5 GERAN LCS Architecture

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

Figure 1 shows the general arrangement of the Location Service feature. This illustrates, generally, the relation of LCS Clients and servers in the core network with the GERAN. The definition and operation of LCS entities operating in the core network is outside the scope of the present document. The LCS entities within the GERAN communicate with the Core Network (CN) across the A, Gb and Iu interfaces.

Communication among the GERAN LCS entities makes use of the messaging and signalling capabilities of the GERAN.

As part of their service or operation, the LCS Clients may request the location information of Mobile Station. There may be more than one LCS client. These may be associated with the core network, associated with the GERAN, operated as part of a MS application or accessed by the MS through its access to an application (e.g. through the Internet).

Within the GERAN, the BSC receives authenticated requests for LCS information from the core network across the A, Gb or Iu interface and passes these to the SMLC. The SMLC may be a standalone network element or functionality that is integrated to the BSC. LCS entities then manage the GERAN resources, including the base station, the LMU, the MS and calculation functions, to estimate the location of the MS and return the result to the Core Network.

Figure 1: Functional LCS Architecture in GERAN

5.1 LCS Operations

The schematic functional description of LCS operations is defined in figure 2.

Upon request from the LCS entities or for internal operations, the GERAN LCS functional entities will:

– request measurements, typically from the MS and/or one or more BTS radio apparatus;

– send the measurement results to the appropriate calculating function within GERAN;

– receive the result from the calculating function within GERAN;

– send the results to the LCS entities in the core network or to application entities within GERAN.

In the event that the client is internal to GERAN the request may be made directly to the GERAN LCS entities as the internal clients are considered to be "pre-authorised".

As part of its operation, the GERAN LCS calculating function may require additional information. This may be obtained by the function directly by communication with a database, or it may be through a request to GERAN LCS entities that will mediate the request and return of information from the appropriate database (or databases if more than one is needed to fulfil the requests).

There may possibly also be available independent information that is able to supply the location information directly, or may be able to supply auxiliary information to the calculation function. The GERAN LCS co-ordination function, as part of its activity to supervise the location process, may query the MS or other elements of the GERAN to determine their capabilities and use this information to select the mode of operation.

This general operation is outlined in the following (generic) sequence diagram Figure 2. This figure is not intended to show the complete LCS operation for GERAN, but to simply to outline the basis for operation. Location measurements may continually be taken in the background.

Figure 2: General sequence for LCS operation

5.2 High-Level Functions

Several functional groupings may be defined to describe the LCS. These groupings occur in both the Core Network and the GERAN. The overall LCS functional grouping is described in the system stage 2, 3GPP TS 23.271. Each grouping encompasses a number of functional components and functions.

The functions within the GERAN are described in more detail in the following clauses of the present document.

Within GERAN the functional entities may be grouped as follows:

– the Internal Client group;

– the GERAN System Handling group;

– the Positioning group.

The LCS functional diagram shown in figure 3 depicts the interaction of the LCS functional entities within the GERAN. The GERAN uses the various LCS components to provide the target MS Location Information to the internal LCS client.

Figure 3: GERAN LCS Capability Functional Diagram

5.2.1 Co-ordination, Measurement and Calculation Functions

These GERAN functions (including functions in the System handling and Positioning groups) provide the co‑ordination, measurement and calculation functions needed to provide a location estimate. The functions interface with the requesting application and select the appropriate location method and speed of response. The functions co‑ordinate the operations of the radio and measurement equipment to transmit the needed signals and to make the needed measurements. The functions may also access databases or other sources of information appropriate for the location method. The functions also provide the calculation functions appropriate for the location method to estimate the MS location and the accuracy of the report. The functions also may record information on the usage of the LCS that may be used for administrative purposes (e.g. forwarded to a billing function in the Core Network). If needed by the location method, the functions will ensure the broadcast of information and gather and update information concerning GERAN operating parameters (e.g. timing of BTS transmissions) needed for LCS operations.

These entities are mainly concerned with the location method, controlling the radio equipment and performing the calculations to determine the location and thus may be associated with the SMLC in the GERAN. These functions may receive location requests from either the core network or from applications internal to the GERAN.

These functions communicate with the core network across the A, Gb and Iu interfaces, and with the BTS and LMU and with the MS across the Um interface.

5.3 GERAN LCS Functional Entities

5.3.1 Internal Client Group

5.3.1.1 Internal GERAN Location Client Function (LCF)

The Location Client Function (LCF) represents a logical interface between the internal GERAN LCS applications and the LCS BSC Handling entities (e.g. the Location System Control Function (LSCF) in the BSC).

