8 Cell ID and Enhanced Cell ID based positioning methods
25.3053GPPRelease 17Stage 2 functional specification of User Equipment (UE) positioning in UTRANTS
The Cell ID and Enhanced Cell ID based positioning method can work in either the RNC centric mode or in an SAS centric mode where the SAS has control over the positioning procedures.
This subclause outlines the procedures for Cell ID and Enhanced Cell ID positioning methods. Subclause 8.1 provides procedures for the determination of the cell ID depending on the operational status of the target UE. Subclause 8.2 provides a procedure for the mapping of the cell ID to a corresponding SAI to be returned to the LCS application in the CN. Subclause 8.3 provides additional information on other Enhanced Cell ID methods.
In the RNC centric Cell ID based method, the SRNC determines the identification of the cell providing coverage for the target UE and may request additional UE and/or UTRAN measurements for Enhanced Cell ID methods. The general flow to determine the cell ID and UE and/or UTRAN measurements for Enhanced Cell ID methods is shown in figure 8.1. For Enhanced Cell ID methods, the SRNC may request measurements either from the Node B(s) (e.g., RTT, Rx Timing Deviation, Angle of Arrival) or from the target UE using RRC measurement procedure (e.g., pathloss, CPICH RSCP, CPICH Ec/No). The available measurements for Enhanced Cell ID are described in Section 8.3. If the location request from the CN is a request for periodic reporting, the general procedure may be repeated and UTRAN sends an LCS response one reporting interval after the previous LCS response message until the desired amount of reports is attained, or until the procedure is cancelled by UTRAN or CN.
Figure 8.1: RNC Centric, Cell ID and Enhanced Cell ID Based Methods
In the SAS centric mode the request for a Cell ID or Enhanced Cell ID based position may be originated by an associated SAS via a PCAP Postion Activation Request message on the Iupc interface as discussed in subclause 6.6.5.1.3. In this case the Cell ID and other measurements for Enhanced Cell ID would be provided to the SAS in a PCAP Position Activation Response message as shown in figure 8.2.
For periodic reporting, the SAS may continue to invoke Cell ID or Enhanced Cell ID based position via PCAP Position Activation Request messages and the SAS sends the periodic location reports to the SRNC in PCAP Position Periodic Result messages until the desired amount of reports is attained (or until the procedure is cancelled by UTRAN or CN) as shown in Figure 8.2. The final location estimate may be provided to the SRNC either in a PCAP Position Periodic Result or in a PCAP Position Initiation Response message, as described in subclause 6.6.5.1.3.
Figure 8.2: SAS Centric, Cell ID and Enhanced Cell ID Based Methods
8.1 Cell ID determination
In order for the SRNC to determine the cell ID when an UE Positioning request is received, additional operations may be needed depending on the operational status of the UE.
When the LCS request is received from the CN the SRNC checks the state of the target UE. If the UE is in a state where the cell ID is available, the target cell ID is chosen as the basis for the UE Positioning. In states where the cell ID is not available, the UE is paged, so that SRNC can establish the cell with which the target UE is associated. The SRNC may also map the cell ID to a corresponding SAI to match the service coverage information available in the CN.
The cell ID based method shall determine the position of the UE regardless of the UE RRC mode (i.e. connected or idle).
8.1.1 UE Cell ID is not known
For UE for which the cell ID is not known at the time the UE Positioning request is received at the SRNC, the UE may be paged to locate its current cell ID. If the UE is in an idle mode and there is a need for it to be paged, then the paging shall be initiated by the CN. If the UE is in URA_PCH state the paging may be initiated by the SRNC in UTRAN. For example, the UE can be forced to perform a transition to a Cell_FACH state to define the cell ID of its current cell.
If the UE is in an idle mode, or in a RRC connected state when there is a need to page for the UE to obtain the cell ID, the CN may initiate paging, authentication and ciphering, as specified in [13].
Alternatively, the cell ID may be determined as the one that was used during the last active connection to the UE. In the case the UE is not reachable, the last known position should include the age of location field.
8.1.2 UE Cell ID is known
8.1.2.1 UE not in Soft handover
The cell ID may be determined as the cell that is providing an active connection for the UE at the time of receiving the UE Positioning request at the SRNC.
8.1.2.2 UE in Soft handover
In order for the SRNC to provide the geographical co-ordinates of a target UE in soft handover, the SRNC combines the information about all cells associated with the UE.
