Handover in LTE Along With in-Depth Call Flow Understanding | TechLTE World

1. Handover in LTE along with in-depth call flow understanding Handover is the process of transferring an active communication session for a UE from one eNodeB to another, ensuring seamless mobility and service continuity. Unlike legacy technologies (e.g., GSM), LTE employs a hard handover mechanism, meaning that the UE breaks its connection with the source eNodeB before establishing it with the target. Handover in LTE is crucial for: ▪Seamless voice and data service during mobility ▪Load balancing across cells ▪Radio link quality optimization ▪Inter-technology (Inter-RAT) mobility Types of LTE Handovers LTE supports three main handover categories based on frequency and technology: There are three types of Handovers in LTE: ▪Intra-Frequency Handover-Occurs between cells operating on the same carrier frequency. It is typically faster and less complex due to reuse of the same frequency context. ▪Inter-Frequency Handover-Takes place when a UE moves between cells operating on different frequencies. This requires measurement gaps to switch and evaluate alternate frequencies. ▪Inter-RAT Handover-Inter-Radio Access Technology (Inter-RAT) handover enables mobility across LTE and other technologies like UMTS, GSM, or even NR (in later releases). Examples: ✓LTE to WCDMA/UMTS (for CS fallback) ✓LTE to GSM (for coverage fallback) ✓LTE to Wi-Fi (with ANDSF support) www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

2. Intra-Frequency Handover An Intra-Frequency Handover occurs when the UE transitions from one LTE cell to another, both operating on the same EARFCN. When a handover occurs between two different cells belonging to the same eNodeB and operating on the same frequency, it is still called an Intra-Frequency Handover. Key Characteristics: ▪No frequency re-tuning is needed by the UE. ▪Measurement reports are generated without measurement gaps because the UE can simultaneously monitor neighbouring cells while being connected to the serving cell. Example Use Case: A UE moving across a city served by a single LTE frequency (e.g., 1800 MHz) where multiple eNodeBs use the same frequency but different PCI (Physical Cell IDs). Handover Trigger: Typically initiated based on Event A3: "Neighbour cell becomes offset better than serving cell." www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

3. Inter-Frequency Handover An Inter-Frequency Handover occurs when a UE transitions between LTE cells operating on different frequencies (different EARFCNs). Why It’s Needed: ▪Load balancing across frequency layers (e.g., moving heavy traffic from 1800 MHz to 800 MHz) ▪Coverage improvement (low-frequency bands provide better penetration in buildings) ▪Mobility optimization in multi-layer LTE deployments Measurement Events Used: ▪Event A4: "Neighbour cell becomes better than a threshold." ▪Event A5: "Serving cell becomes worse than threshold1 AND neighbour becomes better than threshold2." www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

4. Inter-RAT Handover An Inter-RAT Handover is a mobility event where the UE transitions between LTE and other radio access technologies such as: ▪UMTS/WCDMA (3G) ▪GSM (2G) ▪Wi-Fi (IEEE 802.11) ▪5G NR (from LTE anchor) www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

5. When is it Triggered: ▪LTE coverage is poor or unavailable ▪Legacy network (2G/3G) provides better signal or fallback services ▪CS fallback is required for voice (in LTE networks without VoLTE) ▪Preferred access or policy rules (e.g., Wi-Fi preferred indoors) Measurement Events Used: ▪Event B1: "Inter-RAT neighbour cell becomes better than a threshold" ▪Event B2: "Serving cell becomes worse than threshold1 AND Inter-RAT neighbour becomes better than threshold2" Type Frequency Context Same EARFCN Different EARFCNs Different Technologies RAT Change Measurement Gaps Needed No No No Yes Yes Yes Events Intra-Frequency Inter-Frequency Inter-RAT A3 A4, A5 B1, B2 Handover Categorization Based on Core Network Involvement LTE supports two principal handover architectures depending on whether or not the EPC actively participates in the handover signalling and data path management. The LTE handover procedure enables seamless transfer of a UE's connection between eNodeBs while maintaining an active session. This ensures uninterrupted service and optimal performance as the UE moves through the network. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

