Signaling radio bearer (SRB) transmission using small data transmission (SDT)

1 . A method performed by a current serving base station, comprising:

receiving, from a user equipment (UE) in a radio resource control (RRC) inactive state or a RRC idle state, a small data packet that includes a signaling radio bearer (SRB);

sending a retrieve UE context request to a last serving base station of the UE, the retrieve UE context request comprising assisting information for the last serving base station to determine whether anchor relocation is required; and

receiving, from the last serving base station, a UE context for the anchor relocation, or at least one of SRB data forwarding information or a SRB configuration, for transmission of the small data packet.

2 . The method of claim 1 , further comprising: sending SRB data to the last serving base station based on receiving, from the last serving base station, the SRB configuration.

3 . The method of claim 1 , further comprising: sending SRB data to the last serving base station based on receiving, from the last serving base station, the SRB data forwarding information.

4 . The method of claim 3 , wherein the SRB data forwarding information comprises an Internet protocol (IP) address and a general packet radio service (GPRS) tunnelling protocol user plane (GTP-U) tunnel endpoint identifier (TEID) of a dedicated GTP-U tunnel or a common GTP-U tunnel.

5 . The method of claim 2 , further comprising: abstracting a packet data convergence protocol (PDCP) protocol data unit (PDU) of the SRB, and sending the PDCP PDU to the last serving base station via Xn application protocol (XnAP) control plane signaling.

6 . The method of claim 1 , wherein the SRB is included in a media access control (MAC) protocol data unit (PDU), and the method further comprises sending the MAC PDU to the last serving base station via Xn application protocol (XnAP) control plane signaling.

7 . The method of claim 2 , wherein the SRB configuration includes a radio link control (RLC) configuration of the SRB.

8 . The method of claim 1 , further comprising: receiving, from the UE, an SRB small data transmission (SDT) indication indicating that the SRB is included in the small data packet.

9 . The method of claim 1 , wherein the retrieve UE context request comprises an SRB small data transmission (SDT) indication indicating that the SRB is included in the small data packet and a type of the SRB included in the small data packet.

10 . A current serving base station, comprising:

at least one memory;

at least one processor coupled with the at least one memory and configured to cause the current serving base station to:

receive, from a user equipment (UE) in a radio resource control (RRC) inactive state or a RRC idle state, a small data packet that includes a signaling radio bearer (SRB);

send a retrieve UE context request to a last serving base station of the UE, the retrieve UE context request comprising assisting information for the last serving base station to determine whether anchor relocation is required; and

receive, from the last serving base station, a UE context for the anchor relocation, or at least one of SRB data forwarding information or a SRB configuration, for transmission of the small data packet.

11 . The current serving base station of claim 10 , wherein the at least one processor is configured to cause the current serving base station to send SRB data to the last serving base station based on receiving, from the last serving base station, the SRB configuration.

12 . The current serving base station of claim 11 , wherein the at least one processor is configured to cause the current serving base station to abstract a packet data convergence protocol (PDCP) protocol data unit (PDU) of the SRB, and sending the PDCP PDU to the last serving base station via Xn application protocol (XnAP) control plane signaling.

13 . The current serving base station of claim 11 , wherein the SRB configuration includes a radio link control (RLC) configuration of the SRB.

14 . The current serving base station of claim 10 , wherein the at least one processor is configured to cause the current serving base station to send SRB data to the last serving base station based on receiving, from the last serving base station, the SRB data forwarding information.

15 . The current serving base station of claim 14 , wherein the SRB data forwarding information comprises an Internet protocol (IP) address and a general packet radio service (GPRS) tunnelling protocol user plane (GTP-U) tunnel endpoint identifier (TEID) of a dedicated GTP-U tunnel or a common GTP-U tunnel.

16 . The current serving base station of claim 10 , wherein the SRB is included in a media access control (MAC) protocol data unit (PDU), and wherein the at least one processor is configured to cause the current serving base station to send the MAC PDU to the last serving base station via Xn application protocol (XnAP) control plane signaling.

17 . The current serving base station of claim 10 , wherein the at least one processor is configured to cause the current serving base station to receive, from the UE, an SRB small data transmission (SDT) indication indicating that the SRB is included in the small data packet.

18 . The current serving base station of claim 10 , wherein the retrieve UE context request comprises an SRB small data transmission (SDT) indication indicating that the SRB is included in the small data packet and a type of the SRB included in the small data packet.

19 . A last serving base station, comprising:

at least one memory; and

at least one processor coupled with the at least one memory and configured to cause the last serving base station to:

receive, from a current serving base station, a retrieve user equipment (UE) context request based at least in part on reception of a small data packet with signaling radio bearer (SRB) from a UE in a radio resource control (RRC) inactive state or a RRC idle state, the retrieve UE context request comprising assisting information for the last serving base station to determine whether anchor relocation is required;

send, to the current serving base station, a UE context in response to a determination that the anchor relocation is required; and

send, to the current serving base station, at least one of SRB data forwarding information or a SRB configuration in response to the determination that the anchor relocation is not required.

20 . The last serving base station of claim 19 , wherein the SRB configuration includes a radio link control (RLC) configuration of the SRB.