NR SA LTM(L1/L2 Triggered Mobility)

This tutorial shows how to test NR SA to NR SA L1/L2 Triggered Mobility (LTM). This tutorial also shows how to configure LTM candidate cells and measurement reporting, and how to trigger the measurement report and cell switch. LTM is a connected-mode mobility procedure in which the target-cell configuration is prepared through RRC signaling in advance, while the actual cell switch is triggered using faster L1/L2 signaling, as described below.

Step 1 : Network (gNB) configures the UE with the measurement configuration, LTM candidate cells, and the required target-cell configurations

Step 2 : UE continuously performs the configured measurements and sends a measurement report when the configured reporting condition is satisfied

Step 3 : Network selects a target cell based on the measurement report and other mobility criteria

Step 4 : Network sends an L1/L2 mobility command to instruct the UE to switch to the selected target cell

Step 5 : UE applies the previously configured target-cell parameters, accesses the target cell, and completes the mobility procedure

In terms of frequencies of current cell and target/destination cell, the handover can be categorized into two types as below

Intra Frequency Handover : Current Cell and Target Cell frequency is same
Inter Frequency Handover : Current Cell and Target Cell frequency is different

NR SA LTM(L1/L2 Triggered Mobility)

Introduction
Summary of the Tutorial
Test Setup

< Setup A >
< Setup B >

Key Configuration Parameters
Test 1 : SA Intra Frequency - LTM4

Configuration
Perform the test
Log Analysis

Test 2 : SA Intra Frequency - LTM4 and L3 Measurement

Configuration
Perform the test
Log Analysis

RRC / NAS Signaling

RrcReconfiguration

Introduction

5G New Radio (NR) Standalone (SA) networks introduce advanced connected-mode mobility procedures to support seamless user experience and maintain robust connectivity as User Equipment (UE) moves across different cells. Among these, L1/L2 Triggered Mobility (LTM) is a cutting-edge mechanism that leverages lower-layer signaling for faster and more efficient handovers. In the LTM procedure, the network (gNodeB or gNB) prepares the UE in advance with candidate cell configurations and measurement parameters via Radio Resource Control (RRC) signaling. The actual cell switch is then triggered using rapid Layer 1/Layer 2 (L1/L2) commands, reducing handover latency and improving mobility performance. This approach contrasts with traditional RRC-based handovers, where the entire process relies on higher-layer signaling, often resulting in increased delay. LTM plays a crucial role in high-mobility scenarios and dense network deployments by enabling the UE to rapidly transition between cells with minimal service interruption. This tutorial provides a hands-on guide to testing NR SA to NR SA LTM, including the configuration of measurement reporting, LTM candidate cells, and the triggering process for measurement reports and cell switches. By understanding the architectural principles and operational workflow of LTM, practitioners can effectively evaluate and optimize mobility performance in 5G NR networks.

Context and Background
- 5G NR SA networks are designed for enhanced mobility, ultra-reliable low-latency communications (URLLC), and massive connectivity.
- LTM introduces a split between RRC-based preparation and L1/L2-based execution to minimize handover interruption times.
- The gNB manages UE mobility by configuring candidate cells and measurement conditions, and by issuing fast-switch commands at the L1/L2 layers.
Relevance and Importance
- LTM is vital for supporting seamless mobility in high-speed scenarios such as highways, railways, and ultra-dense urban environments.
- By pre-configuring target cell parameters, LTM drastically reduces the time required for handover execution compared to legacy methods.
- Understanding and testing LTM is essential for network engineers, system integrators, and researchers working on 5G NR deployment and optimization.
Tutorial Outcomes
- Learn how to configure LTM candidate cells and measurement reporting in a 5G NR SA environment.
- Gain insights into triggering measurement reports and executing L1/L2-based mobility commands.
- Understand the distinction between intra-frequency and inter-frequency handovers in the context of LTM.
- Acquire practical skills for testing and validating LTM procedures using relevant tools and test equipment.
Prerequisite Knowledge and Skills
- Familiarity with 5G NR architecture, including the roles of gNB, UE, and core network components.
- Understanding of mobility concepts such as handover, measurement reporting, and cell reselection.
- Basic knowledge of RRC, L1/L2 signaling, and 5G NR protocol stack operations.
- Experience with network testing tools and simulation environments is recommended for hands-on exercises.

