2-Step RACH Message A B Procedure for Wireless Access Latency
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Solution Overview
Problem
Current wireless communication systems face challenges in efficiently performing a 2-step or fall-back 4-step random access channel (RACH) procedure, particularly in ensuring reliable data transmission and low latency, especially in applications requiring high mobility and ultra-low latency like autonomous vehicles and smart cities.
Innovation Solution
A method and apparatus for performing a RACH procedure in a wireless communication system, involving the transmission of a message including a physical random access channel (PRACH) preamble and a physical uplink shared channel (PUSCH), with the user equipment (UE) receiving contention resolution information based on the success or failure of the preamble and PUSCH reception, and retransmitting accordingly, to ensure successful access to the base station.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a 2-step RACH procedure is used to reduce latency, then access speed is improved, but transmission reliability may deteriorate due to reduced contention resolution opportunities
Solution Approach 1:
The UE prepares and transmits both the random access preamble and uplink data in Message A simultaneously before receiving any response, enabling faster access without waiting for intermediate acknowledgments. This preliminary action reduces latency while the base station's Message B response provides subsequent reliability confirmation through contention resolution information.
Solution Approach 2:
The base station provides feedback in Message B that includes contention resolution information indicating whether the uplink data transmission was successful. This feedback mechanism allows the UE to determine if retransmission is needed, maintaining reliability while using the faster 2-step procedure. The feedback loop ensures that even though the procedure is abbreviated, transmission reliability is preserved through explicit success/failure indication.
2Reliability
If a 4-step RACH procedure is used to ensure reliable contention resolution, then transmission reliability is improved, but access latency increases
Solution Approach 1:
The patent combines the random access preamble transmission and uplink data transmission into a single Message A, and merges the random access response and contention resolution into Message B. This consolidation reduces the number of separate signaling steps from four to two, cutting access latency while preserving all essential reliability functions through the integrated feedback mechanism in Message B.
Solution Approach 2:
The system dynamically selects between 2-step and 4-step RACH procedures based on current radio conditions and traffic requirements. When conditions favor speed, the 2-step procedure is used with its streamlined Message A/Message B exchange. When reliability is paramount and conditions permit, the system can fall back to the traditional 4-step procedure, providing adaptive optimization of the contradiction between speed and reliability.
3Reliability
If the UE retransmits Message A upon detecting failure, then access reliability is improved, but the complexity of managing retransmission states increases
Solution Approach 1:
The UE autonomously monitors its own transmission status by checking for the presence of its random access preamble identifier in the base station's Message B response. This self-service mechanism allows the UE to determine transmission success or failure without requiring complex external coordination or additional signaling protocols. The UE independently manages its retransmission decisions based on this self-monitored feedback, reducing overall system complexity while maintaining reliability.
Data Source
AI summary
Disclosed is a method by which a terminal performs a random access channel (RACH) procedure in a wireless communication system. In particular, the method includes: transmitting message A including a physical random access channel (PRACH) preamble and a physical uplink shared channel (PUSCH); and receiving message B including contention resolution information, in response to message A, wherein the contention resolution information included in message B can be received on the basis of the terminal having acquired information about a random access preamble identifier (RAPID) for the PRACH preamble, and information providing notification of the successful reception of the PUSCH.