Two-Step RACH Procedure for NR-U LBT Latency
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Solution Overview
Problem
The existing random access channel (RACH) procedures in New Radio Unlicensed (NR-U) communications face challenges with high latency and frequent listen-before-talk (LBT) failures, leading to increased interference and delay in unlicensed channels.
Innovation Solution
The implementation of a two-step RACH procedure with LBT parameter selection based on PUSCH payload, allowing PUSCH transmission even after PRACH LBT failure, and introducing symbol level gaps between RACH occasions to mitigate LBT blocking, along with power ramping adjustments based on channel success.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional multi-step RACH procedures are used in NR-U, then LBT compliance is maintained, but procedure latency increases and access reliability decreases due to frequent LBT failures
Solution Approach 1:
The patent segments the traditional multi-step RACH procedure into a simplified two-step process (MsgA containing PRACH preamble and PUSCH payload, MsgB containing RAR and contention resolution). This segmentation reduces the number of LBT procedures required from multiple steps to just one or two, directly reducing latency while maintaining LBT compliance through selective application at critical transmission points.
Solution Approach 2:
The patent performs preliminary LBT assessment and channel selection before initiating the RACH procedure. The UE determines LBT parameters and performs channel access checks in advance, preparing transmission opportunities beforehand. This preliminary action reduces waiting time during the actual RACH procedure and improves access reliability by selecting favorable channels before transmission attempts.
2Ease of manufacture
If LBT procedures are performed at every RACH transmission step, then spectrum access compliance is ensured, but transmission complexity and failure probability increase
Solution Approach 1:
The patent extracts and consolidates LBT procedures from every transmission step, retaining them only at critical points (MsgA transmission and optionally MsgB transmission). By removing redundant LBT checks from intermediate steps, the implementation becomes simpler while compliance is maintained at essential transmission boundaries. This extraction reduces procedural complexity without sacrificing regulatory compliance.
Solution Approach 2:
The patent creates a universal LBT parameter set that can be applied across different RACH message types and transmission scenarios. The same LBT parameter determination mechanism serves multiple purposes: initial channel access, retransmission timing, and power ramping decisions. This multi-functionality simplifies implementation by using a single consistent approach rather than separate LBT procedures for each message type.
3Reliability
If power ramping is applied after every LBT failure, then transmission reliability improves, but interference to other users increases and energy consumption rises
Solution Approach 1:
The patent implements dynamic power ramping where the power increase decision adapts based on the specific failure scenario. Instead of uniform power ramping after every failure, the system dynamically adjusts power only when necessary (e.g., after MsgA failure but before MsgB transmission). This dynamic approach maintains reliability by applying power compensation selectively while reducing unnecessary interference and energy consumption from redundant power increases.
Solution Approach 2:
The patent incorporates feedback mechanisms where the UE monitors transmission outcomes (ACK/NACK, contention resolution success) and adjusts subsequent transmission parameters accordingly. Power ramping decisions are based on feedback from previous transmission attempts rather than automatic escalation. This feedback-driven approach ensures power ramping occurs only when truly needed, balancing reliability improvement with interference and energy management.
Data Source
AI summary
The present disclosure relates to wireless communications, and more particularly to enhancements to a random access channel (RACH) procedure for use in New Radio (e.g., 5G) communications in the unlicensed spectrum (also referred to as NR-U). The present disclosure provides a user equipment (UE) or network entity that determines an available RACH occasion (RO) within a slot for a message transmission according to one of a two-step RACH procedure or a four-step RACH procedure, wherein the available RO is adjacent to at least one unavailable RO within the slot. The UE or network entity may further perform, during the RO, the message transmission according to one of the two-step RACH procedure or the four-step RACH procedure.