority to the second priority when an emergency situation or a termination of an emergency situation occurs.

3. The method of claim 2 , wherein the priority is changed from the first priority to the second priority when the emergency situation occurs, and

wherein the second priority is higher than the first priority.

4. The method of claim 2 , wherein the priority is changed from the first priority to the second priority due to a termination of an emergency situation, and

wherein the first priority is higher than the second priority.

5. The method of claim 2 , further comprising:

transmitting, to the at least one UE in the group, information that informs the at least one UE about the occurrence of the emergency situation or the termination of the emergency situation.

6. The method of claim 2 , wherein the emergency situation includes at least one of an MCPTT emergency alert, an MCPTT emergency group call, or an MCPTT emergency private call, and

wherein the termination of the emergency situation includes at least one of a termination of an MCPTT emergency alert, a termination of an MCPTT emergency group call, or a termination of an MCPTT emergency private call.

7. The method of claim 1 , wherein the priority is changed from the first priority to the second priority by changing a priority of the UE.

8. The method of claim 7 , wherein the priority of the UE informs at least one of an application layer pre-emption level, whether the UE is allowed to pre-empty a message, an application layer priority value, or a priority assigned to the UE.

9. The method of claim 1 , wherein when the UE has the data to be transmitted in a buffer, and when user data with a higher priority than the second priority is not received, the data is transmitted.

10. The method of claim 1 , further comprising:

storing the first priority.

11. The method of claim 1 , wherein the second priority is transmitted via a PC5 message.

12. The method of claim 1 , further comprising:

transmitting, to the at least one UE in the group, at least one of an ID of the group, an ID of the UE, or a priority of the UE.

13. A user equipment (UE) for changing a priority, the UE comprising:

at least one transceiver;

at least one processor; and

at least one computer memory, operably connectable to the at least one processor, and storing instructions that, when executed, cause the at least one processor to perform operations comprising:

changing the priority of a group for sidelink communication from a first priority to a second priority, wherein the UE belongs to the group;

after changing the priority of the group for sidelink communication from the first priority to the second priority, selecting a first transmission resource pool related to the first priority;

transmitting the second priority to at least one UE in the group, using the selected first transmission resource pool related to the first priority;

after the second priority is transmitted, selecting a second transmission resource pool related to the second priority; and

transmitting data to the at least one UE in the group, using the selected second transmission resource pool related to the second priority.