haul which means that no direct fibre connection is required to the small cell. The architecture options are shown in Figure 8.5. Release 12 solution is based on DC feature and bearer split where the UE is simultaneously connected to macro cell and small cell and it can receive data of the same bearer from each of the two cells. Macro @ f1 Small cell @ f2 ((a)) UE with dual connectivity. UE served by small UE served by macro- PCell on macro and SCell cell only (PCell on only (PCell on macro) on small cell small cell) Figure 8.4 Inter-site carrier aggregation between macro and small cells Small Cell Optimization Small cell without any aggregation in Release 8 X2 with non-ideal backhaul Macro cell Small cell including baseband Carrier aggregation in Release 10 Ideal backhaul Macro cell RF head Inter-site carrier aggregation in Release 12 X2 with non-ideal backhaul Macro cell Small cell including baseband Figure 8.5 HetNet architecture options Figure 8.6 generally illustrates the network and UE functioning with DC and bearer split between a macro and a small cell. Data from the core network are first transferred to the macro cell, which operates as the master eNodeB (MeNB). In the macro the data flow is split, SO some data are transmitted via the macro to the UE, while other data are transferred over X2 to the small cell, which operates as the secondary eNodeB (SeNB). The MeNB and SeNB have independent medium access control (MAC) entities and physical layer processing, including independent hybrid automatic repeat request (HARQ) and link adaptation. In order to support Release 12 inter-site carrier aggregation the UE is required to have multi-carrier transmission capabilities in uplink SO that it can feedback separate channel state information (CSI) and HARQ acknowledgements (ACK) to the macro and small cells. Data from core network (CN) Macro Flow control of data (MeNB) over backhaul (X2) Data flow is split in PCell the macro-eNB Small cell Data needs to be X2 interface (SeNB) buffered in the small cell eNB SCell U