9 OTDOA positioning method

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The primary standard OTDOA measurement is the "SFN-SFN observed time difference" observed at the UE (see [15] and [16]). These measurements, together with other information concerning the surveyed geographic position of the transmitters and the RTD of the actual transmissions of the downlink signals may be used to calculate an estimate of the position and optionally, the velocity of the UE. Each OTDOA measurement for a pair of downlink transmissions describes a line of constant difference (a hyperbola (see note 1)) along which the UE may be located. The UE’s position is determined by the intersection of these lines for at least two pairs of Node Bs. The accuracy of the position estimates made with this technique depends on the precision of the timing measurements, the relative position of the Node Bs involved (see note 2), and is also subject to the effects of multipath radio propagation. This is illustrated in the figure 9.1.

NOTE 1: This is really a figure in three dimensions, a hyperboloid. For convenience here, this will be simplified to the hyperbola representing the intersection of this surface with the surface of the earth. For location service in three dimensions the hyperboloid must be considered.

NOTE 2: The geometry of the Node B positions may affect the accuracy of the position and optional velocity estimates. The best results are when the Node Bs equally surround the UE. If they do not, there is a reduction in accuracy, which is sometimes termed the Geometric Dilution of Position (GDP).

The primary OTDOA measurements (made by the UE) are sent to the SRNC. These measures are sent via signaling over the Uu, Iub (and Iur) interfaces between the UE and the SRNC. In networks with a SAS, the SRNC may forward the measurement results over the Iupc interface to the SAS. The calculation function makes use of the measurements, the known positions of the transmitter sites and the RTD of the transmissions to estimate the UE’s position.

Figure 9.1: OTDOA Positioning Method

The OTDOA method may be operated in two modes: UE-assisted OTDOA and UE-based OTDOA. The two modes differ in where the actual position calculation is carried out.

In the UE-assisted mode, the UE measures the difference in time of arrival of several cells and signals the measurement results to the network, where the SRNC or the SAS carries out the position calculation.

In the UE-based mode, the UE makes the measurements and also carries out the position calculation, and thus requires additional information (such as the position of the measured Node Bs) that is required for the position calculation.

The signalling requirements for the two OTDOA modes are described in subclause 9.6. As the UE Positioning involves measurements, there is always uncertainty in the results. Physical conditions, errors and resolution limits in the apparatus all contribute to uncertainty. To minimise the uncertainty in the UE Positioning result, it is important that as many measurements of OTDOA (and assistance data as RTT in FDD and Rx Timing Deviation in TDD) as are possible for a UE are provided to the UE. Thus it is important that the standard UE Positioning method not be restricted to rely on a single measure. The UE thus provides SFN-SFN observed time difference measurements for as many cells as it can receive. The cells to be measured shall include those in the active set and the monitored set.

In order to support the OTDOA method, the positions of the UTRAN transmitters needs to be accurately known by the calculation function in SRNC or SAS (for UE-assisted method) or UE (for UE-based method). This information may be measured by appropriate conventional surveying techniques (see note 3). The surveyed position should be the electrical centre of the transmitting antenna (and not the position of the radio equipment building). The use of antenna diversity, beamforming or beam steering techniques may cause the effective antenna position to change with time and this information is also needed to perform calculations. The methods of measuring the position of the UTRAN transmitters are outside the scope of the present document.

NOTE 3: These surveying methods may, for example, make use of a GPS receiver.

In order to support the OTDOA method, the RTD of the DL transmissions must also be known to perform the calculation. If the UTRAN transmitters are unsynchronised, the RTD will change over time as the individual clocks drift. Thus, RTD estimations may need to be made regularly and the calculation function updated appropriately.

One convenient method is to make use of an LMU at a fixed position. This unit performs timing measurements (e.g. UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference) of all the local transmitters. The CRNC then may convert these measures into the actual (absolute) relative time difference for each of the transmitters by making use of the known position of the LMU and the transmitters.

In some conditions a sufficient number of cells may not be available for measure at the UE. This may occur, for example, if the UE is located quite close to the UTRAN transmitter and its receiver is blocked by the strong local transmissions. This is referred to as the "hearability" problem.

9.1 Use of Idle Periods

Location based services needs the support of physical layer as a prerequisite, so that the measurements required for the UE position, and optional velocity calculation can be carried out. In UTRAN there are several factors that must be taken into account while considering the physical layer procedures related to location services:

– hearability: a basic consequence of a CDMA radio system is that a terminal near its serving Node B cannot hear other Node Bs on the same frequency. In order to calculate its position the UE should be able to receive at least three Node Bs. To facilitate this some special means are required;

– asynchronous network causes significant uncertainty to the time-difference-of-arrival (TDOA) measurements. To compensate for the effects of this, the relative time difference (the synchronicity) between Node B transmissions must be measured, and used for correcting OTDOA measurement;

– capacity loss: signalling related to position calculation may take capacity from other services. This capacity loss should be minimised.