Summary of the Tutorial

This tutorial demonstrates test procedures for L1/L2 Triggered Mobility (LTM) between two NR SA (StandAlone) cells using the Amarisoft system. Two main tests are described: Test 1 (basic LTM4 procedure) and Test 2 (LTM4 with additional Layer 3 measurement control). The procedures involve configuring two NR cells, preparing the UE and gNB for LTM, and executing mobility by directly changing cell power to trigger the handover.

Test Setup
- Two NR cells (serving and target) are configured on the same frequency band (n78), supporting 2x2 MIMO and TDD operation.
- SIM card and configuration files provided; advanced configuration available in the referenced Configuration Guide.
- Test setups A and B illustrate different hardware combinations (real UE, simulated UE).
Key Configuration Parameters
- LTM-related configuration parameters are highlighted, including measurement triggers, reporting quantities, switching thresholds, and candidate cell preparation.
- Important parameters include:
  - Trigger conditions (activation_trigger, a3_report_type, a3_offset, a4/a5 thresholds, hysteresis, time_to_trigger)
  - Reporting setup (periodic, type, quantity, period)
  - LTM candidate list and target configurations
Test 1: SA Intra Frequency - LTM4
- Configuration Procedure
  - Configure gNB and UE using provided configuration files with LTM enabled and CONDITIONAL_HO disabled.
  - Set N_CELL=2 for a two-cell scenario, N_ANTENNA_DL=2 for MIMO, and prepare both cells as LTM candidates of each other.
  - For the serving cell:
    - Set configuration_trigger: "immediate" to pre-configure the UE with candidate cell information.
    - Enable periodic measurement reporting (quantity: ssb_Index_RSRP, period: 80ms).
    - Switching condition: ltm4 threshold of -55dBm, count=2 (condition met twice before mobility command issued).
    - Candidate list: specifies the pre-configured target cell and radio parameters.
  - For the target cell:
    - Symmetric configuration with the serving cell, allowing bidirectional LTM testing.
    - Csi_resource_type: "csi_ssb" used for candidate evaluation and switching.
    - Precoding and SSB configuration allow per-beam measurement and switching analysis.
  - Common NR cell parameters are set (subcarrier spacing, bandwidth, TDD pattern).
  - UE configured for two NR cells, TDD mode, 20MHz bandwidth, single simulated UE with LTM support enabled.
- Test Execution Steps
  - Start with serving cell at higher power (dl_gain = 0dB), candidate at lower power (dl_gain = -20dB).
  - Reverse cell powers (serving to -20dB, candidate to 0dB) to trigger LTM condition.
  - UE detects new candidate as stronger, meets LTM4 threshold in two consecutive reports, and triggers LTM mobility.
  - Network sends L1/L2 mobility command, UE switches to the target cell, and resumes operation.
- Log Analysis
  - UE capability signaling confirms support for NR band n78 and LTM features.
  - Measurement configuration and reporting confirm correct setup of SSB-based measurements and gap handling.
  - MAC/PHY logs show LTM triggering, cell switch command transmission, context handling, and successful resumption on the target cell.
Test 2: SA Intra Frequency - LTM4 and L3 Measurement
- Configuration Procedure
  - Similar two-cell setup as Test 1, but with explicit Layer 3 measurement control added in the configuration.
  - Layer 3 measurements (A2 and A3 events) are configured in meas_config_desc:
    - A2 event detects serving cell degradation (RSRP falls below threshold).
    - A3 event detects neighbor cell becoming better than serving cell by a configured offset.
    - Measurement gaps are configured as needed for inter-frequency scenarios.
  - LTM candidate preparation and reporting are configured as in Test 1.
- Test Execution Steps
  - Initial downlink power conditions confirmed (serving stronger than candidate).
  - Powers reversed to trigger measurement events:
    - A2 event triggers measurement report when serving cell degrades.
    - A3 event triggers report when candidate cell improves.
    - Measurement gap configuration ensures UE can perform necessary measurements.
  - Upon satisfying LTM4 (L1) condition, network triggers LTM Cell Switch Command.
  - UE switches to target cell, applies pre-configured candidate settings, and confirms switch by sending RRC Reconfiguration Complete on the new cell.
- Log Analysis
  - UE capability signaling and measurement reports confirm correct event handling (A2/A3), measurement gap usage, and candidate evaluation.
  - Advance preparation of target UE context demonstrated (pre-allocating resources and configuration before cell switch).
  - CSI reporting and candidate configuration logs show mapping of SSB resources and TCI states for beam-based switching.
  - MAC/PHY logs confirm successful triggering, execution, and completion of LTM mobility.

