L1/L2 Triggered Mobility (LTM) as Baseline for Mobility in 6G

In current cellular networks, when a UE moves from the coverage area of one cell to another cell, a handover procedure needs to be performed. However, the baseline handover procedure used in 5G networks, as well as in the previous cellular network generations, is triggered by $\mathrm{L 3}$ measurements and involves a series of processes between the network and UE which result in a complete change of network configuration at the UE and reset of L1/L2 protocols. Therefore, the legacy L3 handover procedure leads to larger overhead and longer interruption time than beam switch mobility. To overcome this drawback, this paper highlights the latest progress made by 3GPP in the domain of mobility and handover. In doing that, we provide a comprehensive description of L1/L2 Triggered Mobility (LTM), the fundamental steps involved in LTM execution and LTM enhancements over the legacy L3 handover procedure. This scope of the paper is also complemented by a numerical simulation analysis which emphasizes LTM benefits, in terms of reduced handover interruption, downlink packet delay, and downlink packet drop rate at the UE.