9.6 Multilateration Positioning Methods

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

9.6.1 General

Multilateration Positioning methods do not require the use of LMUs. For devices in PEO or EC operation (see 3GPP TS 43.064 [41] two methods are supported: Multilateration Timing Advance (MTA) and Multilateration Observed Time Difference (MOTD). Based on assistance information from the BSS, targeted accuracy, MS capability and SMLC capability the SMLC decides which method to use, see sub-clause 9.6.2.2 below. When the MS is in idle mode the Core Network initiates the methods by indicating ‘paging for positioning’ as specified in sub-clause 9.1.3a. Both methods are only applicable for the PS domain in A/Gb-mode.

9.6.2 Multilateration Timing Advance

9.6.2.1 General procedure

This positioning method is based on one or more BSS acquiring Timing Advance (TA) information from a set of cells selected by the MS wherein the MS is triggered to perform MTA as a result of receiving an RRLP message from an SMLC indicating the MTA positioning method is to be used. The RRLP message may include network assistance information in order to facilitate and speed up the procedure, see sub-clause 9.6.2.4.2 for more details. Based on signal strength the MS autonomously selects a subset of the cells identified by the network assistance information (if any) to use for MTA. In addition, the MS may also select one or more additional cells not identified within assistance information the triggering RRLP message to perform MTA. In each of the selected cells the MS performs packet access for the BSS to be able to estimate a TA value for that particular cell. Three different methods are supported for the Timing Advance estimation.

– RLC data block method: This method supports Timing Advance Estimation for devices in both PEO and EC operation (CC1-CC4) wherein the MS sends an access burst on the (EC-)RACH and receives a corresponding single block allocation on the (EC-)AGCH for transmission of an RLC data block on a dedicated resource. The RLC data block contains MS accuracy related information and Source Identity of the serving cell see sub-clause 9.6.2.2. The method is suitable for Multilateration both cells internal to the serving BSS as well as for cells belonging to a non-serving BSS. For this method SMLC may indicate the MS to generate MTA Signature at the end of radio access part of MTA positioning procedure which can be used to verify the integrity of the location estimated from the MTA positioning method. Alternatively, for this method the SMLC may choose to direct the MS to include additional information in each RLC data block it sends during the MTA procedure, thereby allowing the serving BSS to verify the integrity of the location estimated from the MTA positioning method.

– Extended Access burst method: This method supports Timing Advance Estimation for devices in both PEO and EC operation (CC1 only) wherein the MS first sends an access burst and receives an assigned timing advance value within a corresponding Acknowledgement received on the (EC-)AGCH. It then sends an Extended Access Burst containing MS accuracy related information and a Short BSS ID that via configuration can be mapped to a serving BSS, see sub-clause 9.6.2.2. The method is suitable for both cells internal to the serving BSS as well as for cells belonging to a non-serving BSS when the MS has been provided with a Short BSS ID in the triggering RRLP message.

– Access burst method: This method supports Timing Advance Estimation for devices in both PEO and EC operation wherein the MS sends an Access Burst on the RACH and receives an Acknowledgement on the (EC-)AGCH. The method is only suitable for cells internal to the serving BSS.

All TA estimations made by a BSS for any given MS performing MTA are relayed to the serving SMLC along with the identity of the reporting cell, the MS and BTS Accuracy thereby allowing the SMLC to estimate the position of the target MS.

9.6.2.2 Multilateration Timing Advance Request

The signalling flow is generic irrespective if the MS is in Packet Idle or Packet Transfer mode. If the MS is in Packet Idle mode, the Multilateration Timing Advance request procedure is performed subsequently to the paging for positioning procedure in sub-clause 9.1.3a. The signaling flow in Figure 38 below applies to integrated and standalone SMLCs in a packet switched network.