6. The Procedure involved 1. Measurement and Evaluation ▪UE continuously measures signal quality (e.g., RSRP, RSRQ) of the serving and neighbouring cells. ▪These measurements are reported to the serving eNodeB periodically or when configured events (e.g., A3, A5) are triggered. ▪The serving eNodeB evaluates the measurement reports to assess if handover conditions are met. 2. Handover Decision ▪The serving eNodeB compares the received measurements with preconfigured thresholds and offsets. ▪If the target cell has better radio conditions (based on triggers like A3, A5), a handover decision is made. ▪The decision considers other factors such as cell load, mobility patterns, and handover hysteresis. 3. Handover Preparation ▪The serving eNodeB sends a Handover Request message to the target eNodeB via the X2 or S1 interface. ▪The message includes UE context, bearer information, and radio parameters. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

7. ▪The target eNodeB allocates necessary resources and replies with a Handover Request Acknowledge. ▪RRC Connection Reconfiguration message is prepared and buffered for the UE. 4. Handover Execution ▪The serving eNodeB sends an RRC Connection Reconfiguration command to the UE with details of the target cell. ▪UE detaches from the serving cell and synchronizes with the target cell based on provided configuration. ▪UE sends RRC Connection Reconfiguration Complete to the target eNodeB. ▪The data path is updated by the EPC (SGW/PGW) if using S1-based HO. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

8. 5. Handover Completion ▪After successful synchronization, the target eNodeB becomes the new serving cell for the UE. ▪Buffered and ongoing downlink data is forwarded from the old eNodeB to the new one (via X2 or S1). ▪UE resumes its communication session with no user-perceivable interruption. ▪The handover context is released at the source eNodeB to free up resources. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

9. X2-Based Handover An X2-Based Handover is a procedure where the source and target eNodeBs communicate directly over the X2 interface to manage the handover. The EPC (MME and SGW) plays only a minor role — mainly involved in the path switch after the handover is complete. ▪Intra-LTE handovers within the same operator network. ▪Both eNodeBs are connected to the same MME and SGW. ▪X2 interface is available and active. ▪X2 interface configured between eNodeBs (direct IP link or via S1-based tunnel). ▪Target and source eNodeBs must trust each other (configured security context). ▪UE remains within the same Tracking Area or MME pool. S1-Based Handover An S1-Based Handover is used when the X2 interface is not available or cannot be used (e.g., in inter-vendor or inter-operator deployments). In this case, all handover signalling and context transfer is routed via the EPC, involving both the MME and the SGW. ▪X2 interface not present or blocked. ▪Inter-MME handovers (target eNodeB is served by a different MME). ▪Inter-PLMN handovers (e.g., during roaming). ▪Inter-RAT handovers, e.g., LTE → UMTS/GSM. ▪Multi-vendor environments without X2 interconnection. ▪S1-MME interface must be available for both source and target eNodeBs. ▪Target eNodeB must support the same bearer configurations. ▪EPC handles bearer modifications and UE context transfer. Comparison: X2 vs S1 Handover Parameter X2-Based Handover S1-Based Handover EPC Involvement Minimal Full involvement (MME, SGW) Latency Low Higher Interface X2 S1-MME Common Use Case Intra-MME, same operator Inter-MME, inter-PLMN, inter-RAT Data Forwarding X2 user-plane tunnel EPC rerouting or buffering Measurement Events A3, A5 A5, B2 (if inter-RAT) www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

10. LTE Measurement Events (3GPP TS 36.331) LTE relies on measurement events to determine when a handover or reconfiguration should be triggered. These event-based triggers are used by the UE to monitor signal quality and report when certain conditions are met. These reports enable the network to make mobility decisions dynamically and efficiently. Event Trigger Condition Typical Use Case A1 Serving > Threshold Stop measuring neighbours A2 Serving < Threshold Start measuring others (IRAT/Inter-Freq) A3 Neighbour > Serving + Offset Intra/Inter-frequency Handover A4 Neighbour > Absolute Threshold Inter-Frequency Handover A5 Serving < T1 AND Neighbour > T2 Inter-Frequency Handover A6 Neighbour > Serving + Offset Carrier Aggregation (SCell add/remove) B1 Inter-RAT Neighbour > Threshold Inter-RAT Measurements B2 Serving < T1 AND Inter-RAT > T2 Inter-RAT Handover www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

