NSA (Non-Standalone) anchor optimization focuses on enhancing LTE (anchor cell) stability to ensure seamless 5G connectivity. Key aspects include reducing LTE call drops, optimizing RRC reestablishment, and improving E-RAB success rates. Techniques involve fine-tuning LTE handover thresholds, minimizing interference, and ensuring stable SCG (Secondary Cell Group) additions. Proper PCI planning, mobility parameter adjustments, and robust LTE coverage help maintain a reliable anchor for NSA UEs, reducing 5G interruptions and improving overall user experience. Below are outlines and steps for abnormal anchoring events troubleshooting and optimization.
- Anchor Call Drop.
- RRC Connection Reestablishment.
- Anchor Handover Issue.
- Anchor Access Failure.
NSA Anchor Optimization
1. Anchor Access Failure Optimization
In NSA mode, the B1 measurement delivery and SCG addition process are triggered only after the UE successfully accesses the LTE network. If the LTE access fails, the 5G service cannot be activated.
Therefore, troubleshooting for LTE RRC and E-RAB establishment follows the same process as LTE-only users. This involves analyzing LTE accessibility issues, such as weak signal conditions, RACH failures, or insufficient resources, to ensure successful LTE attachment before enabling 5G dual connectivity. Below picture shows Signaling flow for SgNB Addition.
2. Anchor Call Drop Optimization
When an LTE call drops, the SgNB is released, causing 5G service interruptions. After the UE reaccesses the LTE network, only the SgNB can be re-added, affecting user experience. Additionally, after re-adding the SgNB, the UE must reaccess NR, requiring RANK and MCS adjustments, temporarily impacting the downlink data rate.
The troubleshooting process for anchor cell call drops follows the same approach as LTE-only users, focusing on LTE RRC stability, interference, handover failures, and coverage issues to prevent disruptions in NSA dual connectivity. Below picture shows LTE Call Drop Signaling Procedure.
3. RRC Connection Reestablishment Optimization on Anchor Cell.
When an LTE RRC connection reestablishment occurs, the eNodeB must release and re-add or reconfigure the SgNB, leading to a 5G service interruption. After the SgNB is re-added or reconfigured, the NSA UE must reaccess the network, requiring RANK and MCS adjustments, which temporarily impact the downlink data rate. The troubleshooting process for LTE RRC connection reestablishment on the anchor cell follows the same approach as LTE-only users, focusing on optimizing RRC stability, reducing interference, and minimizing handover failures to maintain NSA service continuity.
4. Anchor Handover Optimization.
The Impact of LTE Call Drops on the 5G:
If LTE handover fails, the UE initiates a reestablishment or a call drop ( depending on the failure phase , as shown on the table) , as a result , the NR service will be interrupted.
| Scenarios | Symptoms |
| The UE cannot receive the LTE handover command (RRC CONN RECFG). | If the UE cannot receive LTE Handover Command, the handover cannot be performed. As a result , the interference of the neighboring cell becomes more serious. 1. If the UE detect UL Radio Link Fails, then UE initiates RRC connection reestablishment, NR data transmission will be interrupted. 2. If the UE detect DL Radio Link Fails, the LTE serving cell will releases the UEs, and LTE call drops occur. |
| The UE fails to access the target LTE cell. | The UE initiates RRC connection reestablishment, NR data transmission will be interrupted. |
| The target cell cannot receive the handover completion message (RRC CONN RECFG CMP) due to air interface or PDCP Integrity Check Failure. | If the UE does not initiate RRC Connection Reestablishment, the target LTE cell does not send a SgNB Reconfiguration Complete message to the gNodeB . The SgNB initiates a release request after gNodeB timer expires. |