In soft handover, the UE may have several signal branches connected to different cells, reporting different cell IDs. A reference cell ID may be determined by the SRNC based on the coverage area of each cell. The reference cell ID may be selected based on one or more of the following principles:
– the cell ID may be selected based on the parameters defining the quality of the received signal branches. That is, the cell ID with the best quality signal branch is selected as the reference cell ID;
– the cell ID may be selected that was used during connection set-up between the UE and the serving Node B;
– the cell ID of the cell most recently associated with the UE may be selected;
– the cell ID of the latest "new" cell that the UE has started to receive, but has not yet been handed over to may be selected;
– the cell ID may be selected as the cell to which UE has the shortest distance (to the Node B site);
– the cell ID may be selected as the cell that provides an active connection for the UE at the time of receiving the UE Positioning request at the SRNC.
The selection may also be based on RTT measurements, power levels and received signal strengths in UE and related Node B or LMU.
Other relevant mechanisms such as IPDL or SSDT power control should also be taken into account when applying the cell ID selection procedure for UE in a soft handover mode.
8.2 Mapping the Cell ID to Geographic Co-ordinates or a Service Area
A UTRAN cell ID should be mapped to geographical coordinates or a SAI before being sent from UTRAN to the CN. The Service Area Identifier may include one or several cells. The mapping may be accomplished either in the SRNC, in a Network Management System, including Network Management Unit or by co-operation of various access network elements.
The CN may request the geographical co-ordinates or the SAI, or both for the target UE. The SAI may be used for routing of corresponding Emergency calls, or for CAMEL services to correspond to the usage of Cell ID in the core network of GSM. However, the MSC shall not send the Service Area Identity to GMLC.
Although the mapping of the cell(s) associated with the target UE into geographical co-ordinates by the SRNC is not standardised, the response to the CN Location request with geographical co-ordinates shall be as defined in [11].
In order to determine a cell coverage estimate and to map it to the geographical coordinates or Service Area parameter Identity, the SRNC may use parameters such as the best reference signal, RTT in FDD [15] or Rx Timing Deviation [16] and knowledge of the UE timing advance in TDD, as well as antenna beam direction parameters.
Alternatively, the service area coverage of a cell may be determined by using a reference signal power budget. Based on the reference signal power budget it is possible to obtain, for example, the Node B transmitted power, isotropic path loss, coverage threshold at coverage area border for a given location probability, and a cell radius for an indoor and outdoor coverage.
The SRNC may use a reference signal link budget based cell radius estimate, in conjunction with the cell identifier, to make a coverage estimation for the cell(s) related to the target UE.
Additionally, the SRNC may compare the received power levels with the power budget, whereby more accurate information of the position of the UE may be provided.
Also, the interaction between neighbouring cell coverage areas may be used to determine a more exact UE Positioning.
8.3 Enhanced Cell ID Methods
Techniques which use additional UE and/or UTRAN radio resource related measurements are defined under Enhanced Cell ID. Measurements for these ECID methods may include:
UE measurements:
– UE Rx-Tx Time Difference (FDD);
– pathloss;
– CPICH RSCP (FDD);
– CPICH Ec/No (FDD);
– Timing Advance (TDD).
UTRAN measurements:
– RTT (FDD);
– Rx Timing Deviation (TDD);
– Angle of Arrival (1.28 Mcps TDD).
Various techniques exist to use these measurements to estimate the location of the target UE. The specific techniques are beyond the scope of this specification. Examples of other Enhanced Cell ID method include the following:
8.3.1 Cell-ID with RTT
Cell-ID with Round Trip Time measurements:
This variant uses in addition NodeB measurements of the signal Round-Trip-Time (RTT). These measurements can be made for all NodeBs in the active set. If RTT measurements to several geographically dispersed NodeBs are available, the UE location may be found via trilateration. The RTT measurements may be complemented by the UE measurement of the Rx-Tx Time Difference Type 1 and/or Type 2. In case of TDD mode, the distance measurement may be based on RX Timing Deviation and/or Timing Advance.
8.3.2 Cell-ID with Pathloss and Related Measurements
Cell-ID and Pathloss related measurements:
This variant uses in addition several UE measurements for the serving and neighboring cells related to pathloss or SNR. UE measurements which could be used include:
– Pathloss (as defined in [18] subclause 10.3.7.3);
– CPICH RSCP (FDD);
– CPICH Ec/N0 (FDD).
These measurements may be used to obtain distance related information, or the measurements may be used for statistical modeling approaches, etc.
8.3.2.1 RF Pattern Matching (RFPM)
The RF Pattern Matching positioning method is based on radio link measurements collected from the network and/or the UE. The method relies on predictions or models of the radio environment against which it performs an algorithmic comparison of the measurements to determine a best match estimation of the UE’s location. RFPM may utilize measurements other than the pathloss measurements noted above.
8.3.3 Cell-ID with Angle of Arrival
Cell-ID with Angle-Of-Arrival (AOA) measurements (1.28 Mcps TDD only):
This variant uses NodeB angle-of-arrival measurements, usually together with distance related measurements obtained via timing advance to estimate the UE location.