11. Event A1 – “Serving cell becomes better than threshold” UE triggers this event when the RSRP/RSRQ of the serving cell becomes better than a predefined threshold. Indicates good radio conditions; no need to consider handovers. Use Case: ▪Stop unnecessary measurements of neighbouring cells. ▪Saves UE power and reduces measurement overhead. Example: If a UE moves closer to the serving cell and its signal becomes strong (e.g., better than −95 dBm), Event A1 is triggered. Event A2 – “Serving cell becomes worse than threshold” Triggered when the serving cell's signal strength drops below a set threshold. Initiates measurement of neighbouring cells or inter-RAT cells. Use Case: ▪Prepare for possible handover or cell reselection. ▪Used in poor signal areas, e.g., building interiors. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

12. Example: When RSRP of the serving cell drops below −110 dBm. Event A3 – “Neighbour becomes offset better than serving” A neighbouring cell’s signal becomes better than the serving cell’s by a defined offset (e.g., 3 dB). UE reports when a neighbouring cell is significantly better, to trigger a handover. Use Case: ▪Intra-frequency or inter-frequency handovers. Example: When Neighbour Cell = −96 dBm and Serving Cell = −100 dBm with offset = 3 dB. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

13. Event A4 – “Neighbour becomes better than an absolute threshold” A neighbouring cell's RSRP/RSRQ exceeds a predefined absolute threshold. Checks availability of good neighbour cells, regardless of serving cell strength. Use Case: ▪Mainly used in inter-frequency handover scenarios. Example: Neighbour RSRP crosses −100 dBm absolute threshold. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

14. Event A5 – “Serving worse than threshold1 AND Neighbour better than threshold2” Dual condition: ▪Serving cell becomes worse than threshold1 ▪Neighbour becomes better than threshold2 UE only reports when both degradation and better alternative exist. Use Case: ▪Advanced inter-frequency handovers. ▪Prevents ping-pong handovers. Example: Serving RSRP < −110 dBm & Neighbour RSRP > −100 dBm. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

15. Event A6 – “Neighbor becomes better than serving by offset” A Secondary Cell (SCell) becomes better than the Primary Cell (PCell) by a given offset. Used to add or remove SCells for Carrier Aggregation. Use Case: ▪Enhancing throughput by activating additional downlink carriers. Example: When SCell RSRP is better than PCell by 5 dB. Event B1 – “Inter-RAT Neighbor becomes better than threshold” A neighbour cell from a different RAT (e.g., 3G, GSM, NR) exceeds a set threshold. Enables measurement reporting for inter-RAT mobility. Use Case: ▪LTE to WCDMA/GSM/NR for load balancing, coverage fallback. Example: WCDMA RSRP > −102 dBm triggers B1. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

16. Event B2 – “Serving < T1 AND Inter-RAT Neighbour > T2” Dual-condition for inter-RAT: ▪Serving LTE cell < threshold1 ▪Inter-RAT cell > threshold2 Indicates need for handover to a different RAT (e.g., for CS fallback or coverage). Use Case: ▪LTE to GSM or UMTS (e.g., for voice services if no VoLTE). ▪LTE to NR during ENDC deactivation. Example: Serving RSRP < −110 dBm & GSM RSRP > −102 dBm. X2 Handover Call flow X2 handover in LTE enables direct mobility between eNodeBs using the X2 interface without involving the MME during handover signalling. It ensures faster and more efficient handovers, reducing latency and preserving user experience. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

17. 1. Measurement & Handover Triggering The UE periodically measures signal strength (RSRP/RSRQ) of the serving and neighboring cells and sends a Measurement Report to the Source eNodeB, triggering the handover process. ▪UE performs measurements on the serving and neighbouring cells (e.g., A3, A5 events). ▪Measurement reports are sent to the Source eNodeB. ▪Source eNodeB decides a handover is required based on thresholds (not MME in X2, unlike S1 handover). 2. Handover Preparation The Source eNodeB sends a Handover Request to the Target eNodeB over the X2 interface, containing UE context and bearer info. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

