Summary based on the input provided by Intel in RP-220919.
This Work Item specifies solutions for NR Positioning enhancements, including improvement of positioning accuracy and latency of Rel-16 NR positioning methods, improvements of network efficiency (On-Demand PRS transmission), improvement of device efficiency (positioning in RRC_INACTIVE), providing high integrity and reliability requirements (GNSS integrity) and enhancements of A-GNSS positioning (BDS B2a,/B3I and NavIC to NR).
All these aspects are detailed below.
Improvement of positioning accuracy
To improve positioning accuracy, several solutions were considered:
Mitigation of gNB/UE Tx/Rx timing delay errors: For mitigation of gNB/UE Tx/Rx timing delay errors, multiple enhancements are introduced in Rel-17:
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For DL-TDOA, a UE can be requested to provide the Rx TEG IDs together with RSTD measurements and a TRP can be requested to provide TRP Tx TEG association information of DL PRS resources to mitigate UE Rx timing errors and TRP Tx timing errors. A UE can also be requested to measure the same DL PRS resource of a TRP with different UE Rx TEGs and report the corresponding multiple RSTD measurements to mitigate UE Rx timing errors.
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For UL-TDOA, Rel-17 supports a TRP to provide the Rx TEG IDs together with RTOA measurements and a UE to provide UE Tx TEG association information of UL positioning SRS resources to mitigate TRP Rx timing errors and UE Tx timing errors. A TRP can also be requested to measure the same UL SRS resource of a UE with different TRP Rx TEGs and report the corresponding multiple RTOA measurements to mitigate TRP Rx timing errors.
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Multi-RTT, Rel-17 supports a UE/TRP to provide the RxTx TEG IDs, or combination of {Rx TEG ID, Tx TEG ID} together with UE/gNB Rx-Tx time difference measurements and a UE/gNB to provide its Tx TEG association information for mitigating UE/TRP Rx/Tx timing errors;
In addition, Rel-17 supports a UE to report more than one measurement instance of RSTD, DL RSRP, and/or UE Rx-Tx time difference measurements in a single measurement report, and support a TRP to report more than one measurement instance of RTOA, UL RSRP, and/or gNB Rx-Tx time difference measurements in a single measurement report. Each measurement instance is reported with its own timestamp;
UL-AOA enhancements: The new assistance information (expected UL-AOA value and uncertainty range) can be provided by the LMF to facilitate gNB measurements for NR UL-AOA, UL-TDOA and Multi-RTT positioning methods. Support of the first arrival path UL-AOA/ZOA measurement pair per SRS resource were introduced, including reporting of multiple per path AOA value pairs (including additional paths) to cope with possible ambiguity of angle measurements in antenna arrays with larger spacing than a half wavelength, and reporting ZoA only to cope with angle measurements in linear antenna arrays.
The per path UL SRS receive reference signal power (UL SRS-RSRPP) measurement definition was introduced.
The antenna reference point (ARP) location can be associated with UL measurements for NR Positioning (UL AOA, UL-RTOA, UL SRS-RSRP, UL SRS-RSRPP and gNB Rx-Tx time difference measurements).
Finally, to facilitate hybrid RAT dependent positioning, gNB can report to LMF the following set of measurements {one SRS-RSRP, multiple UL-AOAs (AOA/ZoA pairs), one UL-RTOA or one-gNB Rx-Tx time difference}.
DL-AOD enhancements: The enhancements of assistance data (a subset of PRS resources for each PRS resource for the purpose of prioritization of DL-AOD reporting or the boresight direction information for each PRS resource) were defined to improve DL-AOD estimation. In addition, the assistance information on DL-AOD/DL-AOA expected value and uncertainty range can be provided by LMF to UE. In Rel-17, for more accurate DL-AOD measurements, the gNB can provide the beam/antenna information to LMF (and LMF can further share it to UE for UE-based positioning). The per path DL PRS reference signal received power (DL PRS RSRPP) for the first path measurements were defined for DL-AOD estimation, and the maximum number of DL PRS RSRPP for the first path measurements was 24. Finally, the maximum number of DL PRS RSRP measurements per TRP was increased up to 24 compared to 8 in Rel.16.
Multi-path and NLOS mitigation: In Rel-17, multi-path (additional path) report enhancements and LOS/NLOS indication were introduced for NR positioning solutions. The maximum number of additional paths that can be reported is increased (up to 8) with per path RSRP measurements and associated relative timing supported. Multiple UL-AOAs (up to 8) per additional path reporting is supported for the UL-TDOA and Multi-RTT positioning methods. The LOS/NLOS indicator was introduced that can be associated with specific measurements, DL/UL reference signals / resources for positioning.
Improvement of positioning latency
To improve the positioning latency, following solutions were considered:
Preconfigured measurement gap: To reduce latency of procedures for DL PRS processing with measurement gaps, the set of measurement gap patterns can be pre-configured to UE and activated/deactivated by gNB using new DL MAC CE signalling designed to control DL PRS measurement by UE. The UE can request activation and deactivation of the pre-configured MG using new UL MAC CE signalling introduced in Rel-17. The LMF can request activation of pre-configured MG using new NRPPa signalling introduced in Rel-17.
