7 General UTRAN UE Positioning procedures

25.3053GPPRelease 17Stage 2 functional specification of User Equipment (UE) positioning in UTRANTS

7.1 General procedures in UTRAN for UE Positioning

The General UE positioning procedure in UTRAN starts with a request over Iu from the CN. The request from the core network may be a request for direct (on demand) reporting or periodic reporting.UTRAN then determines the UE position by selecting a suitable positioning method. UTRAN then responds to the request with the estimated position and optionally, the estimated velocity, possibly the associated accuracies and if available, the positioning method (or the list of the methods) used to obtain the position and optional velocity estimates. If periodic reporting was requested by the CN, UTRAN responds to the request according to the received periodic reporting criteria and sends the estimated position together with other information (e.g., velocity, associated accuracies, positioning methods used, etc.) one reporting interval after the previous report until the desired amount of reports has been attained or the procedure is cancelled by the UTRAN or CN.

7.2 Common procedures supporting UE Positioning interaction between RNCs

In the case that the positioning information is needed from an associated LMU in a Node B that is not controlled by the SRNC then the transfer of this information needs to be supported on the Iur interface. This information is the same information that is signalled between an associated LMU and the corresponding CRNC in the case when Iur support is not needed.

The SRNC requests the information it requires (e.g. GPS timing of cell measurements) from the CRNC of the Node B, which has the associated LMU. The CRNC in turn requests the information from the Node B and upon success returns the results to the SRNC.

Similarly when the SRNC needs a Node B measurement on a UE when that Node B is not controlled by that SRNC there needs to be support on the Iur. One example is the RTT measurement.

Other information that may need to be signalled over the Iur includes LMU parameters (geographical position, covered cells etc.).

NOTE: Confirmation or FS is needed by R3 experts.

7.2.1 Signalling in case of SRNS relocation

In case of SRNC relocation UE Positioning functionalities may be transferred in order for DRNC to be able to handle the responsibility of SRNC in LCS process. Therefore the Source RNC may transfer the following information to the Target RNC:

– last known position, time stamp and accuracy of the position calculation;

– LCS capabilities of the UE.

If there is a positioning procedure going on in order to estimate the position of the UE (e.g. in case of periodic reporting) and an SRNS relocation occurs, the positioning procedure shall be stopped in the old SRNC. After SRNS relocation, it is under the responsibility of the MSC, MSC Server or SGSN to restart an Iu aborted positioning procedure. In the UE, the positioning procedure is going on and positioning information (e.g. measurement results) may be sent back to UTRAN if the UE was requested to do so. The new SRNC then decides whether it wants to use these information or discard them. If an SAS is used, the SRNC shall send an Abort message to the SAS over the Iupc interface.

7.3 Exception procedures

7.3.1 Procedures in the SRNC

The SRNC can work in either the RNC centric mode or in a SAS centric mode where the SAS has control over the positioning procedures.

7.3.1.1 RNC Centric Mode

When a positioning attempt fails due to failure of a position method itself (e.g. due to inaccurate or insufficient position measurements and related data) and the SRNC is unable to instigate another positioning attempt (e.g. due to a requirement on response time), the SRNC shall return a Location response over the Iu interface containing a less accurate position estimate if available, even if this position estimate is not within the accuracy requested from the CN. In this case, the Location response shall indicate that the position estimate does not satisfy the requested accuracy. If a less accurate estimate is not available, the SRNC shall return a Location response message containing no position estimate and indicating the cause of failure.

When a positioning attempt is interrupted by some other unrecoverable error event inside the SRNC, the SRNC shall immediately terminate the positioning attempt and return a Location Response message containing the reason for the positioning attempt cancellation. In that case, SRNC may also abort any dialogue previously opened with an LMU for the purpose of instigating position measurements for the UE being located.

In networks that include an SAS, the SRNC will receive a PCAP Position Calculation Response message on the Iupc interface and then return a Location response over the Iu interface containing the results that were provided over the Iupc interface (a less accurate position estimate, cause of failure indication, etc). This is described in subclause 7.3.4.

If the uplink channel characteristics for the target UE change while a U-TDOA positioning is ongoing, the SRNC shall send a PCAP Position Parameter Modification message to the SAS containing the updated channel characteristics and cell information.

When a positioning attempt is interrupted by some other unrecoverable error event inside the SRNC, the SRNC may also abort any dialogue previously opened with an LMU for the purpose of instigating position measurements for the UE being located.

When periodic reporting was requested from the CN and when no location estimate is available at the SRNC when the reporting criteria are fulfilled (e.g. due to failure of a position method itself), the SRNC shall return a location response message containing no position estimate and indicating the cause of failure.

