On L1/L2-Triggered Mobility in 3GPP Release 18 and Beyond

5-th generation (5G) has brought mobility robustness and interruption time reduction enhancements to mobility procedures. Advanced beam management procedures were also introduced in 5G, including beam-based mobility. In this paper, we summarize the procedure that fuses beam-based mobility with conventional mobility procedures, namely L1/L2-triggered mobility (LTM). We first provide the Release 18 LTM procedural summary, followed by a detailed explanation of the introduced technical enhancements and an analysis of their expected benefits. Through system-level simulations, we show that having the LTM as the mobility procedure brings two major improvements, namely interruption time reduction and mobility robustness. The lower interruption time is mainly the result of configuring the network and the user equipment (UE) well in advance of the handover, early configuration decoding, early synchronization, and triggering the handover with a lightweight medium access control (MAC) command. The mobility robustness increase is the result of the triggering of the handover based on the layer 1 (L1) measurements, which are more sensitive to channel degradation. This in turn further reduces the interruption time as the radio link failure and reestablishment are avoided. On the other hand, LTM can suffer from excess unnecessary handovers, where the UE is handed over back and forth between two or three cells. Besides, the increased measurement reporting of the L1 measurements can cause additional overhead. Finally, we give a short insight on the on-going Release 19 work that addresses some the known LTM issues.