Simultaneous Connectivity Handover Using SN and HFN State Transfer
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Solution Overview
Problem
Existing solutions for simultaneous connectivity based handover in 3GPP Release-16 LTE and NR mobility enhancement are unclear on how to transfer sequence number (SN) status information and ensure SN continuity during the handover, particularly for RLC acknowledged mode (AM) and RLC unacknowledged mode (UM).
Innovation Solution
A method for determining and transmitting SN and hyper frame number (HFN) information between network devices during handover, ensuring continuous data transmission by maintaining two active protocol stacks and using specific formats for PDU and SDU handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If simultaneous connectivity based handover is implemented to reduce data interruption, then data transmission continuity is improved, but sequence number assignment and transmission complexity increases
Solution Approach 1:
The patent segments the sequence number information into two distinct parts: M (most significant bits) and L (least significant bits). This segmentation allows the source network device to transfer only the necessary M bits to the target network device, while the L bits continue to be managed locally. This division reduces the complexity of sequence number assignment during handover while maintaining data transmission continuity, as the target device can reconstruct complete sequence numbers using the transferred M bits and its own local counters.
2Reliability
If sequence number status information is transferred during handover, then data transmission reliability is improved, but information transfer overhead increases
Solution Approach 1:
The patent extracts only the essential most significant bits (M bits) of the sequence number from the complete sequence number status information. By transferring only these critical M bits rather than the entire sequence number state, the patent reduces information transfer overhead while still ensuring sequence number continuity. The target network device uses these extracted M bits combined with its local least significant bits to reconstruct the complete sequence number, minimizing the information that needs to be transferred.
3Reliability
If two active protocol stacks are maintained during handover, then data transmission seamless is improved, but processing complexity increases
Solution Approach 1:
The patent implements preliminary action by having the source network device prepare and transfer the sequence number most significant bits (M bits) to the target network device before the handover is complete. This advance preparation ensures that when the terminal switches to the target protocol stack, the sequence number state is already synchronized, allowing seamless data transmission without requiring complex real-time coordination between multiple active protocol stacks.
Data Source
AI summary
Embodiments of the present disclosure relate to methods, devices and computer readable media for data transmission during simultaneous connectivity based handover. A method of communication comprises in response to a second connection being established between a second network device and a terminal device having a first connection with a first network device, determining, at the first network device, a first set of SDUs to be transmitted by the second network device to the terminal device; determining first information regarding a SN assigned to each of the first set of SDUs and a HFN associated with the SN; and transmitting the first set of SDUs and the first information to the second network device. The method further comprises receiving, at the second network device, the first set of SDUs and first information; and transmitting the first set of SDUs to the terminal device based on the first information. The method further comprises receiving, at the terminal device, a first set of PDUs corresponding to the first set of SDUs from the second network device; receiving a second set of PDUs corresponding to the second set of SDUs from the first network device; determining the first information; and determining an order of the first and second sets of PDUs based on the first information. Embodiments of the present disclosure can facilitate correct data transmission during a simultaneous connectivity based handover and improve data transmission efficiency.