7.3.1.2 SAS Centric Mode

When the SRNC is unable to accept a PCAP Position Activation Request message due to congestion or any other failure, the PCAP Position Activation Failure message shall be sent to the SAS on the Iupc interface.

If an RRC measurement procedure for periodic UE positioning measurement reporting in the UE has been activated by the SRNC upon reception of a PCAP Position Activation Request message, and if the SRNC or UE terminates the measurement reporting (e.g. after RRC state transition), the SRNC shall send a PCAP Position Periodic Termination message to the SAS indicating the cause of termination.

When the SRNC is unable to continue positioning activity due to cell reselection that results in a different SRNC, loss of contact with the UE or any other reason, the PCAP Abort message shall be sent to the SAS on the Iupc interface.

A PCAP Position Parameter Modification message shall be sent containing updated channel characteristics (including cell information) whenever the RF channel parameters associated with the UE connection change due to cell reselection or handover within the same SRNC, modification of the channel capacity or any other reason within the control of a single RNC. The SRNC shall keep the SAS updated on any change in serving cell status (using PCAP Position Parameter Modification message) so that the SAS can provide correct assistance data to the UE whenever the SAS invokes a positioning method or methods.

If an RRC measurement procedure for periodic UE positioning measurement reporting in the UE has been activated by the SRNC upon reception of a PCAP Position Activation Request message, and if there is a RRC state transition during the RRC measurement procedure, which results not in a termination of the UE measurement reporting, the SRNC shall send a PCAP Position Parameter Modification message informing the SAS about the new UE state. The SRNC shall continue to notify the SAS of such state changes until the periodic position measurement reporting has been completed by the UE, cancelled by the SAS or terminated by the SRNC.

Sending of a PCAP Position Parameter Modification message does not change the status in the SRNC of any position method or position methods that may have been previously activated through receipt of a PCAP Position Activation Request message.

If the SRNC receives a new PCAP Position Activation Request message before it has responded to a previous PCAP request for the same UE or while it is still performing activity for a previous non-periodic request (e.g. in the case of U-TDOA in CELL_FACH state), the SRNC shall terminate all activity for the previous request, including sending any response to the SAS, and shall process the new request.

If the SRNC receives a new PCAP Position Activation Request message for UE position measurement reporting using A-GPS or OTDOA or Barometric Pressure or WLAN or Bluetooth or TBS (periodic or non-periodic) while it is still performing activity for a previous A-GPS or OTDOA or Barometric Pressure or WLAN or Bluetooth or TBS periodic request, the SRNC shall terminate all activity for the previous request, including terminating the periodic measurement reporting in the UE, and shall process the new request.

If the SRNC receives a new PCAP Position Activation Request message for cell ID-RTT or U-TDOA while it is still performing activity for a previous A-GPS or OTDOA or Barometric Pressure or WLAN or Bluetooth or TBS periodic request (but after returning any PCAP Position Activation Response for this request), the SRNC may both continue with the previous request and process the new request.

If an RRC measurement procedure for periodic UE positioning measurement reporting in the UE has been activated by the SRNC upon reception of a PCAP Position Activation Request message, and if the SRNC receives a PCAP Position Periodic Termination message from the SAS, the SRNC shall terminate the periodic UE positioning measurement reporting in the UE.

If the SRNC receives a request for assistance data from a UE positioning procedure (periodic or non-periodic) that uses A-GNSS, the SRNC should indicate this to the SAS by sending a Position Activation Response message. The Position Activation Response message shall include the assistance data types requested by the UE. In case of periodic UE positioning reporting and after a Position Activation Response message has already been returned to the SAS, the request for additional assistance data may be conveyed in a Position Periodic Report message.

7.3.2 Procedures in a LMU

An LMU shall return an error indication to its CRNC when positioning measurements previously ordered by the RNC cannot be provided due to any error condition.

7.3.3 Procedures in the target UE

A target UE shall terminate any positioning procedure or the transfer of RRC positioning assistance data without sending any response to the SRNC if any UE Positioning RRC message is received from the SRNC that starts some other RRC management procedure. The new RRC procedure shall then be executed by the UE.

7.3.4 Procedures in the SAS

The SAS can work in either the RNC centric mode or in a SAS centric mode where the SAS has control over the positioning procedures.

7.3.4.1 RNC Centric Mode

In the RNC centric mode, when a UE positioning attempt fails (e.g. due to inaccurate or insufficient position measurements and related data), the SAS may return a PCAP Position Calculation Response message over the Iupc interface containing a less accurate position estimate. If a less accurate estimate is not available or will not meet the accuracy requirement, the SAS may instead return a PCAP Position Calculation Failure message containing no position estimate and indicating the cause of failure.

When a positioning attempt is interrupted by some other unrecoverable error event inside the SAS, the SAS shall immediately terminate the positioning attempt and return a PCAP Position Calculation Failure message containing the reason for the positioning attempt cancellation.