Figure 38: MTA positioning procedures in the PS domain

1. The SGSN sends a BSSGP PERFORM-LOCATION-REQUEST to the BSC serving the MS and includes, if available, the “Multilateration Timing Advance”, “MS Sync Accuracy”and “BTS Reception Accuracy Level”. If LLC ciphering is not activated at SGSN, the BSSGP PERFORM-LOCATION-REQUEST may indicate “MTA Access Security Required” if integrity protection for the radio access part of the MTA procedure is required.

2. The BSC sends to the SMLC a BSSMAP-LE Perform Location Request message that includes the “LCS capability”, the “BSS Multilateration Capability” and if available also the “Multilateration Timing Advance”, “MS Sync Accuracy”, “BTS Reception Accuracy Level” and an “MTA Access Security Required” indication if included in the BSSGP PERFORM-LOCATION-REQUEST received from the SGSN.

3. The SMLC may invoke the Assistance Information Transfer procedure to request Assistance Information from the serving BSS by sending to it a BSSMAP-LE Assistance Information Request message that includes a set of one or more cells for which assistance information is requested and optionally include a limited set of “Random ID” values (applies to Extended Access Burst method) or a set of “Short ID” values (applies to Access Burst method) associated with each cell for which it has requested Assistance Information. The serving BSS responds with a BSSMAP-LE Assistance Information Response message that contains the “Cell Information List” with the per cell assistance information and the “BTS Reception Accuracy Capability”, see sub-clause 9.6.4.1 for more additional details. The serving BSS also includes “MTA Access Security Method Support” and/or “BSS Duplication Detection Method Support” in the BSSMAP-LE Assistance Information Response message.

4. To meet a targeted positioning accuracy the SMLC uses the “BTS Reception Accuracy Capability” as well as the “MS Sync Accuracy” as provided in the MSRAC IE (see 3GPP TS 24.008 [8]) to determine the number of targeted cells to be used in the MTA procedure and based on the serving cell it includes assistance data in an RRLP Multilateration Timing Advance Request message (see 3GPP TS 44.031 [15]) and sends the message to the MS (tunnelled via the BSS and the SGSN, see sub-step below and sub-clause 8.1a for the general description). If “MTA Access Security Required” is received from the BSS the SMLC decides if it should include an indication of (a) “MTA Access Security Method” in the RRLP Positioning Multilateration Timing Advance Request message if BSS includes “MTA Access Security Method Support” in the BSSMAP-LE Assistance Information Response message or (b) “BSS Duplication Detection Method” in the RRLP Positioning Multilateration Timing Advance Request message if the BSS includes “BSS Duplication Detection Method Support” in the BSSMAP-LE Assistance Information Response message.

4a. The SMLC passes a BSSMAP-LE Connection Oriented Information message to the serving BSS containing an indication of the type of Multilateration method, a Multilateration Positioning Method timer and an embedded BSSLAP MS Position Command with an RRLP message parameter including the RRLP Positioning Multilateration Timing Advance Request message.

4b. The BSS relays the embedded RRLP message, the RRLP flags IE and the MPM timer to the SGSN inside a BSSGP Position Command message. When the BSSGP Position Command message has been transferred, the BSS shall start a Multilateration supervision timer for the associated SCCP connection corresponding to the value indicated by the Multilateration Positioning Method timer field and upon expiry release the connection.

4c. The SGSN receives the RRLP message in the BSSGP Position Command message and relays it to the MS in an LLC UI frame and includes a Positioning Event Indicator in the DL-UNITDATA PDU to indicate that the LLC PDU contains an RRLP Multilateration Timing Advance Request message (this is determined based on SGSN receiving a value for the MPM Timer in the BSSGP Position Command message). When the DL-UNITDATA PDU has been transferred the SGSN starts a supervision timer corresponding to the value indicated by the Multilateration Positioning Method timer for the associated MS and suspends downlink data delivery while the timer is running (see 3GPP 48.018 [25]).