Overall Test Methodology:

Configure two NR SA cells and a compliant UE (real or simulated) for LTM testing, ensuring both cells can serve as LTM candidates for each other.
Pre-configure LTM candidate cell information in the UE using "immediate" configuration trigger for fast Layer 1/2 mobility execution.
Set up measurement reporting and LTM4 switch conditions (threshold and count) to make the handover decision robust against short-term fluctuations.
Use explicit Layer 3 measurement events (A2, A3) in advanced scenarios to control when LTM candidate preparation is activated.
Perform stepwise power adjustments to trigger the mobility conditions and observe the UE's response and network handling through logs and traces.
Verify successful mobility by checking RSRP before/after handover, RNTI changes, and log indications of context movement and random access on the new cell.

Test Setup

Test setup for this tutorial is as shown below.

SIM Card used in this tutorial is the one delivered with the system as it is.
If you want to change the configuration, The tutorial Configuration Guide would help

< Setup A >

TestSetup Callbox UE 2sdr 01

< Setup B >

Key Configuration Parameters

The following are important L1/L2 Triggered Mobility (LTM) configuration parameters for this tutorial. Click an item to open its description in the Amarisoft documentation.

Test 1 : SA Intra Frequency - LTM4

In this test, I will show a simple scenario for triggering L1/L2 Triggered Mobility between two NR SA cells. The mobility is triggered by directly changing the cell power, causing the UE measurement result to satisfy the configured condition. The UE sends a measurement report, and the network then triggers the UE to switch to the selected target cell using L1/L2 signaling.

Configuration

I used the gnb-sa-ltm.cfg which is copied and modified from gnb-sa.cfg

LTE HO Inter Config 01

I also used mme-ims.cfg as it is.

LTE HO Intra Config 02

I used the ue-nr-sa-ltm.cfg which is copied and modified from ue-nr-sa.cfg

Configure gnb-sa-ltm.cfg as below.

This configuration enables a two-cell NR SA LTM test scenario. N_CELL is set to 2 to create the serving and target cells, and N_ANTENNA_DL is set to 2 to configure 2Ã�2 MIMO transmission. CONDITIONAL_HO is disabled because this test uses L1/L2 Triggered Mobility rather than Conditional Handover, while LTM is enabled to activate the LTM-specific configuration and procedure.

This configuration defines the LTM relationship between the two NR cells. Cell 1 is configured as the serving cell, while cell 2 is prepared in advance as an LTM candidate target. The preprocessor first assigns default values of 0 to FR2, CONDITIONAL_HO, and LTM when those variables are not explicitly defined. Because LTM is enabled for this test, the LTM-specific configuration is included instead of relying only on the conventional neighbor-cell configuration.

Within the serving-cell configuration, configuration_trigger: "immediate" instructs the gNB to prepare and configure the LTM candidate without waiting for an additional measurement event to occur first. This allows the UE to receive the candidate-cell configuration in advance, so that the later mobility execution can be triggered through L1/L2 signaling with less RRC processing at the time of the cell switch.

The report section configures periodic measurement reporting. The quantity ssb_Index_RSRP requests RSRP measurements for the individual SSB indexes detected from the candidate cell, rather than only a single cell-level measurement value. The period value of 80 configures the UE to send these reports every 80 ms. These periodic reports allow the serving gNB to continuously evaluate the radio quality of the prepared candidate cell and determine whether the configured LTM switching condition has been satisfied.

The cell_switch section defines the condition used for LTM execution. The ltm4 condition uses a threshold of -55 dBm for the configured measurement quantity. The count value of 2 requires the threshold condition to be satisfied in two measurement evaluations before the switch is triggered. This prevents a single short measurement fluctuation from immediately causing the UE to change cells. Once the condition is satisfied for the required count, the network can send the corresponding L1/L2 mobility command to move the UE to the selected candidate.

The candidate_list identifies cell_id 2 as the cell prepared for LTM from cell 1. The detailed radio and access parameters required for the UE to access cell 2 are therefore provided before the actual mobility command. On the target-cell side, ltm_target marks cell 2 as a cell that can operate as an LTM candidate. The candidate_config section selects csi_resource_type: "csi_ssb", meaning that candidate evaluation and switching are based on CSI measurement resources associated with SSB transmission.

