10 Network-assisted GPS 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

When GPS is designed to inter-work with the UTRAN, the network assists the UE GPS receiver to improve the performance in several respects. These performance improvements will:

– reduce the UE GPS start-up and acquisition times; the search window can be limited and the measurements sped up significantly;

– increase the UE GPS sensitivity; positioning assistance messages are obtained via UTRAN so the UE GPS can operate also in low SNR situations when it is unable to demodulate UE GPS signals;

– allow the UE to consume less handset power than with stand-alone GPS; this is due to rapid start-up times as the GPS can be in idle mode when it is not needed.

The Network-assisted GPS methods rely on signalling between UE GPS receivers (possibly with reduced complexity) and a continuously operating GPS reference receiver network, which has clear sky visibility of the same GPS constellation as the assisted UEs. GPS reference receivers may be connected to the UTRAN to enable derivation of UE assistance signals.

Figures 10.1 and 10.2 provide an example of the end-to-end call flows for UE-assisted and UE-based positioning respectively. The SAS is an optional network element and the call segments do not apply in a network where the UE Positioning resides within the SRNC.

Figure 10.1: Network-assisted GPS methods – UE-Assisted

Figure 10.2: Network-assisted GPS methods – UE-Based

10.1 Timing calibration

Timing calibration is achieved by using UE or UTRAN GPS timing measurements as specified in [15].

10.2 Timing assistance

The UTRAN may derive the estimated UE position using UTRAN parameters (e.g. Cell-ID or IPDL) and may use this information, in conjunction with satellite specific ephemeris data received from the GPS reference receiver network, to derive the estimated times of arrival (code phases) for equivalent GPS satellite signals received by the UE-based GPS receiver functionality. For the UE-assisted method, the estimated code phase data may be conveyed, together with T_UTRAN-GPS (as specified in [15] and [16]), from the UTRAN to the UE using higher layer signalling. The estimated code phase data value is uncertain to a degree depending on the accuracy of the UTRAN timing calibration and initial position determination methods used.

Alternatively, for the UE-based method, the UE itself may derive its location aided by assistance messages. The ephemeris data are transmitted from UTRAN to the UE using higher layer signalling. The UE may use this information, in conjunction with the UE’s reference location, to derive the times of arrival (code phases) for GPS satellite signals received by the UE-based GPS receiver functionality.

10.3 GPS assistance data

The UE may receive GPS information through the UTRAN radio interface, using higher layer signalling. Once a UE Positioning measurement is setup by the SRNC the UE is responsible to maintain valid and up to date GPS assistance data in order to report the requested measurement results. In case that the UE has not sufficient assistance data or the data is out of date then the UE should indicate it to the SRNC and additionally request for assistance data.

When the UE is unable to detect a sufficient number of satellites, the assisted GPS method can be combined with other positioning methods. Altitude assistance can compensate for one satellite measurement.

The assistance data signalled to the UE may include all information listed below or a selected subset:

– data assisting the measurements; e.g. reference time, visible satellite list, satellite signal Doppler, code phase, Doppler and code phase search windows. This data can be valid for a few minutes (e.g., less than 5 minutes) or longer depending on the code phase and Doppler search window size that can be accommodated by the UE;

– data providing means for position calculation; e.g. reference time, reference position, satellite ephemeris, clock corrections. Satellite ephemeris and clock corrections data can be used for up to six hours.

NOTE: Certain types of GPS Assistance data may be derived, wholly or partially, from other types of GPS Assistance data.

If DGPS is utilised, then differential corrections may also be transmitted. If Selective Availability is turned off, these corrections can be valid for a few minutes or more. The DGPS data is valid for a large geographical area, so one centrally located reference receiver can be used to service this large region.

10.4 UE search

Provided that timing assistance, data assistance, and/or frequency reference is available in the UE, they should be applied in the GPS signal search procedure. The UE search procedure involves a three-dimensional search for a satellite pseudorandom code, time of arrival of a signal and the associated carrier Doppler.

"Modulation wipe-off" is defined here to mean a removal of the GPS navigation data bit modulation to GPS signals received at the UE, through the application of UTRAN timing and data assistance provided from the UTRAN to the UE. This process allows the UE to coherently integrate received GPS signals beyond 1 data bit period (i.e., 20 milliseconds).

