Broadcast Handover Command for LEO-NTN Signaling Overload
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Solution Overview
Problem
In Low Earth Orbit (LEO) satellite-based Non-Terrestrial Networks (NTNs), frequent and periodic handovers lead to heavy signaling overhead due to the predictable mobility of satellites and relatively slow user equipment (UE) movements, resulting in frequent handover messages and potential 'Random Access storms' when multiple UEs attempt to send preambles simultaneously.
Innovation Solution
Implementing a broadcast or groupcast downlink handover command message with a backoff indicator to UEs, allowing them to perform random-access procedures at different times and reducing the need for frequent measurement reports, thereby mitigating signaling overload and improving handover efficiency through Conditional Handover and delayed transmission of handover complete messages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional individual handover procedures are used for each UE, then handover reliability is maintained, but signaling overhead increases heavily due to frequent handovers in LEO-NTN
Solution Approach 1:
The patent merges individual handover procedures into a group-based handover mechanism where multiple UEs are handed over simultaneously using a single handover command message. The source gNB groups UEs experiencing similar mobility conditions and sends one broadcast handover command to multiple UEs, consolidating what would otherwise be numerous individual signaling exchanges into a single operation, thereby dramatically reducing signaling overhead while maintaining handover reliability
Solution Approach 2:
The handover command message is designed with universal applicability to serve multiple UEs simultaneously. The single downlink handover command message can be received and executed by multiple UEs in the group, making the signaling mechanism multi-functional and eliminating the need for individualized handover signaling for each UE
2Productivity
If all UEs transmit HO Complete messages simultaneously after receiving handover command, then handover completion is achieved, but Random Access storms occur due to concurrent preamble transmissions
Solution Approach 1:
The patent introduces periodic or staggered action by implementing random backoff timers for UEs before they transmit HO Complete messages. Instead of simultaneous transmissions, UEs wait for different durations based on their individual backoff values, spreading out the uplink transmissions over time and eliminating the Random Access storm while still achieving rapid handover completion
Solution Approach 2:
The source gNB performs preliminary action by including backoff indicator values in the handover command message before UEs execute the handover. This pre-configured backoff information allows UEs to automatically stagger their HO Complete message transmissions without requiring additional coordination signaling, preventing Random Access storms before they occur
3Measurement precision
If frequent measurement reports are required from UEs for handover decisions, then handover accuracy is improved, but signaling load increases
Solution Approach 1:
The patent enables self-service by allowing the network to estimate UE locations and predict handover targets using GNSS data and satellite orbital information, eliminating the need for UEs to perform and report detailed measurement procedures. The network autonomously determines handover decisions based on predictable satellite mobility patterns and UE location data, significantly reducing measurement-related signaling load while maintaining handover accuracy
Data Source
AI summary
This innovation describes methods for an NR-based, LEO Non-Terrestrial Networks (NTN) to improve handover process by sending broadcast or group-cast downlink handover (HO) Command message to all (or group of) UEs in the coverage area (cell or beam-spot) of the LEO satellite. Subsequently, on receiving the broadcast (or group-cast) HO Command message, all the UEs in the source cell transmit HO Complete message to the source cell of the LEO-NTN. In order to reduce or avoid heavy Random Access (RA), generating from all the UEs, the LEO-NTN can either send Contention Free Random Access (CFRA), or instruct the UE to perform a random backoff before sending the uplink HO Complete message. Moreover, improvements to configure Conditional Handover (CHO) and delaying the transmission of HO Complete message are also provided.