R1-2601833
discussion
On PRACH and RACH procedure
From Nokia
Nokia's prior position on
10.5.1.2
at
RAN1#124
· AI-synthesized, paraphrased
verify sources →
Proposes Zadoff-Chu sequences for 6G PRACH preamble due to CAZAC property and frequency offset robustness, and refrains from sequences lacking good cross-correlation under timing errors. Proposes 4-step RACH as the sole baseline procedure, with Msg3-based identification as baseline and Msg1-based identification restricted to limited use cases. Requires short PRACH formats based on 30 kHz SCS (baseline from NR) and long PRACH formats based on 5 kHz SCS for the 7 GHz frequency range. Proposes supporting PRACH and PUxCH overlapping to enhance RACH capacity through increased time-domain allocations. Proposes studying AI/ML-based Spatial-Domain Beam Prediction (BM-Case1) for coverage extension and PRACH attempt reduction, and Temporal-Domain Beam Prediction (BM-Case2) for improving Msg2/3/4 transmission/reception in 4-step RACH.
Summary
Nokia's contribution to 3GPP RAN1 #124bis on 6G PRACH and RACH procedure provides a comprehensive design framework with 27 proposals and 15 observations. The document covers PRACH sequence selection, preamble formats (especially for 7 GHz), configuration mechanisms (including parameterized formats and reduced table-based configurations), RACH procedure simplifications, and the application of AI/ML-based beam prediction to initial access.
Position
Nokia proposes retaining Zadoff-Chu as the PRACH sequence due to its CAZAC properties and robustness against frequency offset, while opposing sequences that lack these characteristics. For PRACH configuration, they recommend studying parametrized PRACH format configuration against static formats definitions and support reduced table-based PRACH configurations with separately configurable parameters to enable RO clustering and time adaptation. They require a unified design for terrestrial and non-terrestrial networks where feasible, and propose that SBFD RA support should not increase complexity of basic TDD RA operation. For RACH procedure, they propose 4-step RACH as the sole baseline, requiring Msg3-based identification as the default and restricting Msg1-based identification to coverage extension and OD-SIB1 use cases. For AI/ML, they propose reusing Rel-18/19 BM-Case1 (Spatial-Domain Beam Prediction) and BM-Case2 (Temporal-Domain Beam Prediction), prioritizing spatial beam prediction over temporal, and present simulation results showing 64-65% relative reduction in MSG1 failures using spatial beam prediction compared to classical methods.
Key proposals
- Proposal 1 (Sec 2.1): RAN1 should consider Zadoff-Chu sequence for PRACH preamble.
- Proposal 2 (Sec 2.1): RAN1 should refrain from choosing sequences that do not demonstrate CAZAC, good cross correlation even under timing errors and tolerance against frequency errors.
- Proposal 3 (Sec 2.2): If RACH capacity is to be further enhanced in 6GR compared to NR, 6GR to consider a simple design by increasing time domain allocations.
- Proposal 5 (Sec 2.3): Existing PRACH formats defined in NR for FR1 (long formats based on 1.25 and 5 kHz SCSs, short formats based on 15 and 30 kHz SCSs) and FR2 (short formats based on 120 kHz SCS) are baseline for 6GR in corresponding FRs.
- Proposal 7 (Sec 2.3): Support long PRACH formats based on 5 kHz subcarrier spacing at around 7 GHz.
- Proposal 8 (Sec 2.4.1): RAN1 to study further the RACH design under MRSS scenario and to consider separate/dedicated RACH resources as starting point.
- Proposal 10 (Sec 2.4.2): RAN1 to prioritize a unified design for TN and NTN where feasible.
- Proposal 11 (Sec 2.4.3): In 6GR, RA support in SBFD should be designed such that it does not increase the complexity of basic RA operation in TDD.
- Proposal 12 (Sec 2.4.4): Study the merits of parametrized PRACH format configuration against static formats definitions.
- Proposal 13 (Sec 2.4.5): Study support of reduced table-based PRACH configurations considering the deployed configuration in NR.
- Proposal 15 (Sec 2.4.6): Retaining the three SSB-to-RO mappings rules in 6GR: N SSBs to 1 RO, 1 SSB to 1 RO, 1 SSB to M ROs.
- Proposal 19 (Sec 3): Avoid multiple options for RACH procedure in 6GR and support 4-step RACH as baseline.
- Proposal 20 (Sec 3): Consider Msg3 based identification as a baseline. Restrict use Msg1 based identification to limited use cases, focus on coverage extension and on-demand procedures (OD-SIB1).
- Proposal 24 (Sec 4): RAN1 should consider studying AI/ML in initial access and RA procedures by reusing the Rel-18/19 study/work on AIML beam management use cases (BM-Case1: Spatial-Domain DL Tx Beam Prediction and BM-Case2: Temporal-Domain DL Tx Beam Prediction).
- Proposal 25 (Sec 4.1): Study a unified initial-access beam operation that supports coexistence of AI/ML-based and non-AI/ML-based random access with a focus on spatial beam prediction in initial access and its required enablers.