Based on the results in [29] a solution for the above mentioned hearability problem is the IPDL method. In this method each Node B ceases its transmission for short periods of time (idle periods). Therefore the Node B is configured appropriately by the CRNC using the NBAP protocol [22]. During an idle period of a Node B, terminals within the cell can measure other Node Bs and the hearability problem is reduced. The UEs are made aware of the occurrences of IPDLs by using RRC signalling over the Uu interface [18]. Also, during idle periods the real time difference measurements can be carried out. Because the IPDL method is based on downlink the location service can be provided efficiently to a large number of terminals simultaneously.

The specification and operation of the IPDL technique are provided in [14] and [26].

9.2 Relative Time Difference (RTD)

In order to calculate the estimate of the position of the UE, the calculation function needs to know:

– the OTDOA measurements;

– the surveyed geographic positions of the Node Bs that have had their signals measured; and

– the actual relative time difference between the transmissions of the Node Bs at the time the OTDOA measurements were made.

The accuracy of each of these measurements contributes to the overall accuracy of the position estimate. The measurement of the RTD is described in the following.

There are several approaches to determining the RTD. One is to synchronise the transmissions of Node B. In this technique the RTD are known constant values (see NOTE) that may be entered in the database and used by the calculation function when making a position estimate. The synchronisation must be done to a level of accuracy of the order of tens of nanoseconds (as 10 nanoseconds uncertainty contributes 3 metres error in the position estimate). Drift and jitter in the synchronisation timing must also be well controlled as these also contribute uncertainty in the position estimate. Synchronisation to this level of accuracy is currently only readily available through satellite based time-transfer techniques. Generally in the TDD operating mode, the Node Bs are synchronised.

NOTE: The transmission times may all be aligned to a common reference (such as UTC) in which case all RTD have a common value. However, in a more general case the transmissions may have a fixed offset with reference to UTC, and thus the RTD values are non-zero and may be stored in the database for use by the calculation function.

Alternatively (typically in FDD mode), Node Bs may be left to free run within some constraint of maximum frequency error. In this scenario, the RTD will change (slowly) with time. The rate of change will depend on the frequency difference and jitter between Node Bs. If, for example, the maximum frequency difference between two Node Bs is ±10^-9, then the start of transmission of a 10 millisecond code sequence will drift through a cycle in about 1 390 hours (or 57 days). With this relatively slow rate of drift the measurements needed to obtain the RTD can be performed by fixed LMUs at known positions and stored in the database for use by the calculation function. The jitter and drift of the individual oscillators in each Node B may cause the change of timing to slow, remain constant or reverse direction over time. Ongoing measurements may be made to assure the most current values are available for the calculation function. The measurement units needed to support RTD estimation may be co-located with the Node Bs or installed at other convenient positions in the UTRAN coverage area, and report their results through the UTRAN signalling.

The LMUs may directly measure the SFN-SFN Observed Time Difference between neighbouring and reference cells and return the measurements to the CRNC. Alternatively the LMUs may measure the UTRAN GPS timing of cell frames of the neighbouring and reference cells and return the measurements to the CRNC. If the CRNC is not the SRNC the information is also forwarded from CRNC to SRNC. In networks with a SAS the information may also be forwarded to the SAS.

The SRNC or SAS then uses the UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference measurements to calculate the RTD values. The information to be transferred in each case is listed in 7.4.3.

9.3 Time of Day (ToD)

If there are frequency drifts between the (unsynchronised) Node Bs, as noted in subclause 9.2, the OTDOA measurements must be reported together with the time-of-day they were made (timestamp). This is necessary so that the appropriate value of the RTD may be used by the calculation function.

In order to assure less than a 20 nanosecond uncertainty in the RTD value, the time of day must be known to better than 10 seconds (if the maximum frequency difference between the Node Bs is ±10^-9). The method by which the ToD is measured is the system the frame number, which provides a 10 millisecond resolution and can be unambiguous up to 40.95 seconds.

9.4 Node B Synchronisation

It is preferable that the positioning methods do not require the Node B to be synchronised. The needed level of synchronisation accuracy for UE Positioning is not by any means straightforward to achieve. The necessary information to obtain RTD between Node Bs can be measured by LMU (e.g. UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference) and distributed in the network (e.g. as broadcast information). Also, these measurements may benefit from the IPDL option.

