R1-2601811
discussion
Discussion on downlink transmission scheme(s) for downlink shared channels for 6GR
From Spreadtrum
Spreadtrum's prior position on
10.5.2.2
at
RAN1#124
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Proposes unified 6G downlink transmission schemes to minimize UE and NW implementation complexity, explicitly opposing multiple schemes for the same scenario and scheme enhancements yielding less than 10% performance gain. Retains NR baselines for maximum layers (8), maximum codewords (2), DMRS port count, and PN sequence for CP-OFDM, while requiring a single DMRS type for both DL and UL. For AI/ML-based DMRS overhead reduction, presents simulation results showing AI-based channel estimation achieving similar BLER with reduced DMRS overhead (2-symbol DMRS matching 3-symbol non-AI performance in time domain, 1/3 RE reduction in frequency domain) and proposes prioritizing sparse orthogonal DMRS (sub-case A) over superimposed pilots (SIP), which they consider insufficiently evaluated for multi-user interference and standardization impact. For NR-6G multi-RAT spectrum sharing, proposes semi-static FDM/TDM and rate matching mechanisms while opposing SDM between NR and 6G due to negative performance impacts.
Summary
Spreadtrum presents a comprehensive analysis of downlink transmission schemes for 6G, containing 21 proposals and 5 observations covering PDSCH design, DMRS, PTRS, and MRSS aspects. The document argues for simplifying 6G designs relative to 5G NR by unifying schemes, minimizing configurations, and avoiding marginal enhancements.
Position
Spreadtrum proposes maintaining 5G NR layer limits (max 8 layers, 2 codewords) and a baseline layer mapping rule where the first half layers map to CW0 and remaining to CW1, with a single MCS per codeword. They require a single PDSCH mapping type to reduce receiver complexity and oppose multiple transmission schemes for the same scenario, arguing that only Rel-15 schemes achieved good commercialization while later enhanced schemes were not implemented. For DMRS, they propose defining a single DMRS type for DL and supporting up to 32 ports via increased CDM groups or longer FD-OCC, and present technical case against SIP (superimposed pilot) by deprioritizing its study due to insufficient demonstrated gains and increased AI model complexity requiring 1.48M parameters and 80M FLOPs. For MRSS, they oppose signals/channels sharing between NR and 6G as well as SDM between the two RATs, and propose semi-static FDM/TDM and rate matching of 6GR PDSCH around NR signals/channels.
Key proposals
- Proposal 1 (Sec 2.1.1): For PDSCH scheduling, the maximum number of layers is 8 and the maximum number of codewords is 2.
- Proposal 5 (Sec 2.1.2): To design 6GR downlink transmission scheme(s), strive for unified transmission scheme(s) to minimize schemes and configurations, avoid multiple schemes for the same scenario, and avoid unnecessary enhancements pursuing gains less than 10%.
- Proposal 7 (Sec 2.1.3): For time-domain resource allocation for PDSCH, support a single mapping type.
- Proposal 10 (Sec 2.1.4): For frequency-domain resource allocation for PDSCH, support both RBG-based and RIV-based mapping.
- Proposal 11 (Sec 2.1.5): For frequency interleaving for PDSCH, support at least non-interleaved mapping.
- Proposal 12 (Sec 2.2.1): For DMRS design, strive to define a single DMRS type for DL.
- Proposal 13 (Sec 2.2.1): Support up to 32 DMRS ports for DL.
- Proposal 14 (Sec 2.2.2): For DMRS port multiplexing, FDM, TDM and CDM can be considered.
- Proposal 15 (Sec 2.2.3): For DMRS sequence design, consider PN sequence for CP-OFDM as starting point.
- Proposal 16 (Sec 2.2.4): For AI/ML based DMRS overhead reduction, the trade-off between UE implementation complexity and the performance gain needs to be fully considered.
- Proposal 17 (Sec 2.2.4): Deprioritize the study of SIP.
- Proposal 18 (Sec 2.3): For phase noise estimation, support dedicated PT-RS design.
- Proposal 19 (Sec 2.3): For PT-RS design, consider to reuse PT-RS in 5G NR as baseline.
- Proposal 20 (Sec 2.4): High-level aspects to consider for NR-6GR MRSS include UE/NW implementation complexity, resource allocation coordination, radio resource utilization, operating bands at least existing FR1, aligned numerology and waveform as NR in shared band, unified design between MRSS and non-MRSS scenario, and no impact to legacy NR UE behavior.
- Proposal 2 (Sec 2.1.1): Consider the number of scheduled codeword(s) for each rank: rank=1 supports 1 codeword, rank 2-4 studies 1 or 2, rank 5-8 supports 2.