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full Table of Contents for  TS 23.273  Word version:   16.2.0

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4.3  Functional description of LCS per network function
4.3.1  Access Network
The Access Network is involved in the handling of various positioning procedures including positioning of a target UE, provision of location related information not associated with a particular target UE and transfer of positioning messages between an AMF or LMF and a target UE. The Access Network shall support determination of location estimates in geographical co-ordinates as defined in TS 23.032.
In this version of the specification, location services are supported for NG-RAN, trusted non-3GPP access and untrusted non-3GPP access.
The LCS specific functionalities of the radio access network elements are specified in TS 38.305 for NG-RAN.
4.3.2  LCS Clients, Application Functions and Network FunctionsWord-p. 17
AFs and NFs may access LCS services from a GMLC in the same trust domain (e.g. in the same PLMN) using the Ngmlc interface or Event Exposure with location information from an AMF in the same trust domain using the Namf interface.
LCS Clients may access LCS services from a GMLC (e.g. HGMLC) using the Le reference point.
External AFs may access LCS services from an NEF using Nnef interface or CAPIF API. The CAPIF and associated API provider domain functions are specified in TS 23.222.
4.3.3  Gateway Mobile Location Centre, GMLC
The Gateway Mobile Location Centre (GMLC) contains functionality required to support LCS. In one PLMN, there may be more than one GMLC.
A GMLC is the first node an external LCS client accesses in a PLMN (i.e. the Le reference point is supported by the GMLC). AFs and NFs may access GMLC directly or via NEF. The GMLC may request routing information and/or target UE privacy information from the UDM via the Nudm interface. After performing authorization of an external LCS Client or AF and verifying target UE privacy, a GMLC forwards a location request to either a serving AMF using Namf interface or to a GMLC in another PLMN using the Ngmlc interface in the case of a roaming UE.
The target UE's privacy profile settings shall always be checked in the UE's home PLMN prior to delivering a location estimate.
The "Visited GMLC" (VGMLC) is the GMLC, which is associated with the serving node of the target UE.
The "Home GMLC" (HGMLC) is the GMLC residing in the target UE's home PLMN, which is responsible for the control of privacy checking of the target UE.
4.3.4  Location Retrieval Function, LRFWord-p. 18
The Location Retrieval Function (LRF) may be collocated with a GMLC or separate and is responsible for retrieving or validating location information, providing routing and/or correlation information for a UE which has initiated an IMS emergency session. The information is provided by an LRF to an E-CSCF. For more details, refer to TS 23.167.
4.3.5  UE
A target UE may support positioning according to four different modes:
  • UE assisted mode (the UE obtains location measurements and sends the measurements to another entity (e.g. an LMF) to compute a location);
  • UE based mode (the UE obtains location measurements and computes a location estimate making use of assistance data provided by serving PLMN);
  • standalone mode (the UE obtains location measurements and computes a location estimate without making use of assistance data provided by serving PLMN);
  • network based mode (a serving PLMN obtains location measurements of signals transmitted by a target UE and computes a location estimate).
4.3.6  UDM
The UDM contains LCS subscriber LCS privacy profile and routing information. The UDM is accessible from an AMF, GMLC or NEF via the Nudm interface.
4.3.7  Access and Mobility Management Function, AMFWord-p. 19
The AMF contains functionality responsible for managing positioning for a target UE for all types of location request. The AMF is accessible to the GMLC and NEF via the Namf interface, to the RAN via the N2 reference point and to the UE via the N1 reference point.
4.3.8  Location Management Function, LMF
The LMF manages the overall co-ordination and scheduling of resources required for the location of a UE that is registered with or accessing 5GCN. It also calculates or verifies a final location and any velocity estimate and may estimate the achieved accuracy. The LMF receives location requests for a target UE from the serving AMF using the Nlmf interface. The LMF interacts with the UE in order to exchange location information applicable to UE assisted and UE based position methods and interacts with the NG-RAN, N3IWF or TNAN in order to obtain location information.