18. Source eNodeB sends an X2 Handover Request to Target eNodeB via the X2 interface. IEs involved: ▪UE Context (RRC, security, DRB config) ▪E-RABs to Be Setup List X2AP-PDU: initiatingMessage (0) └── initiatingMessage ├── procedureCode: id-handoverRequest (0) ├── criticality: reject (0) └── value └── HandoverRequest └── protocolIEs: 7 items ├── Item 0: id-Old-eNB-UE-X2AP-ID ├── Item 1: id-New-eNB-UE-X2AP-ID ├── Item 2: id-HandoverType ├── Item 3: id-Cause ├── Item 4: id-TargetCellID ├── Item 5: id-E-RABs-ToBeSetupList └── Item 6: id-UE-ContextInformation 3. Target eNodeB Admission & Resource Allocation The Target eNodeB allocates necessary radio resources for the UE and prepares to receive the handover. Target eNodeB: ▪Allocates necessary radio resources. ▪Prepares to receive the UE. ▪Sends X2 Handover Request Acknowledge to Source eNodeB. IEs involved: ▪UE Context ▪DRB setup response www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

19. X2AP-PDU: initiatingMessage (0) └── initiatingMessage ├── procedureCode: id-handoverRequest (0) ├── criticality: reject (0) └── value └── HandoverRequest └── protocolIEs: 8 items ├── Item 0: id-Old-eNB-UE-X2AP-ID ├── Item 1: id-New-eNB-UE-X2AP-ID ├── Item 2: id-HandoverType ├── Item 3: id-Cause ├── Item 4: id-TargetCellID ├── Item 5: id-E-RABs-ToBeSetupList ├── Item 6: id-UEContextInformation └── Item 7: id-GUMMEI-ID 4. Handover Command Source eNodeB sends RRC Connection Reconfiguration (Handover Command) to UE.The Source eNodeB sends an RRC Connection Reconfiguration (handover command) to the UE, directing it to move to the target cell. ▪Contains TargetCellID and configuration details for access to Target eNodeB. RRC: DL-DCCH Message └── message: c1 └── rrcConnectionReconfiguration ├── rrc-TransactionIdentifier: 2 └── criticalExtensions: c1 └── rrcConnectionReconfiguration-r8 ├── measConfig: (optional) ├── mobilityControlInfo │ ├── targetPhysCellId: 82 www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

20. │ ├── carrierFreq: 6350 │ ├── additionalSpectrumEmission: 1 │ ├── t304: ms50 │ └── newUE-Identity: 0x00001234 ├── securityConfigHO │ ├── keyChangeIndicator: true │ └── nextHopChainingCount: 2 ├── radioResourceConfigDedicated │ ├── srb-ToAddModList: 1 item │ │ └── SRB ID: 1 (SRB1) │ └── mac-MainConfig │ └── rach-ConfigDedicated │ ├── ra-PreambleIndex: 38 │ ├── ra-PRACH-MaskIndex: 1 │ ├── ra-ResponseWindowSize: sf10 │ ├── mac-ContentionResolutionTimer: sf64 │ └── maxHARQ-Msg3Tx: 4 └── nonCriticalExtension: present 5. Handover Execution The UE detaches from the Source eNodeB and synchronizes with the Target eNodeB, beginning communication with the new cell.The UE sends an RRC Connection Reconfiguration Complete message to the Target eNodeB, confirming successful handover. ▪UE detaches from Source eNodeB, synchronizes with Target eNodeB, and performs random access. ▪Upon successful access, UE sends RRC Connection Reconfiguration Complete to Target eNodeB. 6. SN Status Transfer (optional but important) SN Status Transfer in X2 handover is used to exchange uplink and downlink sequence number (SN) information between the Source and Target eNodeBs. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