10.5 Position determination

There are two types of network-assisted GPS methods, namely UE-based and UE-assisted, which differ according to where the actual position and optional velocity calculation is carried out.

Computation of the position and optional velocity fix can either be performed in UTRAN (i.e. SRNC or SAS) for UE-assisted or in the UE for UE‑based.

The UE-based method maintains a full GPS receiver functionality in the UE, and the position and optional velocity calculation is carried out by the UE, thus allowing stand-alone position and optionally, velocity fixes.

In the UE-assisted method, the UE employs a reduced complexity GPS receiver functionality. This carries out the pseudorange (code phase) measurements. These are signalled, using higher layer signalling, to the specific network element that estimates the position of the UE and carries out the remaining GPS operations. In this method, accurately timed code phase signalling (as specified in [15] and [16]) is required on the downlink. If DGPS is performed in the UE, then differential corrections must be signalled to it. On the other hand, DGPS corrections can be applied to the final result in the network to improve the position accuracy without extra signalling to the UE.

10.5.1 Information to be transferred between UTRAN elements

Table 10.1 lists information for both UE-assisted and UE-based modes that may be sent from the network to UE. This information can be signalled to the UE either in a broadcast channel or as dedicated signalling.

Table 10.1: Information that may be transferred from the network to UE(‘Yes’ = information applicable ‘No’ = information not applicable

Information	UE-assisted	UE-based
Number of satellites for which assistance is provided	Yes	Yes
reference time for GPS (T_UTRAN-GPS) (specified in [15] and [16])	Yes	Yes
3-d reference position (specified in [11])	No	Yes
ionospheric corrections	No	Yes
satellite ID for identifying the satellites for which assistance data is provided	Yes	Yes
Ephemeris & clock corrections	Yes	Yes
UTC offset	No	Yes
DGPS corrections	No	Yes
almanac data	Yes	Yes
real-time integrity (e.g. a list of unusable satellites)	No	Yes
doppler (0^th order term)	Yes	No
Doppler Search Window width	Yes	No
doppler (1^st order term)	Yes	No
azimuth	Yes	No
elevation	Yes	No
code phase	Yes	No
code phase centre and search window width	Yes	No

The information that may be signalled from UE to the network is listed in table 10.2.

Table 10.2: Information that may be transferred from UE to the network

Information	UE-assisted	UE-based
reference time for GPS (T_UE-GPS) (specified in [15] and [16])	Yes	Yes
serving cell information	No	Yes
Latitude/Longitude/Altitude/Error ellipse	No	Yes
velocity estimate in the UE	No	Yes
satellite ID for which measurement data is valid	Yes	No
Whole/Fractional chips for information about the code-phase measurement	Yes	No
C/N₀ of the received signal from the particular satellite used in the measurements	Yes	No
doppler frequency measured by the UE for the particular satellite	Yes	No
pseudorange RMS error	Yes	No
multipath indicator	Yes	No
number of Pseudoranges	Yes	No

Table 10.3 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 10.3: Information that may be transferred from Node B/LMU to CRNC and between RNCs

Information	UE-assisted	UE-based
reference time for GPS (T_UTRAN-GPS) (specified in [15] and [16])	Yes	Yes

10.5.1.1 Almanac data

The almanac parameters specify the coarse, long-term model of the satellite positions and clocks. These parameters are a subset of the ephemeris and clock correction parameters in the Navigation Model, although with reduced resolution and accuracy. The almanac model is useful for receiver tasks that require coarse accuracy, such as determining satellite visibility. The model is valid for up to one year, typically. Since it is a long-term model, the field should be provided for all satellites in the GPS constellation.

Optionally, "SV Global Health" information may accompany this almanac information. This additional information is composed of the sequence of all non-parity data bits contained in words 3-10 of page 25 of subframe 4 of the GPS navigation message followed by the sequence of all non-parity bits contained in words 3-10 of page 25 of subframe 5 of the GPS navigation message. The following GPS navigation message fields are excluded when constructing these sequences: "Data ID", "SV (Page) ID", and "t".

10.5.1.2 DGPS corrections

In order to allow a UE to estimate its position more accurate, biases in the pseudorange measurements may be provided to the UE.

Status/Health

This information indicates the status of the differential corrections contained in the message.

