Two-Step RACH Downlink Response for Low-Latency UE Access
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Solution Overview
Problem
Existing wireless communication systems face challenges in reducing latency and network inefficiencies in random access procedures, particularly in unlicensed spectra, due to the use of multiple messages and listen-before-talk procedures.
Innovation Solution
A two-step random access procedure is introduced, comprising an access request message from a user equipment (UE) and a downlink response message, which reduces message count and listen-before-talk procedures, allowing multiplexing of responses for multiple UEs and omitting acknowledgement processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a four-step random access procedure is used, then reliability is improved through multiple handshake messages and acknowledgements, but latency increases and network efficiency decreases
Solution Approach 1:
The patent combines multiple random access messages into fewer transmissions. Specifically, it merges the functionality of four separate messages (preamble, random access response, contention resolution, and contention resolution indicator) into a two-step procedure where the UE transmits a first message containing both preamble and payload, and the gNB responds with a second message containing both random access response and contention resolution information, thereby reducing latency while maintaining reliability
Solution Approach 2:
The patent segments the random access procedure into distinct phases handled by different message types. The first message is segmented into preamble portion and payload portion, allowing the gNB to identify the procedure type (two-step or four-step) and process accordingly. This segmentation enables flexible handling of different access scenarios while reducing overall procedure steps
2Productivity
If multiple downlink response messages are sent to multiple UEs, then network capacity is improved, but message complexity and processing overhead increase
Solution Approach 1:
The patent merges multiple downlink response messages into a single multiplexed transmission. The gNB can multiplex multiple second messages (containing random access responses and contention resolution information) into one physical downlink shared channel transmission, allowing multiple UEs to receive their responses simultaneously, thereby improving network capacity while reducing per-message processing complexity
Solution Approach 2:
The second message is designed with multi-functionality to serve multiple purposes: it contains both the random access response (providing timing advance, uplink grant, and preamble identifier) and the contention resolution information (providing contention resolution identifier and uplink grant). This universal message structure handles multiple functions in a single transmission, reducing overall system complexity
3Loss of time
If a two-step random access procedure is implemented, then latency is reduced and network efficiency improves, but compatibility with existing four-step procedures becomes challenging
Solution Approach 1:
The patent implements dynamic procedure selection where the gNB can identify whether a received first message is part of a two-step or four-step random access procedure based on the message structure and preamble identification. The gNB then dynamically applies appropriate processing rules for each procedure type, allowing the system to adapt between different access modes based on UE capability and network conditions, thereby maintaining compatibility while enabling low-latency two-step access
Data Source
AI summary
Methods, systems, and devices for wireless communications are described. The method includes identifying that the user equipment (UE) is to use a two-step random access channel (RACH) procedure that includes a first message (e.g., uplink request message) and a second message (e.g., downlink response). The methods include sending, as a first message of the two-step RACH procedure, the uplink request message to a base station, receiving, from the base station, a physical downlink control channel comprising an indication of resources of a broadcast or multicast physical downlink shared channel on which the downlink response is to be received, and receiving the downlink response from the base station on the broadcast or multicast physical downlink shared channel. The physical downlink shared channel may be configured to provide downlink responses for one or multiple UEs.