5. The MS acknowledges reception of the RRLP MULTILATERATION REQUEST message on RR layer by means of a PDAN message and therein includes MS accuracy related information (“MS Sync Accuracy” and “MS Transmission Offset”). The BSS uses this PDAN transmission to estimate the timing advance applicable to the serving cell (i.e. a value used as the “Multilateration Timing Advance”) as well as the associated BTS Reception Accurcy Level before the MS leaves the serving cell. The “Multilateration Timing Advance” value as well as the “BTS Reception Accuracy Level” and “MS Sync Accuracy” are forwarded from the BSS to the SMLC (see 3GPP TS 45.010 [42]).

6. The RRLP application in the MS examines the RRLP Positioning Multilateration Timing Advance Request message and if assistance data is present it may use the assistance data to autonomously select which subset of these sets of cells it shall use to perform the radio access part of the MTA procedure. The MS may also select one or more additional cells not provided by the RRLP message.

7. The MS starts a supervision timer corresponding to the value indicated by timer contained in the RRLP mesage and sends an Multilateration access request indicating in each of the cells it selects for performing the Multilateration Timing Advance procedure.

– New SI and EC-SI information shall indicate whether or not a cell supports the Multilateration Timing Advance procedure using either the RLC Data Block Transmission method, the Extended Access Burst method or the Access Burst method. This indication may also be included for each of the cells identified by the assistance information sent to the MS in the RRLP Multilateration message.

– Information included within the RRLP Positioning Multilateration Timing Advance Request message indicates the MTA method is to be used when performing Multilateration Timing Advance for those cells (if they are used in the procedure). An MS in PEO operation shall perform the Multilateration Timing Advance procedure in a cell using the method indicated by the RRLP Positioing Multilateration Timing Advance Request message if (a) if the indicated method is supported by that cell, (b) the MS supports the indicated method and (c) the RRLP message provides provides sufficient assistance information for performing the indicated method in that cell. An MS that has enabled EC operation shall follow these same rules with the restriction that the use of the Extended Access Burst method is limited to an MS in coverage conditions corresponding to CC1 or better.

– When performing the radio access part of the Multilateration Timing Advance procedure using the RLC Data Block method a MS sends a single MCS-1 coded RLC data block that conveys the MS identity (TLLI), “Source Identity” information (i.e. the “Cell Identity” and “Routing Area Identity”received in EC SI2/SI13 of the cell in which it was triggered to perform Multilateration), “MS Sync Accuracy”, “MS Transmission Offset” (applicable to when it transmitts the RLC data block) as well as a “Random ID” (see 3GPP TS 44.031 [15] and 3GPP TS 49.031 [22]). The RLC Data Block also includes “MTA Signature” if the MS is performing the radio access part of the MTA Procedure with the last cell selected for the MTA procedure and the “MTA Access Security Method” was indicated by the RRLP Positioning Multilateration Timing Advance Request message . If the “BSS Duplication Detection Method” was indicated by the RRLP Positioning Multilateration Timing Advance Request message each RLC Data Block sent by the MS during the MTA procedure includes a “MTA Report Instance” field and a “Final MTA Report Indicator” field. The RLC Data Block sent during the Multilateration Timing Advance procedure does not contain an LLC PDU. The BSS estimates the timing advance based on UL transmissions of the MS and adjusts the estimated timing advance value using the MS Transmission Offset (see 3GPP 45.010[42]).

– When performing the Multilateration Timing Advance procedure using the Extended Access Burst method an MS sends an Extended Access Burst wherein the payload space supported by the set of two EC/EGPRS MULTILATERATION REQUEST messages used to perform the radio access part of the MTA procedure together convey the MS identity (“Short BSS ID” and “RandomID”), “MS Sync Accuracy” and “MS Transmission Offset” (applicable to when it transmits the Extended Access Burst). The BSS estimates the timing advance based on UL transmissions of the MS and adjusts the estimated timing advance value using the MS Transmission Offset (see 3GPP 45.010[42]).

– When performing the radio access part of the Multilateration Timing Advance procedure using the Access Burst method an MS sends an access burst wherein the payload space conveys the MS identity (Short ID). The Access Burst method is only used for cells belonging to the serving BSS. The BSS estimates the timing advance based on an MS transmitting a single EC/EGPRS MULTILATERATION REQUEST on UL.