The alternative ncell_list block is used when LTM is disabled. In that case, cell 2 is configured only as a normal neighbor cell. The conditional_handover_target parameter is included only when CONDITIONAL_HO is enabled, so it has no effect in this LTM test. Therefore, this configuration clearly separates the LTM procedure from the conventional Conditional Handover procedure while allowing the same two-cell setup to support either test through the preprocessor options.

This block configures cell 2, identified by cell_id 0x02 and physical cell ID 502, as the second NR cell in the two-cell LTM test. It operates on NR band n78 with DL-ARFCN and SSB-ARFCN set to 626400. Since this cell is configured symmetrically with cell 1, it can operate either as the current serving cell or as the prepared LTM target, depending on the UE�s present connection and the direction of mobility.

The ssb_pos_bitmap value enables four SSB positions. The ssb_precoding configuration assigns a different two-antenna precoding vector to each enabled SSB. SSB 0 is transmitted mainly through the first antenna, SSB 1 uses a balanced combination of both antennas, SSB 2 is transmitted mainly through the second antenna, and SSB 3 again uses both antennas with equal weighting. This creates different effective SSB signal levels at the UE, allowing the test to verify SSB-index-specific RSRP reporting and to evaluate how LTM reacts when the quality of a particular SSB changes. The cell_gain value of -20 dB initially reduces the overall signal level of cell 2, so its power can later be increased to satisfy the configured LTM switching condition.

The ltm block defines the LTM operation when cell 2 is the serving cell. configuration_trigger: "immediate" causes the gNB to provide the candidate-cell configuration without waiting for a separate measurement-trigger event. As a result, the UE is prepared in advance with the information required to switch from cell 2 to cell 1. This advance preparation is one of the main differences between LTM and a conventional RRC-based handover, because the final mobility execution can be initiated through lower-layer signaling without sending the full target configuration at the switching moment.

The report section requests periodic measurement reporting using ssb_Index_RSRP. This quantity reports the RSRP measured for individual SSB indexes, which is important in this test because the enabled SSBs use different precoding vectors and may therefore be received with different power levels. The period value of 80 configures the reporting interval to 80 ms. These periodic reports provide the serving gNB with frequent updates about the SSB-level radio quality of the candidate cell.

The cell_switch section defines the LTM4 execution condition. The threshold is set to -55 dBm, meaning that the relevant candidate-cell measurement must reach the configured threshold before the switching condition is considered satisfied. The count value of 2 requires the condition to be met twice before the cell switch is triggered. This provides basic filtering against a temporary measurement spike and avoids triggering mobility based on a single isolated report.

The candidate_list identifies cell_id 1 as the LTM candidate for cell 2. Therefore, when the UE is connected to cell 2, cell 1 is prepared as the possible target cell. Together with the corresponding configuration in cell 1, this creates a bidirectional LTM relationship: cell 1 can switch to cell 2, and cell 2 can switch back to cell 1.

The ltm_target section defines how cell 2 is prepared when it is used as an LTM target by the other cell. The candidate_config selects csi_resource_type: "csi_ssb", indicating that SSB-based CSI resources are used for candidate-cell evaluation and switching. This matches the ssb_Index_RSRP reporting configuration and allows the network to evaluate the candidate using measurements associated with the configured SSB resources.

When LTM is disabled, the configuration falls back to the ncell_list block. In that case, cell 1 is configured only as a normal neighboring cell. The conditional_handover_target parameter is enabled only when CONDITIONAL_HO is separately set to 1. Since Conditional Handover is disabled in this test, that parameter is not applied, and the mobility procedure is controlled exclusively by the LTM configuration.

This block(nr_cell_default) defines the common radio configuration for both NR cells. It uses 30 kHz subcarrier spacing, 20 MHz bandwidth, two downlink antennas, and one uplink antenna. For TDD operation, the default pattern uses a 5 ms period with seven downlink slots, two uplink slots, and one special slot containing six downlink symbols and one uplink symbol.

The csi_rs block configures the CSI reference-signal resources and CSI reporting. Inside resource_auto, nzp_csi_rs_period is set to 80 ms for NZP-CSI-RS transmission. When LTM is enabled, ssb_set is set to true to include an SSB-related CSI resource set for candidate-cell beam measurement. Inside csi_report_config, report_config_type is set to periodic, period is set to 80 ms, and report_quantity is set to CRI_RI_PMI_CQI. For FR2, trs_presence is set to false, so TRS is not included in the resource_auto configuration.

