Handover Target Cell Selection Using Expected Service Time in NTN
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Solution Overview
Problem
In Non-Terrestrial Networks (NTNs), the rapid movement of satellites creates challenges for cell selection and handover processes, leading to increased cell reselections and potential service interruptions due to the volatile nature of satellite cells and the lack of fixed correspondence between satellite beams and ground cells.
Innovation Solution
The proposed solution involves using the expected time to be served in a cell as a criterion for selecting a target cell during handover procedures in the RRC_CONNECTED state, allowing network nodes and wireless devices to maximize coverage time with sufficient channel quality, thereby reducing the frequency and number of cell reselections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional handover criteria (signal strength, quality) are used in NTN, then handover decisions can be made based on current channel conditions, but frequent cell reselections occur due to satellite movement and volatile cell availability
Solution Approach 1:
The patent applies preliminary action by calculating and providing the expected time to be served (ETS) for each candidate target cell before the handover decision is made. The network node determines ETS based on satellite ephemeris data, beam coverage areas, and UE position, then includes this information in the handover request. This allows the target cell to be selected not only based on current signal quality but also on how long the service is expected to continue, preventing frequent reselections caused by satellite movement.
2Reliability
If frequent cell reselections are performed to maintain connectivity in NTN, then coverage continuity can be maintained, but control signaling overhead and energy consumption increase
Solution Approach 1:
The network node performs preliminary calculation of the expected time to be served for each candidate cell using satellite ephemeris data and beam coverage information before the handover execution. This advance preparation allows the selection of a target cell that will maintain service for a longer duration, reducing the frequency of future handovers and thereby decreasing energy consumption and control signaling overhead while maintaining coverage continuity.
3Speed
If handover decisions are made rapidly based on current signal conditions in NTN, then responsiveness to channel changes is improved, but handover failures increase due to volatile cell availability
Solution Approach 1:
The network node calculates the expected time to be served for each candidate target cell in advance, using satellite ephemeris data, beam coverage areas, and UE position information. This pre-calculated ETS information is included in the handover request message sent to the target gNB. The target cell is then selected based on both current signal conditions and the predicted service duration, making the handover decision more robust and less prone to failures caused by rapid satellite movement and cell availability changes.
Data Source
AI summary
A method is performed by a wireless device in a connected mode. The method includes obtaining, by the wireless device, information that includes an expected time for the wireless device in the connected mode to be served in at least one cell. The at least one cell includes a serving cell currently serving the wireless device and/or at least one target cell associated with a conditional handover procedure. The wireless device performs the conditional handover procedure based on the information.