8 TA based Positioning

03.713GPPFunctional descriptionLocation Services (LCS)Release 1999Stage 2TS

8.1 Definition of TA states

8.1.1 MS in IDLE State

In IDLE state the GSM 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 VMSC or BSC along with the current serving cell ID in the BSSMAP‑LE Perform Location request (see section 7). This enables TA based positioning in the SMLC without any further interactions.

8.1.2 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 a TA_RESPONSE carrying the TA value. The associated procedure is described in sections 8.2 and 8.3.

8.2 TA Positioning Procedure for an NSS based SMLC

The TA positioning procedure for an NSS based SMLC makes use of the generic SMLC to BSC Information transfer procedure defined in section 7.

Figure 50: TA Positioning Procedure for an NSS based SMLC

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

2) The VMSC transfers the BSSMAP message received in step 1 to the serving BSC for the target MS.

3) The BSC returns the current TA value and current serving cell for the target MS to the VMSC in a TA response contained within a BSSMAP Connection Oriented Information message. The TA response may also include the latest measurement results received from the target MS for the serving and neighboring cells.

4) The VMSC forwards the BSSMAP-LE message received in step 3 to the SMLC. The SMLC then derives a location estimate for the target MS based on the received serving cell ID, TA value and other measurement results if included.

8.3 TA Positioning Procedure for a BSS based SMLC

The TA positioning procedure for a BSS based SMLC makes use of the generic SMLC to BSC Information transfer procedure defined in section 7.

Figure 51: TA Positioning Procedure for a BSS based SMLC

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

2) The BSC returns the current TA value and current serving cell for the target MS to the SMLC in a TA response contained within a BSSMAP-LE Connection Oriented Information message. The TA response may also 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, TA value and other measurement results if included.

8.4 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 . Please refer to GSM 8.71 for details.

After receiving the BSSLAP TA Request in BSC (message 2 in section 8.2 and message 1 in section 8.3), a Reject will be sent with proper cause value from BSC to SMLC in "BSSMAP Connection Oriented Information" if TA positioning cannot be performed in BSC at that time for reasons other than handover or another ongoing RR management procedure.

Between messages 2 and 3 (in section 8.2) and between messages 1 and 2 in section 8.3, 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. 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.