MT-SDT Paging Handover for Inactive UE Cell Mobility

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Solution Overview

Problem

The challenge in implementing Mobile Terminated Small Data Transmission (MT-SDT) for NR in RRC_INACTIVE state is that the receiving gNB cannot trigger the procedure since it does not receive the DL data at the time of paging, especially when the UE has moved to a different cell or NG-RAN node.

Innovation Solution

Enhancements to existing signaling protocols (NGAP, XnAP, and F1AP) are introduced to enable the transfer of buffered mobile-terminated small data from the Core Network to the RAN node, allowing the RAN node to initiate the MT-SDT procedure by triggering RAN Paging for UEs with pending DL data.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the receiving gNB triggers MT-SDT procedure, then DL data can be delivered to UE in RRC_INACTIVE state, but the procedure cannot be triggered when UE has moved to a different cell or NG-RAN node

Engineering Contradiction:

ImproveMT-SDT procedure initiation reliabilityVSAvoidAbility to handle UE mobility to different cells

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent introduces the Core Network (CN) as an intermediary entity that receives DL data and manages the forwarding to the appropriate RAN node. When UE moves to a different cell, the CN acts as a mediator that has knowledge of the UE's current location and can trigger the MT-SDT procedure through the correct RAN node, resolving the limitation where the receiving gNB cannot trigger the procedure for mobile UEs

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If signaling overhead is reduced for small data transmission, then transmission efficiency improves, but the complexity of signaling protocols increases

Engineering Contradiction:

ImproveSmall data transmission efficiencyVSAvoidSignaling protocol complexity

Core Design Contradiction:

ProductivityVSDevice complexity

Solution Approach 1:

The patent applies preliminary action by pre-configuring SDT parameters and resources in the RRCRelease message before the UE enters RRC_INACTIVE state. This allows the UE to have all necessary information stored locally, enabling efficient small data transmission without requiring complex real-time signaling exchanges, thus improving transmission efficiency while managing protocol complexity

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent segments the signaling process into distinct phases: initial configuration during RRC connection release, and simplified data transmission during RRC_INACTIVE state. By separating the complex configuration phase from the simple data transmission phase, the system achieves high transmission efficiency for small data while confining protocol complexity to the initial setup stage

Inventive Principle:

Principle #1Segmentation

Data Source

PatentUS20250386391A1Mobile Terminated Small Data Transmission

Publication Date: 2025.12.18 TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)

AI summary

In an example, a method performed by a network node for forwarding data to a RAN node for transmitting a Mobile Terminated Small Data Transmission (MT-SDT) to a User Equipment (UE) is disclosed. The method comprises determining data to be sent to a User Equipment (UE) in a Mobile Terminated Small Data Transmission (MT-SDT), and sending, to a first Radio Access Network (RAN) node associated with a last serving cell for the UE, a paging request and an indication that the data is pending. The method also comprises determining that the UE has responded to the paging request, and forwarding the data to a second RAN node for transmitting the MT-SDT to the UE, wherein the second RAN node is associated with a cell that received a response to the paging request from the UE.