R1-2600195
discussion
Discussion on uplink based CSI acquisition
From OPPO
Summary
This OPPO contribution proposes a streamlined 6G SRS framework based on NR Rel-15, arguing many post-Rel-15 features lack practical value. The document contains 21 proposals and 4 observations covering SRS framework, detailed SRS design, and uplink codebook design.
Position
OPPO proposes basing the 6G SRS framework primarily on NR Rel-15, asserting that many features introduced after Rel-15 are not valuable for 6GR and should be pruned. They argue for a unified SRS configuration eliminating the distinction between codebook-based and non-codebook-based UL CSI acquisition, using a single SRS resource set with TPMI indication applied to both schemes. The company opposes using MAC CE to update RRC-configured SRS parameters such as spatial relation or pathloss reference, citing marginal latency gains and unnecessary inter-group complexity between RAN1 and RAN2. For uplink codebooks, OPPO presents technical case that 3Tx fully-coherent codebook provides significant precoding gain over non-coherent codebook, and proposes studying downloadable or mixed codebooks as candidate solutions for diverse UE antenna locations. They argue frequency selective precoding cannot bring gain for 2/4 ports transmission or large subband size, limiting its applicability to larger than 4 ports with CP-OFDM only.
Key proposals
- Proposal 1 (Sec 2.1.1): Consider unified SRS configuration for CB and NCB based transmission if both are supported for 6GR, i.e. single SRS resource set for UL CSI acquisition of S-TRP regardless of the transmission schemes.
- Proposal 4 (Sec 2.1.2): Consider the guard period between SRS resources/resource sets and between SRS and PUSCH in 6G.
- Proposal 6 (Sec 2.2.1): Support the following functionalities for SRS pattern design in 6GR day 1: Flexible SRS symbol location, Comb+CS based port and resource multiplexing, Intra-slot frequency hopping, Inter-slot frequency hopping, Intra-slot repetition. FFS for day 1: RPFS, Cyclic Shift (CS) hopping, Comb Offset hopping and cross-Slot SRS.
- Proposal 8 (Sec 2.2.2): The SRS sequence in Rel-15 based on ZC sequences can be reused for 6GR.
- Proposal 10 (Sec 2.2.3): Dynamic SRS triggering offset can be deprioritized in 6GR day 1.
- Proposal 11 (Sec 2.2.3): Avoid to use MAC CE to update the RRC configuration for SRS (e.g. spatial relation or pathloss reference) in 6G.
- Proposal 12 (Sec 2.2.4): The SRS power control procedure in Rel-15 can be the baseline for 6GR.
- Proposal 13 (Sec 2.2.4): Support two separate closed-loop power control adjustment states in addition to PUSCH power control adjustment states for SRS in 6G day 1.
- Proposal 14 (Sec 2.2.4): Support PHR report based on reference SRS transmission and actual SRS transmission as in NR.
- Proposal 15 (Sec 2.3.1): Support unified/fixed uplink codebook for 1,2,3,4,[8] ports in 6G day 1.
- Proposal 17 (Sec 2.3.1): At least fully-coherent and non-coherent CB should be supported for each port number.
- Proposal 19 (Sec 2.3.1): Further study downloadable UL codebook or mixed codebook in 6G.
- Proposal 20 (Sec 2.3.2): Consider frequency selective precoding only for larger than 4 ports and CP-OFDM, taking TPMI overhead, channel estimation performance and PAPR into count.
- Proposal 21 (Sec 2.3.3): Consider TPMI overhead reduction for uplink especially for large number of antenna ports or frequency selective precoding.