MT-SDT Paging and Context Retrieval in CU-DU Split gNBs
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Solution Overview
Problem
The challenge in 3GPP LTE and NR systems is supporting Mobile-Terminated Small Data Transmission (MT-SDT) when the Central Unit (CU) and Distributed Unit (DU) are separated, particularly in scenarios where the CU is further split into Control Plane (CP) and User Plane (UP), requiring efficient methods to inform User Equipment (UE) about MT-SDT and manage data transmission without unnecessary state transitions.
Innovation Solution
A method is provided for a receiving gNB to transmit a Radio Access Network (RAN) paging message with an MT-SDT indication, followed by a Retrieve UE Context Request message with a Downlink (DL) Transport Network Layer (TNL) address, enabling the gNB to receive and deliver DL data to the UE, while maintaining the UE in the RRC_INACTIVE state when applicable.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the CU and DU are separated in the NG-RAN, then the network architecture becomes more flexible and scalable, but the complexity of supporting MT-SDT increases due to additional coordination requirements between CU-CP, CU-UP, and DU
Solution Approach 1:
The gNB is segmented into CU-CP, CU-UP, and DU components with distinct responsibilities. CU-CP handles control plane functions including RAN paging message generation and MT-SDT indication, while CU-UP manages user plane data transmission. This segmentation allows flexible network architecture while distributing the complexity of MT-SDT support across specialized components rather than concentrating it in a single entity.
Solution Approach 2:
The RAN paging message acts as an intermediary mechanism that carries the MT-SDT indication from CU-CP to the UE. This intermediary message enables coordination between separated network components by conveying critical information (MT-SDT support status) without requiring direct complex interaction between all components at all times, thus reducing overall system complexity.
2Reliability
If the UE transitions to RRC_CONNECTED state for DL data reception, then complete data delivery is ensured, but unnecessary state transitions increase signaling overhead and delay
Solution Approach 1:
The system performs preliminary action by including the MT-SDT indication in the RAN paging message before the UE needs to receive DL data. This advance notification allows the UE to prepare for small data reception in the inactive state, preventing unnecessary transitions to connected state and eliminating the associated signaling overhead and delay. The preliminary indication ensures that when small data arrives, the UE is already prepared to receive it efficiently.
Solution Approach 2:
The system dynamically adapts the UE state transition based on the MT-SDT indication. When the indication is present, the UE remains in RRC_INACTIVE state for small data reception. When absent or for larger data, the UE transitions to RRC_CONNECTED state. This dynamic behavior optimizes the balance between reliability and time loss by selecting the appropriate state transition path based on real-time conditions.
3Productivity
If the gNB sends RAN paging message with MT-SDT indication, then the UE can quickly receive DL data in inactive state, but the paging message structure becomes more complex
Solution Approach 1:
The MT-SDT indication is merged into the existing RAN paging message structure rather than being transmitted as a separate message. This combining approach conveys additional information (MT-SDT support status) within the already-necessary paging infrastructure, achieving fast DL data delivery while minimizing the increase in message structure complexity. The indication is integrated as an additional field or element within the paging message framework.
Data Source
AI summary
A method and apparatus for support of Mobile-Terminated Small Data Transmission (MT-SDT) considering Central Unit (CU)-Distributed Unit (DU) split is provided. A receiving gNB receives a Radio Access Network (RAN) paging message from a last serving gNB. The RAN paging message includes an MT-SDT indication informing that a MT-SDT is supported. The receiving gNB transmits a Retrieve UE Context Request message to the last serving gNB based on an Inactive Radio Network Temporary Identity (I-RNTI). The Retrieve UE Context Request message includes information about a Downlink (DL) Transport Network Layer (TNL) address associated with the MT-SDT. The receiving gNB receives DL data associated with the MT-SDT from the last serving gNB based on the DL TNL address, and transmits the DL data to the UE.