4 Main concepts

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

LCS utilizes one or more positioning mechanisms in order to determine the location of a Mobile Station. Positioning a target MS involves two main steps: signal measurements and location estimate computation based on the measured signals.

Three positioning mechanisms are proposed for LCS: Uplink Time of Arrival (TOA), Enhanced Observed Time Difference (E-OTD), and Global Positioning System (GPS) assisted.

4.1 Assumptions

– Support an SMLC that can be either BSS based or NSS based. While the SMLC is considered to be a separate logical entity, it may still be physically part of an MSC or BSC.

– Standardize a similar open interface to the SMLC whether it is NSS or BSS based. This simplifies migration from an NSS to a BSS based location architecture and avoids two different types of SMLC.

– Support "Type A" LMUs accessed over the GSM air interface using the same signaling protocols for both BSS and NSS based SMLC interaction. A type A LMU supports the RR and MM signaling procedures defined in GSM 04.08. A type A LMU may have a subscription profile in the HLR and may support certain CM services – e.g. outgoing data calls for SW download and SMS for SIM card download.

– Support "Type B" LMUs accessed over the Abis interface. The LMU may be either free standing (support Abis signaling) or associated with a BTS – either integrated or connected by proprietary means. If free standing, a type B LMU could be identified using a pseudo cell ID.

– Employ the same application protocol defined in GSM 04.71 for all types of LMU.

– Use MTP, SCCP, BSSAP as the basis for all LCS signaling between the SMLC, BSC, MSC and (for GPRS) SGSN, since these are the only protocols that are all supported in a BSC, MSC and SGSN. Substitution of TCP/IP or FR could be used in 3G. An important consequence of this change is that TCAP and MAP are no longer needed for signaling to an SMLC (since retention of TCAP and MAP would only be feasible for an NSS based SMLC, thereby producing two distinct types of SMLC).

– Provide enough flexibility to enable usage of transport protocols other than MTP/SCCP to support LCS for GPRS and 3G.

– Employ SCCP connection oriented signaling in the NSS and BSS to access a type A LMU or target MS to enable LCS messages to be easily relayed through an MSC and BSC.

– Add signaling between peer SMLCs to enable an SMLC to request or receive E-OTD, TOA or GPS positioning and assistance measurements obtained by an LMU belonging to another SMLC.

– Enable migration from an NSS based SMLC to BSS based SMLCs.

– Provide positioning procedures through the circuit-switched domain are also applicable to GPRS mobile stations which are GPRS and IMSI attached.

4.2 Timing Advance (TA)

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.

TA is used to assist all positioning mechanisms and as a fall-back procedure.

4.3 Time of Arrival (TOA) positioning mechanism

The uplink TOA positioning method is based on measuring the Time of Arrival (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. The method requires additional measurement unit (LMU) hardware in the network at the geographical vicinity of the mobile to be positioned to accurately measure the TOA of the bursts. Since the geographical coordinates of the measurement units are known, the mobile position can be calculated via hyperbolic triangulation. This method will work with existing mobiles without any modification.

4.4 Enhanced Observed Time Difference (E-OTD) positioning mechanism

The E-OTD method is based on measurements in the MS of the Enhanced Observed Time Difference of arrival of bursts of nearby pairs of BTSs. For E-OTD measurement synchronization, normal and dummy bursts are used. When the transmission frames of BTSs are not synchronized, the network needs to measure the Relative or Absolute Time Differences (RTDs or ATDs) between them. To obtain accurate triangulation, E-OTD measurements and, for non-synchronized BTSs, RTD or ATD measurements are needed for at least three distinct pairs of geographically dispersed BTSs. Based on the measured E-OTD values the location of MS can be calculated either in the network or in the MS itself, if all the needed information is available in MS. See Annex C for a detailed description of E-OTD.

4.5 Global Positioning System (GPS) positioning mechanism

The Global Positioning System (GPS) method refers to any of several variants that make use of GPS signals or additional signals derived from GPS signals in order to calculate MS position. These variants give rise to a range of optional information flows between the MS and the network. One dimension of variation is where position calculation is performed: a) MS-based PCF or b) network-based PCF. Another dimension is whether "assistance data" is required – irrespective of where position calculation is performed. Examples of assistance data include differential GPS data; lists of satellites in view based on approximate MS position, etc. A third dimension of variation is closely related to the preceding, namely, the origin and distribution of any assistance data. For example, even while assistance data may be required of a GPS method, it may be optional that the assistance data originates from and is distributed within and by the PLMN, VPLMN, etc.