UE Random Access Handling with Multi-Preamble TA Responses
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Solution Overview
Problem
Existing random access procedures in Non-Terrestrial Networks (NTNs) face challenges due to large propagation delays, making it difficult for user equipment (UE) to establish a successful connection with communication satellites.
Innovation Solution
A network node refrains from immediately responding to a first RA preamble, instead transmitting a Random Access Response (RAR) with estimated Timing Advance (TA) values for multiple possible preambles, and subsequently checks for matching TA values in retransmitted preambles, allowing for efficient random access handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the network node transmits RAR immediately after receiving RA preamble in traditional random access procedure, then the random access response time is short, but the procedure fails in NTN due to large propagation delays
Solution Approach 1:
The network node transmits RAR messages in advance before actually receiving the RA preambles from the UE. The RAR messages contain TA commands based on estimated timing values. This preliminary action allows the UE to have the RAR ready in its buffer when it eventually transmits the preamble, resolving the contradiction between quick response and reliability in NTN environments with large propagation delays.
2Measurement precision
If the network node responds to each RA preamble individually, then the timing alignment is precise, but the complexity increases when handling retransmitted preambles
Solution Approach 1:
The network node transmits one RAR message that can serve multiple purposes: it contains TA commands that can be used by the UE regardless of which specific preamble the UE transmits. The RAR is designed to be universally applicable to multiple preamble scenarios, including initial preambles and retransmitted preambles, thereby reducing complexity while maintaining timing precision.
Solution Approach 2:
The network node creates RAR messages based on estimated timing values and transmits them in advance. These RAR messages are essentially copies or predictions of what the timing alignment commands should be. When the UE transmits the actual preamble (whether initial or retransmitted), it uses these pre-prepared RAR copies, simplifying the network node's handling of retransmissions.
3Reliability
If the RAR window is configured for long duration to accommodate NTN delays, then the UE can receive RAR successfully, but the resource occupation and latency increase
Solution Approach 1:
By transmitting RAR messages in advance before the UE actually transmits the preamble, the network node makes the RAR available early in the RAR window. This preliminary action allows the UE to quickly acquire the necessary timing alignment information without having to wait through the entire RAR window, thereby reducing the effective latency while maintaining reliability for NTN scenarios.
Data Source
AI summary
There is provided mechanisms for random access handling of a UE. A method is performed by a network node. The method comprises receiving, from the UE during a first RAO, a first RA preamble, whilst refraining from responding to the first RA preamble. The method comprises transmitting, towards the UE and without the network node first having received any retransmitted RA preamble from the UE, one RAR for each of N possible RA preambles, where each RAR comprises a TA command corresponding to a TA value estimated for the first RA preamble. The method comprises receiving, from the UE during a further RAO, a retransmitted RA preamble. The method comprises determining whether the TA value for the retransmitted RA preamble matches the TA value for the first RA preamble or not.