8. If the BSS has a context for the indicated MS Identity (“Short ID”, “Random ID”, or TLLI) i.e. it triggered the current Multilateration Timing Advance procedure for the indicated MS Identity it realizes it is the serving BSS and sends its default SMLC a “BSSMAP-LE Connection Oriented Information” message containing “Multilateration Timing Advance”, “MS Sync Accuracy”, “BTS Reception Accuracy Level”, “Cell Identifer” and “Random ID” information. If the Access Burst method has been used only the “Multilateration Timing Advance” and “Short ID” are provided by the BSS to the SMLC. The BSS includes “MTA Signature” as part of the final BSSMAP-LE Connection Oriented Information to SMLC if available. If “MTA Signature” is included BSS also includes “MTA Sequence” which is generated by concatenation of Cell identifier and MS Transmission Offset parameters received from MS in each successful radio access part of the MTA procedure. If the BSS receives an “MTA Report Instance” and a “Final MTA Report Indicator” in a RLC Data Block received for the MTA procedure associated with the “Random ID” value also received in that RLC Data Block, it shall use them in an attempt to determine if a MTA reporting anomoly exists for that MTA procedure. If an anomoly is detected the BSS aborts the MTA procedure and informs the SMLC of the anomoly, thereby causing the SMLC to also abort that MTA procedure.

If the BSS has no context for the indicated MS Identity, it realizes it is a non-serving BSS and depending on which transport mechanism (CS or PS domain) is used to pass information to the serving BSS it proceeds as follows:

a) CS Domain Used: The non-serving BSS sends the MSC a “BSSAP-LE Connectionless Information” message containing “Multilateration Timing Advance”, “MS Sync Accurcy”, “BTS Reception Accuracy Level” and “Random ID” parameters. The MSC then forwards the “BSSAP-LE Connectionless Information” message to the serving BSS (identified by the “Network Element Identity (target)” IE in the “BSSAP-LE Connectionless Information” message).

b) PS Domain Used: The non-serving BSS sends the serving BSS a RAN-INFORMATION/Single Report (via the serving SGSN) containing “Multilateration Timing Advance”, “MS Sync Accurcy”, “BTS Reception Accuracy Level” and “Random ID” parameters (see 3GPP TS 48.018 [25].

After receiving information from a non-serving BSS the serving BSS sends its default SMLC a “BSSMAP-LE Connection Oriented Information” message containing “Multilateration Timing Advance”, “MS Sync Accuracy”, “BTS Reception Accuracy Level”, “Cell Identifer” and “Random ID” information.

9. For the RLC Data Block and Extended Access Burst methods, the serving SMLC then collects and validates all “Multilateration Timing Advance”, “BTS Reception Accuracy Level”, “MS Sync Accuracy” and “Cell Identifier” information associated with a given MS, for up to the length of the Multilateration Positioning Method timer after triggering the Multilateration Timing Advance procedure, using the provided “Random ID” parameter (see 3GPP TS 49.031 [22]).

For the Access Burst method the serving SMLC collects and validates all “Multilateration Timing Advance” information associated with a given MS, for up to the length of the Multilateration Positioning Method timer after triggering the Multilateration Timing Advance procedure, using the provided “Short ID” value.

If the SMLC receives an instance of MTA information from the BSS wherein either the value of “Random ID” or “Short ID” parameters has a value previously received for that MS for the ongoing MTA procedure the SMLC shall discard that instance of MTA information.

If the SMLC has included “MTA Access Security Method” IE in the RRLP message sent for a given MTA procedure and does not receive a corresponding “MTA Signature” from BSS, SMLC abandons the estimate of the MS location based on the received information from BSS and starts abnormal case handling procedure.

Using validated information, the SMLC then determines the MS location.

10. The SMLC then sends the serving BSS a “BSSMAP-LE Perform Location Response” message and includes the determined MS location information in the “Location Estimate” IE. The SMLC also includes “MTA Signature” and “MTA Sequence” in this message to SGSN, if it has received these IEs from BSS and has included “MTA Access Security Method” IE in the RRLP message sent for the ccorresponding MTA procedure.