The meas_config_desc block defines the general NR measurement configuration. It includes A1, A2, and A3 RSRP-based measurement conditions for normal measurement reporting and conventional handover operation. In this LTM test, the nr_ltm block is not configured inside meas_config_desc. Therefore, LTM candidate preparation is not controlled by a separate Layer 3 measurement-trigger condition. The candidate configuration is provided immediately by the gNB, and the actual cell switch is evaluated using the configured LTM Layer 1 measurement condition.

The nr_conditional_handover block is included only when CONDITIONAL_HO is set to 1 and is not used in this test. The meas_gap_config block sets pattern_id to 0, configuring the measurement-gap pattern available for inter-frequency or other measurements that require measurement gaps.

Configure ue-nr-sa-ltm.cfg as below

N_CELL is set to 2, NR_TDD is set to 1 for TDD operation, and CELL_BANDWIDTH is set to 20 MHz. N_ANTENNA_DL is set to 2 for 2x2 downlink MIMO, while N_ANTENNA_UL is set to 1. UE_COUNT is set to 1, and CHANNEL_SIM is set to 0 because the channel simulator is not used in this test.

The cell_groups block defines one NR cell group containing the RF configurations for both cells used in the LTM test. The parameter group_type is set to nr, and multi_ue is set to 1.

Inside the cells block, the first cell uses rf_port 0, band 78, dl_nr_arfcn 625400, and ssb_nr_arfcn 625400. The second cell uses rf_port 1, band 78, dl_nr_arfcn 626400, and ssb_nr_arfcn 626400. Both cells use the bandwidth defined by CELL_BANDWIDTH, 30 kHz SSB and data subcarrier spacing, and the antenna numbers defined by N_ANTENNA_DL and N_ANTENNA_UL.

The ue_list block defines the simulated UE identity and capability settings used for the LTM test. The parameter as_release is set to 18 so that the UE advertises an NR access-stratum release that supports LTM. The parameter ue_category is set to nr, ue_count uses the value defined by UE_COUNT, and ltm_support is set to true to enable LTM capability in the simulated UE. The parameter rrc_initial_selection is set to false so that the UE does not perform automatic initial cell selection based on the strongest detected cell.

Perform the test

The cell_phy command displays the physical-layer configuration of the two NR cells used in the LTM test. Cell 0x001 uses dl_nr_arfcn 625400 and ssb_nr_arfcn 625400, while cell 0x002 uses dl_nr_arfcn 626400 and ssb_nr_arfcn 626400. Both cells operate in band n78 with 20 MHz bandwidth and 30 kHz subcarrier spacing. Since both ARFCNs belong to the same NR band, this is an intra-frequency LTM scenario.

The cell command first confirms the initial downlink power condition. Cell 0x001 with PCI 501 has dl_gain set to 0.0 dB and acts as the serving cell, while cell 0x002 with PCI 502 has dl_gain set to -20.0 dB and is initially weaker.

The cell_gain 1 -20 command reduces the dl_gain of cell 0x001 to -20 dB, and the cell_gain 2 0 command increases the dl_gain of cell 0x002 to 0 dB. This reverses the relative received power of the two cells and creates the triggering condition for LTM. The UE then detects cell 0x002 as the stronger candidate and the configured LTM measurement condition can be satisfied.

The UE trace shows the LTM cell switch from cell 0x00 with RNTI 4601 to cell 0x01 with RNTI 4602. Before the switch, the UE remains connected to the original serving cell while its RSRP decreases to approximately -58.8 dBm. After the configured LTM condition is satisfied, the UE changes to the target cell and receives the new RNTI 4602. The RSRP then improves to approximately -37.1 dBm, confirming that the UE has completed the L1/L2 triggered mobility procedure and is operating on the stronger target cell.

Log Analysis

Sample Log

The UE capability enquiry message is sent by the gNB after the security procedure to request the UE radio capability required for the LTM test. In the ueCapabilityEnquiry block, rat-Type is set to nr, and frequencyBandListFilter specifies bandNR 78 so that the UE reports its NR capabilities for band n78. The nonCriticalExtension block includes rrc-SegAllowed-r16 enabled, indicating that segmented RRC capability transfer is supported when the UE capability information is too large for a single message.

The UE capability information message reports the UE radio capabilities for NR. In the featureSetDownlinkPerCC-r18 block, the parameter ltm-MCG-IntraFreq-r18 is set to supported, indicating that the UE supports Release 18 intra-frequency L1/L2 Triggered Mobility within the Master Cell Group. This capability allows the gNB to configure and trigger LTM between NR cells operating on the same frequency layer.

