Content for  TS 23.273  Word version:  19.4.0

Based on UE user plane positioning capability, control plane congestion status (e.g. AMF load status) and other implementation factors, LMF decides whether to use the positioning procedure via a user plane connection between UE and LMF. LMF may invoke Nnrf_NFDiscovery service operation to retrieve control plane congestion status (e.g. AMF load information). LMF may also invoke Nnrf_NFManagement_NFStatusSubscribe service to subscribe specific AMF load information. Based on AMF load information, LMF may determine to use user plane positioning, if there is available user plane connection between UE and LMF. Steps 2-8 are skipped if there is already a user plane connection context of the target UE in LMF and LMF determines to utilize the user plane connection for positioning.

[Conditional] If LMF decides to utilize user plane for positioning and there is no established secure user plane connection between the UE and the LMF, LMF invokes Namf_communication_N1N2MessageTransfer service operation to send the user plane information to AMF in a NAS container to indicate UE to utilize user plane over TLS for positioning. The user plane information includes the user plane positioning address of the LMF and a LMF identifier for user plane connection association in UE. The LMF allocates LCS-UP binding ID to be used to associate the user plane connection to be established with the target UE and includes this LCS-UP binding ID in the user plane information. The LMF associates the target UE identity (SUPI and/or GPSI) with this LCS-UP binding ID. If the UE supports the user plane positioning capability and AMF has not subscribed the status of LCS user plane connection, the AMF may subscribe from LMF the status of LCS user plane connection. If user plane connection context exists for this UE with a different LMF and UE does not indicate in the 5GMM capabilities the support of multiple LCS-UPP connections, then AMF rejects the request.

[Conditional] When AMF receives the user plane information from LMF in step 2, AMF sends it to UE via a DL NAS TRANSPORT message.

[Conditional] If there is no established applicable PDU session for the user plane positioning, the UE uses the URSP as defined in TS 23.503 which includes user plane positioning related PDU session parameters, e.g. a dedicated DNN and S-NSSAI, to establish the PDU session for user plane positioning. UE establishes a secured user plane connection with LMF if there is no established secure user plane connection towards the LMF. If LMF send its FQDN to the UE, a DNS server/resolver is used to resolve the IP address of LMF (e.g. EASDF or local DNS for local LMF address resolution). After the secured user plane connection been established successfully, the UE sends the LCS-UP binding ID received in step 3 to LMF via the secured user plane connection to enable LMF to perform the correlation of the UE with the secured user plane connection. The LCS-UP binding ID will be released once the correlation is complete.

[Conditional] UE sends an acknowledgement to LMF through AMF to indicate a success of user plane connection establishment for positioning service or a failure to utilize the user plane connection as defined in TS 24.572.

[Conditional] AMF sends the acknowledgement received in step 5 to the LMF via Namf_N1messageNotify service.

[Conditional] LMF indicates AMF in the Nlmf_Location_UPNotify message that user plane connection between the UE and LMF has been established.

[Conditional] The AMF stores the LCS-UP connection context as part of UE context.

If LMF or UE determines to utilize the user plane connection for positioning and the secure user plane connection is established, LPP messages are transferred between UE and LMF for UE based positioning, UE assisted positioning and delivery of assistance data. Supplementary services messages including event report messages, periodic triggered invoke messages and MS cancel deferred location messages may also be transferred between LMF and UE via the established user plane connection.

UE sends a user plane connection establishment request to AMF via NAS Message, if there is no established secure user plane connection between the UE and LMF and the UE decides to request a user plane connection for upcoming positioning requests.

[Conditional] AMF selects an LMF which is capable to establish a user plane session for positioning with the UE if the UE is authorized based on UE Subscription to use the user plane positioning and either has no existing user plane connection context with another LMF or has indicated in the 5GMM capabilities the support of multiple LCS-UPP connections. Otherwise, the AMF rejects the request. AMF may either query the NRF or based on local configuration to discover and select a proper LMF.

[Conditional] The AMF sends a Nlmf_Location_UPConfig Request towards LMF to request set up of an LCS-UP connection. The AMF shall include the target UE identity (see TS 29.572) (SUPI and/or GPSI) in the request.

[Conditional] If LMF accepts to utilize user plane for positioning and there is no established secure user plane connection between the UE and LMF, LMF sends a user plane information to AMF to indicate UE to accept and utilize user plane for positioning. The user plane information includes the user plane positioning address of the LMF and a LMF identifier for user plane association in UE. The LMF allocates a LCS-UP binding ID to associate the user plane connection to be established with the target UE and includes the LCS-UP binding ID in the user plane information. The LMF associates the target UE identity (SUPI and/or GPSI) with the LCS-UP binding ID.

[Conditional] When AMF receives the user plane information from LMF in step 4, AMF forwards it to UE via a DL NAS TRANSPORT message.

[Conditional] If there is no established secure user plane connection towards the LMF, UE establishes a secured user plane connection with LMF. UE uses the user plane positioning address of the LMF, together with the information in the URSP, to determine the PDU session parameters including DNN+S-NSSAI. UE uses the PDU session parameter to establish PDU session. When SMF receives the request, it selects a proper UPF based on the DNN+S-NSSAI, and establishes the connection between the UPF and LMF. After the secured user plane connection has been established successfully, the UE sends the LCS-UP binding ID received in step 4 to LMF via the secured user plane connection to enable LMF to perform the correlation of the UE with this secured user plane connection. The LCS-UP binding ID will be released once the correlation is complete.

