Method and user equipment for small data transmission

1. A method performed by a User Equipment (UE) for small data transmission, the method comprising:

receiving, when operating in a Radio Resource Control (RRC)_CONNECTED state, an RRC release message from a Base Station (BS), the RRC release message indicating at least one downlink (DL) Reference Signal (RS), at least one Configured Grant (CG) resource and information indicating an association between the at least one DL RS and the at least one CG resource;

transitioning to an RRC_INACTIVE state in response to receiving the RRC release message;

initiating an attempt for the small data transmission;

measuring the at least one DL RS to obtain a measurement result;

selecting, from the at least one CG resource, a particular CG resource for the small data transmission according to the measurement result;

determining whether a Timing Advance (TA) value for the particular CG resource is valid for the small data transmission according to (i) an amount of Reference Signal Received Power (RSRP) change of a DL RS associated with the particular CG resource, the DL RS being one of the at least one DL RS, and (ii) whether a TA-related timer is running; and

performing the small data transmission on the particular CG resource after determining that the TA value is valid, wherein:

the TA-related timer defines a time interval during which the TA value is allowed to be determined as valid;

the TA-related timer is configured by a TA-related timer configuration provided by the BS;

the TA-related timer is started when the TA value is received; and

the at least one CG resource is released when the TA-related timer expires.

2. The method according to claim 1 , further comprising:

obtaining an RSRP threshold from the RRC release message;

obtaining an RSRP value of the DL RS associated with the particular CG resource from the measurement result; and

selecting the particular CG resource from the at least one CG resource after determining that the RSRP value of the DL RS associated with the particular CG resource is larger than the RSRP threshold.

3. The method according to claim 1 , wherein the association information includes at least one RS identifier (ID) and at least one CG configuration ID, the at least one RS ID indicates the at least one DL RS, and the at least one CG configuration ID indicates at least one CG configuration that configures the at least one CG resource.

4. The method according to claim 3 , wherein the at least one RS ID includes a first RS ID and a second RS ID, and the first RS ID and the second RS ID are associated with a same CG configuration indicated by the at least one CG configuration ID.

5. The method according to claim 3 , wherein the at least one RS ID includes a first RS ID and a second RS ID, and the first RS ID and the second RS ID are associated with different CG configurations indicated by the at least one CG configuration ID.

6. The method according to claim 1 , further comprising:

obtaining an RSRP offset threshold from the RRC release message;

measuring the DL RS associated with the particular CG resource at a first time point to obtain a first RSRP value of the DL RS;

measuring the DL RS associated with the particular CG resource at a second time point to obtain a second RSRP value of the DL RS;

determining the amount of RSRP change of the DL RS by calculating a difference between the first RSRP value and the second RSRP value; and

determining that the TA value for the particular CG resource is valid in a case that the amount of RSRP change is less than, or equal to, the RSRP offset threshold; and

determining that the TA value for the particular CG resource is invalid in a case that the amount of RSRP change is larger than the RSRP offset threshold.

7. A User Equipment (UE) for small data transmission, the UE comprising:

a processor; and

a memory coupled to the processor, wherein the memory stores at least one computer-executable program that, when executed by the processor, causes the processor to:

receive, when operating in a Radio Resource Control (RRC)_CONNECTED state, an RRC release message from a Base Station (BS), the RRC release message indicating at least one downlink (DL) Reference Signal (RS), at least one Configured Grant (CG) resource and information indicating an association between the at least one DL RS and the at least one CG resource;

transition to an RRC_INACTIVE state in response to receiving the RRC release message;

initiate an attempt for the small data transmission;

measure the at least one DL RS to obtain a measurement result;

select, from the at least one CG resource, a particular CG resource for the small data transmission according to the measurement result;

determine whether a Timing Advance (TA) value for the particular CG resource is valid for the small data transmission according to (i) an amount of Reference Signal Received Power (RSRP) change of a DL RS associated with the particular CG resource, the DL RS being one of the at least one DL RS, and (ii) whether a TA-related timer is running; and

perform the small data transmission on the particular CG resource after determining that the TA value is valid, wherein:

the TA-related timer defines a time interval during which the TA value is allowed to be determined as valid;

the TA-related timer is configured by a TA-related timer configuration provided by the BS;

the TA-related timer is started when the TA value is received; and

the at least one CG resource is released when the TA-related timer expires.

8. The UE according to claim 7 , wherein when the at least one computer-executable program is executed by the processor, the computer-executable program further causes the processor to:

obtain an RSRP threshold from the RRC release message;

obtain an RSRP value of the DL RS associated with the particular CG resource from the measurement result; and

select the particular CG resource from the at least one CG resource after determining that the RSRP value of the DL RS associated with the particular CG resource is larger than the RSRP threshold.

9. The UE according to claim 7 , wherein the association information includes at least one RS identifier (ID) and at least one CG configuration ID, the at least one RS ID indicates the at least one DL RS, and the at least one CG configuration ID indicates at least one CG configuration that configures the at least one CG resource.

10. The UE according to claim 9 , wherein the at least one RS ID includes a first RS ID and a second RS ID, and the first RS ID and the second RS ID are associated with a same CG configuration indicated by the at least one CG configuration ID.

11. The UE according to claim 9 , wherein the at least one RS ID includes a first RS ID and a second RS ID, and the first RS ID and the second RS ID are associated with different CG configurations indicated by the at least one CG configuration ID.

12. The UE according to claim 7 , wherein when the at least one computer-executable program is executed by the processor, the computer-executable program further causes the processor to:

obtain an RSRP offset threshold from the RRC release message;

measure the DL RS associated with the particular CG resource at a first time point to obtain a first RSRP value of the DL RS;

measure the DL RS associated with the particular CG resource at a second time point to obtain a second RSRP value of the DL RS;

determine the amount of RSRP change of the DL RS by calculating a difference between the first RSRP value and the second RSRP value; and

determine that the TA value for the particular CG resource is valid in a case that the amount of RSRP change is less than, or equal to, the RSRP offset threshold; and

determine that the TA value for the particular CG resource is invalid in a case that the amount of RSRP change is larger than the RSRP offset threshold.