R1-2601788
discussion
Discussion on downlink transmission schemes for downlink shared channels for 6GR air interface
From FUTUREWEI
FUTUREWEI's prior position on
10.5.2.2
at
RAN1#124
· AI-synthesized, paraphrased
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Proposes adopting the existing 5G NR MIMO/RS/CSI framework as a 6G baseline while requiring AI/ML integration from day-1 of the MIMO framework design. Proposes a UE-assisted DMRS pattern selection scheme where UEs report minimum DMRS density requirements (D_F, D_T, D_J) rather than preferred patterns, claiming over 70% reporting overhead reduction compared to direct pattern reporting. Proposes studying per-subcarrier matched-filter frequency-selective precoding that simultaneously achieves finest precoding granularity and single wideband channel estimation, addressing what they identify as a fundamental limitation of 5G NR subband precoding. Proposes cooperative MIMO schemes leveraging TDD reciprocity with DL interference probing via SRS, presenting capacity gains of 64.8% in Dense Urban and 26.9% in Urban Macro scenarios with 45Mbps XR traffic. Emphasizes hybrid antenna architectures for UMB with large antenna element counts (2048) and moderate TXRU numbers, distinguishing them from fully-digital architectures based on subarray size and dynamic analog beamforming adjustability.
Summary
This Futurewei contribution to the 3GPP RAN1 #124bis meeting on FS_6G_Radio presents 8 Observations and 12 Proposals covering downlink transmission schemes for 6G shared channels, addressing DMRS adaptation, AI/ML-based DMRS enhancement, MU-MIMO for hybrid antenna architectures, frequency-selective precoding, cooperative MIMO via SRS probing, transmit diversity, QCL/TCI enhancements, and EVM assumptions.
Position
Futurewei proposes a UE-centric DMRS adaptation framework where the UE reports minimum DMRS density requirements (D_F, D_T, D_J) rather than selecting preferred patterns, achieving over 70% reporting overhead reduction compared to direct pattern index reporting. For hybrid antenna architectures in UMB, Futurewei proposes distinguishing two MU-MIMO transmission cases: when full CSI is available, adjusting both analog beams (subject to phase-only constraints) and digital precoders; when only analog-beam-specific CSI is available, adjusting only digital precoders without modifying analog beams. Futurewei presents a technical case for per-subcarrier matched-filter precoding that simultaneously achieves finest precoding granularity and wideband channel estimation, collapsing the effective channel impulse response to one dominant zero-lag tap. For TDD cooperative MIMO, Futurewei proposes DL interference probing via aperiodic SRS where the network configures UEs to transmit SRS with parameters tied to associated PDSCH transmissions (PRB allocation, port allocation), enabling distributed inter-cell interference nulling without inter-gNB information exchange.
Key proposals
- Proposal 1 (Sec 2.1.1): For 6GR DMRS enhancements, support UE reports DMRS densities values to facilitate DMRS pattern selection.
- Proposal 2 (Sec 2.1.2): Consider including AI/ML-based DMRS configuration optimization as a candidate approach for DL DMRS enhancement in 6G study.
- Proposal 3 (Sec 2.2): For UMB base stations, hybrid antenna architecture with a large number of antenna elements and moderate number of TXRUs supporting hybrid beamforming with a combination of adjustable analog beamforming and digital precoding, should be a key focus area.
- Proposal 4 (Sec 2.2): Study potential MU MIMO transmission schemes for hybrid antenna architectures, distinguishing cases where full CSI is available versus only analog-beam-specific CSI.
- Proposal 5 (Sec 2.3): To support wider bandwidth PDSCH transmission, study fine-granularity frequency-selective precoding in DL transmissions without the need of separate/multiple UE-side channel estimations for multiple precoding resource groups (i.e., PRB bundles).
- Proposal 6 (Sec 2.3): To support wider bandwidth PDSCH transmission with reduced DMRS overhead in the frequency domain, study reduced DMRS frequency domain densities than 5G NR without reduced channel estimation performance.
- Proposal 7 (Sec 2.4): Study cooperative MIMO via DL interference probing based on SRS enhancements to improve 6G system capacity for TDD.
- Proposal 8 (Sec 2.4): Study cooperative MIMO for TDD with downlink interference probing via SRS, associating PRB/port allocation of a PDSCH transmission with a corresponding aperiodic SRS transmission.
- Proposal 9 (Sec 2.5): Study how to achieve desired tradeoff between capacity and reliability in 6GR by utilizing many degrees of freedom (DoFs) in spatial/frequency/time domains, supporting transmit diversity schemes and allowing multiplexing of transmit diversity and MIMO spatial multiplexing schemes.
- Proposal 10 (Sec 2.6): In 6GR, consider the unified TCI framework developed in 5G NR as a starting point for 6GR QCL/TCI framework.
- Proposal 11 (Sec 2.6): In 6GR, support QCL/TCI enhancements to enable the use of UL signal (e.g., SRS) as QCL source RS of DL TCI states.
- Proposal 12 (Sec 2.7): For SLS and LLS EVM assumptions for studying PDSCH and RS for PDSCH, support the following BS antenna configurations: 128 TXRUs with 2048 AEs and 512 TXRUs with 2048 AEs.
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