[Conditional] UE sends an acknowledgement to LMF through AMF to indicate a success of user plane connection establishment for positioning service or a failure to utilize the user plane connection as defined in TS 24.572.

[Conditional] AMF sends the acknowledgement received in step 7 to the LMF via Namf_N1messageNotify service.

[Conditional] LMF responds to AMF that user plane connection between the UE and LMF has been established.

[Conditional] The AMF stores the LCS-UP connection context as part of UE context

[Conditional] After the secure user plane connection is established, if LMF determines to utilize the user plane connection for positioning after receiving a positioning request from AMF, or UE determines to utilize the user plane connection for positioning, LPP messages are transferred between UE and LMF for UE based positioning, UE assisted positioning and delivery of assistance data. Supplementary services messages including event report messages, periodic triggered invoke messages and MS cancel deferred location messages may also be transferred between LMF and UE via the established user plane connection.

[Conditional] The LMF discovers a need to change LMF or decides to terminate the user plane connection, e.g., after receiving an event report via user plane from UE or after receiving an AMF relocation information from a target AMF (target AMF obtains from source AMF the UE LCS-UP context which indicates UE has maintained LCS-UP connection with the source LMF, the target AMF may inform the source LMF about the AMF change using the Nlmf_Locatoion_UPConfig request).

[Conditional] The LMF sends an Nlmf_Location_UPNotify message that includes an indication of connection change or termination and if connection change is requested then the message may include a target LMF identification. The address of the AMF was provided to LMF as a "Notification Target Address" in latest Nlmf_Location_UPConfig message or Nlmf_Location_UP Subscribe message. If the LMF is going to terminate the user plane connection, only step 1a and step 6 are performed and other steps are skipped. If the request is to notify the termination of the user plane connection, the AMF releases the LCS-UP context.

[Conditional] If step 1a includes an indication of connection change, the AMF should use target LMF identification received from source LMF in step 1a or select a target LMF if source LMF does not provide a target LMF identification for user plane connection change. AMF may perform LMF reselection if a UE established a LCS UP connection with the source LMF to a new location (which may be out of serving area of source LMF and in serving area of target LMF) and select the target LMF for the current UE location based on LMF service area, LMF user plane positioning capability information and other information listed in clause 5.1. The target LMF needs to be capable to establish a user plane session for positioning with the UE. After AMF relocation, the target AMF may trigger the LMF reselection and LCS-UP connection modification procedure, e.g. if the source LMF does not perform LMF reselection.

The AMF sends an Nlmf_Location_UPConfig Request towards the source LMF. If this step is to terminate the LCS-UP connection of the LMF, the message includes an indication of LCS-UP connection termination for the LMF to terminate the user plane connection to the UE. If this step is to trigger LMF relocation, the message should include the target LMF identifier, which is selected by source LMF in step 0 or by AMF in step 2, to indicate source LMF to transfer location context(s) to the selected target LMF. Alternatively, it may include information about AMF reallocation.

[Conditional] Based on the target LMF identification in the request of step 3, the source LMF should invoke an Nlmf_Location_LocationContextTransfer Request service operation towards the target LMF to provide an indication of user plane connection existing, source LMF Identifier and the current user plane based location context(s) of the UE to the target LMF if there is user plane based periodic and triggered UE location events report context(s).

[Conditional] If target LMF decides to perform user plane positioning for the UE, Steps 2-8 of Figure 6.18.1-1 are performed between AMF (or Target AMF), UE, and Target LMF. If UE supports multiple LCS-UPP connection:

• In step 2, if Target LMF does not have user plane connection with the UE, user plane information also needs to include source LMF identifier; if Target LMF already has user plane connection with the UE, step 2 to step 8 are skipped. Instead, the Target LMF needs to send Source LMF identifier for the user plane connection and the UE sends back the acknowledge for the user plane connection.
• After step 4, UE identifies old user plane connection using the source LMF identifier. The UE associates all deferred location sessions associated with old user plane connection to the new user plane connection and stop using old user plane connection.

If UE does not support multiple LCS-UPP connection:

• The step 4a in this procedure is performed with addition that UE also stops using the old user plane connection to Source LMF.
• To guarantee that only LCS-UPP connection establishment message from target LMF can be sent to the UE which has already an LCS-UPP connection, AMF forwards LCS-UPP connection establishment message only from Target LMF to the UE and AMF should reject the user plane connection management message from other LMF with proper failures.

The target LMF informs source LMF of the location context transfer operation results.

[Conditional] If the user plane connection to source LMF is still active, the source LMF terminates the connection to the UE.

The Source LMF sends Nlmf_Location_UPConfig Response message to AMF (or Target AMF) to respond to the request of LCS-UP connection change or confirm connection termination or acknowledge change of AMF. If this step is used for connection termination confirmation, the AMF will release the LCS-UP context after receiving the response message.