R1-2601792
discussion
Aspects of uplink-based CSI acquisition
From FUTUREWEI
FUTUREWEI's prior position on
10.5.3.2
at
RAN1#124
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Proposes that the 5G NR MIMO/RS/CSI framework serves as the starting point for 6G with enhancements across five distinct areas. For upper midband (UMB) hybrid antenna architectures, proposes studying specific antenna combinations (2048 elements with 256 TXRUs at 7 GHz; 2048 elements with 32 or 128 TXRUs at 15 GHz) and enhanced SRS repetition schemes for fast full CSI acquisition. Proposes a general carrier switching framework that reduces or eliminates switching-back by moving all transmissions to the switching-to carrier, potentially combined with DL carrier switching. For cooperative MIMO, proposes studying DL interference probing based on SRS enhancements, presenting simulation results showing capacity gains of 64.8% in dense urban and 26.9% in urban macro scenarios for 45Mbps XR traffic. Urges RAN1 to discuss high-level MIMO framework principles enabling AI/ML-based functionality from day-1 and to identify AI/ML use cases to facilitate assessment. Proposes QCL/TCI enhancements to enable UL signals such as SRS as QCL source RS for DL TCI states.
Summary
This Futurewei contribution proposes 10 specific proposals and 2 observations focused on uplink-based CSI acquisition for 6G, covering hybrid antenna architectures for upper midband, enhanced SRS-based cooperative MIMO with downlink interference probing, and QCL/TCI enhancements. The document emphasizes using 5G NR designs as a baseline while introducing significant enhancements for fast full CSI acquisition, efficient UL carrier switching, and interference-aware precoding.
Position
Futurewei proposes that hybrid antenna architectures with large antenna element counts and moderate TXRU counts be a key focus for UMB base stations, specifically defining hybrid beamforming as a combination of adjustable analog beamforming (via phase shifters) and digital precoding. They propose studying simultaneous multi-beam codebooks that allow multiple orthogonal beams per OFDM symbol to reduce full CSI acquisition latency—for example, enabling 128 RF chains driving 2048 elements to acquire full CSI in 16 OFDM symbols instead of 2048. They present quantitative performance results for cooperative MIMO via downlink interference probing (BiT), showing capacity improvements from ~8 to ~13.1 UEs in Dense Urban and from ~5.2 to ~6.6 UEs in Urban Macro for 45Mbps XR traffic with [DDDUU] slot configuration, and propose SRS trigger enhancements that associate SRS transmission parameters with PRB/port allocation of corresponding PDSCH. They propose extending 5G SRS carrier-based switching into a general UL switching framework that eliminates switching-back by moving all transmissions to the switching-to carrier, and additionally propose enabling UL signals as QCL source RS for DL TCI states.
Key proposals
- Proposal 1 (Sec 2.1.1): For UMB base stations, hybrid antenna architecture with a large number of antenna elements and moderate number of TXRUs supporting hybrid beamforming should be a key focus area for panels with >512 elements, carriers ≥100 MHz bandwidth, or frequencies at the high upper midband.
- Proposal 2 (Sec 2.1.2): For hybrid BS antennas, study the issue of how to acquire full CSI faster for efficient MU MIMO support.
- Proposal 3 (Sec 2.1.3): Study enhanced SRS transmission (e.g., repetition schemes) for fast full CSI acquisition by hybrid BS antennas.
- Proposal 4 (Sec 2.2): For 6G UL switching, use 5G SRS carrier-based switching as a starting point, incorporate SRS carrier-based switching into a general UL switching framework that reduces/eliminates switching-back, moves all transmissions to the switching-to carrier, and maintains one set of signals/channels over n carriers.
- Proposal 5 (Sec 2.3.1): For 6G SRS, use 5G NR SRS designs as baseline, including SRS carrier-based switching, SRS hopping, 1/2/4/8 ports, SRS physical antenna switching, codebook-based and non-codebook-based SRS, and SRS frequency-domain and time-domain resource allocation.
- Observation 1 (Sec 2.3.2): TDD MIMO performance can be significantly improved by DL interference probing based on flexible SRS triggering with dynamically indicated partial frequency sounding.
- Proposal 6 (Sec 2.3.2): Study cooperative MIMO via DL interference probing based on SRS enhancements to improve 6G system capacity for TDD.
- Observation 2 (Sec 2.3.2.1): DU scenario experiences higher gains than UMa scenario with downlink interference probing via SRS relative to Zero-Forcing precoding, due to shorter inter-cell distance where interference is more dominant than noise.
- Proposal 7 (Sec 2.3.2.2): Study cooperative MIMO for TDD with SRS trigger enhancement to dynamically signal parameters with low overhead, SRS transmission parameters associated with PRB/port allocation of corresponding PDSCH transmission, and SRS beamforming based on DL channel and interference measurement.
- Proposal 8 (Sec 2.3.3): For 6GR UL-based CSI acquisition using SRS, study non-codebook-based precoded SRS, with SRS precoding based on CSI-RS with network indicated CSI-RS precoding matrix.
- Proposal 9 (Sec 2.4): In 6GR, consider the unified TCI framework developed in 5G NR as a starting point for 6GR QCL/TCI framework.
- Proposal 10 (Sec 2.4): In 6GR, support QCL/TCI enhancements to enable the use of UL signal (e.g., SRS) as QCL source RS of DL TCI states.