5.43 Support for 5G Satellite Backhaul

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

5.43.1 General

Satellite may be used as part of the backhaul between (R)AN and 5GC. The 5G System supports to report of usage of satellite backhaul as described in clause 5.43.2.

For some deployments, UPF may be deployed on the satellite. In these cases, edge computing or local switch via UPF deployed on the satellite may be performed as described in clauses 5.43.2 and 5.43.3. Deployments with satellite backhaul and edge computing with UPF on the ground is supported as described in clause 5.13, i.e. without satellite backhaul specific requirements.

5.43.2 Edge Computing via UPF deployed on satellite

This clause only applies to the case where Edge Computing is deployed with UPF and Edge Computing services on-board the satellite. The UPF deployed on satellite can act as UL CL/BP/local PSA UPF or act as PSA UPF.

NOTE 1: In this Release, Edge Computing via UPF deployed on satellite only applies to GEO satellite backhaul.

If the UPF deployed on satellite is acting as PSA, the following enhancements apply:

– If the UE is accessing gNB with satellite backhaul, and AMF is aware the satellite backhaul category, the AMF sends the satellite backhaul category to the PCF. If GEO satellite backhaul category is indicated, the PCF may take it into account to generate or update the URSP rule as defined in clause 6.1.2.2 of TS 23.503 [45] to including an appropriate Route Selection Descriptor for services deployed on GEO satellite, which further enable PDU Session Establishment with PSA UPF on the satellite.

If the UPF deployed on satellite is acting as UL CL/BP and local PSA, the SMF performs UL CL/BP/local PSA selection and insertion during the PDU Session Establishment procedure as described in clause 4.3.2 of TS 23.502 [3] or PDU Session Modification procedure as described in clause 4.3.3 of TS 23.502 [3] based on GEO satellite ID provided by the AMF, which includes:

– Based on configuration, the AMF may determine the GEO Satellite ID serving the UE and send it to the SMF. If GEO satellite ID changes, e.g. due to UE handover to an gNB using different GEO satellite as part of backhaul, the AMF may update the latest GEO Satellite ID to the SMF.

NOTE 2: It is assumed that AMF determines the GEO Satellite ID based on local configuration, e.g. based on Global RAN Node IDs associated with satellite backhaul.

– The SMF determines DNAI based on local configuration and the GEO Satellite ID received from AMF.

NOTE 3: It’s assumed that a DNAI value is assigned for each GEO Satellite ID by the operator. SMF is locally configured with mapping relationship between DNAI and GEO Satellite ID.

– If the UE is allowed to access the service(s) according to the EAS Deployment Information as described in clause 6.2.3.4 of TS 23.548 [130], the SMF selects the UL CL/BP/local PSA based on the DNAI corresponding to the GEO Satellite ID and other factors as described in clause 6.2.3.2 of TS 23.548 [130].

5.43.3 Local switch for UE-to-UE communications via UPF deployed on GEO satellite

5.43.3.1 General

The UE to UE traffic may be locally routed by UPF(s) deployed on satellite (i.e., through local switch) to the target UE without traversing back to the satellite gateway on the ground.

Local switching via UPF(s) deployed on satellite in this clause only applies on GEO satellite backhaul case and considers only DNNs and slices for 5G VN.

N19 tunnel may be established between two UPFs deployed on different satellites to locally switch UE to UE traffic.

Only a single SMF is supported for local switching and N19 forwarding, i.e. both UEs are served by the same SMF.

NOTE: The latency optimisation that can be gained by inter-satellite N19 between UPFs on different GEO satellites depends on the distance between the satellites that can be substantial, depending on the number of deployed satellites.

Clause 5.43.3.2 describes the case of PSA UPF deployed on satellite, clause 5.43.3.3 describes the case of ULCL and local PSA deployed on satellite (PSA UPF is on the ground). Selection of PSA UPF or UL CL/BP/local PSA on satellite is described in clause 6.3.3.

5.43.3.2 Local switch with PSA UPF deployed on satellite

A combination of DNN/S-NSSAI is assigned by the operator to the services deployed on GEO satellite, the URSP configuration is described in TS 23.503 [45].

If SMF selects the UPF deployed on satellite as PSA of UE’s PDU Session, the SMF configures the UE’s N4 session to forward/detect packet to/from the internal interface same as the configuration for the 5GVN group member’s N4 Session in clause 5.8.2.13.1 (Support for unicast traffic forwarding of a 5G VN).

Based on pre-configuration, the SMF may establishes the N19 tunnel between PSA UPFs and set corresponding group-level N4 session rules utilizing IP address(s) in UPF address pool to forward/detect packet to/from the N19 tunnel interface.

