work of a radio planning engineer. This is partly because, at the time of writing, the algorithms introduced in literature have not been implemented in real-world products, but also because the basic operation of the algorithms relies on information that is not directly reported by UE to the network (e.g. downlink SINR). In this section the focus is on inter-layer mobility in a classical macro-cell network. Specif- ically, small cells, carrier aggregation and coordinated multipoint are beyond the scope of discussion. In the long term, carrier aggregation and coordinated multipoint techniques are expected to drive a paradigm shift from classical 'slow' handover-based mobility to 'fast' millisecond scale mobility, although the transition period is likely to take some years. To facilitate discussion it is useful to define some terminology for inter-layer mobility. Towards this purpose, Figure 15.6 illustrates a three-layer network with three different prior- ities. The highest priority layer could be 20 MHz LTE2600, while the second highest layer could be 10 MHz LTE1800, for example. The coverage layer - 2G or 3G - has the lowest priority and is to be used as the last resort when LTE service level is insufficient. In spirit of the priority-based layering, the idle and connected mode mobility thresholds are set SO that the UE is moved to a higher priority layer whenever the service level offered by that layer is better than some minimum requirement; this is called the entry point of the layer. In reverse direction, the exit point for a layer should reflect a service level that is below an acceptable level. Obviously, some hysteresis is required in order to avoid unnecessary layer changes. The design problem is to choose initial entry and exit thresholds to satisfy the given service-level requirement, while the optimization problem is to tune those thresholds based on actual monitored performance. To be able to discuss the mobility threshold design in a practically meaningful way, definition of service level is needed