7.3.4.2 SAS Centric Mode

If the SAS invokes a positioning method by sending a PCAP Position Activation Request message to the SRNC, it may subsequently receive a PCAP Position Activation Failure or PCAP Abort message from the SRNC, while positioning in the SRNC is active, according to subclause 7.3.1.2. At any time after receiving a PCAP Position Initiation Request message and whether positioning in the SRNC is active or not active, the SAS may receive a PCAP Abort message or PCAP Position Parameter Modification message from the SRNC, according to subclause 7.3.1.2.

If the SAS receives a PCAP Abort message it shall immediately cease positioning attempts and return a PCAP Position Initiation Response message to the SRNC carrying any location estimate already obtained or a Position Initiation Failure message containing a cause of failure indication. If the SAS receives a PCAP Postion Activation Failure message, the SAS may either terminate positioning as for receipt of a PCAP Abort or continue the positioning attempt using the same or other positioning methods, e.g. if permitted by the QoS response time requirement. If the SAS receives a PCAP Position Parameter Modification message, the SAS shall take the received information into account in either performing ongoing positioning (e.g. reconfigure LMUs for U-TDOA) or invoking new positioning (e.g. use new serving cell to provide A-GPS timing assistance).

If the SAS is unable to accept or continue processing a PCAP Position Initiation Request from the SRNC due to capacity constraints or equipment failure, a PCAP Position Initiation Failure message shall be sent to the SRNC on the Iupc interface carrying an appropriate cause of the failure indication.

7.4 Radio interface timing procedures

The Radio Interface Timing determination system consists of functions in LMUs in the SRNC, and in the SAS for networks that include an SAS. The system runs continuously offering cell timing information for UE Positioning.

7.4.1 LMU Functions

The Radio Interface Timing functionality in the CRNC associated LMU should be capable of performing the following functions:

– The LMU performs necessary radio interface measurements from signals transmitted by Node Bs;

– If the LMU contains a common reference clock, e.g. GPS TOW, it time stamps reception of Node B signals by performing measurements of UTRAN GPS timing of cell frames;

– If there is no reference clock available, the LMU may make SFN-SFN Observed Time Difference or measurements, i.e. measures the time difference between arrival of SFNs from neighbouring Node Bs and a reference Node B;

– The LMU may perform some processing of measurements, like averaging and filtering, using parameters delivered to it, or in their absence using default settings.

7.4.2 CRNC Functions

The CRNC must be capable of performing the following functions related to Radio Interface Timing determination:

– The CRNC sends to LMUs requests for Radio Interface Timing measurement information;

– The CRNC will communicate regularly with LMUs; thus, the CRNC can monitor operation of LMUs. If a LMU fails to send Radio Interface Timing information, the CRNC shall try to restart the LMU, and if this restarting fails, the CRNC shall inform O&M system. CRNC can use also diagnostics messages to query the status of LMUs;

– The CRNC receives Radio Interface Timing measurement results from LMUs;

– The CRNC stores or queries extra information required for Node B synchronization determination, like Node B and LMU coordinates, Node B identity information;

– The CRNC determines synchronization differences between different downlink signals using LMU measurements and other information;

– Synchronization information is delivered for UE Positioning purposes.

7.4.3 LMU-CRNC Interactions

The request for Radio Interface Timing measurement information from the CRNC to a LMU contains the following parameters:

– Measurement type. This indicates whether the CRNC wants the LMU to perform UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference;

– Measurement result reporting frequency. This indicates how often the LMU should send Radio Interface Timing measurement results;

– Measurement duration. This indicates how long the LMU should make measurements and report results;

– Instructions about filtering of raw measurement data;

– Instructions about Primary CPICH signals to be measured. The LMU unit can measure autonomously a certain number of most strongly received signals. Another possibility is that the CRNC tells which Node B signals it should measure;

– In the SFN-SFN Observed Time Difference case, which common Primary CPICH the LMU should use as a reference in the measurements;

– Instruction of how the measurement quality should be reported.

In case a SFN-SFN Observed Time Difference measurement was requested by the CRNC, the LMU returns the following information to CRNC:

– Identity of the Node B at which the associated LMU is residing;

– Primary CPICH info of the measured signals;

– SFN-SFN observed time difference between neighbour cells and reference cell;

– Identity of the neighbour cells;

– SFN-SFN drift between neighbour cells and reference cell;

– Time stamp of the measurement (e.g. SFN);

– Accuracy of the measurement.

In case a UTRAN GPS timing of cell frames measurement was requested by the CRNC, the LMU returns the following information to the CRNC:

– Cell id of the measured cell;

– SFN;

– Time stamp (e.g. GPS TOW) of the SFN;

– Node B clock drift;

– Accuracy of the measurement.