5G Lower Layer Triggered Mobility | L1/L2 Mobility

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5GLTM | Lower Layer Triggered Mobility

In 5G networks, during the mobility a small interruption time for UE is extremely crucial for ultra-reliable low latency communication (URLLC) applications as well eMBB traffic. Similar to legacy cellular technology, 5G allows handover Mobility decisions based on the measurement report of neighboring cells received from the UE. The performance of the baseline handover mechanism highly depends on the time-line latency of measurement report transmission and/or HO command reception.

Following up from 3GPP Release 17 discussion, one of the objectives of 3GPP Release 18 mobility enhancements work item identified to facilitate any new algorithm for lower layer cell change based on L1 measurements where the handover is triggered by Medium Access Control (MAC) using lower layer signaling. This new solution for 5G Advanced network will be supported for FR1 and FR2 and is referred as L1/L2 inter-cell mobility. Lower Layer Triggered Mobility goal to reduce interruption during handover (HO) execution compared to baseline HO.

Key Pointers

  • LTM also know as L1/L2 Mobility
  • It is a 3GPP Release 18/ 5G Advance Feature
  • In LTM handover is executed based on Layer 1 (L1) measurements so called Lower Layer Triggered Mobility (LTM) .
  • It has been shown that lower layer triggered mobility procedure outperforms the existing handover mechanisms with respect to radio communication reliability at the expense of higher number of handovers and ping-pongs.
  • To tackle these drawbacks, additional filtering for the L1 measurements used in handover decision is introduced to reduce the fluctuations caused by fast fading and measurement errors.
  • Lower layer mobility procedure is enhanced with dynamic switching mechanism enabling the UE to change cells without being reconfigured by the network.
  • Such techniques is beneficial in reducing the number of ping-pongs and signaling overhead at the expense of an increase in the delay to react to rapid signal degradation and resource reservation overhead, respectively

5G LTM Procedure Details

LTM is a procedure in which a gNB receives L1 measurement report(s) from a UE, and on their basis the gNB changes UE serving cell by a cell switch command signaled via a MAC CE.

The cell switch command indicates an LTM candidate configuration that the gNB previously prepared and provided to the UE through RRC signaling. Then the UE switches to the target configuration according to the cell switch command. The LTM procedure can be used to reduce the mobility latency.

When configured by the network, it is possible to activate TCI states of one or multiple cells that are different from the current serving cell. The TCI states of the LTM candidate cells can be activated in advance prior to any of those cells become the serving cell, allowing the UE to be DL synchronized with those cells, thereby facilitating a faster cell switch to one of those cells when cell switch is triggered. When configured by the network, it is possible to initiate UL TA acquisition (called early TA) procedure of one or multiple cells that are different from the current serving cells. The network may request the UE to perform early TA acquisition of a candidate cell before a cell switch.

The early TA acquisition procedure is triggered by PDCCH order or realized through UE-based TA measurement as configured by RRC. In the former case, the gNB/gNB-DU to which the candidate cell belongs calculates the TA value and sends it to the gNB/gNB-DU to which the serving cell belongs via gNB-CU.

The serving cell sends the TA value in the LTM cell switch command MAC CE when triggering LTM cell switch. In the latter case, the UE performs TA measurement for the candidate cells after being configured by RRC but the exact time the UE performs TA measurement is up to UE implementation. The UE applies the TA value measured by itself and performs RACH-less LTM upon receiving the cell switch command. The network may also send a TA value in the LTM cell switch command MAC CE without early TA acquisition.

Depending on the availability of a valid TA value, the UE performs either a RACH-less LTM or RACH-based LTM cell switch. If the valid TA value is provided in the cell switch command, the UE applies the TA value as instructed by the network. In the case where UE-based TA measurement is configured, but no valid TA value is provided in the cell switch command, the UE applies the valid TA value by itself if available.

Meanwhile, the UE performs RACH-less LTM cell switch upon receiving the cell switch command. If no valid TA value is available, the UE performs RACH-based LTM cell switch. Regardless of whether the UE is configured for UE-based TA measurement for a certain candidate cell, it will still follow the PDCCH order, which includes requesting a random access procedure towards the candidate cells. This also applies to the candidate cells for which the UE is capable of deriving TA values by itself. Additionally, regardless of whether the UE has already performed a random access procedure towards the candidate cells, it will still follow the UE-based measurement configuration if configured by the network.

For RACH-less LTM, the UE accesses the target cell using either a configured grant or a dynamic grant. The configured grant is provided in the LTM candidate configuration, and the UE selects the configured grant occasion associated with the beam indicated in the cell switch command. Upon initiation of LTM cell switch to the target cell, the UE starts to monitor PDCCH on the target cell for dynamic scheduling. Before RACH-less LTM procedure completion, the UE shall not trigger random access procedure if it does not have a valid PUCCH resource for triggered SRs.

LTM supports both intra-gNB-DU and intra-gNB-CU inter-gNB-DU mobility, along with both intra frequency and inter-frequency mobility, including mobility to inter-frequency cell that is not a current serving cell. LTM is supported only for licensed spectrum. The following scenarios are supported:

  • PCell change in non-CA scenario and non-DC scenario;
  • PCell and SCell(s) change in CA scenario;
  • Dual connectivity scenario: including PCell and MCG SCell(s) change and intra-SN PSCell and SCG
  • SCell(s) change without MN involvement. LTM for simultaneous PCell and PSCell change is not supported.

Signaling procedure for LTM

Following figure depicts the LTM Signaling procedure.

5G-Lower-Layer-Triggered-Mobility

  • Step #1: UE sends a Measurement Report message to the gNB.
  • Step#2: gNB transmits an RRC Reconfiguration message to the UE including LTM candidate configurations.
  • Step#3: UE stores the LTM candidate configurations and transmits an RRC Reconfiguration Complete message to the gNB.
  • Step#4 (a): UE performs DL synchronization with LTM candidate cells before receiving the cell switch command
  • Step#4 (b):UE performs UL synchronization with LTM candidate cells before receiving the cell switch command.
  • Step#5: UE performs L1 measurements on configured LTM candidate cells and transmits L1 measurement reports to the gNB.
  • Step#6: gNB decides to execute cell switch to a target cell and transmits an LTM cell switch command MAC CE.
  • Step#7: UE performs the random access procedure towards the target cell if UE does not have a valid TA of the target cell.
  • Step#8: UE completes the LTM cell switch procedure by sending an RRC Reconfiguration Complete message to the target cell.

In LTM, the UE may perform partial or full MAC reset, may re-establish radio link control (RLC), may perform data recovery with packet data convergence protocol (PDCP) during cell switch.

References :

  • 3GPP TS 38.300: ” NR; NR and NG-RAN Overall Description; Stage 2″.
  • RP-192534, Revised WID on NR mobility enhancements, Intel Corporation, RAN#86.
  • RP-201274 Summary of WI on NR mobility enhancements.
  • 3GPP TS 38.331: “NR; Radio Resource Control (RRC); Protocol specification”.
  • RP-213565 New WID on Further NR mobility enhancements.
  • R2-2300375 38.300 running CR for introduction of NR further mobility enhancements.

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