Two-Step Random Access MAC Procedure for 5G NR Latency
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Solution Overview
Problem
The 5G NR system faces challenges in improving radio access efficiency due to increasing demand, particularly in accommodating various use cases like eMBB, mMTC, and URLLC, which require enhanced data rate, latency, and reliability.
Innovation Solution
A two-step random access procedure is implemented in the MAC Layer, involving a message A (MSGA) and a message B (MSGB), where the UE transmits MSGA, monitors for MSGB-RNTI within a specific window, receives a success random access response (RAR) containing HARQ Feedback Timing Indicator and PUCCH Resource Indicator, and performs HARQ feedback on an uplink resource indicated by these indicators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a traditional 4-step random access procedure is used, then the random access process is relatively simple to implement, but the access latency increases and efficiency decreases
Solution Approach 1:
The patent segments the traditional 4-step random access procedure into a compressed 2-step procedure by merging Message 2 (Random Access Response) and Message 3 (RRC Connection Request) into a single MSGB message. This segmentation reduces the number of interaction steps between UE and gNB, directly decreasing access latency and improving random access efficiency for 5G NR systems.
Solution Approach 2:
The patent merges multiple message exchanges into consolidated messages: MSGA combines the random access preamble and uplink data, while MSGB combines the random access response and contention resolution. This merging reduces the total number of message transmissions required, thereby reducing access time and improving overall random access productivity.
2Productivity
If resource allocation is optimized for high data rate, then data rate improves, but system complexity increases
Solution Approach 1:
The patent introduces new parameters for the compressed random access procedure including msgA-FirstUplinkMessage for uplink grant configuration, msgB-ResponseWindow for response timing, and pucch-ResourcesForFallbackRAR for fallback indication. These parameter changes enable optimized resource allocation for high data rate while maintaining manageable system complexity through structured configuration.
3Ease of operation
If the random access procedure is simplified, then implementation ease improves, but reliability and adaptability for diverse use cases deteriorate
Solution Approach 1:
The patent introduces dynamic fallback mechanisms where the gNB can indicate fallback to the traditional 4-step procedure via fallback indication in MSGB, and the UE can transition between 2-step and 4-step procedures based on radio conditions and network configuration. This dynamic adaptability ensures reliability across diverse use cases including eMBB, mMTC, and URLLC while maintaining implementation simplicity.
Solution Approach 2:
The patent creates a universal random access framework that supports both 2-step compressed procedure and traditional 4-step procedure within the same system. The gNB can configure and indicate which procedure to use based on service requirements, making the system universally applicable to various 5G use cases with different reliability and latency requirements.
Data Source
AI summary
A method performed by a UE for implementing a random access procedure is provided. The method transmits a MSGA, monitoring an MSGB-RNTI within an MSGB time window starting from an earliest symbol of an earliest PDCCH occasion after the MSGA transmission. The method receives the MSGB in a first slot. The MSGB includes a success RAR that contains a HARQ Feedback Timing Indicator, a Physical Uplink Control Channel (PUCCH) Resource Indicator, and a UE Contention Resolution Identity. The method determines, by a MAC entity of the UE, to instruct a lower layer to transmit a HARQ feedback in a second slot in response to the reception of the success RAR. The method delivers, by the MAC entity, the HARQ Feedback Timing Indicator and the PUCCH Resource Indicator to the lower layer, and performs, by the lower layer, a HARQ feedback delivery on an uplink (UL) resource.