5G NSA Inter-gNB HO Failures

Cheatsheet Content

### Overview of Inter-gNB Handover in 5G NSA Inter-gNB Handover (HO) in 5G Non-Standalone (NSA) architecture involves the transfer of a User Equipment (UE) connection between two different gNBs (Next Generation Node B) while anchored to an LTE eNB (Evolved Node B) for the control plane. This process relies heavily on X2 (between eNBs) and Xn (between gNBs) interfaces. **Phases of Handover:** 1. **Measurement Report:** UE sends measurements to source eNB/gNB. 2. **Handover Decision:** Source eNB/gNB decides to initiate HO. 3. **Handover Preparation:** Source and target eNB/gNB exchange information. 4. **Handover Execution:** UE connects to target gNB, releases source. 5. **Handover Completion:** Target gNB updates core network. ### Common Failure Causes Inter-gNB HO failures can stem from various issues across different network layers. 1. **RF/Physical Layer Issues:** * Poor signal quality (RSRP/RSRQ) at the target gNB. * Interference, leading to poor measurement reports or connection loss. * Incorrect antenna tilt/azimuth or power settings. 2. **X2/Xn Interface Issues:** * X2/Xn link down or congested. * Incorrect X2/Xn setup (e.g., IP address, port configuration). * Missing or incorrect PCI/CGI configurations for neighboring cells. 3. **UE Behavior/Capability:** * UE not supporting specific 5G NSA features or bands. * UE reporting inaccurate measurements. * Battery saving features impacting measurement reporting. 4. **Core Network (EPC) Issues:** * S1-MME/S1-U path issues between eNB and MME/S-GW. * MME/S-GW overload or misconfiguration. * P-GW issues impacting data plane forwarding. 5. **eNB/gNB Configuration:** * Incorrect handover parameters (e.g., A3 event thresholds, hysteresis). * Missing neighbor relations (ANR failures). * Load balancing or admission control preventing HO. * Incorrect security context transfer. ### Troubleshooting Steps Follow these steps to diagnose and resolve inter-gNB HO failures. #### 1. Analyze Call Traces/Logs * **UE Traces:** Check UE measurement reports (MR), RRC connection re-establishment attempts. * **eNB/gNB Traces:** Focus on HO command messages, HO failure indications, X2/Xn messages, RRC messages. * **Core Network Traces:** Verify S1 messages, MME/S-GW interactions. #### 2. Check RF Conditions * **RSRP/RSRQ:** Ensure target gNB has adequate signal strength and quality. * **SINR:** Check Signal-to-Interference-plus-Noise Ratio. * **Interference Scan:** Identify potential interference sources. * **Physical Inspection:** Verify antenna alignment, cabling, and power. #### 3. Verify X2/Xn Connectivity and Configuration * **Ping Tests:** Confirm IP reachability between source/target eNBs and gNBs. * **Interface Status:** Check X2/Xn link status on network elements. * **Neighbor Relations:** * Confirm correct **ANR (Automatic Neighbor Relation)** functionality. * Manually verify PCI/CGI (Physical Cell ID/Cell Global Identifier) of neighbor cells. * Ensure NRT (Neighbor Relation Table) is up-to-date. * **X2/Xn Setup:** Confirm correct IP addresses, SCTP ports, and security associations. #### 4. Review Handover Parameters * **A3 Event Thresholds:** Are they configured appropriately? (e.g., too high/low). * **Hysteresis:** Is it preventing early/late handovers? * **Time-to-Trigger (TTT):** Is it too short/long for the environment? * **Admission Control:** Is the target gNB rejecting HO requests due to capacity? #### 5. Check eNB/gNB & EPC Configuration * **Cell Barring:** Ensure target cell is not barred for HO. * **Security Context:** Verify security parameters are correctly transferred. * **MME/S-GW Logs:** Look for errors related to S1-MME/S1-U setup or context transfer. * **UE Capability:** Confirm UE supports required 5G frequency bands and features. #### 6. Specific Failure Scenarios & Solutions * **HO Failure with `RRC Connection Re-establishment`:** Often due to target gNB not receiving HO command or UE losing connection during HO execution. Check Xn/S1-U path. * **HO Failure due to `Target Not Able to Prepare Handover`:** Indicates issues at target gNB (e.g., congestion, missing configuration, Xn issue). * **HO Failure during `Data Forwarding`:** S1-U or X2-U/Xn-U path issue. Check S-GW/UPF and data plane connectivity. ### Key Performance Indicators (KPIs) and Counters Monitoring these counters can help identify trends and specific failure points. * **Handover Attempt Rate:** Total number of HO attempts. * **Handover Success Rate (HOSR):** Percentage of successful HOs. * **HO Preparation Success Rate:** Success rate of the HO preparation phase. * **HO Execution Success Rate:** Success rate of the HO execution phase. * **X2/Xn Setup Success Rate:** Success rate of establishing X2/Xn connections. * **RRC Connection Re-establishment Rate:** High values might indicate frequent HO failures. * **Dropped Call Rate (DCR):** Can increase if HO failures lead to dropped calls. **Specific Counters to Monitor:** * `HO_FAILURE_X2_LINK_DOWN` * `HO_FAILURE_TARGET_CONGESTION` * `HO_FAILURE_MISSING_NR` * `HO_FAILURE_UE_CAPABILITY_MISMATCH` * `HO_ATTEMPTS_INTRA_GNB` (For comparison) * `HO_ATTEMPTS_INTER_GNB` ### Recommended Tools * **Drive Test Tools:** For RF measurements and UE behavior analysis (e.g., TEMS, Nemo, Accuver). * **OSS/NMS (Operations Support System/Network Management System):** For alarms, performance counters, configuration management. * **Packet Analyzers:** For X2/Xn/S1 interface message decoding (e.g., Wireshark with appropriate dissectors). * **UE Simulators/Emulators:** For controlled testing of HO scenarios. * **Trace Tools:** Built-in eNB/gNB/MME tracing functionalities.