In the TDD operating mode the Node Bs will typically be synchronised and this may be of assistance to the UE Positioning technique.

9.5 OTDOA-IPDL and OTDOA Modes

There are two modes of operation for the OTDOA-IPDL and OTDOA methods.

In the UE-assisted mode, the UE measures the difference in time of arrival of several cells and signals the measurement results to the network, where the SRNC or SAS carries out the position calculation.

9.5.1 Information to be transferred between UTRAN elements

Table 9.1 lists the required information for both UE-assisted and UE-based modes that may be sent from UTRAN to UE. The required information can be signalled to the UE either in a broadcast channel or partly also as dedicated signalling.

Table 9.1: Information to be transferred from UTRAN to UE (‘Yes’ = information required, ‘No’ = Information not required)

Information	UE- assisted	UE-based
Intra frequency Cell Info (neighbour list)	Yes	Yes
Ciphering information for UE Positioning (see note 1)	No	Yes
Measurement control information (idle period locations)	Yes	Yes
Sectorisation of the neighbouring cells	No	Yes
Measurements results needed for RTD values for Cells mentioned at Intra frequency Cell Info	No	Yes
RTD accuracy	No	Yes
Measured roundtrip delay for primary serving cell	No	Yes
Geographical position of the primary serving cell	No	Yes
Relative neighbour cell geographical position	No	Yes
Accuracy range of the geographic position values	No	Yes
IPDL parameters	Yes	Yes
IPDL-Alpha parameter for Open Loop Power Control when using IPDLs in TDD (see note 2)	Yes	Yes
Maximum Power increase the UE may use when using IPDLs in TDD (see note 2)	Yes	Yes
NOTE 1: The idea behind UE Positioning specific ciphering information is e.g. that the operator can sell information that the UE needs for calculating its position. For reference in the GSM world see [4]. NOTE 2: These parameters are not required for 1.28 Mcps TDD in this release.

When IPDLs for 3.84/7.68 Mcps TDD are applied and the IPDLs occur in the slot carrying the PCCPCH, a special alpha parameter needs to be signalled from SRNC to the UE in order to take the impact of the IPDLs on the Open Loop Power Control into account. Additionally the UE shall not increase the transmit power by a certain value between an IPDL slot and the next slot carrying the PCCPCH when IPDLs are applied within a cell. OTDOA for 1.28 Mcps TDD is based on DwPTS and so these requirements do not apply.

The information that may be signalled from UE to SRNC is listed in table 9.2.

Table 9.2: Information to be transferred from UE to SRNC

Information	UE- assisted	UE-based
OTDOA measurement results	Yes	No
OTDOA measurement accuracy	Yes	No
UE geographical position	No	Yes
Position accuracy indicator (based on the signalled and measurement accuracies)	No	Yes

Table 9.3 shows the information that may be transferred from Node B to its CRNC. If the CRNC is not the SRNC the information is also forwarded from CRNC to SRNC.

Table 9.3: Information to be transferred from Node B/LMU to CRNC and between RNCs

Information	UE assisted	UE based
Measured UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference values for Cells mentioned at Intra frequency Cell Info	Yes	Yes
UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference accuracy	Yes	Yes

Table 9.4 shows the information that may be transferred from CRNC to Node B. If the CRNC is not the SRNC the information may also be sent from CRNC to SRNC.

Table 9.4: Information to be transferred from CRNC to Node B/LMU and between RNCs

Information	UE assisted	UE based
IPDL parameters	Yes	Yes

Table 9.5 shows the information that may be transferred between RNCs.

Table 9.5: Information to be transferred between RNCs

Information	UE assisted	UE based
Geographical position of the primary serving cell	Yes	Yes
Relative neighbour cell geographical position	Yes	Yes
Accuracy range of the geographic position values	Yes	Yes
IPDL-Alpha parameter for Open Loop Power Control when using IPDLs in 3.84/7.68 Mcps TDD	Yes	Yes
Maximum Power the UE may use when using IPDLs in 3.84/7.68 Mcps TDD	Yes	Yes

9.6 OTDOA network positioning procedures

OTDOA may be selected by the SRNC as the method to be used on receipt of an LCS positioning request from the CN or, optionally, when the SRNC receives a PCAP Position Activation Request message indicating the OTDOA positioning method from a SAS that is configured to select the positioning method. In the SAS initiated case the OTDOA measurement information will be returned to the SAS in a PCAP Position Activation Response message, or – if periodic reporting was requested – in one or more PCAP Position Periodic Report messages as discussed below.

