Centralized Unit PDCP Retransmission for Lossless Handover
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Solution Overview
Problem
In 5G NR RAN architectures, there is a challenge in ensuring seamless user equipment (UE) mobility across different distributed units (DUs) without packet loss, particularly when UE moves from one DU to another, as existing technologies struggle to transparently and losslessly retransmit unacknowledged Protocol Data Units (PDUs).
Innovation Solution
Implementing a centralized retransmission mechanism where the centralized unit (CU) stores and retransmits downlink PDCP PDUs to the target DU, ensuring that unacknowledged PDUs are retransmitted without impacting the current RLC procedure, and utilizing sequence numbers to maintain RLC layer state continuity during handovers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a centralized retransmission mechanism is implemented at the CU, then packet loss during handover is reduced, but the complexity of the network architecture increases
Solution Approach 1:
The CU acts as an intermediary between the DU and the core network, centralizing the retransmission function at the CU level. This mediator approach allows the DU to focus on radio access functions while the CU handles packet retransmission, resolving the contradiction by improving reliability through centralized control while managing architecture complexity through functional separation.
Solution Approach 2:
The gNB is segmented into two functional units: the DU (Distributed Unit) handling lower layer functions and the CU (Centralized Unit) handling upper layer functions including retransmission. This segmentation allows independent optimization of each unit, improving overall reliability while distributing complexity across modular components rather than concentrating it in a single monolithic structure.
2Reliability
If PDCP PDUs are buffered at the CU for retransmission, then handover success rate is improved, but the buffering resource requirements increase
Solution Approach 1:
The CU performs preliminary actions by buffering PDCP PDUs before handover occurs. This advance preparation ensures that when handover happens, the necessary packets are already available for retransmission to the target DU, improving handover success rate while allowing controlled buffering resource allocation based on predicted handover scenarios.
Solution Approach 2:
The system dynamically adjusts buffering parameters such as buffer size, retention time, and retransmission thresholds based on network conditions, UE mobility patterns, and handover predictions. This parameter optimization allows the system to maintain high handover success rates while minimizing the quantity of buffering resources required through adaptive resource management.
Data Source
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Figure 3A~3B
AI summary
A centralized RAN architecture that includes lower layer transmission nodes (referred to as distributed units (DUs) herein) that connect to an upper layer RAN node (referred to as a centralized unit (CU) herein). A centralized RAN architecture may include a functional split, between the CU and DU, corresponding to the Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) layers. In one embodiment, a CU may store copies of downlink PDCP PDUs that are transmitted to a first DU. The CU may retransmit, to a second DU, those of the PDCP PDUs which were previously transmitted to the source DU but have not been acknowledged as successfully delivered by the source DU.