R1-2600136
discussion
Discussion on NR-NTN GNSS resilience
From SageRAN
Summary
This SageRAN document for the 3GPP RAN1 5G-Advanced NTN SI discusses GNSS-resilient operation across pre-compensation, PRACH, connected mode, and positioning. It presents 7 observations and 15 proposals focusing on UE-side time/frequency estimation using reference locations, minimal physical layer impact for PRACH, closed-loop frequency control, de-prioritization of DL-TDOA positioning, and GNSS status reporting.
Position
SageRAN argues that assistance information of common TA/Doppler in the service link is inapplicable due to diverse UE positions and velocities, proposing instead that a reference location be used for time/frequency pre-compensation. They present a technical case that the last UE positioning information, including altitude, should be considered for TA/Doppler coarse estimation alongside the reference location to improve accuracy. For initial access, SageRAN requires minimal physical layer impact, supporting multiple PRACH transmissions (Solution 1A/2B) but requiring evaluation of gNB processing overhead for solutions that demand per-RACH-occasion complexity (Solution 1C/1D), while supporting the introduction of Frequency Adjustment Commands and DL-timestamp-based estimation (Solution 2C) only when local time error is limited. They oppose enhanced TA commands for connected mode, citing inability to provide accurate timing due to unknown UE velocity and position, and instead support closed-loop frequency control via MAC CE. SageRAN further proposes lowering the priority of DL-only UE-based DL-TDOA positioning, arguing it is inapplicable for single-satellite deployment, requires all gNBs to be regenerative and timing-synchronized, and presents higher complexity than reference location and last GNSS position-based methods.
Key proposals
- Proposal 1 (Sec 2.1): Assistance information of (moving) reference location is applicable for (quasi) earth fixed cell or moving cell.
- Proposal 6 (Sec 2.2): Solution 1A / 2B for PRACH should be supported.
- Proposal 11 (Sec 2.3): Enhanced TA command (e.g., TA drift rate and TA drift variation rate) is not recommended.
- Proposal 13 (Sec 2.4): It is suggested to lower the priority for DL-only UE-based positioning method by DL-TDOA in R20 study.
- Proposal 14 (Sec 2.5): GNSS status reporting from UE to network should be supported.
- Proposal 2 (Sec 2.1): The last UE positioning information should be considered for TA/Doppler coarse estimation based on reference location.
- Proposal 3 (Sec 2.1): A new IE should be introduced in SIB19 to indicate the corresponding GSCN.
- Proposal 5 (Sec 2.1): UE can derive uplink frequency offset by receiving downlink frequency, transmitting downlink frequency (indicated GSCN), and scaling factor from DL and UL carrier frequency.
- Proposal 8 (Sec 2.2): The mechanism of FAC should be introduced for random access and normal UL transmission.
- Proposal 10 (Sec 2.2): Solution 2D / 2E can both be used for service link time/frequency pre-compensation.
- Proposal 12 (Sec 2.3): Frequency adjustment command in MAC CE i.e. closed-loop frequency control in connected mode can be supported.
- Proposal 15 (Sec 2.5): GNSS status can be reported via MAC CE Timing Advance Reporting by redefining a reserved bit.