The measAndMobParametersCommon block reports the UE measurement and mobility capabilities. The parameter ltm-InterFreq-r18 is set to supported, indicating that the UE supports Release 18 inter-frequency L1/L2 Triggered Mobility between NR cells operating on different frequencies.

The UE Caps window provides a simplified summary of the decoded UE capability information. Selecting UE Caps opens the capability view, where the NR bands section confirms support for band n78 and the CA combination section lists the supported downlink and uplink band combinations.

In this example, Comb. 1 shows a two-carrier NR combination using DL n78A + UL n78A together with another DL n78A + UL n78A component. Comb. 2 shows DL n78B + UL n78B, while Comb. 3 shows DL n78A + UL n78A. These entries confirm the UE bandwidth-class and carrier-combination capabilities reported for band n78.

The measConfig block in the RRC reconfiguration message configures the NR measurement object used for LTM candidate-cell measurement. Inside measObjectToAddModList, measObjectId is set to 1 and measObjectNR defines the target NR carrier. The parameter ssbFrequency is set to 625400, ssbSubcarrierSpacing is set to kHz30, and freqBandIndicatorNR is set to 78, identifying the candidate carrier as band n78 with 30 kHz SSB subcarrier spacing.

Inside smtc1, periodicityAndOffset is set to sf20 with offset 0, so the UE searches for the candidate-cell SSB every 20 ms starting from offset 0. The parameter duration is set to sf1, defining a 1 ms SSB measurement window.

Inside referenceSignalConfig, the ssb-ConfigMobility block sets deriveSSB-IndexFromCell to FALSE. This means that the UE does not derive the SSB index automatically from the physical cell identity and instead uses the configured SSB measurement information.

The parameter quantityConfigIndex is set to 1, linking this measurement object to the first entry in the quantityConfig block. Inside quantityConfigNR-List, the quantityConfigCell block provides the filtering configuration for SSB-based and CSI-RS-based measurement results through ssb-FilterConfig and csi-RS-FilterConfig.

The measGapConfig block contains gapUE release NULL, indicating that no dedicated UE measurement gap is configured. The UE is therefore expected to perform the configured candidate-cell measurements without using a separate measurement gap.

The RRC reconfiguration complete message confirms that the UE successfully applied the measurement configuration and reports its measurement-gap requirements.

Inside needForGapsInfoNR-r16, the intraFreq-needForGap-r16 block sets servCellId-r16 to 0 and gapIndicationIntra-r16 to no-gap. This indicates that the UE can perform the configured intra-frequency NR measurements using serving cell 0 as the reference without a dedicated measurement gap.

The interFreq-needForGap-r16 block sets bandNR-r16 to 78 and gapIndication-r16 to gap. This indicates that the UE requires a measurement gap when performing inter-frequency measurements on NR band n78.

The masterCellGroup block in the RRC reconfiguration message updates the UE radio configuration for the serving Master Cell Group. In the rlc-BearerToAddModList block, the gNB configures the signalling radio bearers, including logicalChannelIdentity 4 mapped to servedRadioBearer srb-Identity 1 and logicalChannelIdentity 2 mapped to servedRadioBearer srb-Identity 2.

The rlc-Config block defines the AM RLC parameters for each bearer. The ul-AM-RLC and dl-AM-RLC blocks configure values such as sn-FieldLength, t-PollRetransmit, pollPDU, pollByte, maxRetxThreshold, t-Reassembly, and t-StatusProhibit.

The mac-LogicalChannelConfig block configures uplink logical-channel parameters such as priority, prioritisedBitRate, bucketSizeDuration, logicalChannelGroup, schedulingRequestID, logicalChannelSR-Mask, and logicalChannelSR-DelayTimerApplied.

The spCellConfig block contains the serving-cell-specific configuration, including uplinkConfig and initialUplinkBWP. This RRC reconfiguration prepares the UE with the updated bearer, MAC, RLC, and serving-cell settings required before the LTM candidate-cell configuration and subsequent mobility operation.

The csi-MeasConfig block adds the CSI measurement and reporting configuration used for L1/L2 Triggered Mobility. Inside ltm-CSI-ReportConfigToAddModList-r18, ltm-CSI-ReportConfigId-r18 is set to 1 to identify this LTM CSI report configuration, while ltm-ResourcesForChannelMeasurement-r18 is set to 0 to associate the report with LTM channel-measurement resource configuration 0.

