bearer over X2-type non-ideal back- haul connections can achieve approximately 80% of the gain available with ideal fibre-based fronthaul connections. The gain mechanism with inter-site carrier aggregation and DC is multifold. First, terminals configured with carrier aggregation/DC benefit from higher transmission bandwidth by being able to utilize the radio resources in both macro and small cell layers. This is especially visible in low load conditions in Figure 8.10, in which case the average number of allocated physical resource blocks (PRBs) per UE with inter-site carrier aggregation almost doubles as compared to the case with neither carrier aggregation nor DC. This is because the probability of having a single user accessing all the available radio resources in both the macro and the small cells higher at low load. At higher load the gain from larger allocated bandwidth becomes marginal as the average number of users in a system starts to increase. Second, the system can benefit from increased multi-user diversity order and faster inter- layer load balancing if terminals are configured with DC/carrier aggregation between macro and small cells. Contrary to the bandwidth gain, the load balancing gains become more noticeable as the load increases. As clearly illustrated in Figure 8.11, the load is more balanced among the cells when either carrier aggregation or DC is used. This is especially evident in the macro layer, resulting in a better utilization of the available radio resources, as shown in Figure 8.12. Small Cell Optimization Macro - high load 95 percentile 50 percentile Without CA/DC CA with ideal DC with non-ideal backhaul backhaul Small cell - high load 95 percentile 50 percentile Without CA/DC CA with ideal DC with non-ideal backhaul backhaul Figure 8.11 Number of UEs served in each cell with and without DC at high offered load of 50 Mbps 8.3.2 Mobility with Dual Connectivity DC can be utilized for improving mobility robustness. We will next analyse the impact of DC on the number of primary cell ha