Called Terminal Paging Fallback for 5G SA IMS Call Failures

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Solution Overview

Problem

In a 5th generation (5G) standalone (SA) network, failed incoming calls occur due to the absence of a circuit switching (CS) domain, causing terminal devices to fail in fallback to the CS domain for receiving calls when an Internet Protocol (IP) multimedia subsystem (IMS) link is abnormal, and network faults prevent normal voice bearer processes.

Innovation Solution

A paging method where the called terminal device initiates a timer upon establishing a data radio bearer (DRB) for IMS data, and if the timer expires without receiving the INVITE message, it falls back to LTE or performs a circuit switch fallback (CSFB) to receive the call, ensuring it can receive the incoming call through the CS domain.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If the terminal device operates in 5G SA network with IMS domain only, then the network architecture is simplified and modernized, but the terminal device cannot fall back to CS domain for receiving incoming calls when IMS link is abnormal

Engineering Contradiction:

Improvenetwork architecture modernizationVSAvoidincoming call reception reliability

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The terminal device performs preliminary actions by establishing a data radio bearer (DRB) for IMS data before the call arrives, starts a timer to monitor for the INVITE message, and prepares the fallback mechanism in advance. When the timer expires without receiving the INVITE, the terminal proactively triggers the fallback to LTE or CSFB procedure, ensuring call reception capability is maintained despite the IMS link abnormality.

Inventive Principle:

Principle #10Preliminary action

2Device complexity

If the network uses only IP multimedia subsystem (IMS) domain without circuit switching (CS) domain, then the network structure is streamlined, but failed incoming calls occur when IMS link is abnormal

Engineering Contradiction:

Improvenetwork structure complexityVSAvoidcall completion rate

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The invention changes the network domain parameter dynamically. When IMS link abnormality is detected (timer expiry without INVITE reception), the terminal device changes from operating in IMS-only domain to falling back to LTE or CS domain. This parameter change enables the terminal to receive incoming calls through alternative domains when the primary IMS domain is unavailable, thereby improving call completion rate while maintaining a streamlined network structure.

Inventive Principle:

Principle #35Parameter changes

3Ease of operation

If the terminal device does not register with CS domain in SA network, then the terminal operates purely in IMS domain, but the terminal cannot receive incoming calls when network fault prevents normal EPS Fallback voice bearer process

Engineering Contradiction:

Improveterminal operation simplicityVSAvoidincoming call reception

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The invention introduces an intermediary mechanism (timer-based detection and fallback procedure) between the terminal device and the network. When the terminal detects IMS link abnormality through timer expiry, it uses the fallback to LTE or CSFB as an intermediary path to receive incoming calls, rather than directly attempting to receive calls through the abnormal IMS link.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentEP4329368B1Called paging method and apparatus

Publication Date: 2026.01.21 HONOR DEVICE CO LTD

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AI summary

Embodiments of this application relate to the communication field and provide a paging method and apparatus for a called terminal, which can resolve a problem of a failed incoming call in an SA network. The method is as follows: A called terminal device camping on a 5G SA network receives a paging message. The called terminal device may start a first timer when detecting that a data radio bearer DRB for transmitting the IMS data is established, and detect whether internet protocol multimedia subsystem IMS data is received in an on period of the first timer. If the first timer expires and the called terminal device does not receive INVITE or other IMS data, it is considered that the network is abnormal and therefore the IMS data (such as INVITE) cannot be received. In this case, the called terminal device may actively fall back to LTE to receive an incoming call. In this way, the called terminal device can receive an INVITE message or a paging message in an IMS domain on the LTE, thereby avoiding a failed incoming call.