R1-2600145
discussion
PRACH and RACH procedure for 6GR
From Huawei
Summary
This Huawei/HiSilicon contribution discusses 6G PRACH and RACH procedure enhancements, making 12 observations and 12 proposals across five main areas: PRACH capacity, resilience to large Doppler, coverage enhancement, procedure enhancement, and evaluation methodology. The document argues for fundamental redesigns beyond 5G NR limitations, including new preamble sequences for capacity, handling large Doppler for TN/NTN harmonization, early MIMO operation, and UE-dedicated preambles.
Position
Huawei proposes studying fundamentally new preamble sequences to break the Zadoff-Chu capacity limit of 64 preambles, citing a 72× reduction in sequence capacity for short formats and a 4× further reduction due to 6G Doppler requirements. They present a mathematical case against Zadoff-Chu sequences for combined large cell and high-speed scenarios, arguing that the product of maximum round-trip time and frequency offset exceeding 1/2 makes detection impossible at 7 GHz (product = 1.38). They require introducing early MIMO operation during initial access to acquire CSI and apply precoded transmission, specifically identifying Msg3 as the bottleneck channel. They propose UE-dedicated PRACH preambles for contention-free fast transition from sub-state to connected mode and for association with grant-free data transmission, requiring a significantly larger preamble sequence pool. They support I/Q-offset DFT-s-OFDM SE factor 1/2 (pi/2 BPSK) for Msg3, claiming up to 2.8 dB MPR gain over QPSK. They propose studying SBFD PRACH, clustering of RACH occasions with common signals for 23%/17% NES gain (CAT1/CAT2+ BSs), and flexible spectrum utilization with anchor/non-anchor CCs for initial access.
Key proposals
- Proposal 1 (Sec 2): Study new preamble sequence for PRACH capacity enhancement.
- Proposal 2 (Sec 3): Study PRACH resilient to large Doppler (e.g., two-way Doppler corresponding to 8 ppm residual CFO) targeting harmonized design for TN and NTN.
- Proposal 3 (Sec 4): Introduce early MIMO operation in the initial access procedure to acquire CSI earlier and apply precoded MIMO transmission for random access messaging, thereby enhancing the coverage performance of RA messages in ~7 GHz.
- Proposal 4 (Sec 4.1): 6GR needs to investigate narrow-beam RACH other than simple beam sweeping, to avoid excessive beam training overhead.
- Proposal 5 (Sec 4.2): Low-overhead methods for determining the repetition numbers of multiple Msg types should be studied.
- Proposal 6 (Sec 4.3): If an uplink low PAPR waveform is introduced in waveform agenda, it also applies to Msg3.
- Proposal 7 (Sec 5.1): Study UE dedicated PRACH preamble for fast transition from sub-state to connected mode.
- Proposal 8 (Sec 5.2): Study UE dedicated PRACH preamble association with contention-based grant-free data transmission.
- Proposal 9 (Sec 5.3): Support clustering RO with the rest of common signals, so as not to interrupt BS sleep for RO monitoring.
- Proposal 10 (Sec 5.4.1): Study efficient initial access mechanism for flexible DL/UL pairing and fragmented spectrum by taking into account access latency, system overhead (sync/SIB), NW/UE energy saving, and RACH/paging capacity.
- Proposal 11 (Sec 5.4.2): Study SBFD PRACH and related procedures taking SBFD into consideration.
- Proposal 12 (Sec 6): For 6GR PRACH evaluation assumption, the following parameters can be enhanced to meet 6G scenarios and requirements: UE speed: 500 km/h, 1000 km/h; Number of UEs: 4, 8 UEs per RACH occasion; Number of preamble sequences: 256, 512, 1024 preamble sequences per RACH occasion.