The LMF shall determine the result of the positioning in geographical co-ordinates as defined in TS 23.032. If requested and if available, the positioning result may also include the velocity of the UE.
4.3.9  Network Exposure Function, NEF
An NEF provides a means of accessing location services by an external AF or internal AF. AFs access location services from an NEF using an API. Depending on QoS requirements, an NEF can forward a location request to a GMLC or request an event exposure for location information from serving AMF (optionally via a UDM). When event exposure via AMF is used, an NEF may request routing information and/or target UE privacy information from the UDM via the Nudm interface.
4.3.10  Unified Data Repository, UDRWord-p. 20
The UDR contains privacy data information for target UEs and may be updated by a serving AMF via UDM with new privacy information received from a UE.
4.4  Reference Point to Support Location Services
4.4.1  Le Reference Point
The Le reference point supports location requests sent by an LCS Client to a GMLC or LRF. The Le reference point may be supported using the Mobile Location Protocol (MLP) defined by OMA.
4.4.2  NL3 Reference Point
The NL3 reference point supports location requests forwarded by an HGMLC to a VGMLC.
4.4.3  N1 Reference Point
The N1 reference point supports transfer of supplementary services messages between a serving AMF and target UE to support privacy notification and verification and change of UE privacy preference. The N1 reference point also supports transfer of positioning protocol messages and location event reports between a target UE and an LMF via a serving AMF. The N1 reference point supports the transfer of ciphering keys from an AMF to a suitably subscribed UE to enable the UE to receive ciphered broadcast assistance data. All messages sent over the N1 reference point for support of location services are encapsulated in NAS Transport messages as defined in TS 24.501.
4.4.4  N2 Reference Point
The N2 reference point supports transfer of positioning messages, via an AMF, between an LMF and a RAN node, or N3IWF in the case of untrusted non-3GPP access. The N2 reference point also supports transfer of messages, via an AMF, from an LMF to an NG-RAN node, which carry assistance data to be broadcast by the NG-RAN node. Positioning messages relevant to the N2 interface are defined in TS 38.455.
4.4.5Void
4.4.6  NL5 Reference PointWord-p. 21
The NL5 reference point supports location requests sent by an NEF or other NF to a GMLC.
4.4.7  NL2 Reference PointUp
The NL2 reference point supports location requests sent by a GMLC to a serving AMF for a target UE. Messages for the NL2 reference point are defined in TS 29.518.
4.4.8  NL6 Reference Point
The NL6 reference point supports queries from an HGMLC to a UDM for privacy subscription information for a target UE and routing information for a target UE.
4.4.9  N51 Reference Point
The N51 reference point supports queries from an NEF to a serving AMF for the location of a target UE. Messages for the N51 reference point are defined in TS 29.518.
4.4.10  NL1 Reference Point
The NL1 reference point supports location requests for a target UE sent from a serving AMF for the target UE to an LMF. Location requests are supported for immediate location and for deferred location for periodic or triggered location events.
The NL1 reference point also supports the transfer from an LMF to an AMF of ciphering keys and associated data that enable deciphering by suitably subscribed UEs of ciphered broadcast assistance data.
Messages for the NL1 reference point are defined in in TS 29.518 and TS 29.572.
4.4.11  N52 Reference Point
The N52 reference point supports queries from an NEF to a UDM for privacy subscription information for a target UE and routing information for a target UE. The N52 interface also supports a request from an NEF to a UDM to forward a location request from the NEF to a serving AMF for the target UE.
4.4.12  NL7 Reference Point
The NL7 reference point supports location context transfer between two LMFs.
4.5  Service Based Interfaces to Support Location Services
The 5GS LCS architecture contains the following service-based interfaces for Location Services:
  • Nlmf: Service-based interface exhibited by LMF.
  • Ngmlc: Service-based interface exhibited by GMLC.

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