The ltm-ReportConfigType-r18 block configures periodic reporting. reportSlotConfig-r18 is set to slots80, meaning that the UE transmits the LTM CSI report once every 80 slots. With the configured 30 kHz subcarrier spacing, one slot is 0.5 ms, so this corresponds to a 40 ms reporting interval. uplinkBandwidthPartId is set to 0, selecting uplink BWP 0 for report transmission, and pucch-Resource is set to 14, selecting PUCCH resource 14 to carry the report.

The ltm-ReportContent-r18 block limits the report content. nofReportedCells-r18 is set to n1, so the UE reports one candidate cell, while nofReportedRS-PerCell-r18 is set to n1, so one measured reference signal, such as one SSB, is reported for that candidate cell.

The ltm-CandidateToAddModList-r18 block adds the Release 18 LTM candidate-cell configuration that prepares the target cell before the actual L1/L2 cell switch. The parameter ltm-CandidateId-r18 is set to 1 to identify this candidate configuration, while ltm-CandidatePCI-r18 is set to 502 to identify the target cell by its physical cell ID.

Inside ltm-SSB-Config-r18, ssb-Frequency-r18 is set to 626400, subcarrierSpacing-r18 is set to kHz30, and ssb-Periodicity-r18 is set to ms20. These parameters define the candidate-cell SSB carrier, subcarrier spacing, and SSB burst periodicity used by the UE for detection and measurement. The parameter ssb-PositionsInBurst-r18 is set to mediumBitmap f0H to indicate the configured SSB positions within the candidate-cell SSB burst. The parameter ssb-PBCH-BlockPower-r18 is set to -56 dBm and provides the reference SSB/PBCH transmission power used when evaluating candidate-cell measurements.

The ltm-CandidateConfig-r18 block contains the candidate-specific RRC configuration that will be applied when the UE switches to this cell. The rrc-TransactionIdentifier is set to 0 to identify the embedded RRC configuration transaction.

Inside radioBearerConfig, srb-ToAddModList prepares SRB1 and SRB2 for operation on the candidate cell. The parameter discardOnPDCP is set to true for both signalling radio bearers, allowing pending PDCP data associated with these SRBs to be discarded during the mobility transition.

The drb-ToAddModList block prepares the data radio bearer that will continue on the candidate cell. The parameters pdu-Session and drb-Identity are both set to 1, mapping DRB1 to PDU session 1. The mappedQoS-FlowsToAdd value of 1 associates QoS flow 1 with this DRB.

Inside pdcp-Config, pdcp-SN-SizeUL and pdcp-SN-SizeDL are set to len18bits, so 18-bit PDCP sequence numbers are used in both uplink and downlink. The parameter statusReportRequired is set to true to support PDCP status reporting and data continuity during the cell switch. The parameter t-Reordering is set to ms3000, configuring the PDCP reordering timer to 3000 ms.

The csi-SSB-ResourceSetToAddModList block configures the SSB resource set used for CSI and LTM candidate-cell measurements. The parameter csi-SSB-ResourceSetId is set to 0 to identify this resource set.

Inside csi-SSB-ResourceList, SSB indexes 0, 1, 2, and 3 are included. This allows the UE to measure and compare the four configured SSB beams of the candidate cell. The measured SSB resources can then be referenced by the LTM CSI reporting configuration for beam-quality reporting and candidate-cell switch evaluation.

The ltm-ConfigComplete-r18 parameter is set to true, indicating that the UE has received the complete candidate-cell configuration required for the later LTM cell switch.

The ltm-TCI-Info-r18 block provides the transmission configuration indication and beam-related information prepared for the candidate cell. Inside ltm-DL-OrJointTCI-StateToAddModList-r18, tci-StateId-r18 values 0 through 3 define four candidate-cell TCI states.

For each TCI state, referenceSignal-r18 identifies one candidate-cell SSB index from 0 through 3. The qcl-Type-r18 parameter is set to typeD, indicating spatial QCL. This allows the UE to derive the receive beam direction and related spatial properties from the corresponding candidate-cell SSB.

The pathlossReferenceRS-Id-r18 parameter associates each TCI state with a pathloss reference signal configuration. Inside pathlossReferenceRS-ToAddModList-r18, pathlossReferenceRS-Id-r17 values 0 through 3 are mapped to referenceSignal-r17 SSB indexes 0 through 3. These SSBs are used as pathloss references when the UE determines the uplink transmission parameters after switching to the candidate cell.