21. It ensures in-sequence delivery and lossless data forwarding during handover by transferring the status of PDCP packet delivery. Source eNodeB → Target eNodeB sends: ▪UE ID ▪E-RAB ID ▪UL SN, DL SN ▪Ensures continuity in user-plane forwarding. X2AP-PDU: initiatingMessage (0) └── initiatingMessage ├── procedureCode: id-SNStatusTransfer (9) ├── criticality: ignore (1) └── value └── SNStatusTransfer ├── id-UE-X2AP-ID (unique UE context identifier) └── id-SNStatusTransfer-IEs └── Bearer Context List ├── E-RAB ID ├── PDCP SN (Uplink) ├── PDCP SN (Downlink) └── Hyper Frame Number (HFN) – optional 7. Path Switch Request Target eNodeB sends a Path Switch Request to the MME to update the bearer path via the S1 interface.Target eNodeB sends Path Switch Request to MME. IEs: ▪Target Cell ID, Bearer IDs S1AP-PDU: initiatingMessage (0) └── initiatingMessage ├── procedureCode: id-PathSwitchRequest (28) ├── criticality: reject (0) www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

22. └── value └── PathSwitchRequest ├── id-eNB-UE-S1AP-ID ├── id-MME-UE-S1AP-ID ├── id-E-RABToBeSwitchedDLList │ └── [E-RAB ID, GTP TEID, Transport Layer Address] ├── id-UESecurityCapabilities └── id-SubscriberProfileIDforRFP (optional) 8. Modify Bearer Procedure MME sends a Modify Bearer Request to the SGW to switch the data path; SGW updates path and responds with Modify Bearer Response. ▪MME sends Modify Bearer Request to SGW. ▪SGW updates bearer paths and responds with Modify Bearer Response. 9. Path Switch Ack & Optional Context Release The MME informs the Target eNodeB that the path switch is successful via Path Switch Acknowledge. ▪MME sends Path Switch Request Ack to Target eNodeB`. ▪Optional: UE Context Release IE may be included to request Source eNodeB to release resources. 10. UE Context Release ▪Target eNodeB sends UE Context Release to Source eNodeB. ▪Source eNodeB responds with UE Context Release Ack. 11. Data Forwarding (optional) ▪If required, data packets buffered at Source eNodeB are forwarded to Target eNodeB until UE is fully served by the new path. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

23. 12. Resource Release Once handover is complete and path switch is successful: ▪Source eNodeB releases all radio and bearer resources. ▪The handover is now complete. S1 Handover Call flow S1 handover in LTE is performed when the X2 interface between eNodeBs is unavailable or not configured. It involves the MME and SGW for signaling, bearer modification, and context transfer. Although slightly slower than X2 handover, S1 handover ensures mobility continuity across different eNodeBs or MME pools. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

24. 1.Measurement Report & Trigger The UE sends an RRC Measurement Report to the source eNodeB based on configured event triggers (e.g., Event A3, A5). This report includes signal metrics like RSRP/RSRQ for neighbouring cells. RRC: UL-DCCH Message └── message: c1 └── measReport ├── rrc-TransactionIdentifier: 1 └── criticalExtensions: c1 └── measReport-r8 └── measResults ├── measId: 1 ├── measResultPCell │ └── rsrpResult: 40 └── measResultNeighCells └── measResultListEUTRA └── measResult ├── physCellId: 82 └── rsrpResult: 55 2.Handover Required (Source eNodeB → MME) Source eNodeB informs MME about the need for handover with target cell info.Upon deciding a better cell is available, the source eNodeB sends a Handover Required message to the MME. This includes the target cell ID and bearer information to initiate handover preparation. Key IEs: ▪TargeteNB-ID ▪E-RABs to be setup www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

25. ▪Cause HandoverRequired └─ Cause: radioNetwork (handover-desirable-for-radio-reason) └─ TargeteNB-ID: ECGI (PLMN + Cell ID) └─ E-RAB To Be Setup List: └─ E-RAB ID = 5 └─ QoS Params 3.Handover Request (MME → Target eNodeB) The MME forwards the Handover Request to the target eNodeB with UE context, bearer parameters, and security credentials, instructing it to prepare resources for the incoming UE. Key IEs: ▪E-RABs to be setup ▪UE security context ▪Target Cell ID HandoverRequest └─ E-RABs To Be Setup: └─ E-RAB ID: 5 └─ GTP TEID & Transport Layer Address └─ UE Security Capabilities └─ Target Cell ID 4.Handover Request Acknowledge (Target eNodeB → MME) The target eNodeB accepts the handover by allocating radio resources and responding with the TEID and IP for GTP-U. This confirms it's ready to receive the UE. Key IEs: ▪E-RAB Setup Response ▪Downlink Tunnel Info (TEID, IP) www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

