In an earlier post we looked at the 3GPP Release 18 Description and Summary of Work Items. One of the key areas was Further NR mobility enhancements, where a new feature called L1/L2-triggered mobility (LTM) has been introduced. This procedure aims to reduce mobility latency and improve handover performance in 5G-Advanced.

Mobility has always been one of the most important areas in cellular networks. The ability of a user equipment (UE) to move between cells without losing service is essential for reliability and performance. Traditional handover procedures in 4G and 5G rely on Layer 3 (L3) signalling, which is robust but can result in high signalling overhead and connection interruption times of 50 to 90 milliseconds. While most consumer services can tolerate this, advanced use cases with strict latency demands cannot.

3GPP Release 18 takes a significant step forward by introducing the L1/L2 Triggered Mobility (LTM) procedure. Instead of relying only on L3 signalling, LTM shifts much of the handover process down to Layer 1 (physical) and Layer 2 (MAC), making it both faster and more efficient. The goal is to reduce interruption to around 20 to 30 milliseconds, a level that can better support applications in ultra-reliable low latency communication, extended reality and mobility automation.

The principle behind LTM is straightforward. The UE is preconfigured with candidate target cells by the network. These configurations can be provided in two ways: either as a common reference with small delta updates for each candidate or as complete configurations. Keeping the configuration of multiple candidates allows the UE to switch more quickly without requiring another round of reconfiguration after each move.

Measurements are then performed at lower layers. The UE reports reference signal measurements and time and phase information to the network. Medium Access Control (MAC) control elements are used to activate or deactivate target cell states, including transmission configuration indicator (TCI) states. This ensures the UE is already aware of beam directions and reference signals in the target cells before the actual switch.

A particularly important innovation in LTM is the concept of pre-synchronisation. Both downlink and uplink pre-synchronisation can take place while the UE is still connected to the serving cell. For downlink, the network instructs the UE to align with a candidate cell’s beams. For uplink, the UE can transmit a random-access preamble towards a target cell, and the network calculates a timing advance (TA) value. This TA is stored and delivered only at the moment of execution, allowing the UE to avoid a new random access procedure. In cases where TA is already known or equal to the serving cell, the handover becomes RACH-less, eliminating a significant source of delay.

The final step is the LTM cell switch command. This MAC control element carries the chosen target configuration, TA value and TCI state indication. Since synchronisation has already been achieved, the UE can break the old connection and resume data transfer almost immediately in the new cell.

Compared to earlier attempts such as Dual Active Protocol Stack (DAPS) handover, which required maintaining two simultaneous connections and faced practical limitations, LTM offers a more scalable solution. It can be applied across frequency ranges, including higher bands above 7 GHz where beamforming is critical, and it works for both intra-DU and inter-DU mobility within a gNB.

The Release 18 specification restricts LTM to intra-gNB mobility, but work has already begun in Release 19 to expand it further. Future enhancements are expected to cover inter-gNB mobility and to refine measurement reporting for even greater efficiency.

Looking beyond 5G Advanced, new concepts are being explored for 6G. At the Brooklyn 6G Summit 2024, MediaTek introduced the idea of L1/L2 Triggered Predictive Mobility (LTPM), where predictive intelligence could play a role in mobility decisions. While this is still at an early research stage, it points to how mobility management will continue to evolve.

For now, the introduction of LTM marks a practical and important milestone. By reducing handover latency significantly, it brings the network closer to meeting the demanding requirements of next generation services while maintaining efficiency in signalling and resource use.

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