Editor’s note: The difference of the group-level N4 session rules utilizing IP address(s) in UPF address pool in this clause and the group-level N4 session rules in clause 5.8.2.13.1 is for further discussion.

For establishing N19 tunnel between the UPFs onboard the satellite, the UPFs are controlled by the same SMF.

– To process packets between UE and servers residing in DN, SMF configures rules to route traffic via N6 as described in clause 5.8.2.13.1.

The group-level N4 session is per DNN and slice. The SMF can create, update or delete the group-level N4 Session, i.e., add or delete N4 rules, allocate or release the N19 tunnel resources based on operator deployment, e.g. based on GEO satellite’s planed obsolescence or new GEO satellite setup.

5.43.3.3 Local switch with ULCL/BP and local PSA UPF deployed on satellite

For UPF deployed on satellite acting as ULCL/BP/local PSA case, the PSA UPF anchored by UE PDU sessions is on the ground, if the UEs are served by the same SMF, the SMF determines to activate local switching and N19 forwarding for the UEs using the GEO satellite backhaul, based on:

1) AF request including IP addresses of UEs which require UE-to-UE communications;

Editor’s note: UE list or collection provision will reuse a NEF API name defined by the Edge_Ph2 topic.

2) Target IPs reported by on-ground PSA UPF as current reporting mechanism in clause 5.8.2.11.7. SMF configures the on-ground PAS UPF to detect UL packets with destination IP addresses which belong to the current UPF address pool.

After SMF receiving IP addresses from AF or on-ground PAS UPF, if SMF determines that UEs in communications are using the GEO satellites backhaul and UEs are allowed to access the DNAIs corresponding to the GEO satellites, for each UE communicating with target UE(s) in the communication group, the SMF selects the UPF deployed on GEO satellite according to the DNAI as UL-CL/BP and L-PSA, and configures UL-CL/BP with the following rule:

− Route the data traffic received from the UE and destined to IP address(es) of the target UE(s) to the L-PSA.

− Route other data traffic received from the UE to the PSA of the UE’s PDU Session.

The SMF configures the Local PSA with local forwarding rules to forward the data traffic to the target UEs directly. If the selected L-PSAs are different for the UEs in the communication group, N19 tunnel is established between the L-PSAs. For establishing N19 tunnel between the UPFs onboard the satellite, the UPFs are controlled by the same SMF. The SMF regards the UEs in the communication group as members of a 5G VN group, and configures the local data forwarding rules on L-PSA(s) based on 5GVN user plane handing mechanism in clause 5.8.2.13.1 (Support for unicast traffic forwarding of a 5G VN).

Editor’s note: It is for further discussion whether or not the configurations are different and needs additional description, if the UL-CL/BP and L-PSA are combined.

NOTE: The selected UPF deployed on satellite can be inserted as ULCL/BP/L-PSA reusing existing ULCL/BP insertion procedures defined in TS 23.502 [3] or TS 23.548 [130].

5.43.4 Reporting of satellite backhaul to SMF

If the AMF is aware that a satellite backhaul is used towards 5G AN, the AMF may report this to SMF as part of the PDU Session establishment procedure as described in clause 4.3.2 of TS 23.502 [3]. If AMF is aware that satellite backhaul category changes (e.g. at handover), the AMF reports the current satellite backhaul category and indicates the satellite backhaul category change to SMF.

Satellite backhaul category refers to the type of the satellite (i.e. GEO, MEO, LEO or OTHERSAT, DYNAMIC_GEO, DYNAMIC _MEO, DYNAMIC _LEO, DYNAMIC _OTHERSAT) used in the backhaul. Only a single backhaul category can be indicated.

If dynamic satellite backhaul is used by the NG-RAN, i.e. capabilities (latency and/or bandwidth) of the satellite backhaul change over time due to e.g., use of varying inter-satellite links as part of backhaul, the AMF notifies the SMF of the corresponding dynamic satellite backhaul category to serve the PDU Session.

NOTE: It is assumed that the AMF can determine the Satellite backhaul category for the notification to the SMF based on local configuration, e.g. based on Global RAN Node IDs associated with satellite backhaul.

5.43.5 QoS monitoring when dynamic Satellite Backhaul is used

If dynamic satellite backhaul is used, QoS monitoring can be used to obtain packet delay as specified in clause 5.33.3.

If AMF informs SMF that the dynamic satellite backhaul is used, the SMF informs this to PCF. The PCF may trigger QoS monitoring as specified in clause 5.33.3 if dynamic satellite backhaul is used.

Editor’s note: In this case, whether PCF can request QoS monitoring for the backhaul only, i.e. without receiving delay values including RAN part delay, is FFS.