26. HandoverRequestAcknowledge └─ E-RAB Admitted List └─ TEID = 0x58ab1234 └─ IP = 10.11.22.5 5.Step 5: Handover Command (MME → Source eNodeB) The MME informs the source eNodeB to initiate the handover, including the transparent RRC container from the target eNodeB that will be forwarded to the UE. Key IEs: ▪Target-to-Source Transparent Container HandoverCommand └─ Target-To-Source Transparent Container: └─ RRC Connection Reconfiguration Info 6.Step 6: RRC Connection Reconfiguration (Source eNodeB → UE) The source eNodeB sends the RRCConnectionReconfiguration message to the UE, containing the handover command and target cell configuration. The UE then starts accessing the target cell. RRCConnectionReconfiguration (DL-DCCH) └─ mobilityControlInfo └─ Target Cell ID └─ Carrier Frequency └─ Handover Command (MAC/RLC config) 7.Step 7: Handover Execution (UE → Target eNodeB) The UE synchronizes with the target eNodeB via RACH and completes the handover. Once successful, it sends RRCConnectionReconfigurationComplete to confirm its attachment to the new cell. RRC: UL-DCCH Message ├── message: c1 │ └── rrcConnectionReconfigurationComplete www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

27. 8.Step 8: Path Switch Request (Target eNodeB → MME) The target eNodeB notifies the MME with a Path Switch Request to redirect the user-plane data path from the SGW to the new eNodeB’s GTP tunnel. PathSwitchRequest └─ E-RAB ID = 5 └─ GTP-U TEID & DL IP Address 9.Step 9: Modify Bearer Request (MME → SGW) The MME sends a Modify Bearer Request to the Serving Gateway (SGW), asking it to update the downlink GTP-U tunnel toward the new target eNodeB. ModifyBearerRequest └─ Bearer Context: └─ E-RAB ID = 5 └─ New DL TEID 10.Step 10: Modify Bearer Response (SGW → MME) The SGW updates the bearer context and responds with Modify Bearer Response, confirming that downlink packets will now be forwarded to the target eNodeB. ModifyBearerResponse └─ Bearer Status = Successful 11.Step 11: Path Switch Request Acknowledge (MME → Target eNodeB) The MME sends a Path Switch Request Acknowledge back to the target eNodeB, completing the user-plane redirection and confirming the successful switch. S1AP: initiatingMessage (id-PathSwitchRequest) ├── id-MME-UE-S1AP-ID ├── id-eNB-UE-S1AP-ID ├── id-E-RABToBeSwitchedDLList 12.Step 12: UE Context Release (Source eNodeB → MME) After a successful handover, the source eNodeB releases the UE context and radio resources, informing the MME that it is no longer responsible for that UE. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

28. UEContextRelease └─ Cause: Handover Successful Measurement Configuration in LTE RRC Reconfiguration for Handover During handover, RRC Connection Reconfiguration messages include detailed measurement configurations that guide the UE (User Equipment) in evaluating neighboring cells. There are 3 main things to focus for measurement related rrc reconfiguration message in handover: 1. Measurement Object ID Defines where (on which frequency or cell) the UE should perform measurements. Key Information Elements: ▪Measurement Object ID Unique identifier that refers to a specific frequency or cell group for measurement. ▪Carrier Frequency Specifies the downlink EARFCN where measurements are to be performed. ▪Allowed Measurement Bandwidth Bandwidth range in which UE must collect signal quality metrics. ▪Offset Frequency Adjusts the target frequency for fine-tuning inter-frequency measurements. ▪System Frame Number (SFN) Provides synchronization reference for when measurements are to be taken. ▪Antenna Port Presence Indicates whether measurements are aligned with specific antenna port (e.g., port 1 in CRS-based measurements). ▪Measurement Bandwidth Indicates how wide the measurement scan should be across frequency. ▪Purpose Explains why this object is being measured (e.g., handover, load balancing). www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