9.6.1 RNC based selection of OTDOA

The following diagram illustrates the operations for the OTDOA method for UE Positioning when the request for positioning information is initiated by an LCS application from the CN.

This illustration only includes the information flow related to UE Positioning operations and does not indicate other operations that may be required, for example, to establish a signalling connection between the UE and the SRNC. Also not illustrated is the signalling used to initiate the location service request from the CN or a UE-based application.

Figure 9.2: OTDOA Signalling Operations

1. The operation begins with an authenticated request for positioning information about a UE from an application in the CN being received at the SRNC. The request from the CN may be a request for on-demand or periodic reporting. The SRNC considers the request and the UTRAN and UE capabilities.

2. The SRNC requests from the UE the measurement of the OTDOA for the signals in the active and neighbourhood sets. These measurements are made while the UE is in connected mode CELL_DCH state. If the location request from the CN in step 1 contained periodic reporting information (reporting interval and amount of reports), the SRNC may at this step request periodic OTDOA measurement reporting from the UE according to subclause 6.6.4.1.

3. If it is considered advantageous to do so, the SRNC requests the UE Rx-Tx timing difference (FDD only) or UE timing advance, T_ADV, (1.28 Mcps TDD) information from the UE. If the location request from the CN in step 1 contained periodic reporting information (reporting interval and amount of reports), the SRNC may at this step request periodic measurement reporting from the UE according to subclause 6.6.4.1.

4. The UE returns the OTDOA measures to the SRNC. The SRNC receives the OTDOA information and co-ordinates obtaining other information to support the calculation request.

5. The UE returns the UE Rx-Tx timing difference (FDD only) or UE timing advance, T_ADV, (1.28 Mcps TDD) information to the SRNC, together with a time stamp of when the value was obtained.

6. If there are insufficient OTDOA measures, or it is otherwise considered advantageous to do so, the SRNC requests the RTT (in FDD) or Rx timing deviation (in TDD) and/or angle of arrival (in 1.28 Mcps TDD) measure for the UE from the serving Node B.

7. In FDD, the SRNC requests the RTD values for the associated transmitters from the associated database. These may be stored locally if they are constant over time, otherwise they must be updated to represent the RTD timing at the time-of-day the OTDOA measurements were made.

8. The Node B returns the RTT (in FDD) or Rx Timing Deviation (in TDD) and/or angle of arrival (in 1.28 Mcps TDD) measures to the SRNC if they were requested.

9. The SRNC performs a position, and optional velocity calculation using the OTDOA, RTD and, if necessary, RTT (in FDD) or Rx timing deviation and UE timing advance (in TDD) information and angle of arrival information (1.28 Mcps TDD). The calculation may include a co-ordinate transformation to the geographic system requested by the application. The position estimate includes the position and the estimated accuracy of the results. The optional velocity estimate may also include an estimated accuracy. In networks that include the SAS, the SAS may perform the position calculation and then pass the position estimate to the SRNC.

10. The SRNC passes the position estimate to the CN including, if available, the positioning method (or the list of the methods) used to obtain the position and optional velocity estimate. If the CN has requested an accuracy for the position estimate, the Location response shall include an indication whether the position estimate satisfies the requested accuracy or not.

11. If periodic reporting was requested form the CN in step 1, and steps 2 or 3 requested periodic UE reporting according to subclause 6.6.4.1, steps 4 to 10 are repeated. If periodic reporting was requested form the CN in step 1, but the SRNC does not request periodic UE reporting at steps 2 or 3, the SRNC repeats steps 2 to 10. The SRNC sends a location estimate to the CN one reporting interval after the previous location estimate until the desired amount of reports has been attained (or until the procedure is cancelled by UTRAN or CN).

9.6.2 SAS based selection of OTDOA

The following describes the signalling for the optional selection of the OTDOA positioning procedure by the SAS when the CN has requested UE positioning.

Figure 9.3: OTDOA Signalling Operations for SAS based positioning method selection

2. The SRNC forwards the information contained in the RANAP Location Reporting Control message, including any periodic reporting information, plus the Cell ID and UE capability information to the SAS in a PCAP Position Initiation Request message.

3. The SAS sends a PCAP Position Activation Request message to the SRNC that requests the OTDOA positioning method and may also request the UE Rx-Tx timing difference (FDD only) or UE timing advance, T_ADV, (1.28 Mcps TDD) information from the UE. The PCAP Position Activation Request message may include periodic reporting information (amount of reports and reporting interval).

