 based on UE measurements. This implies that all handover decisions for RRC connected mode are taken by the network, often triggered by radio resource management (RRM) reporting events from the UE. The most commonly used UE RRM reporting for triggering a handover is event A3, which is defined as the target cell being offset decibels stronger than the serving source cell. For RRC Idle mode, the network can provide additional guidance (a.k.a. Idle mode traffic steering) by broadcasting cell reselection parameters and setting different priorities per frequency layer. Figure 8.1 pictures a typical HetNet environment with different cell types, as well as a mixture of terminals that are either semi-stationary or moving along certain trajectories at different velocities. Typically, the path loss slope and the spatial correlation distance of shadow fading are different from macro and small cells to UEs, generally resulting in a steeper UE trajectory crossing the UE path crossing several peripheries of multiple small macro and small cells with ((Q)) cells time-variant velocity ((a)) ((p)) ((g)) Indoor UEs with no movement ((o)) ((Q)) Small cell deployed for High-speed UE with macro- hotspot coverage of many only coverage semi-stationary UEs Figure 8.1 High-level sketch of LTE HetNet scenario with a 3-sector macro base station and multiple small cells and UEs Small Cell Optimization gradient of the received signal strength from small cells as compared to macro cells. Timely and accurate handover between co-channel-deployed macro and small cells is therefore essential for ensuring acceptable mobility performance. Especially small cell outbound handover is challenging for high-velocity users, while good mobility performance for users at lower speed can be achieved with basic LTE handover methods [1]. In this context, the following aggregates are worth highlighting for optimizing the co-channel HetNet mobility: Mobility state estimation and time-to-trigger (TTT) scaling [Release 8]: Based on the past number of experienced han