11. The serving BSS sends the SGSN a “BSSGP PERFORM-LOCATION-RESPONSE PDU” message that includes the MS location information in the “Location Estimate” IE , “MTA Signature” IE and “MTA Sequence” IE if received from SMLC. The SGSN verifies the integrity of the location estimate by verifying the received MTA Signature against the Message Authentication Code (MAC) value derived based on the Integrity protection algorithm and keys associated with MS using “MTA Sequence” as MESSAGE [43].

9.6.2.3 Error Handling for MTA

Case 1: When the SMLC sends the BSSAP-LE Connection Oriented Information message containing the RRLP Positioning Multilateration Timing Advance Request message to the BSS and the BSS cannot support the procedure it will send a BSSLAP reject message to the SMLC.

Case 2: When the BSC forwards the RRLP Multilateration Timing Advance Request message in an BSSGP Position Command message (sent to the SGSN) the Multilateration supervision timer is started. After this, if the BSS determines that the current location procedure cannot be continued, the BSS sends an abort message to the SMLC. The determination that the MS has performed a reselection to a new cell is not a reason for the BSS to abort the procedure.

Case 3: If the Multilateration supervision timer times out in the BSS before any “Multilateration Timing Advance”, “MS Sync Accuracy”, “BTS Reception Accuracy Level”, “Cell Identifier” and “Random ID” information for the target MS is sent to the SMLC, the BSS shall send a BSSLAP Abort to the SMLC. The location service attempt may then be restarted from the LCS Client, VMSC, or SGSN.

Case 4: If the Multilateration supervision timer times out in the MS before the MS has been able to perform the Multilateration Timing Advance procedure to all selected cells the MS shall abort the procedure and return to idle mode.

Figure 39: Error Handling for the MTA procedure in PS domain

9.6.2.4 Assistance Information

9.6.2.4.1 Assistance Information Transfer

The purpose of this procedure is for the SMLC to obtain assistance information from the serving BSS for subsequent inclusion in the RRLP Positioning Multilateration Timing Advance Request message sent from the SMLC to the MS to trigger the Multilateration Timing Advance procedure. In the procedure the SMLC sends to the serving BSS a BSSMAP-LE Assistance Information Request message containing the TLLI of the MS, a list of cells for which assistance information is requested and optionally a set of cell specific “Short ID” values assigned by the SMLC, and a limited set of “MTA Reference ID” values. The “Short ID” field specific to a given cell is included when the SMLC anticipates using the Access Burst method in that cell (if that cell is selected by the MS for performing MTA). Similarly, the limited set of “Random ID” values is included when the SMLC anticipates using Extended Access Burst method in that cell (if that cell is selected by the MS for performing MTA). The BSS responds with a BSSMAP-LE Assistance Information Response message and includes one or more instances of the (EC-)RACH control parameters/Coverage class thresholds (Coverage class thresholds are only provided if the serving BSS determines that the target MS is in EC operation) and BTS Reception Accuracy Capability applicable to the set of cells for which assistance information has been requested. The BTS Reception Accuracy Capability is used by the SMLC when determining the targeted number of cells to be selected by the MS in the Multilateration Timing Advance procedure.

