R1-2601858
discussion
Discussion on PRACH and RACH procedure for 6GR
From NEC
Summary
NEC presents 19 proposals across 8 technical sections covering PRACH and RACH procedure design for 6G radio, using NR as baseline while proposing studies for increased capacity, simplified signaling, energy efficiency, coverage enhancement, diverse device support, and NTN polarization.
Position
NEC proposes Zadoff-Chu (ZC) based sequences as the baseline for 6GR PRACH preamble and considers both long and short NR PRACH preamble formats as starting points. They propose studying configuration of multiple PRACH formats per cell/carrier and increasing PRACH capacity from 64 to 128 candidate sequences. NEC questions whether PRACH configuration should remain under the BWP framework and proposes studying a simplified SSB-RO mapping mechanism or separate PRACH resource allocation per SSB. They require 4-step RACH procedure as baseline and prioritized, while proposing studies on reducing Msg2 payload size and introducing group common Msg4 to handle increased preamble numbers. For energy efficiency, NEC proposes a reference time (SSB or SIB1 transmission time) for clustered PRACH transmission with two options: relative configuration or default availability of only the first resource set(s). They propose studying PRACH resource adaptation with granularities including association period, PRACH periodicity, and SSB index. For coverage, NEC proposes native support of Msg1/2/3/4/5 repetition with joint repetition scheme to avoid resource fragmentation, and separate RSRP threshold configuration per SSB index for PRACH repetition.
Key proposals
- Proposal 4 (Sec 2.1.1): RAN1 may need to further study how to increase the capacity of PRACH for 6GR, the following aspects can be considered: Increasing the number of candidates PRACH sequences, e.g., from 64 to 128; Limiting the number of features which requires early indication by PRACH preamble transmission; Improved PRACH multiplexing mechanism compared to NR.
- Proposal 6 (Sec 2.1.2): RAN1 may need to study whether PRACH configuration is still under BWP framework or new configuration mechanism/reference for PRACH resources should be introduced for 6GR.
- Proposal 7 (Sec 2.1.3): Simplified SSB-RO mapping mechanism compared to 5G NR or a new mechanism for allocating separate PRACH resources to each SSB can be studied in 6GR.
- Proposal 9 (Sec 2.1.4): For RACH procedure, 4-step RACH procedure can be baseline and prioritized to be supported in 6GR.
- Proposal 10 (Sec 2.1.4): The following aspects can be studied for Msg2/3/4 in terms of 6GR RACH procedure: How to reduce the Msg2 payload size regarding the number of preambles in one RO may be increased compared to NR; A more efficient Msg3 resource allocation mechanism scheduled by UL grant in Msg2; A more efficient Msg4 transmission scheme, e.g., group common Msg4 for multiple UEs.
- Proposal 12 (Sec 2.2.1): To support the clustered PRACH transmission with other common signaling, a reference time could be defined, e.g., the transmission time of SSB or SIB1 and the following two options can be considered: Option 1: PRACH time domain resource configuration is implemented based on the reference time in a relative way; Option 2: After the PRACH configuration, only the first set(s) of resources right after the reference time are regarded as available by default.
- Proposal 14 (Sec 2.2.1): RAN1 can study PRACH resource adaptation mechanism with the following candidate granularities: Enable or disable one of the PRACH resource sets when there are multiple resource sets are configured; Enable or disable a subset of resources within one single PRACH resource set: The granularities could be association period (in association pattern period), PRACH periodicity, SSB index or PRACH mask indicating the RO(s) within the RO set corresponding to a single SSB index, etc.
- Proposal 15 (Sec 2.2.2): RAN1 can study the native support of Msg1/2/3/4/5 repetition during random access procedure.
- Proposal 17 (Sec 2.2.2): For PRACH repetition mechanism, some parameters can be separately configured for each SSB index, e.g., RSRP threshold, for 6GR.
- Proposal 18 (Sec 2.2.3): To support the coexistence of diverse device type in 6GR, RAN1 can further study the following aspects during random access procedure: Whether/how to share the resources for Msg1 among different device types; Whether common or sperate Msg2 should be transmitted for different device types; The huge number of LPWA devices in 6GR and the performance of EMBB UE needs to be considered in priority.
- Proposal 19 (Sec 2.2.4): Study the utilization of LHCP and RHCP as a resource dimension for the RACH resource for 6GR NTN.