3GPP43.059Functional stage 2 description of Location Services (LCS) in GERANRelease 16TS
This procedure is common to all positioning methods in the CS domain.
Figure 28: Common Positioning Procedure for CS Domain
1) The BSC sends the BSSAP-LE Perform Location Request message to request the SMLC to start the positioning procedure. The Location Type and Current Cell Identifier IEs are always included. Depending on the type of location request, additional parameters may be included to provide Measurement Report (in BSSLAP APDU IE), LCS Client Type, Classmark Information Type 3, Chosen Channel, LCS Priority, LCS Quality of Service, and A-GNSS Assistance Data. In addition, the IMSI and / or the IMEI of the mobile station involved in the positioning procedure may be provided by the BSC in order to allow the SMLC to maintain a context between subsequent requests for a mobile station, to tune positioning for each mobile station and to add more options for SMLC diagnostics.
2) If location information is requested and the location accuracy within the QoS, if provided, can be satisfied by the reported cell ID and, if available, TA value, the SMLC may send a BSSAP-LE Perform Location Response message immediately. Otherwise, the SMLC determines the positioning method and instigates the particular message sequence for this method defined in subsequent clauses. If the position method returns position measurements, the SMLC uses them to compute a location estimate (with optional velocity estimate). If there has been a failure to obtain position measurements, the SMLC may use the current cell ID and, if available, TA value to derive an approximate location estimate. If a computed location estimate is returned for an MS based position method, the SMLC may verify consistency with the current cell ID and, if available, TA value. If the location estimate so obtained does not satisfy the requested accuracy or the location attempt failed, e.g. due to missing data, and sufficient response time still remains, the SMLC may instigate a further location attempt using the same (e.g. providing more assistance data to MS) or a different position method. If there is insufficient response time for another position attempt, the SMLC shall return any location estimate already obtained even if not satisfying the requested accuracy. If a vertical location co-ordinate is requested but the SMLC can only obtain horizontal co-ordinates, these may be returned. Requirements on the geographic shape encoded within the ‘position information’ parameter may exist for certain LCS client types. The SMLC shall comply with any shape requirements defined in 3GPP. The SMLC in a certain country should attempt to comply with the shape requirements defined for a specific LCS client type in the relevant national standards of that country. If location assistance data is requested, the SMLC transfers this data to the MS as described in subsequent clauses. The SMLC determines the exact location assistance data to transfer according to the type of data specified by the MS, the MS location capabilities and the current cell ID. If deciphering keys are requested the SMLC obtains the current keys.
3) The SMLC sends the BSSAP-LE Perform Location Response message to the BSC containing any location estimate (with optional velocity estimate) or deciphering keys. In case of failure the cause value may be included.