9.6.2.4.2 Provisioning of Assistance Information to the MS

The MS is triggered to perform the Multilateration Timing Advance procedure through reception of a new RRLP Positioning Multilateration Timing Advance Request message (see 3GPP TS 44.031[15] sent by the SMLC where the MS is assumed to be in the best serving cell upon reception of the RRLP message. The RRLP Positioning Multilateration Timing Advance Request message may include network assistance information that consists of multiple sets of co-sited cells which the MS may take into account when performing the Multilateration Timing Advance procedure as well as cells co-sited with the serving cell which the MS shall avoid using when performing Multilateration Timing Advance procedure. The assistance information included in the RRLP Positioning Multilateration Timing Advance Request message and may be generated by the SMLC using implementation specific methods or acquired from the serving BSS using the Assistance Information Transfer procedure, see sub-clause 9.6.4.1. Examples of additional assistance information include information about suitable neighbour cells, an indication of which of the indicated neighbour cells are co-sited cells, requested accuracy of the positioning procedure in the form of number of cells involved, the requested MS Sync Accuracy (see 3GPP TS 45.010 [42]), neighbour cell specific RACH/EC-RACH control/Coverage class parameters. The SMLC may also optionally provide an assignment of cell specific “Short ID” values (i.e. when the RRLP MTA message indicates Access Burst method is to be used),or a limited set of “Random ID” values (i.e. when the when the RRLP MTA message indicates Extended Access Burst method or RLC Data Block method is to be used) to be used during the TA estimation procedure (see 3GPP TS 49.031[22]). The MS autonomously selects the set of cells for performing the Multilateration Timing Advance procedure which may include a subset of the cells identified by the assistance information. The provisioning of assistance data improves the location accuracy for the Multilateration Timing Advance procedure and decreases the energy consumed in the MS to perform the positioning.

9.6.3 Multilateration OTD

9.6.3.1 General Procedure

This procedure uses MS measured observed time difference between serving cell and one or more neighbour cell and the timing advance reported by serving base station to estimate the position of the MS using multilateration. The observed time difference measurement is triggered at MS on reception of RRLP Multilateration OTD Request message containing measurement request for multilateration. The message may also include the network assistance information for OTD measurements such as list of co-sited neighbour cells and list of co-sited cells which share the same site as serving cell. On reception of the RRLP Multilateration OTD Request MS autonomously searches for best neighbour cell and after synchronisation with this cell it measures the time difference between TDMA frame timing of this neighbour cell and the serving cell. While selecting cell for OTD measurements the MS shall use the assistance information provided in the RRLP message to identify the co-sited cells and measures the OTD only for the best cell of the co-sited cells. In case the MS detects cell which is co-sited with serving cell the OTD measurement on this cell shall be skipped. After completion of OTD measurements with all the detected neighbour cells as per network assistance info or after measurement made on targeted number of cells provided in the measurement command, MS establishes TBF and sends RRLP measurement report for OTD multilateration. SMLC estimates the position of MS based on serving cell timing advance value reported from serving BSS and the OTD measurements reported from MS. The serving cell timing advance is estimated and reported to SMLC before MS leaves the serving cell for MOTD measurements. If the MS can find only cells of one neighbour cell site, it reports in addition to the OTD measurement of the best cell of the neighbour cell site also the received signal strength of all neighbour cells that it can receive. The Multilateration OTD procedure is only applicable for the PS domain in A/Gb-mode.

9.6.3.2 Multilateration Observed Time Difference Measurements

The Multilateration OTD is performed subsequently to the paging for positioning procedure described in sub-clause 9.1.3a. The signaling flow in Figure 40 below applies to integrated and standalone SMLCs in a packet switched network and is described below.

Figure 40: MOTD positioning procedure in the PS domain

1. The SGSN sends a BSSGP PERFORM-LOCATION-REQUEST to the BSC serving the MS. SGSN also includes the serving cell Timing advance, MS/BTS accuracy information in the message if available.

2. The BSC sends a BSSMAP-LE Perform Location Request message to the SMLC that includes the LCS capability IE and the BSS Multilateration Capability IE.

3. Based on the serving cell TA parameters reported, targeted MS position accuracy SMLC decides the Multilateration method to be used. The SMLC includes assistance data, if any, in an RRLP MULTILATERATION OTD REQUEST message and sends to the MS (tunnelled via the BSS and the SGSN, see sub-step below and sub-clause 8.1a for the general description).

3a. The SMLC sends a BSSMAP-LE Connection Oriented Information message to the serving BSS containing an embedded BSSLAP MS Position Command with an RRLP message parameter including the RRLP MULTILATERATION OTD REQUEST message.SMLC includes the multilateration method and multilateration positioning timer information in this message.