The ltm-CSI-ResourceConfigToAddModList-r18 block adds the CSI resource configuration used for LTM candidate-cell measurement. The parameter ltm-CSI-ResourceConfigId-r18 is set to 0 to identify this LTM CSI resource configuration.

Inside ltm-SSB-ResourceSet-r18, the ltm-SSB-ResourceList-r18 includes SSB indexes 0, 1, 2, and 3. This allows the UE to measure all four configured SSB beams of the candidate cell.

The ltm-CandidateIdList-r18 contains candidate ID 1 for each SSB resource. This maps all four SSB indexes to LTM candidate 1, allowing the UE to measure the candidate-cell beams and report the best beam through the configured periodic LTM CSI report.

The MAC and PHY logs show the LTM execution after the UE reports the candidate-cell CSI measurement. The log entry LTM triggering condition is met indicates that the configured LTM4 threshold and count condition have been satisfied based on the received CSI report.

The gNB then issues the LTM cell-switch command through lower-layer signaling. The following log entry LTM cell switch command transmitted confirms that the command has been sent to the UE, after which the UE is placed in a stopped state on the source cell.

The UE then accesses the prepared target cell using PRACH. The target cell detects the random access preamble, allocates a new UE context, and assigns a new C-RNTI. The subsequent PDSCH, PUSCH, and MAC control-element exchanges confirm that the UE has completed access to the target cell and resumed communication after the L1/L2 triggered cell switch.

NOTE : If you enable csi decoding option, you can see more detailed information from the csi message. The decoded CSI log provides additional visibility into the LTM measurement and triggering process. The CSI entries with quantity ssb-Index-RSRP and ltm 1 identify periodic LTM CSI reports from the UE. These entries show the reported candidate-cell SSB index and its measured RSRP value.

In this example, the reported RSRP for the LTM candidate improves from approximately -58 dBm to -44 dBm. After the configured threshold and count requirements are satisfied, the MAC log displays LTM triggering condition is met, issue LTM cell switch command. This confirms that the decoded CSI report was used to evaluate the LTM4 condition and trigger the lower-layer cell-switch command.

The LTM Cell Switch Cmd MAC control element instructs the UE to switch to the prepared target configuration. Target Config ID is set to 0, identifying the candidate configuration that the UE must apply. TCI state ID is set to 3, selecting the prepared beam associated with TCI state 3 for the target-cell access.

Timing Advance Command indicates that no valid timing adjustment is available. Therefore, the UE cannot reuse a preconfigured timing-advance value for the target cell and must establish uplink timing through the subsequent random access procedure.

The LTM cell switch command transmitted log confirms that the MAC-layer LTM cell-switch command was successfully delivered to the UE. After this confirmation, the source-cell UE context is moved to the stopped state and the UE begins random access on the prepared target cell.

The set UE to stopped state log indicates that the source cell stops scheduling the UE after transmitting the LTM cell-switch command. This prevents further user-plane and control-plane transmission on the old serving cell while the UE moves to the prepared target cell.

The following PRACH log on cell 2 shows that the UE begins random access toward the target cell. The target cell detects the preamble, allocates a temporary UE context, sends the Random Access Response, and assigns a new C-RNTI. Subsequent PUSCH, PDSCH, MAC, and RRC activity confirms that the UE resumes communication on the target cell and completes the LTM transition.

The PRACH log shows that the UE starts random access on target cell 2 immediately after leaving the source cell. The detected preamble uses sequence_index 6 and was received with ssb_index 3, which matches the TCI state selected in the LTM cell-switch command. The UE therefore accesses the prepared target cell through the selected candidate-cell beam before receiving the Random Access Response and completing the remaining RACH procedure.

The RRC reconfiguration complete message confirms that the UE has successfully applied the prepared target-cell RRC configuration after accessing cell 2. The message is transmitted on DCCH-NR through the target cell, confirming that the LTM cell switch and subsequent RRC configuration activation have completed successfully.

The needForGapsInfoNR-r16 block again reports the UE measurement-gap capability. Inside intraFreq-needForGap-r16, servCellId-r16 is set to 0 and gapIndicationIntra-r16 is set to no-gap, indicating that intra-frequency measurements can be performed without a dedicated gap. Inside interFreq-needForGap-r16, bandNR-r16 is set to 78 and gapIndication-r16 is set to gap, indicating that inter-frequency measurements on band n78 require a measurement gap.