CS Fallback PLMN Redirection in Evolved Packet Systems

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Solution Overview

Problem

In mobile communication networks, particularly in 4G-LTE systems, the existing mechanisms for CSFB (Circuit Switched Fallback) face challenges when moving User Equipment (UE) back to E-UTRAN from legacy RANs, leading to issues such as loss of IP connectivity, excessive signaling, and poor user experience due to unpredictable PLMN selection during inter-operator mobility.

Innovation Solution

The solution involves providing the Identity of the PLMN that was serving the UE over E-UTRAN to the RAN node serving the UE when legacy radio resources are released, allowing the RAN to redirect the UE back to the original PLMN, thereby maintaining continuity and reducing signaling overhead.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If existing mechanisms are used to move UE back to E-UTRAN from legacy RAN, then UE can return to packet-switched domain, but IP connectivity is lost and excessive signaling occurs

Engineering Contradiction:

ImproveIP connectivity continuityVSAvoidPLMN selection information

Core Design Contradiction:

ReliabilityVSLoss of information

Solution Approach 1:

The MME performs preliminary action by storing the E-UTRAN PLMN identity in the SGs context before the UE falls back to legacy RAN. This pre-stored information is later used to guide the UE's return to the correct PLMN, preventing connectivity loss and reducing signaling overhead.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The SGs interface acts as an intermediary between the EPC and CS domain, carrying the PLMN identity information from the MME to the MSC/VLR. This intermediary mechanism ensures that PLMN selection information is preserved and transmitted correctly during the CS fallback process.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of operation

If UE is redirected without PLMN information, then redirection can be performed, but unpredictable PLMN selection degrades user experience

Engineering Contradiction:

ImproveUE redirection capabilityVSAvoidPLMN selection accuracy

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The system implements feedback by providing the stored E-UTRAN PLMN identity to the legacy RAN node, which then uses this information to redirect the UE to the correct PLMN. This feedback loop ensures accurate PLMN selection and maintains consistent user experience across network transitions.

Inventive Principle:

Principle #23Feedback

3Adaptability or versatility

If CSFB is implemented without PLMN tracking, then circuit-switched services can be accessed, but network signaling increases and operational efficiency decreases

Engineering Contradiction:

ImproveCS domain service accessVSAvoidNetwork signaling efficiency

Core Design Contradiction:

Adaptability or versatilityVSProductivity

Solution Approach 1:

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS11553380B2Support of CS fallback in an evolved packet system

Publication Date: 2023.01.10 NOKIA TECHNOLOGIES OY

AI summary

An embodiment of the present invention is a method for support of Circuit Switched (CS) fallback in an Evolved Packet System (EPS), said support including moving a User Equipment (UE) from an evolved Radio Access Network (RAN) to a legacy RAN for access to CS domain services, and moving back said UE from said legacy RAN to said evolved RAN at termination of said CS domain services, said support including moving back said UE to said evolved RAN in the Public Land Mobile Network (PLMN) that was serving said UE over said evolved RAN before moving said UE to said legacy RAN.