Preconfigured PRS processing window: To further reduce latency of DL PRS processing, UEs can perform DL PRS measurement outside measurement gaps and inside the active DL BWP with PRS having the same numerology as the active DL BWP. The gNB can use RRC signalling to pre-configure PRS processing window and DL MAC CE signalling for activation of PRS processing window, respectively. gNB can indicate the DL PRS processing priority relative to other DL signals/channels within the PRS processing window for PRS measurement outside MG.
M-sample measurement (M = 1): In Rel-17, LMF can request UE and/or TRP to perform measurement over either a single RS transmission period (M=1) or four RS transmission periods (M=4). Configuring M=1 for a UE reduces UE measurement period comparing to Rel.16 (In Rel-16, UE is expected to measure DL PRS over four periods (M = 4)). An AGC sample, in addition to the M=1 sample may be required at the UE subject to measurement conditions.
Lower Rx beam sweeping factor: In Rel-17, a new UE capability on lower Rx beam sweeping factor (<8) is introduced to reduce the PRS measurement latency for FR2 positioning frequency layers.
Storing LPP capability in AMF: The LMF may interact with the AMF to provide (updated) UE Positioning Capability to AMF and to receive stored UE Positioning Capability from AMF as described in
TS 23.273. The LPP procedures to transfer UE LPP positioning capabilities may be skipped if the LMF already obtained the UE positioning capabilities from the AMF.
Preconfigured assistance data: Preconfigured assistance data is the DL-PRS assistance data (with associated validity criteria, i.e. area ID) that can be provided to the UE (before or during an ongoing LPP positioning session), to be then utilized for potential positioning measurements at a future time (e.g. for deferred MT-LR). Pre-configured DL-PRS assistance data may consist of multiple instances, where each instance is applicable to a different area within the network.
Scheduled location time: During positioning procedure, the LMF may obtain the scheduled location time from the AMF. Based on the obtained scheduled location time, the LMF may schedule location measurements by the UE and/or location measurement by the NG-RAN to occur at or near to the scheduled location time.
On-Demand PRS transmission
On-Demand PRS transmission procedure allows the LMF to control and decide whether PRS should be transmitted or not and whether the characteristics of an ongoing PRS transmission should be changed or not.
In case of UE-initiated On-Demand PRS, the LMF may configure the UE with pre-defined PRS configurations via LPP Provide Assistance Data message or via posSI. The UE sends an On-Demand PRS request to the LMF via LPP Request Assistance Data message. The On-Demand PRS request can be the request for a defined PRS configuration with PRS configuration ID or explicit parameter for PRS configuration and may be a request for PRS transmission or change to the PRS transmission characteristics for positioning measurements.
In case of LMF-initiated On-Demand PRS, the LMF and the UE may exchange LPP messages e.g., to obtain UE measurements or the DL-PRS positioning capabilities of the UE, etc).
The actual PRS changes are requested by the LMF irrespective of whether the procedure is UE- or LMF-initiated.
Positioning in RRC_INACTIVE state
Positioning may be performed when a UE is in RRC_INACTIVE. Any uplink LCS or LPP message can be transported in RRC_INACTIVE. If the UE initiated data transmission using UL SDT, the network can send DL LCS, LPP and RRC message (e.g. to configure SRS for UL positioning, if it is supported) to the UE. UE may also receive PRS or transmit SRS in RRC_INACTIVE. Support of all NR positioning measurements and support of NR positioning methods such as NR E-CID, DL-TDOA, DL-AOD, UL-AOA, UL-TDOA, Multi-RTT and RAT-independent positioning methods are extended for UEs in RRC_INACTIVE state.
GNSS Integrity
Positioning integrity is a measure of the trust in the accuracy of the position-related data and the ability to provide associated alerts. UE based GNSS integrity is supported in Rel-17. It allows the UE to determine and report the integrity results of the calculated location, where only protection level (PL) reporting mode is supported; the UE can use the integrity requirements and assistance data obtained via NG-RAN, together with its own measurements, to determine the integrity results of the calculated location.
For integrity operation, the network will ensure that:
P(Error > Bound for longer than TTA | NOT DNU) ≤ Residual Risk + IRallocation
for all values of IRallocation in the range irMinimum ≤ IRallocation ≤ irMaximum
for all the errors in Table 8.1.2.1b-1 as specified in
clause 8.1.2.1b of TS 38.305, which have corresponding integrity assistance data available and where the corresponding DNU flag(s) are set to false.
Enhancements of A-GNSS positioning
Enhancements of A-GNSS positioning specified: BDS B2a signal; BDS B3I signal; NavIC to NR.
References
[6.2.2.1-1]
RP-220803: "Status Report to TSG on NR Positioning Enhancements", Intel Corporation
[6.2.2.1-2]
TR 38.857: "Study on NR positioning enhancements" v17.0.0