3b. The BSS relays the embedded RRLP message and the RRLP flags IE to the SGSN inside a BSSGP Position Command message. The BSS also includes the multilateration positioning timer information in this message which was received from SMLC.

3c. The SGSN receives the RRLP message in the BSSGP Position Command message and relays it to the MS in an LLC UI frame. The SGSN starts the Multilateration positioning timer. SGSN does not send any downlink data towards MS while this timer is running. SGSN also includes additional parameter in the DL UNITDATA PDU which contains the RRLP message that the LLC PDU contains multilateration message.

4) The MS acknowledges the reception of the RRLP MULTILATERATION REQUEST message on RR layer by means of a PDAN message and therein includes MS accuracy related information (“MS Sync Accuracy” and “MS Transmission Offset”). The BSS uses this transmission to estimate the timing advance to the serving cell before the MS leaves the cell.

5) BSS sends the estimated timing advance along with BTS/MS accuracy related information to SMLC.

6. The RRLP application in MS receives the message and if it contains the network assistance information MS uses this information for selecting the neighbour cells for OTD measurements as part of its autonomous neighbour cell selection for OTD measurements. The RRLP message may also contain the targeted number of measurement reports.

6a. MS synchronises to each of the detected neighbour cells and compares the TDMA frame timing of the neighbour cell with the TDMA frame timing of serving cell. The difference between the TDMA frame timing is considered as OTD of the neighbour cell with serving cell. MS uses the assistance information provided to select the neighbour cells for which OTD measurements are required.

6b. MS also reports the downlink synchronisation accuracy of each cell along with each OTD measurement.

6c. After completing the OTD measurements on required number of cells or on all the detected neighbour cell sites, MS sends the RRLP MULTILATERATION OTD MEASUREMENT RESPONSE message through serving cell. (For the special case that the MS can provide an OTD measurement for only one neighbour cell site, see subclause 9.6.1.1.)

7) SMLC receives the OTD measurement reports from MS via RRLP MULTILATERATION OTD MEASUREMENT RESPONSE. SMLC calculates the position of the MS based on timing advance of MS estimated at serving cell and the OTD measurements of neighbour cells.

NOTE: If the MS reports only one OTD measurement, the SMLC can perform only a bilateration. If the bilateration results in 2 intersections of 2 circles, the reported signal strength values of the neighbour cells can be used to determine which of these 2 possible MS locations is more plausible.

8. The SMLC then sends the serving BSS a “BSSMAP-LE Perform Location Response” message and includes the determined location information in the “Location Estimate” IE.

9. The serving BSS sends to the SGSN a “BSSGP PERFORM-LOCATION-RESPONSE PDU” message that includes the identified MS location in the “Location Estimate” IE.

9.6.3.3 Error Handling for MOTD

Case 1: When the SMLC sends the BSSAP-LE Connection Oriented Information message containing the RRLP Multilateration OTD Request message to the BSS and the BSS cannot support the procedure it will send a BSSLAP reject message to the SMLC.

Case 2: If the Multilateration supervision timer times out in SMLC before receiving the RRLP Multilateration OTD Measurement Response from MS SMLC aborts the procedure.Case 3: If the RRLP Multilateration OTD Measurement Response does not contain sufficient information for estimating the position, SMLC estimates the position based on serving cell ID +Timing advance and report this position in the Location Estimate IE.

9.6.3.4 Network Assistance Information for Multilateration OTD

The OTD measurements for multilateration can be optimised based on assistance information given from network as part of the Multilateration OTD measurement message. SMLC may include the list of co-sited cells at neighbour cell sites in the network assistance information which can be used by MS to optimise the OTD measurements among the co-sited cells. SMLC may also include the co-sited cells of serving base station as additional parameter. If the required positioning accuracy is known to SMLC, SMLC may also indicate the target number of measurement reports in the assistance information to optimise the MOTD measurements at MS. SMLC may also provide the requested downlink sync accuracy (see 3GPP TS 45.010) to MS as part of the assistance information.