R1-2601787
discussion
Discussion on 6G NTN
From FUTUREWEI
FUTUREWEI's prior position on
10.7.1
at
RAN1#124
· AI-synthesized, paraphrased
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Proposes studying Very Low Earth Orbit (VLEO) NTN scenarios at an orbital height of 300 km, while defaulting to Rel-17 NR-NTN LEO and GEO parameters for other cases. To mitigate increased UE power consumption from GNSS-based operation, proposes considering GNSS position fix time intervals and satellite elevation angles in the 6G PRACH format design. Recommends studying network-controlled timing advance adjustments in both positive and negative directions during random access to correct GNSS position errors, and proposes studying closed-loop frequency control for the network to adjust UE carrier frequency in RRC connected mode.
Summary
This Futurewei contribution addresses 6G NTN open issues from RAN1#124, containing 2 formal proposals and 3 observations. It focuses on the satellite beam footprint coverage ratio during initial access and the impact of beam hopping patterns on UE performance.
Position
Futurewei proposes that RAN1 agree on a 100% satellite beam footprint coverage ratio before proceeding with studies on beam footprint versus active beam mismatch, and suggests that simultaneous active beams may be as low as 1.5% of total beams per satellite across all frequency bands. They present a technical case showing that with current assumptions (160 ms SSB periodicity, one SSB/PBCH block per half radio frame), only 1024 out of 2134 beams can be covered, yielding 48% coverage—insufficient for full footprint coverage. They propose studying the impact of both static and dynamic beam hopping patterns on UE performance in idle and connected modes, specifically identifying the SSB index / PRACH preamble collision problem in adjacent beams as a performance-affecting factor. They also indicate that 6GR NTN uplink time-frequency synchronization should follow the Rel-19 NR NTN GNSS-based pre-compensation methodology as baseline, allowing RAN1 to focus on 6GR NTN-specific issues rather than duplicating Rel-20 GNSS resilient NR-NTN study work.
Key proposals
- Proposal 1 (Sec Discussion): RAN1 should agree on the satellite beam footprint coverage ratio being [100%] before studying aspects related to having more beam footprints than simultaneously active beams, noting that simultaneous active beams may be as low as [1.5%] of total beams per satellite.
- Proposal 2 (Sec Discussion): RAN1 should study the impact of beam hopping patterns (e.g., static vs dynamic) on the performance of UE in idle and connected modes.
- Observation 1 (Sec Discussion): Using the beam parameter assumption with 2134 total beams and 32 simultaneous active beams, and assuming SSB periodicity of 160 ms with one SSB/PBCH block per half radio frame, the satellite can only cover 1024 beams out of 2134, resulting in approximately 48% satellite footprint coverage ratio.
- Observation 2 (Sec Discussion): RAN1 has not yet agreed on the expected coverage ratio for 6GR NTN during initial access, and this must be resolved before studying aspects of beam footprint versus active beam mismatch.
- Observation 3 (Sec Discussion): Beam hopping, which may follow fixed or dynamic patterns, determines active beams at a given time and affects UE performance in idle and connected modes, including the problem where two adjacent beams broadcasting the same SSB index associated with the same random-access occasion prevent the network from distinguishing PRACH preambles between those beams.