IODE

This is the sequence number for the ephemeris for the particular satellite. The UE can use this information to determine if new ephemeris is used for calculating the corrections that are provided in the broadcast message. This eight-bit IE is incremented for each new set of ephemeris for the satellite and may occupy the numerical range of [0, 239] during normal operations. More information about this field can be found from [24].

User Differential Range Error (UDRE)

The UDRE provides an estimate of the uncertainty (1-s) in the corrections for the particular satellite. The value in this field shall be multiplied by the UDRE Scale Factor in the common Corrections Status/Health field to determine the final UDRE estimate for the particular satellite. More information about this field can be found from [24].

Pseudo-Range Correction (PRC)

The PRC indicates the correction to the pseudorange for the particular satellite at the GPS Reference Time, t0. The PRC definition here is different from the one given in [24].

Pseudo-Range Rate Correction (PRRC)

This information indicates the rate-of-change of the pseudorange correction for the particular satellite, using the satellite ephemeris identified by the IODE IE.

10.5.1.3 Ionospheric corrections

The Ionospheric Model contains information needed to model the propagation delays of the GPS signals through the ionosphere. Proper use of these information allows a single-frequency GPS receiver to remove approximately 50% of the ionospheric delay from the range measurements. The Ionospheric Model is valid for the entire constellation and changes slowly relative to the Navigation Model.

10.5.1.4 Ephemeris data and clock correction

Ephemeris data and clock corrections provide an accurate model of the satellite positions to the UE.

10.5.1.5 Real Time integrity monitor function

An Integrity Monitor (IM) function in the network should detect unhealthy (i.e., failed/failing) satellites. Excessively large pseudo range errors, as evidenced by the magnitude of the corresponding DGPS correction determined by the IM, may be used to detect unhealthy satellites. Unhealthy satellites should be detected very close to the occurrence of the satellite failure (e.g. 10 seconds) and marked in an unhealthy satellite list as unusable/bad. When unhealthy satellites are detected, the assistance and/or DGPS correction data should not be supplied for these satellites. Upon receiving the list of unhealthy satellites from the SRNC, the UE shall consider the data associated with these satellites to be invalid.

The IM function should also inform the UE of measurement quality in DGPS modes when satellites are healthy. This can be done by computing the position of the DGPS reference receiver using its derived pseudo ranges and differential corrections at the IM, and differencing the IM computed position with the known location of the DGPS reference receiver to compute a position error. When the error in the IM computed position is excessive for solutions based upon healthy satellites only, DGPS users should be informed of measurement quality through the supplied User Differential Range Error (UDRE) adjusted values based on the operation of the IM. The UE should use the measurement quality as a factor in weighing data obtained from associated satellites in its position calculation.

NOTE: UDRE is one of the IEs contained in the DGPS information ([19]).

The real-time Integrity Monitor function provides the following information to a UE:

– BadSATid;

– UDRE value adjusted based on the measurement quality.

BadSATid is a lit of unhealthy (i.e., failed/failing) satellites. The UE shall consider any assistance or DGPS data of these satellites as invalid.

Adjusted UDRE value reports the measurement quality of the corresponding satellites. The UE should consider the quality while calculating its position.

10.5.1.6 GPS reference time

GPS reference time may be used to provide a mapping between UTRAN and GPS time.

GPS TOW Assist

This information contains several fields in the Telemetry (TLM) Word and Handover Word (HOW) that are currently being broadcast by the respective GPS satellites. Combining this information with GPS TOW helps the UE with time-recovery needed to predict satellite signal.

TLM Message

This information contains a 14-bit value representing the Telemetry Message (TLM) being broadcast by the GPS satellite identified by the particular SatID, with the MSB occurring first in the satellite transmission.

Anti-Spoof/Alert

These information contain the Anti-Spoof and Alert flags that are being broadcast by the GPS satellite identified by SatID.

TLM Reserved

These information contain the two reserved bits in the TLM Word being broadcast by the GPS satellite identified by SatID, with the MSB occurring first in the satellite transmission.

10.5.1.7 UTC

UTC parameters may be used to provide Coordinated Universal Time to the UE.

10.5.1.8 Reference Location

The Reference Location contains a 3-D location (with uncertainty) specified as per [11]. The purpose of this field is to provide the UE with a priori knowledge of its position in order to improve GPS receiver performance.

