Method for small data transmission in RRC inactive state and related devices

1 . A method for small data transmission (SDT) in RRC_INACTIVE state, performed by a user equipment (UE) in a network, the method comprising:

receiving a Radio Resource Control (RRC) release message used for providing SDT configuration, and a first timing advance (TA) command transmitted along with the RRC release message;

applying the first TA command upon the reception of the RRC release message;

starting or restarting Timing Alignment Timer (TAT) upon the reception of the first TA command transmitted along with the RRC release message, for keeping uplink (UL) time alignment during SDT in RRC_INACTIVE state; and

restarting the TAT upon reception of a second TA command in the RRC_INACTIVE state, wherein the reception of the second TA command in the RRC_INACTIVE state is carried out by:

transmitting a subsequent SDT indication to the network for indicating whether subsequent SDT is waiting to be transmitted to the network or not, wherein the subsequent SDT indication is also used for requesting the TAT to be restarted in the RRC_INACTIVE state; and

receiving the second TA command which is in response to the subsequent SDT indication.

2 . The method of claim 1 , wherein the second TA command is carried by Media Access Control (MAC) Control Element (CE).

3 . The method of claim 1 , wherein the subsequent SDT indication is multiplexed with RACH (random access channel)-based SDT.

4 . The method of claim 1 , wherein the subsequent SDT indication is multiplexed with CG (configured grant)-based SDT.

5 . The method of claim 1 , further comprising:

stopping the TAT used in RRC CONNECTED state and associated with one or more Timing Advance Groups (TAGs).

6 . The method of claim 1 , further comprising:

performing, after entering the RRC_INACTIVE state, initial UL configured grant (CG)-based SDT while the TAT started or restarted upon the reception of the first TA command is running; and

performing subsequent CG-based SDT after the initial CG-based SDT.

7 . The method of claim 6 , further comprising:

performing RACH-based SDT after the initial CG-based SDT in response to missing transmission timing on CG resource.

8 . The method of claim 6 , further comprising:

after missing transmission timing on CG-based resource, performing CG-based SDT following RACH-based SDT,

wherein the CG-based resource on which the CG-based SDT is transmitted is reactivated after the TAT is updated through the RACH-based SDT.

9 . The method of claim 1 , further comprising:

performing, after entering the RRC_INACTIVE state, initial RACH-based SDT no matter the TAT is running.

10 . The method of claim 1 , wherein the subsequent SDT indication is included in MSGA of a 2-step RACH SDT or MSG3 of a 4-step RACH SDT.

11 . A method for small data transmission (SDT) in RRC_INACTIVE state, performed by a base station (BS) in a network, the method comprising:

transmitting to a user equipment (UE) a Radio Resource Control (RRC) release message used for providing SDT configuration, and a first timing advance (TA) command transmitted along with the RRC release message;

expecting the UE to apply the first TA command upon the reception of the RRC release message;

expecting the UE to start or restart Timing Alignment Timer (TAT) upon the reception of the first TA command transmitted along with the RRC release message, for keeping uplink (UL) time alignment during SDT in RRC_INACTIVE state; and

expecting the UE to restart the TAT upon reception of a second TA command in the RRC_INACTIVE state, wherein the reception of the second TA command in the RRC_INACTIVE state is carried out by:

receiving from the UE a subsequent SDT indication for indicating whether subsequent SDT is waiting to be transmitted to the network or not, wherein the subsequent SDT indication is also used for requesting the TAT to be restarted in the RRC_INACTIVE state; and

transmitting the second TA command which is in response to the subsequent SDT indication.

12 . A user equipment (UE), comprising a memory, a transceiver and a processor coupled to the memory and the transceiver, the processor configured to call and run program instructions stored in a memory, to execute the method of claim 1 .

13 . A base station (BS), comprising a memory, a transceiver and a processor coupled to the memory and the transceiver, the processor configured to call and run program instructions stored in a memory, to execute the method of claim 11 .

14 . The method of claim 1 , further comprising:

transmitting a subsequent SDT MAC CE including one or more subsequent SDT indications to the network in the RRC_INACTIVE state.

15 . The method of claim 10 , wherein the MSGA contains a RRCResumeRequest message multiplexed with the subsequent SDT indication.

16 . The method of claim 15 , wherein the MSGB contains a RRCRelease message for reactivating CG-based resource.

17 . The method of claim 1 , further comprising:

performing downlink (DL) SDT in the RRC_INACTIVE state upon reception of a paging cause or access type carried in a SDT paging message.

18 . The method of claim 1 , wherein the subsequent SDT indication is indicated by a preamble group used for SDT in the RRC_INACTIVE state.

19 . The method of claim 1 , wherein the TAT is maintained in a carrier aggregation (CA) duplication scenario.

20 . The method of claim 11 , wherein upon reception of the subsequent SDT indication from the UE, the network determines a potential size of the subsequent SDT.