The general LCS control plane architecture in the EPS applicable to a target UE with E-UTRAN access is defined in TS 23.271.

The LTE-Uu interface, connecting the UE to the eNode B over the air, is used as one of several transport links for the LTE Positioning Protocol.

The S1-MME interface between the eNode B and the MME is transparent to all UE-positioning-related procedures. It is involved in these procedures only as a transport link for the LTE Positioning Protocol. For eNode B related positioning procedures, the S1-MME interface transparently transports both positioning requests from the E-SMLC to the eNode B and positioning results from the eNode B to the E-SMLC. For delivery of broadcast location assistance data information, the S1-MME interface transparently transports the assistance data information from the E-SMLC to the eNode B for broadcasting and feedback information from the eNode B to the E-SMLC. The S1-MME interface is also used by an MME to provide ciphering keys to UEs for use in deciphering broadcast location assistance data information which was ciphered by an E-SMLC.

The SLs interface, between the E-SMLC and the MME, is transparent to all UE related and eNode B related positioning procedures. It is then used only as a transport link for the LTE Positioning Protocols LPP and LPPa. The SLs interface supports location sessions instigated by the MME as defined in TS 23.271. LPP and LPPa transport are then supported as part of any location session.

The SLm interface between the E-SMLC and an LMU is used for uplink positioning. It is used to transport SLmAP protocol messages over the E-SMLC-LMU interface. Network sharing should be supported. (Details FFS).

The LTE Positioning Protocol (LPP) is terminated between a target device (the UE in the control-plane case or SET in the user-plane case) and a positioning server (the E-SMLC in the control-plane case or SLP in the user-plane case). It may use either the control- or user-plane protocols as underlying transport. In this specification, only control plane use of LPP is defined. User plane support of LPP is defined in [17] and [18]. LPP is a point to point positioning protocol with capabilities similar to those in UMTS RRC (TS 25.331) and GERAN RRLP (TS 44.031). Whereas RRLP supports positioning of a target MS accessing GERAN and RRC supports positioning of a target UE accessing UTRAN, LPP supports positioning and location related services (e.g. transfer of assistance data) for a target UE accessing E-UTRAN. To avoid creating new positioning protocols for future access types developed by 3GPP, and to enable positioning measurements for terrestrial access types other than E-UTRAN, LPP is in principle forward-compatible with other access types, even though restricted to E-UTRAN access in this specification. LPP further supports the OMA user plane location solution SUPL 2.0, as defined in the OMA SUPL 2.0 standards ([17], [18]), and is intended to be compatible with the successor protocols of SUPL 2.0 as well. LPP messages are carried as transparent PDUs across intermediate network interfaces using the appropriate protocols (e.g., S1-AP over the S1-MME interface, NAS/RRC over the Uu interface). The LPP protocol is intended to enable positioning for LTE using a multiplicity of different position methods, while isolating the details of any particular positioning method and the specifics of the underlying transport from one another. The protocol operates on a transaction basis between a target device and a server, with each transaction taking place as an independent procedure. More than one such procedure may be in progress at any given moment. An LPP procedure may involve a request/response pairing of messages or one or more "unsolicited" messages. Each procedure has a single objective (e.g., transfer of assistance data, exchange of LPP related capabilities, or positioning of a target device according to some QoS and use of one or more positioning methods). Multiple procedures, in series and/or in parallel, can be used to achieve more complex objectives (e.g., positioning of a target device in association with transfer of assistance data and exchange of LPP related capabilities). Multiple procedures also enable more than one positioning attempt to be ongoing at the same time (e.g., to obtain a coarse location estimate with low delay while a more accurate location estimate is being obtained with higher delay). An LPP session is defined between a positioning server and the target device, the details of its relation with transactions are described in clause 4.1.2 of TS 36.355. A single LPP transaction may be realised as multiple procedures; e.g., a single transaction for provision of assistance data might comprise several Provide Assistance Data messages, with each such message constituting a separate procedure (since there is no "multiple unsolicited messages" procedure type). For the 3GPP EPS Control Plane solution defined in TS 23.271, the UE is the target device and the E-SMLC is the server. For SUPL 2.0 support, the SUPL Enabled Terminal (SET) is the target device and the SUPL Location Platform (SLP) is the server. The protocol does not preclude the possibility of future developments in control plane and user plane solutions (e.g., possible successors of SUPL 2.0, as well as possible future 3GPP control plane solutions). All LPP operations and procedures are defined with respect to the target and server, and thus the LPP operations and procedures defined here with respect to a UE and an E-SMLC can also be viewed in this more generic context by substituting any target for the UE and any server for the E-SMLC. LPP further supports multiple positioning methods as defined in clause 4.3. LPP supports hybrid positioning, in which two or more position methods are used concurrently to provide measurements and/or a location estimate or estimates to the server. LPP is forward compatible with the later addition of other position methods in later releases (e.g., position methods associated with other types of terrestrial access). LPP also supports RRC broadcast of location assistance data information using data types defined in relation to LPP which are embedded in positioning SIBs. This enables an E-SMLC and a UE to support broadcast location assistance data using the same data structures which are used for point to point location. The operations controlled through LPP are described further in clause 7.1.

The RRC protocol is terminated between the eNode B and the UE. In addition to providing transport for LPP messages over the Uu interface, it supports transfer of measurements that may be used for positioning purposes through the existing measurement systems specified in TS 36.331. The RRC protocol also supports broadcasting of location assistance data via positioning System Information messages.

The LTE Positioning Protocol Annex (LPPa) carries information between the eNode B and the E-SMLC. It is used to support the following positioning functions:

• E-CID cases where assistance data or measurements are transferred from the eNode B to the E-SMLC;
• data collection from eNodeBs for support of downlink OTDOA positioning;
• retrieval of UE configuration data from the eNodeBs for support of uplink (e.g., UTDOA) positioning;
• exchange of information between E-SMLC and eNodeBs for the purpose of assistance data broadcasting.

The LPPa protocol is transparent to the MME. The MME routes the LPPa PDUs transparently based on a short Routing ID corresponding to the involved E-SMLC node over S1 interface without knowledge of the involved LPPa transaction. It carries the LPPa PDUs over S1 interface either in UE associated mode or non-UE associated mode.

The S1-AP protocol, terminated between the MME and the eNode B, is used as transport for LPP and LPPa messages over the S1-MME interface. The S1-AP protocol is also used to instigate and terminate eNode B related positioning procedures.