10.5.1.9 Additional non-GPS related information

Additional non-GPS measurements performed by UTRAN or UE may be used by the SRNC to improve the performance of the UE-assisted GPS method. This information may be RTT in FDD or Rx Timing Deviation in TDD, UE receiving transmitting time (UE Rx-Tx), SFN-SFN observed time difference or CPICH Ec/No. All the additional measurements are defined in [15] and [16] and can be made available through RRC signalling for UE measurements or NBAP signalling for UTRAN measurements.

Furthermore, to those UE technologies requiring externally provided sensitivity and time aiding data, some navigation bits may be sent from UTRAN to UE for sensitivity assistance and time recovery.

10.6 RNC based Network Assisted GPS positioning Procedure

The diagram in Figure 10.3 and Figure 10.2 illustrate the operations for the network assisted GPS when the request for position information is initiated by a LCS application signalled from the Core Network. A detailed description of the positioning procedure is given as follows. Note that the procedure is for illustration purpose and actual implementations may vary.

Figure 10.3: RNC based Network-assisted GPS methods

1. The operation begins with an authenticated request for positioning information about a UE from an application in the core network being received at the SRNC. The request from the CN may be a request for on-demand or periodic reporting. The SRNC acts as interface between the Core Network and the UE Positioning entities in the UTRAN. The SRNC considers the request and the capabilities of the UE and the UTRAN. In networks that include the SAS, the SRNC may invoke the SAS via the Iupc interface.

2. Depending on the UE capabilities, the network sends to the UE certain GPS assistance information. This information may include: the reference time for GPS, the satellite IDs, the Doppler frequency, the search window and its centre, the ephemeris and clock corrections, the almanac, and other information specified in 10.5.1. Depending on the UE capabilities, the network sends a request for GPS positioning to 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. If the UE has not enough assistance data to perform the measurements, the UE should indicate it to the SRNC and additionally request for assistance data.

For UE-based method, jump to step 8.

3. For UE-assisted method, the SRNC may optionally request the following information before the assistance message(s) is (are) sent to the UE: the LMU update (see NOTE), the RTT measurements (from the Node Bs in the active set) to compensate for the one-way propagation delays. The LMU (associated or stand-alone) returns the information containing the time difference between the Node B and the GPS (e.g. UTRAN GPS timing of cell frames or SFN-SFN Observed Time Difference) to the CRNC. The Node B returns its RTT measurement to the CRNC. If the CRNC is not the SRNC, the CRNC forwards these information to SRNC.

4. The network requests from the UE the measurement of GPS satellite pseudoranges and other information specified in 10.5.1. These measurements may be made while the UE is in RRC connected mode CELL_DCH state. The SRNC may request SFN-SFN Observed Time Difference measurements and Rx-Tx timing difference information from the UE to support the processing related to the RTT measurements.

5. The UE returns to the network the measurement of GPS satellite pseudoranges and other information specified in 10.5.1. If requested, the UE returns to the SRNC SFN-SFN measurements and the Rx-Tx time difference information, together with a time stamp of when these values were obtained.

6 The UE position and optional velocity is calculated in the network.

7. If there is insufficient information to yield a UE positioning estimate, the SRNC may start a new process from step 3.

8. In case of UE based method, UE returns the position and optional velocity estimate to the SRNC. This estimate includes the position and optionally, velocity, the estimated accuracy of the results and the time of the estimate.

9. In networks that include the SAS, the SAS passes the position estimate to the SRNC.

10. The SRNC passes the position and optional velocity estimate to the CN including, if available, the positioning method (or the list of the methods) used to obtain the position 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 the SRNC requested periodic UE reporting in step 2 according to subclause 6.6.4.1, steps 5 to 10 are repeated and the UE reports new measurements one reporting interval after the previous measurement report. If periodic reporting was requested form the CN in step 1, but the SRNC does not request periodic UE reporting in step 2, 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).

NOTE: The LMU update (of the time difference between the GPS and the Node B) may be performed on a per-request basis (with respect to each UE Positioning request) or be performed timely that is independent of individual UE Positioning request. The latter is preferable when there is a large volume of UE Positioning requests.

10.7 SAS initiated Network Assisted GPS positioning Procedure

The following describes the signalling for the optional initiation of the network assisted GPS positioning procedure by the SAS.

Figure 10.4: Network-assisted GPS methods when initiated by the SAS

2. The SRNC sends parameters received in the location request, including any periodic reporting information, together with the Cell ID and UE capability information to the SAS in a PCAP: Position Initiation Request message via the Iupc interface.

