NR-U RACH Msg3 Retransmission Using Multiple Uplink Opportunities
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Solution Overview
Problem
In New Radio in Unlicensed Spectrum (NR-U), the handover of User Equipment (UE) from a source cell to a target cell faces delays and failures in the Random Access Channel (RACH) procedure due to noise, interference, and Listen-Before-Talk (LBT) issues, particularly in the transmission of Msg3, which existing HARQ retransmission methods do not effectively address.
Innovation Solution
Implementing multiple transmission opportunities for RACH messages (Msg1, Msg2, and Msg3) and extending the monitoring window for Msg2 responses, allowing flexible UE behavior to handle LBT failures and interference, with mechanisms to ensure consistent transport block parameters and efficient HARQ combination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the UE follows the standard RACH procedure with single Msg3 transmission opportunity, then the procedure is simpler to implement, but the latency increases due to failures in transmitting RACH messages and inability to perform retransmissions
Solution Approach 1:
The base station pre-configures multiple Msg3 transmission opportunities and associated uplink grants in the random access response message, allowing the UE to have multiple chances to transmit Msg3 without requiring complex real-time decisions or additional signaling for retransmissions
Solution Approach 2:
The UE dynamically selects which Msg3 transmission opportunity to use based on channel conditions and LBT outcomes, adapting its transmission strategy to improve success probability while maintaining procedural simplicity
2Reliability
If the UE monitors for multiple Msg2 transmissions and has multiple Msg3 opportunities, then the reliability of data transmission improves, but the device complexity increases
Solution Approach 1:
The random access procedure is segmented into multiple independent Msg3 transmission opportunities, each with its own uplink grant and timing, allowing the UE to attempt transmission across different time instances to overcome channel access failures
Solution Approach 2:
The base station provides feedback through the random access response message indicating multiple valid Msg3 transmission opportunities, enabling the UE to reliably determine when and how to transmit without requiring complex autonomous decision-making
3Productivity
If the random access response window is extended to accommodate multiple Msg3 transmissions, then the productivity of the RACH procedure improves, but the loss of time increases due to longer monitoring period
Solution Approach 1:
Multiple Msg3 transmission opportunities are distributed periodically within the random access response window, allowing the UE to attempt transmissions at regular intervals while the base station can efficiently process and respond to successful transmissions
Solution Approach 2:
The system changes the time parameter by extending the random access response window duration to accommodate multiple transmission opportunities, but compensates by providing multiple uplink grants that enable productive use of the extended time period
Data Source
AI summary
Apparatuses, methods, and systems are disclosed for efficient RACH behavior. One method performed by a UE includes transmitting a first PRACH preamble to a base unit to start a RACH procedure and receiving a RAR (Msg2) from the base unit during a random access response window. The method includes transmitting a second PRACH preamble in response to being unable to transmit a RACH Msg3 within an indicated transmission opportunity. The UE may obtain multiple transmission opportunities for transmitting a RACH message such as RACH Msg1 or RACH Msg3 or monitor for multiple RACH Msg2 where transmission opportunity data are conveyed via a RACH Order, an RAR, or DCI for the RACH message. An apparatus or system may perform steps of the method.