ows changing layer via idle mode if needed. However, heavy data consumers which in practice generate most of the network load tend to stay in connected mode for extended periods of time. It is thus important to move such users to the best frequency layer by intelligently activating measurement gaps to allow inter-layer connected mode mobility. Already a 3 dB improvement in average SINR increases scheduled throughput by tens of percents or, for a fixed traffic demand, reduces PRB consumption by the same amount. 15.5 Inter-layer Mobility for Capacity-Limited Networks In the case of non-congested network discussed in the previous section, the problem was approached by first defining a minimum acceptable UE throughput and then mapping the throughput target to layer entry and exit thresholds (RSRQ or RSRP). As the offered traffic in the network increases, at some point the PRB utilization hits maximum and multiple UEs have to share the radio resources in every TTI which in turn reduces the UE throughput and increases the packet scheduling delay. Ultimately with too many UEs sharing the radio resources this leads to a situation where the minimum acceptable UE throughput in LTE cannot be satisfied even when SINR or RSRQ is high, which was the design assumption for the unloaded network discussed in the previous section. If all layers within a coverage area are congested, then there is not much that can be done besides increasing network capacity, for example, by deploying small cells in the centre of the traffic hot spots. Therefore, in such a case of both 'vertical and horizontal congestion' it is not possible to improve the situation by off-loading traffic to other cells or layers as all target cell candidates are also congested. On the other hand, if the congestion is vertical-only or Inter-layer Mobility Optimization horizontal-only, it may be possible to utilize the unused resources in other cells by mobility threshold tuning or congestion-triggered load balancing handover. Dealing with intra-layer (horizontal) cong