9.3 TA Based Positioning in CS Domain for A/Gb-mode

3GPP43.059Functional stage 2 description of Location Services (LCS) in GERANRelease 17TS

The TA is based on the existing Timing Advance (TA) parameter. The TA value is known for the serving BTS. To obtain TA values in case the MS is in idle mode a special call, not noticed by the GSM subscriber (no ringing tone), is set up. The cell-ID of the serving cell and the TA is returned as the result of the TA.

9.3.1 Definition of TA states

9.3.1.1 MS in IDLE State

In IDLE state the MS may be paged or may request an originating (e.g. emergency) call. The paging response message or CM Service Request, in each case respectively, received in COMPLETE_LAYER_3 message may contain location information that includes the TA value. If available, the TA value and other location information shall be provided to the SMLC by the requesting BSC along with the current serving cell ID in the BSSAP-LE Perform Location request. This enables TA based positioning in the SMLC without any further interactions.

9.3.1.2 MS in DEDICATED State

In DEDICATED state the SMLC shall send a TA_REQUEST to request the TA value from the serving BSC. The BSC shall respond with either a TA_RESPONSE carrying the TA value and possibly other radio measurements from the MS or a Reset. The associated procedure is described in the next clause.

9.3.2 TA Positioning Procedure

This TA positioning procedure is generic for a standalone SMLC or integrate SMLC in the BSC.

Figure 29: TA Positioning Procedure for the SMLC in the CS Domain

1) The SMLC sends a BSSAP-LE Connection Oriented Information message to the BSC serving a particular target MS. The APDU parameter in this message contains a BSSLAP TA Request.

2) Provided the location area for the target MS has not changed or, if changed, was previously reported to the SMLC, the BSC returns the current TA value and current serving cell for the target MS to the SMLC in a BSSLAP TA response contained within a BSSAP-LE Connection Oriented Information message. The TA response may also include the latest measurement results received from the target MS for the serving and neighbouring cells. If the location area for the target MS has changed since the location request was first sent to the SMLC and if the new location area has not yet been reported to the SMLC, the BSC shall return a BSSLAP Reset to the SMLC within a BSSMAP-LE Connection Oriented Information message. The Reset shall include the current serving cell, the current location area code, a cause value indicating “intra-BSC handover” and the current TA value and may include the latest measurement results received from the target MS for the serving and neighboring cells. The SMLC then derives a location estimate for the target MS based on the received serving cell ID, location area code if included, TA value and other measurement results if included.

NOTE: In the case of a previous intra-BSC handover to a cell in a different PLMN (different MCC, MNC combination), the SMLC would receive a BSSLAP Reset containing the new location area code and new cell ID. The SMLC may then deduce the new MCC and MNC from the uniqueness, when this applies, of the combined values for the new location area code and cell ID compared to the corresponding values for neighbouring cells.

9.3.3 Unsuccessful TA positioning procedure in BSC

There are three messages defined to handle error scenarios during positioning procedure in BSC. The messages are 1) Reject, 2) Abort and 3) Reset. Refer to 3GPP TS 48.071 [21] for details.

After receiving the BSSLAP TA Request in BSC, a Reject will be sent with proper cause value from BSC to SMLC in "BSSAP-LE Connection Oriented Information Message" if TA positioning cannot be performed in BSC at that time for reasons other than handover or another ongoing RR management procedure.

An Abort or Reset is possible if the TA positioning cannot be done in BSC during that time. Reset is sent to SMLC to indicate when the positioning needs to be restarted after temporary interruption due to intra BSC HO or other intra-BSC RR management. The contents of a Reset shall be as defined in step 2 in section 9.3.2. Abort is used to indicate to SMLC the failure of the current TA positioning attempt (e.g. due to inter-BSC handover) and allowing a new one from application level.