3. Depending on the UE capabilities, the SAS initiates an A-GPS positioning procedure by sending a PCAP: Position Activation Request message containing A-GPS assistance data to the SRNC via the Iupc interface. The PCAP Position Activation Request message may include periodic reporting information (number of reports and reporting interval). The SAS may provide all or some A-GPS assistance data needed by the UE. This may include timing assistance data that the SAS may have obtained from associated LMUs or from another source (e.g. GPS Reference Network or measurements from UEs previously positioned by the SAS using A-GPS).

4. The SRNC forwards to the UE the A-GPS positioning request received from the SAS using RRC signalling. The SRNC also forwards in the RRC signalling message(s) the SAS request for either A-GPS measurements, in the case of UE assisted A-GPS, or an A-GPS position and optional velocity estimate, in the case of UE based A-GPS. The RRC signalling may include a request for periodic reporting as described in subclause 6.6.4.1 if this was received in step 3. For a description of UE based A-GPS, jump to step 9.

5. For UE assisted A-GPS, the SRNC requests from the UE the measurement of GPS satellite pseudoranges and other information specified in subclause 10.5.1. These measurements may be made while the UE is in RRC connected mode CELL_DCH state. The SRNC may request SFN-SFN Observed Time Difference measurements and Rx-Tx timing difference information from the UE to support the processing related to the RTT measurements.

6. The UE returns to the SRNC the measurement of GPS satellite pseudoranges and other information specified in 10.5.1. If requested, the UE returns to the SRNC SFN-SFN measurements and the Rx-Tx time difference information, together with a time stamp of when these values were obtained.

7. The information obtained in step 6 is sent from the SRNC to the SAS in a PCAP: Position Activation Response message.

8. The SAS calculates the UE position and optional velocity. If periodic reporting was not requested in step 2, the SAS returns the UE position and optional velocity to the SRNC in a PCAP: Position Initiation Response message. If periodic reporting was requested in step 2, the SAS forwards the position information to the SRNC in a PCAP Position Periodic Result message. The PCAP Position Initiation Response or PCAP Position Periodic Result message may include the positioning method(s) used and an indication of whether the position estimate satisfies the requested accuracy or not.

9. In case of UE based method, the UE returns the position and optional velocity estimate to the SRNC via RRC signalling. The SRNC forwards the position and optional velocity estimate to the SAS in a PCAP: Position Activation Response message. This estimate includes the position and optionally, velocity, the estimated accuracy of the results and the time of the estimate.

10. The SAS may verify the UE position estimate received in step 9 (e.g. using cell ID information). If periodic reporting was not requested in step 2, the SAS returns the resulting estimate to the SRNC in a PCAP: Position Initiation Response message. If periodic reporting was requested in step 2, the SAS forwards the position estimate to the SRNC in a PCAP Position Periodic Result message. The PCAP Position Initiation Response or PCAP Position Periodic Result message may include the positioning method(s) used and an indication of whether the position estimate satisfies the requested accuracy or not.

11. If there is insufficient information to yield a UE positioning estimate satisfying the requested accuracy, the SAS may start a new process from step 3.

12. The SRNC passes the position estimate received from the SAS to the CN including the positioning method (or the list of the methods) used to obtain the position 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.

13. If periodic UE reporting was requested in step 4, the UE continues to send GPS measurement reports one reporting interval after the previous measurement report. The SRNC forward the GPS measurement report information to the SAS in a PCAP Position Periodic Report message. The SAS may calculate the UE position and optional velocity and may verify the calculated UE position estimate or the received UE position estimate (e.g. using cell ID information) and sends the 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 13 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.

14. If periodic UE reporting was not requested in step 4, but was requested in step 2, the SAS may repeat the steps 3 to 12 as for the first request until the desired amount of reports has been attained or the procedure is cancelled by the CN or UTRAN. When repeating step 10 for the final request, the SAS returns the resulting final position estimate to the SRNC in a PCAP: Position Initiation Response message.

NOTE: An update to the SAS from an associated LMU, of the time difference between GPS and the Node B, may be performed on a per-request basis (with respect to each UE Positioning request) or be performed in a timely manner that is independent of individual UE Positioning requests. The latter is preferable when there is a large volume of UE Positioning requests.