29. 2. Report Configuration ID Defines how and when the UE should report measurement results back to the network. Key Information Elements: ▪Report Config ID A unique ID that links a measurement trigger to reporting behavior. ▪Trigger Type Specifies what condition causes a report to be sent (e.g., Event A3, A5, B2). ▪Report Interval Time interval between consecutive reports when trigger is met. ▪Report Amount Total number of reports to send once triggered. ▪Report Quantity Specifies measurement parameters to report, such as RSRP, RSRQ, or both. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

30. ▪Event Trigger Quantity The metric (e.g., RSRP) that is used to evaluate event condition (e.g., neighbour becomes better than serving). ▪Report Quantity per Cell Refines which parameter(s) should be reported per neighbour cell. 3. Measurement ID Serves as a linkage between Measurement Object and Report Config, forming a complete measurement instruction. Key Information Elements: ▪Measurement ID Unique reference ID that binds a measurement object to its report config. ▪Measurement Purpose Indicates why this particular measurement is configured (e.g., intra-frequency HO). ▪Report Config ID (Linked) Links to the report trigger definition associated with this measurement ID. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

31. ▪Quantity Configuration Specifies which quantity (RSRP, RSRQ, SINR) the UE must measure and report. ▪Measurement Timing Configuration Includes details on timing windows and scheduling of measurements. ▪Measurement Control Purpose Higher-level objective, such as radio optimization or mobility. ▪Measurement Object List Lists all Measurement Object IDs tied to this configuration. ▪Measurement Gap Configuration Indicates if and how measurement gaps are configured (e.g., MGL, MGR), allowing UE to scan other frequencies without disrupting active transmissions. What is Measurement GAP in LTE? Measurement GAP is a scheduled silent period (gap) during which the UE temporarily stops transmitting and receiving on the current serving frequency so that it can perform measurements on other frequencies or RATs (like GERAN, UTRAN, CDMA2000). www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

32. Why GAP is needed? ▪In LTE FDD systems, UEs are generally not capable of transmitting and receiving on different frequencies simultaneously. ▪So, to measure inter-frequency LTE or inter-RAT neighbours (e.g., 3G, 2G), the UE requires dedicated time where it pauses normal activity — this is the GAP. ▪During the GAP: ✓UE tunes to the target frequency. ✓Performs measurement (like RSRP/RSRQ). ✓Switches back to the serving frequency. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

33. Measurement GAP Profiles (gp0 and gp1) GAP ID Length (ms) Period (ms) Use Case gp0 6 40 High mobility (e.g., UE in vehicles) gp1 6 80 Low mobility (e.g., indoor, small cell) How the GAP is Scheduled From 3GPP TS 36.331 Section 5.5.2.9: GAP occurs when: SFN mod T = FLOOR(gapOffset / 10) subframe = gapOffset mod 10 If gapOffset = 5, using gp0: ▪T = 40/10 = 4 ▪GAP will be scheduled in every 4th frame at subframe 5. ▪This gives us GAPs at SFN 0, 4, 8… subframe 5. So, every 40ms (every 4 SFNs), in subframe 5, the UE goes "silent" and measures neighbour cells. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855

34. Reference: ▪3GPP TS 36.331 – Defines RRC procedures like Handover Command and Measurement GAP setup. ▪3GPP TS 36.300 – Describes LTE overall architecture including S1 and X2 handover mechanisms. ▪3GPP TS 36.410/420/423 – Specifies S1AP and X2AP protocols and message structures. ▪3GPP TS 36.133 – Provides measurement requirements and GAP timing (MGL, MGRP) for inter-frequency/inter-RAT. ▪Measurement GAP Diagram (36.331/36.133) – Explains gapOffset, SFN alignment, and subframe scheduling. www.techlteworld.com For Protocol Testing Course Contact us: +919140348108/+918660602855