UE IMS Call Handling via TAU Reject State Detection
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Solution Overview
Problem
Existing methods for initiating Internet protocol (IP) multimedia subsystem (IMS) calls and circuit switch fallback (CSFB) calls in user equipment (UE) fail to effectively handle abnormal states, leading to call drops, delays, and poor user experience due to network congestion and timer-related issues, especially in dual SIM dual standby (DSDS) and dual SIM dual active (DSDA) scenarios.
Innovation Solution
A mechanism is introduced that allows the UE to transmit a tracking area update (TAU) service request and respond to a TAU service reject message by stopping a circuit switch fallback call in legacy radio technologies, prioritizing IMS calls and handling abnormal states by moving to a legacy radio access technology for call completion when necessary, thus preventing unnecessary SIP packet re-transmissions and ensuring successful call establishment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the UE initiates an IMS call while in abnormal NAS states (such as timer running, TAU initiated, or TAU reject), then the call may be established, but the call drops or delays occur due to network congestion and abnormal service states
Solution Approach 1:
The patent applies preliminary action by having the UE check its NAS layer state (including timer status and TAU state) before initiating an IMS call. The UE determines whether it is in an abnormal state (such as timer T3411/T3402/T3430/T3440 running, or TAU_INITIATED with TAU reject) and takes appropriate pre-action accordingly. This prevents attempting to establish calls in abnormal states that would lead to drops or delays, thereby improving call establishment reliability while avoiding time loss from repeated failed attempts.
2Productivity
If the UE sends SIP data packets during abnormal NAS states, then data transmission may occur, but unwanted packet re-transmissions cause confusion in the network and lead to call drop
Solution Approach 1:
The patent applies feedback by implementing a mechanism where the UE continuously monitors its NAS layer state (including timer status and TAU state) and uses this feedback information to determine whether to transmit SIP data packets. When the UE detects it is in an abnormal state (timer running, TAU initiated with reject), it suppresses SIP packet transmission. This feedback loop prevents unwanted packet re-transmissions that cause network confusion and call drops, thereby improving both data transmission efficiency and call stability.
3Reliability
If the UE prioritizes IMS calls in dual SIM scenarios, then IMS call success rate improves, but legacy radio technology calls may be affected
Solution Approach 1:
The patent applies local quality by treating IMS calls and legacy radio technology calls differently based on their specific characteristics and the UE's current state. The UE checks whether it is in an abnormal NAS state and applies different handling policies: for IMS calls, it suppresses transmission during abnormal states to avoid drops; for legacy calls, it maintains normal operation. This differentiated approach improves IMS call success rate while preserving adaptability for legacy calls, as each call type receives appropriate local handling according to its requirements.
Data Source
AI summary
Methods and apparatuses are provided for controlling a voice call at a user equipment (UE) in a wireless network. A method includes transmitting, to a network entity, a tracking area update (TAU) service request for a subsystem (IMS) initiation; receiving, from the network entity, a TAU service reject message; and in response to the TAU service reject message, stopping a circuit switch (CS) fall back call in at least one legacy radio technology (RAT).