7 GERAN Network Location Procedures

3GPP43.059Functional stage 2 description of Location Services (LCS) in GERANRelease 16TS

7.1 State description for SMLC

7.1.1 SMLC States

7.1.1.1 NULL State

This is a conceptual rather than actual state in which a certain location request from a particular Location Client, VMSC, SGSN or BSC either has not yet been received or has been completed.

7.1.1.2 LOCATION State

This state exists after the SMLC has received a location request from a BSC and persists while the SMLC is obtaining position measurements for a particular positioning method until such time as positioning measurements have been received and a location estimate (with optional velocity estimate) has been computed and returned to the BSC.

When sufficient positioning measurement results have been received, the SMLC either evaluates them, if they include an already computed location estimate, or uses them to compute a location estimate. The SMLC then has the option of either reinitiating another positioning attempt, e.g. if the location estimate did not satisfy the required QoS and the requirement on response time permits another position attempt, or returning the location estimate to the BSC.

7.1.2 State Functionality

7.1.2.1 State Transitions

Figure 12: State Transitions in the SMLC

Moving from NULL to LOCATION state: After a location request is received from the BSC, the SMLC enters the LOCATION state. It then chooses a positioning method and initiates the appropriate position measurements.

Moving from LOCATION to NULL state: When the SMLC has obtained a location estimate that best meets the requested QoS parameters, it returns this to the BSC and re-enters the NULL state.

7.1.2.2 LOCATION Timer Function

The SMLC runs a timer while in the LOCATION state to limit the total amount of time that positioning can be active. This timer should be related to any response time indicated in the location request QoS parameters. If the timer expires before a final location estimate has been produced, the SMLC either returns the best existing location estimate (with optional velocity estimate) to the BSC (e.g. an estimate based on the current cell ID) or returns a failure indication. It then re-enters the NULL state.

7.2 State Description for the BSC

7.2.1 BSC States

7.2.1.1 IDLE State

In this state, the BSC location service is inactive for a particular MS.

7.2.1.2 LOCATION State

In this state, the BSC is awaiting a response from an SMLC after requesting the location for a particular MS. In this state, a Radio Resource connection to the target MS will be active – allowing the SMLC and MS to exchange positioning related messages for mobile based and mobile assisted position methods. For certain position methods (e.g. network based position methods), the SMLC may invoke substates in the BSC during which other types of association or procedure are supported with the MS (e.g. temporary call establishment, handover). In this state, the BSC may transfer positioning related messages between the SMLC and the target MS and/or between the SMLC and certain LMUs served by the BSC.

7.2.2 State Functionality

7.2.2.1 State Transitions

Figure 13: State Transitions in the BSC

Moving from IDLE to LOCATION state: After a request has been received to locate a particular MS served by the BSC, a location request is sent to the SMLC associated with the serving cell: the BSC then enters the LOCATION state. Before entering this state, a Radio Resource connection to the MS must have been already established by the VMSC.

Moving from LOCATION to IDLE state: After the return of a location estimate result from the SMLC, the BSC shall re-enter IDLE state.

7.2.2.2 LOCATION Timer Function

The BSC runs a timer while in the LOCATION state to limit the amount of time waiting for a location response from the SMLC. If the timer expires before such information is received, the BSC indicates a location failure to the original requesting entity and re-enters IDLE state.

7.3 Usage of SCCP Connection on the Lb interface in the CS domain in A/Gb mode

SCCP connection oriented signalling between an SMLC and a BSC is used to support SMLC signalling to a Type A LMU, a serving BSC, or a target MS. The types of SCCP connections are described below.

7.3.1 SCCP Connection for positioning of a target MS

The BSC establishes this connection when a request is received for a location estimate for a target MS in the CS domain. The BSC sends the BSSAP-LE Perform Location Request to the SMLC inside an SCCP Connection Request message. Signaling between the SMLC and target MS, if required, is then relayed by the BSC between this SCCP connection and the main signaling link to the MS. The same SCCP connection is also used to transfer BSSLAP messages between the SMLC and serving BSC. See Figure 14.

Figure 14: SCCP based signalling for MS positioning with an SMLC in CS domain

7.3.2 SCCP connection to access a Type A LMU

The BSC or SMLC establishes this connection to enable LCS messages to be transferred to or from a Type A LMU. The BSC or SMLC sends a BSSAP-LE LMU Connection Request message inside an SCCP Connection Request message. Signaling is subsequently relayed through the BSC using this SCCP connection as shown in figure 15.

Figure 15: SCCP based signalling to access a Type A LMU with an SMLC

7.4 Usage of SCCP Connection on the Lb interface in the PS domain in A/Gb mode

SCCP connection oriented signalling between an SMLC and a BSS is used to support SMLC signalling to a serving BSS or a target MS. The types of SCCP connections are described below.

7.4.1 SCCP Connection for positioning of a target MS

The BSS establishes this connection when a request is received for a location estimate for a target MS in the PS domain. The BSS sends the BSSAP-LE Perform Location Request to the SMLC inside an SCCP Connection Request message. Signaling between the SMLC and target MS is then relayed by the BSS via the SGSN to the MS. The same SCCP connection is also used to transfer BSSLAP messages between the SMLC and serving BSS. See figure 15a below. RRLP Messages between the SMLC and the MS are carried in BSSGP Position Command/Response messages (see 3GPP TS 48.018) across the Gb interface and in TOM messages in LLC UI frames (see 3GPP TS 44.064 [26]) across the Um interface. Because a connectionless mode of communication is used to transport BSSGP and LLC, an MS identity is included in each message for those protocols.

Figure 15a: SCCP based signalling for MS positioning with an SMLC in PS domain