NOTE: There is not necessarily a requirement for a LCCF (Location Client Control Function) for the GERAN Internal Client as is described for external clients in the system stage 2 specification, 3GPP TS 23.271.

The GERAN may make use of location information for internal operations such as location assisted handover. In such a case, a LCF representing the internal GERAN LCS application may communicate with the LSCF to request and receive the location information.

5.3.2 GERAN System Handling group

5.3.2.1 GERAN Location System Control Function (LSCF)

The GERAN Location System Control Function is responsible for co-ordinating location requests. This function manages call-related and non-call-related location requests and allocates network resources for handling them. This function "insulates" the Location clients in the Core Network from the detailed operation of the location method in order that the GERAN may be used by several types of core network and with several location methods.

The LSCF provides flow control between simultaneous location requests. Simultaneous location requests must be queued in a controlled manner to account for priority requests (e.g. for Emergency Clients). The details of the flow control, priority selection and queuing are beyond the scope of the present document.

The LSCF will select the appropriate location method based on the availability of resources and parameters of the location request. The LSCF coordinates resources and activities needed to obtain data (e.g. base station geographic coordinates) needed for the location method. It also records LCS usage data for the location service request that may be passed to a Location System Recording Function (LSRF) or O&M function in the Core Network.

5.3.2.2 GERAN Location System Operations Function (LSOF)

The Location System Operations Function (LSOF) is responsible for provisioning of data, location capabilities, data related to clients and subscription (LCS client data and MS data), fault management and performance management of LCS within the BSC.

An LSOF may be associated with each entity. The LSOF interacts with Internal (O&M) Clients for administration and maintenance of the data.

5.3.2.3 Location System Broadcast Function (LSBcF)

The Location System Broadcast Function (LSBcF) provides broadcast capability. The LSBcF capability is only used when broadcast data is required for E-OTD or A-GNSS positioning methods. Broadcast information (such as the geographic coordinates of the base stations) may be required, for example, to support a Position Calculation Function (PCF) located in the mobile station. These broadcasts may also include other information (such as currently observable satellites) that may assist a MS in the use of external location services.

The information to be broadcast is selected based on the location techniques offered for use by the LCS and the needs of the MS. This broadcast information may be specially coded (i.e. encrypted) to ensure its availability only to subscribers of the service. The use of broadcasts or other methods for signalling to the MS may be selected based on the chosen location method.

The information to be broadcast includes for example:

1. E-OTD Assistance:

– Reference Time;

– Neighbour Channel Time Slot Scheme;

– Information about sectored neighbour channels;

– Neighbour channel 51 Multiframe Offset values;

– Neighbour channel BCC values;

– RTD Drift Factor values (ciphered if active);

– Neighbour channel RTD values (ciphered if active);

– Serving cell and neighbour cell location information (ciphered if active).

2. A-GNSS Assistance Data:

– Broadcasted assistance data may be the same as in A-GNSS point-to-point signalling.

5.3.3 Positioning group

5.3.3.1 GERAN Position Radio Co-ordination Function (PRCF)

The GERAN Position Radio Control Function manages a location request for a MS through overall co-ordination and scheduling of resources to perform location measurements. This function interfaces with the PSMF, the PRRM and the PCF. The PRCF determines the location method to be used based on the location request, the QoS, the capabilities of the GERAN, and the MS’s capabilities. The PRCF also manages the needed radio resources through the PRRM. It determines which PSMFs are to be involved, what to measure, and obtains processed signal measurements from the PSMF.

Some location methods may involve measurements made at the MS. In this case the PRCF interfaces with the MS to obtain the measurements (or the location results if they have been determined by the MS). Some location methods may involve measurements or information from several sources, including radio units at several BTSs and involve a series of transmissions and receptions. The PRCF entity also provides ancillary measurements in case of network-assisted location method. Ancillary information may be extracted from navigating systems like GNSS.

The PRCF forwards the signal measurement data to the PCF.

It is the function of the PRCF to co-ordinate the sequence of activities and compensate for failures (if they occur) to provide the location estimate.

5.3.3.2 GERAN Position Calculation Function (PCF)

The GERAN Position Calculation Function is responsible for calculating the location of the MS. This function applies an algorithmic computation on the collected signal measurements to compute the final location estimate and accuracy.

The PCF would also be responsible for calculating the speed and bearing of the MS when reported. This function applies an algorithmic computation on the collected signal measurements to compute the final velocity estimate and uncertainty.

It may obtain related data (e.g.: base station geographic coordinates) needed for the calculation. There may be more than one calculating function available within, or associated with, the calculation function of the GERAN.