4. The SRNC requests from the UE the measurement of the OTDOA for the signals in the active and neighbourhood sets. The OTDOA measurement request may include a request for periodic reporting as described in subclause 6.6.4.1.These measurements are made while the UE is in connected mode CELL_DCH state.

5. If the SAS has requested Rx-Tx timing information, the SRNC requests the UE Rx-Tx timing difference (FDD only) or UE timing advance, T_ADV, (1.28 Mcps TDD) information from the UE. The measurement request may include a request for periodic reporting as described in subclause 6.6.4.1.

6. The UE returns the OTDOA measures to the SRNC. The SRNC receives the OTDOA information and co-ordinates obtaining other information to support the calculation request.

7. The UE returns the UE Rx-Tx timing difference (FDD only) or UE timing advance, T_ADV, (1.28 Mcps TDD) information to the SRNC, together with a time stamp of when the value was obtained.

8. The SRNC forward the OTDOA measurement report information and, if available, the UE Rx-Tx timing measurement report information to the SAS in a PCAP Position Activation Response message.

9. If there are insufficient OTDOA measures, or it is otherwise considered advantageous to do so, the SAS requests the RTT (in FDD) or Rx timing deviation (in TDD) and/or angle of arrival (in 1.28 Mcps TDD) measure for the UE from the serving Node B by sending a PCAP Position Activation Request message that requires the SRNC to execute the Cell ID positioning method toward the UE being positioned.

10. In FDD, the RTD values for the associated transmitters are calculated and stored in the SAS based upon input from its associated LMUs.

11. The Node B returns the RTT (in FDD) or Rx Timing Deviation (in TDD) and/or angle of arrival (in 1.28 Mcps TDD) measures to the SRNC if they were requested.

12. The SRNC forwards Cell ID and RTT (in FDD) or Rx timing deviation and UE timing advance (in TDD) information and angle of arrival information (1.28 Mcps TDD) to the SAS in a PCAP: Position Activation Response message.

13. The SAS performs the OTDOA based or Cell ID based position calculation. If periodic reporting was not requested in step 1, the SAS forwards the position information to the SRNC in a PCAP: Position Initiation Response message. If periodic reporting was requested in step 1, the SAS forwards the position information to the SRNC in a PCAP Position Periodic Result message. The calculation may include a co-ordinate transformation to the geographic system requested by the application. The position estimate includes the position and the estimated accuracy of the results. The optional velocity estimate may also include an estimated accuracy.

14. The SRNC passes the position estimate to the CN including, if available, the positioning method (or the list of the methods) used to obtain the position and optional velocity estimate. If the CN has requested accuracy for the position estimate, the Location response shall include an indication whether the position estimate satisfies the requested accuracy or not.

15. If periodic UE reporting was requested in step 4 or 5, the UE continues to send OTDOA measurements or UE Rx-Tx timing difference (FDD only) or UE timing advance, T_ADV, (1.28 Mcps TDD) information to the SRNC, together with a time stamp of when the value was obtained, one reporting interval after the previous measurement report. The SRNC forwards the OTDOA measurement report information and, if available, the UE Rx-Tx timing measurement report information to the SAS in a PCAP Position Periodic Report message. The SRNC may aggregate the OTDOA measurement report information and the UE Rx-Tx timing measurement report information, if both are received from the UE in separate RRC messages, into the same PCAP Position Periodic Report message or may send this information in two separate PCAP Position Periodic Report messages. Steps 9 to 12 may be repeated for each new position estimate, and the SAS performs OTDOA based or Cell ID based position calculation and forwards each new position information to the SRNC in a PCAP Position Periodic Result message. The SRNC passes the new position estimate to the CN including, if available, the positioning method (or the list of the methods) used to obtain the position and optional velocity estimate. If the CN has requested accuracy for the position estimate, the Location response shall include an indication whether the position estimate satisfies the requested accuracy or not. This step 15 is repeated until the desired amount of reports has been attained or the procedure is cancelled by the CN or UTRAN. The SAS may send the final location estimate in a PCAP Position Initiation Response message to the SRNC, and the SRNC forwards the final location information to the CN.

16. If periodic UE reporting was not requested in step 4 or 5, but was requested in step 2, the SAS may repeat the steps 3 to 14 as for the first request until the desired amount of reports has been attained or the procedure is cancled by the CN or UTRAN. When repeating step 13 for the final request, the SAS returns the resulting final position estimate to the SRNC in a PCAP: Position Initiation Response message.