5.4.14.3 Support for UE-satellite-UE communication when serving satellite changes

NOTE 1: The following mechanism assumes that the source gNB, UL CL/L-PSA, and IMS-AGW are onboard the same source satellite, and the target gNB, UL CL/L-PSA, and IMS-AGW are onboard the same target satellite.

In the UE-SAT-UE communication, if the UE's serving satellite changes (e.g. due to UE movement or the satellite movement), the Xn/N2-based handover procedure can be performed to change the serving gNB from that on the source satellite to that on the target satellite. The AMF includes the target satellite ID in the Nsmf_PDUSession_UpdateSMContext Request to the SMF. The SMF decides whether to route IMS user plane traffic to the PSA on the ground or retain the UL CL/L-PSA on the satellite based on local configuration and other factors as specified in clause 6.3.3.

NOTE 2: The SMF can decide to route the IMS user plane traffic to the PSA on ground, e.g., due to no connection between the source satellite and the target satellite.

If the SMF decides to route the IMS user plane traffic to the PSA on the ground and the procedure for removing the UL CL/L-PSA as described in clause 4.3.5.5 of TS 23.502 [3] is to be performed, the SMF provides an early notification without the DNAI associated with satellite ID via Nsmf_EventExposure_Notify to PCF. The PCF forwards this notification to the P-CSCF via Npcf_PolicyAuthorisation_Notify, indicating that the 5GC user plane needs to fall back to the ground.

If the SMF decides to retain the UL CL/L-PSA on the satellite, the principle in clause 5.6.7.2 is used to enable the coordination between SMF and P-CSCF. Based on the early notification and associated indication of "AF acknowledgment to be expected" on AF (P-CSCF) subscription as described in clause AE.5.2.1 of TS 23.228 [15], the SMF provides an early notification with the value of DNAI derived from the target satellite ID via Nsmf_EventExposure_Notify to PCF and the PCF forwards this DNAI value to the P-CSCF via Npcf_PolicyAuthorisation_Notify. In this case, the IMS informs 5GC whether the UE-SAT-UE communication can or cannot continue, as outlined in clause AE.5.2.1 of TS 23.228 [15], i.e. the PCF receives a response from the P-CSCF and forwards it to the SMF:

• If the response (i.e. a positive AF (P-CSCF) acknowledgement as defined in clause 5.6.7.2) indicates that the UE-SAT-UE communication can continue, the SMF allocates the UL CL/BP and L-PSA on the target satellite by performing a simultaneous change of UL CL/BP and additional PSA for the PDU Session, as described in clause 4.3.5.7 of TS 23.502 [3], where the UL CL/BP and L-PSA rule shall be updated with the IMS-derived transport address and optional N6 traffic routing information associated with target DNAI as received in the response. A late notification is sent by SMF to the P-CSCF via PCF afterwards, as outlined in clause 4.3.6.3 of TS 23.502 [3].
• If the response (i.e. a negative AF (P-CSCF) acknowledgement as defined in clause 5.6.7.2) indicates that the UE-SAT-UE communication cannot continue, the SMF removes the UL CL/BP and L-PSA on the source satellite by following the procedure described in clause 4.3.5.5 of TS 23.502 [3].

NOTE 3: The event that the satellite serving the UE has changed is represented by a change of the DNAI value, and the DNAI value can be derived from the satellite identifier.