Terminal apparatus that efficiently changes connection-target cells, and control method

1 . A terminal apparatus that communicates with a base station apparatus using a primary cell and a secondary cell through carrier aggregation, the terminal apparatus comprising:

one or more processors; and

one or more memories that store a computer-readable instruction for causing, when executed by the one or more processors, the one or more processors to execute operations including:

receiving both of parameters required when one or more cells are used as primary cells, respectively, and parameters required when the one or more cells are used as secondary cells, respectively;

changing a first cell among the one or more cells to a primary cell in a case where the first cell is being used as a secondary cell; and

receiving, from the base station apparatus, information regarding whether or not to continue to use, as a secondary cell, a second cell that will be no longer a primary cell after the first cell is changed to a primary cell,

wherein, when changing the first cell to a primary cell, the terminal apparatus executes random access control on the primary cell,

wherein the operations further include performing determination of whether the random access control was successful or failed,

wherein in the determination of whether the random access control was successful or failed:

a timer is started at a timing when the terminal apparatus instructs a change,

a timer is stopped upon success of random access control, and

it is determined whether or not a started timer expires, and

wherein the operations further includes determining that a switch of a primary cell was successful or failed, based on a result of successful or failed random access control.

2 . The terminal apparatus according to claim 1 , wherein the operations further include

when an instruction to continue to use the second cell as a secondary cell is received, determining whether or not a CDRX (connected discontinuous reception) control parameter of the second cell is the same parameter in a case where the second cell is used as a primary cell and in a case where the second cell is used as a secondary cell; and

continuing a timer related to CDRX control that is running, without stopping the timer, in a case where it is determined that the CDRX control parameter is the same and the second cell is changed to a secondary cell.

3 . The terminal apparatus according to claim 1 , wherein the operations further include

determining whether or not a parameter related to beam control of the second cell is the same parameter in a case where the second cell is used as a primary cell and a case where the second cell is used as a secondary cell, when an instruction to continue to use the second cell as a secondary cell is received; and

continuing beam recovery processing instead of stopping the beam recovery processing upon a change of the second cell to a secondary cell, in a case where it is determined that the parameter related to beam control is the same, and the beam recovery processing had been started when the second cell was being used as a primary cell.

4 . The terminal apparatus according to claim 1 , wherein the operations further include

determining whether or not HARQ-related setting for the first cell is the same in a case where the first cell is used as a primary cell and a case where the first cell is used as a secondary cell; and

holding data held in a HARQ soft buffer of the terminal apparatus instead of erasing the data, if it is determined that the HARQ (hybrid automatic repeat request)-related setting is the same.

5 . The terminal apparatus according to claim 1 , wherein

wherein the terminal apparatus receives a parameter using an RRC Reconfiguration message, and

when changing the first cell to a primary cell, the terminal apparatus obtains a value of an RNTI (a radio network temporary identifier) that is used after a change of the first cell to a primary cell, from the base station apparatus using MAC CE (media access control/control element) or DCI (downlink control information).

6 . The terminal apparatus according to claim 1 , wherein

the terminal apparatus receives, as parameters required when the one or more cells are used as primary cells, an RRC Reconfiguration message that includes a value of RNTI (radio network temporary identifier) that is used after the one or more cells are set as primary cells and the primary cells are changed,

the terminal apparatus changes the first cell to a primary cell in accordance with receiving an instruction given from the base station apparatus using MAC CE (media access control/control element) or DCI (downlink control information), and

a value of RNTI received by the terminal apparatus is used with respect to the first cell after the first cell has changed to a primary cell.

7 . The terminal apparatus according to claim 1 , wherein

if it is determined that a timer has expired without random access control being successful, the terminal apparatus determines that a switch to a primary cell failed.

8 . The terminal apparatus according to claim 7 , wherein the operations further include

starting reconnection processing in accordance with determining that a switch to a primary cell failed.

9 . A control method performed by a terminal apparatus that communicates with a base station apparatus using a primary cell and a secondary cell through carrier aggregation, the method comprising:

receiving both of parameters required when one or more cells are used as primary cells, respectively, and parameters required when the one or more cells are used as secondary cells, respectively;

changing a first cell among the one or more cells to a primary cell in a case where the first cell is being used as a secondary cell; and

receiving, from the base station apparatus, information regarding whether or not to continue to use, as a secondary cell, a second cell that will be no longer a primary cell after the first cell is changed to a primary cell,

wherein, when changing the first cell to a primary cell, the terminal apparatus executes random access control on the primary cell,

wherein the operations further include performing determination of whether the random access control was successful or failed,

wherein in the determination of whether the random access control was successful or failed:

a timer is started at a timing when the terminal apparatus instructs a change,

a timer is stopped upon success of random access control, and

it is determined whether or not a started timer expires, and

wherein the operations further includes determining that a switch of a primary cell was successful or failed, based on a result of successful or failed random access control.