5.4.15 Support of UE-Satellite-UE communication for 5G VN services

To support 5G VN services in UE-Satellite-UE communication, network functions (e.g. UPF) transferring user plane traffic shall be deployed onboard the satellites with regenerative payload that provide service link to the multiple UEs which are in the same 5G VN group. The UPF(s) onboard the serving satellite shall support a local switching function, enabling direct routing of UE-Satellite-UE PDU Session traffic without involvement of the ground segment. For local switching over multiple satellites, N6 or N19 tunnel between UPFs deployed on different satellites are used to transfer UE-Satellite-UE traffic between UEs.

To trigger UE-Satellite-UE communication for 5G VN services, the mechanism for local switching for UE-to-UE communications via UPF deployed on satellites described in clause 5.43.3 is used for UE-Satellite-UE communication via UPF(s) deployed onboard NGSO satellite(s).

Only a single SMF or a single SMF set is supported for the 5G VN group. Only unicast communication between two UEs is supported. Both IP PDU Session type and Ethernet PDU Session type are supported.

To support service and session continuity when serving satellite changes, the following applies:

• In the case that PSA UPF is on ground and UL CL/BP/local PSA UPF is deployed on satellite, the SMF determines whether to continue or to fallback the UE-Satellite-UE communication when serving satellite changes based on the availability of Inter-Satellite Link(s) between satellites serving the UEs and the 5G VN group configuration supporting UE-Satellite-UE communication. The SMF obtains the information about the availability of Inter-Satellite Link(s) based on implementation.
• If the SMF determines to continue the UE-Satellite-UE communication, the SMF reuses procedures for simultaneous change of Branching Point or UL CL and additional PSA for a PDU Session described in clause 4.3.5.7 of TS 23.502 [3] and update the group-level N4 Session as specified in clause 4.4.1.3 of TS 23.502 [3].
• If the SMF determines to fallback the UE-Satellite-UE communication to ground network, the SMF coordinates the procedures specified in clause 4.3.5.5 of TS 23.502 [3] for removal of additional PDU Session Anchor and Branching Point or UL CL for both UEs and update the group-level N4 Session as specified in clause 4.4.1.3 of TS 23.502 [3]. The SMF first removes the UL CL/L-UPF of the peer UE and configures PDR, FAR for N6/N19 routing in the ground PSA UPF(s) (when two UEs use different PSA UPFs), e.g. during satellite handover of the local UE. After the satellite change of the local UE, the SMF removes the UL CL/L-UPF of the local UE.
• In the case of PSA UPF deployed on satellite, session and service continuity, SSC mode 2 and SSC mode 3 are applicable to N6-based traffic forwarding of 5G VN communication within the associated 5G VN group. All the UEs in the 5G VN group are accessing NTN without any DN connectivity during UE-Satellite-UE communication with PSA UPF. The procedures specified in clauses 4.3.5.1 and 4.3.5.2 of TS 23.502 [3] are used to change the PDU Session Anchor when the serving satellite changes.

NOTE: Either for UL CL/BP/local PSA UPF onboard satellite or PSA UPF on satellite case, the UL CL/BP/local PSA UPF or PSA UPF and gNB are onboard the same satellite to enable UE-satellite-UE communication for non-IMS service.