The Position Calculation Function is also responsible for estimating the accuracy of the location estimate.

5.3.3.3 GERAN Position Signal Measurement Function (PSMF)

The GERAN Position Signal Measurement Function (PSMF) is responsible for performing and gathering uplink or downlink radio signal measurements for use in the calculation of a MS’s location and velocity. These measurements can be location related or ancillary.

There may be one or more PSMF within a GERAN and they may be located at the MS or a Location Measurement Unit (LMU). The PSMF, generally, may provide measurement of signals (i.e. satellite signals) in addition to measurements of the GERAN radio transmissions. The measurements to be made will depend on the selected location method.

5.3.3.4 GERAN Position Radio Resource Management (PRRM)

The GERAN Position Radio Resource Management entity is responsible for managing the effect of LCS operations on the overall performance of the radio network. This may ensure, for example, that the operation of the PSMF does not degrade the QoS of other calls.

5.4 Assignment of LCS Functional Entities to GERAN Elements

The following Table 1 shows the generic configuration for different location methods, including network-based, mobile-based, mobile-assisted and network-assisted methods. With this approach both the network and the mobiles are able to measure the timing of signals and compute the mobile’s location and velocity estimate. Depending on the applied location method it is possible to utilise the corresponding configuration containing all needed entities. For instance, if a network-based location method is applied, the entities that are involved in measuring the mobile’s signal and calculating its location estimate are allocated to the network elements of the access network. On the other hand, in case mobile-based or network-assisted methods are used these entities should be allocated to the mobile station.

Table 1: Example Allocation of LCS Functional Entities to Network Elements

	MS	LMU	SMLC	BSC
LCF	X			X
LSBcF			X
LSCF				X
PRCF			X
PCF	X		X
PRRM				X
PSMF	X	X
LSOF		X	X	X

5.5 Functional Description of GERAN LCS Network Elements

5.5.1 BSC

The BSC receives authenticated requests for LCS information from the CN across A, Gb and Iu interfaces and passes these to the SMLC.

5.5.2 SMLC

The SMLC is either a separate network element of GERAN or integrated functionality in BSC that contains functionality required to support LCS. The SMLC manages the overall co-ordination and scheduling of resources required for the location of a mobile. It also calculates the final location and velocity estimate and estimates the achieved accuracy. The SMLC may control a number of LMUs for the purpose of obtaining radio interface measurements to locate or help locate MS subscribers in the area that it serves. The SMLC is administered with the capabilities and types of measurement produced by each of its LMUs (only applicable for positioning methods that make use of LMUs).

5.5.3 CBC

For Location Services, when a Cell Broadcast Center (CBC) is associated with a BSC, the SMLC may interface to a CBC in order to broadcast assistance data using existing cell broadcast capabilities. The SMLC shall behave as a user, Cell Broadcast Entity, to the CBC (refer to 3GPP TS 23.041).

5.5.4 LMU

The LCS Measurement Unit (LMU) entity makes measurements (e.g. of radio signals) and communicates these measurements to the SMLC (e.g. the PRCF). The LMU contains a PSMF and may also perform calculations associated with the measurements. The LMU may make its measurements in response to requests (e.g. from the SMLC), or it may autonomously measure and report regularly (e.g. timing of BTS transmissions) or when there are significant changes in radio conditions (e.g. changes in the RTD). There may be one or more LMUs associated with the SMLC and an LCS request may involve measurements by one or more LMUs. LMU functionality may be integrated in a BTS. The Multilateration based positioning methods does not make use of LMUs.

An LMU makes radio measurements to support one or more positioning methods; these assistance measurements are specific to all MSs in a certain geographic area. All location and assistance measurements obtained by an LMU are supplied to the SMLC associated with the LMU. Instructions concerning the timing, the nature, and any periodicity of these measurements are either provided by the SMLC or are pre-administered in the LMU.

The following assistance measurement obtained by an LMU has a generic status usable by more than one position method:

– Radio Interface Timing measurements – comprise Absolute Time Differences (ATDs) or Real Time Differences (RTDs) of the signals transmitted by Base Stations, where timing differences are measured relative to either some absolute time difference (ATD) or the signals of another Base Station (RTD).

5.5.5 MS

The MS interacts with the measurement co-ordination functions to make measurements of downlink signals. The measurements to be made will be determined by the chosen location method.

The MS may also contain LCS applications, or access an LCS application through communication with a network accessed by the MS or an application residing in the MS and/or satellite signals. The MS may include the needed measurement and calculation functions to determine the MS’s location with or without assistance of the GERAN LCS entities.