e dual connectivity (also known as inter-site carrier aggregation), intended to be used between a macro and small cell, the connection is now between two eNodeBs (master eNodeB and secondary eNodeB) that are not co-located. The UE needs in this case to be able to provide physical layer feedback signal to both eNodeBs separately (as non-ideal backhaul is assumed) thus the UE has always connection to both macro eNodeB and small cell eNodeB. This allows aggregating the maximum data rate from the macro and small cells and enables the use of small cells more efficiently as the macro cell is used to maintain the connectivity even if the small cell layer does not offer necessary continuous coverage. Small cells may also due potentially higher frequency, have clearly smaller coverage area and may not necessary be always usable as primary cell. 4.3 Dual Connectivity Architecture Principle Following the 3GPP study phase for small cell enhancements, several different architecture options were considered, as reported in 3GPP TR 36.842 [1], with two of the architecture options agreed to be standardized for Release 12 as shown in Figure 4.2. The selected dual connectivity architecture options have the following main characteristics: In Architecture 1A, the user plane data (i.e. S1-U) is split at the serving gateway (S-GW) between the master eNode (MeNodeB) and the secondary eNodeB (SeNodeB). This means the user plane protocol stacks are identical in both eNodeBs. Architecture 1A was motivated with the potential limitations in the backhaul connectivity between the macro and small cells. That is why a single bearer is only provided either via the small cell or via the macro cell. Small Cell Enhancements in Release 12/13 MeNodeB SeNodeB MeNodeB SeNodeB Alternative 1A Alternative 3C Figure 4.2 Dual connectivity downlink user plane architectures In the case of only a single service being active, there is no possibility aggregate of the data from both cells and respectively the dual connectivity will neither increase peak data rate 