03.713GPPFunctional descriptionLocation Services (LCS)Release 1999Stage 2TS
Figure A.2: Call Related MT-LR
1. An MS requests a voice or data call to some external Location Application (LA).
2. The call is routed from the VMSC through the PSTN to the LA. The MSC stores the original dialed number.
3. The external LA obtains the MSISDN of the calling MS – either verbally or using calling line ID presentation. The LA may also need to verify the number dialed by the MS – e.g. if the LA can be reached by any of several dialed numbers. The external LA sends a Location Request to a GMLC in its serving GPLMN requesting the location of the MS and providing both the MSISDN and the number dialed by the MS.
4. The GMLC queries the HLR of the MS to be located by sending a MAP query to the HPLMN of this MS. In order to route the query to the HLR, translation of the MSISDN of the MS to be located will be required. This translation may be performed within the GMLC and/or may be performed by intermediate STPs.
5. The HLR returns the E.164 address of the VMSC currently serving the MS in the VPLMN.
6. The GMLC forwards the location request to the VMSC and includes within it the number of the LA dialed by the MS. In order to route the request to the VMSC, translation of the E.164 address of the VMSC will be required. This translation may be performed within the GMLC and/or may be performed by intermediate STPs.
7. The VMSC verifies that the MS allows a call related MT-LR in its privacy exception list, that it currently has an originated call established and that the LA number supplied by the GMLC matches the number dialed by the MS. The VMSC then interacts with an SMLC and possibly one or more LMUs in the VPLMN to perform positioning of the MS.
8. The resulting location estimate is returned by the VMSC to the GMLC. The VMSC uses the E.164 address or SS7 signaling point code of the GMLC, provided in step 4, to correctly route the location estimate to the GMLC in the GPLMN.
9. The GMLC returns the location estimate to the requesting LA.
Annex B (Informative):
Description of TOA
The uplink TOA positioning method is based on measuring the Timeof rival (TOA) of a known signal sent from the mobile and received at three or more measurement units. The known signal is the access bursts generated by having the mobile perform an asynchronous handover. This method will work with existing mobiles; i.e. there is no modification to the handset required.
The Serving Mobile Location Center (SMLC) calculates TimeDifferenceofrrival (TDOA) values by pair-wise subtracting the TOA values. The mobile position is then calculated via hyperbolic trilateration assuming that:
a) the geographical coordinates of the measurement units are known, and
b) the timing offset between the measurement units involved in the measurement are known, e.g. by the use of absolute GPS time at the measurement units, or by using reference measurement units (sometimes referred to as "reference mobiles") situated at known locations to determine RTD (Real Time Difference) values.
Access bursts are used for detecting the TOA at the listening measurement units. At a positioning request, the units which should measure the TOA of the MS signal are selected and configured to listen at the correct frequency. The MS is then forced to perform an asynchronous handover.. Under such circumstances, the MS is transmitting up to 70 access bursts (320 ms) with specified power on a traffic channel (which may be frequency hopping).
The TOA measurements are performed at each measurement unit by integrating the received bursts to enhance the sensitivity, and therefore increasing the detection probability and measurement accuracy, and by applying a multipath rejection technique to accurately measure the arrival time of the Line of Sight component of the signal. The presence of diversity, e.g. antenna diversity and frequency hopping will improve the multipath rejection capability and therefore the measurement accuracy.
When an application requires the position of a mobile, it has to send a request to SMLC the identifacation of the mobile and the accuracy level parameter. Depending on this accuracy level, SMLC decides how many measurement units to be included in the positioning request. The measured TOA values together with the accuracy parameter of the TOA value are collected and transmitted to the SMLC. The SMLC utilizes the TOA measurements in combination with information about the coordinates of the measurement units and the RTD values (a and b above) to produce a position estimate. The SMLC delivers the position estimate together with an uncertainty estimate to the application.
The uplink TOA method requires additional hardware (LMUs) to accurately measure the arrival time of the bursts. Different implementation options exist for this positioning method. For instance, it is possible to either integrate the measuring units in the BTSs or implement them as stand-alone units. In case the measurement units are implemented as stand alone units, the communication between the measurement units and the network is preferably carried out over the air interface. The stand-alone units may have separate antennas or share antennas with an existing BTS.
Annex C (informative):
Description of E-OTD