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

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

• transmitting to a user equipment (UE) a SDT configuration, which is used for configuring SDT UL grant, wherein the SDT configuration comprises a SDT threshold;
• expecting the UE to determine, based on the SDT threshold, which type of SDT procedures is to be applied in RRC_INACTIVE state;
• upon transmission of a Timing Advance (TA) command in the RRC_INACTIVE state, expecting the UE to apply the TA command, and start or restart Timing Alignment Timer (TAT) for keeping uplink (UL) time alignment during SDT in the RRC_INACTIVE state;
• and
• expecting the UE to apply the determined SDT procedure based on the started or restarted TAT,
• wherein the BS expects the UE to sustain the SDT configuration or the SDT UL grant even if the TAT expires,
• wherein the method further comprises:
• expecting the UE to perform, after entering the RRC_INACTIVE state, initial RACH-based SDT to trigger the TAT;
• and
• expecting the UE to perform, after the initial RACH-based SDT, the CG-based SDT during the RRC_INACTIVE state upon maintenance of the TAT.