How does LTE use TAU Reject Causes to provide specific rejection reasons to UEs?

In LTE (Long-Term Evolution), the Tracking Area Update (TAU) Reject Causes are specific codes used to communicate rejection reasons to User Equipment (UE) when a TAU procedure is unsuccessful. These causes help the UE understand why its attempt to update its location in the network has been rejected. Let's explore in technical detail how LTE utilizes TAU Reject Causes to provide specific rejection reasons to UEs:

1. TAU Reject Message:
   • When the network decides to reject a TAU request from a UE, it sends a TAU Reject message back to the UE.
2. Cause field in TAU Reject Message:
   • The TAU Reject message includes a "Cause" field, and this field carries a specific numerical code known as the TAU Reject Cause.
   • The Cause field is part of the TAU Reject message structure and is used to indicate the reason for rejecting the TAU request.
3. Numerical Codes for Various Reasons:
   • Each TAU Reject Cause is associated with a unique numerical code. These codes are standardized in the LTE specifications and are defined by the 3rd Generation Partnership Project (3GPP).
4. Information Elements:
   • The TAU Reject message may contain additional information elements providing more details about the rejection. These elements can include parameters related to the cause, helping the UE and network troubleshoot and address the specific issue.
5. Signaling Flow:
   • When the UE initiates a TAU, it sends a TAU Request message to the Mobility Management Entity (MME) in the core network.
   • The MME processes the TAU Request, and if it decides to reject the TAU, it constructs a TAU Reject message.
6. Inserting TAU Reject Cause:
   • The MME inserts the appropriate TAU Reject Cause code into the Cause field of the TAU Reject message, indicating the specific reason for rejection.
7. Sending TAU Reject to UE:
   • The TAU Reject message is then sent back to the UE over the radio interface, utilizing the Radio Resource Control (RRC) protocol.
8. UE Interpretation:
   • Upon receiving the TAU Reject message, the UE interprets the Cause field and identifies the TAU Reject Cause code.
9. Error Handling by UE:
   • The UE uses the TAU Reject Cause code to determine the specific reason for rejection. This information guides the UE in taking appropriate actions to address the issue.
10. Handling Different Scenarios:
    • TAU Reject Causes cover various scenarios such as insufficient resources, authentication failure, PLMN not allowed, and more.
    • The UE adapts its behavior based on the TAU Reject Cause. For example, it may retry the TAU after a delay, resolve authentication issues, or choose a different PLMN.
11. Logging and Troubleshooting:
    • The TAU Reject Cause is logged for network management and troubleshooting purposes. Network operators can analyze these logs to identify patterns of TAU rejections and take corrective actions.
12. Retransmission and Recovery:
    • Depending on the cause, the UE may attempt to retransmit the TAU Request after addressing the identified issue or when the network conditions change.

In summary, LTE uses TAU Reject Causes as part of the TAU Reject message to inform UEs about the specific reasons for rejecting their location update requests. This enables UEs to adapt their behavior, troubleshoot issues, and improve the efficiency of mobility management within the LTE network. The standardized codes and signaling flow ensure consistency and interoperability across different LTE networks and devices.