RACH-less Handover Timing Alignment in NTN
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Solution Overview
Problem
Current 3GPP communication systems face challenges in efficiently managing handover procedures, particularly in non-terrestrial networks (NTN) where long round trip delays result in high handover latency, affecting real-time applications.
Innovation Solution
The proposed solution involves a RACH-less handover procedure where the user equipment (UE) receives a common timing advance value and a timing indication from the source base station, allowing it to determine a UE-specific timing advance value for the target radio cell, thereby aligning uplink transmissions without the need for a random access procedure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a random access procedure (RACH) is performed during handover in non-terrestrial networks, then uplink timing alignment can be achieved, but handover latency increases significantly due to long round trip delays
Solution Approach 1:
The source base station pre-calculates and provides the common timing advance value for the target cell to the UE before handover execution. This preliminary provision of timing information eliminates the need for the UE to perform random access procedures in the target cell to obtain timing advance, thereby resolving the contradiction by achieving timing alignment preparation in advance while avoiding the latency penalty during actual handover
Solution Approach 2:
The common timing advance value acts as an intermediary that bridges the timing synchronization gap between source and target cells. By providing this pre-calculated timing advance value from the source base station, the system avoids the need for direct random access-based timing acquisition in the target cell, thus reducing handover latency while maintaining timing alignment accuracy
2Reliability
If a random access procedure is performed during handover, then uplink synchronization can be established, but data interruption increases due to RACH delay
Solution Approach 1:
The common timing advance value is provided to the UE in advance during the handover preparation phase, enabling the UE to maintain uplink synchronization continuity. This preliminary provision eliminates the need to pause data transmission for random access procedures in the target cell, thus maintaining both synchronization reliability and data transmission productivity
Solution Approach 2:
By providing the common timing advance value beforehand, the system enables continuous uplink transmission without interruption for random access procedures. The UE can seamlessly continue data transmission while maintaining synchronization with the target base station, thus preserving the continuity of useful action during handover
3Measurement precision
If timing advance information is obtained through random access in the target cell, then UE-specific timing can be determined, but handover complexity increases
Solution Approach 1:
The handover procedure extracts and separates the timing advance information provision from the random access procedure. By providing the common timing advance value through dedicated signaling in advance, the system removes the need for the UE to perform random access procedures in the target cell, thereby simplifying the handover procedure while still achieving accurate UE-specific timing determination
Solution Approach 2:
The common timing advance value serves as an intermediary that provides timing information without requiring the complex random access interaction. This intermediary approach simplifies the handover procedure by replacing the multi-step random access sequence with a direct timing advance provision, reducing handover complexity while maintaining timing precision
Data Source
AI summary
The present disclosure relates to a user equipment (UE) that comprises a receiver of the UE receives, from a source base station of a source radio cell, a common timing advance value for a target radio cell. The UE is connected to the source radio cell and is involved in a handover procedure to hand over the UE from the source radio cell to the target radio cell. Further, the common timing advance value is received from the source base station in a first message of the handover procedure, which also comprises a timing indication for transmitting a second message from the UE to the target base station. Then, a processor of the UE determines a first uplink timing of uplink transmissions to the target base station with respect to downlink transmissions from the target base station, based on the received common timing advance value and the timing indication. A transmitter of the UE transmits a second message of the handover procedure to the target base station based on the determined uplink timing. The processor determines a UE-specific timing advance value, specific to the UE and the target radio cell, to be used by the UE for performing uplink transmissions in the target radio cell.