ase SCell release Modification request DRB addition/release SCG release Modifying split/SCG DRB SCG Release request SCG/split DRB release Figure 4.4 Dual connectivity control and user plane architecture for the bearer split The layer 2 structure with dual connectivity and bearer split in use is shown in Figures 4.4 and 4.5. The PDCP layer does the split when aggregation from macro and small cells is possible. 4.5 Dual Connectivity Physical Layer Impacts and Radio Link Monitoring In the LTE Releases 8-11 PUCCH has always been sent on the PCell, but in the case of dual connectivity, this is not possible any more. Because of delays in the backhaul, the operation where HARQ-ACK feedback related to downlink transmission from the SCell of SeNodeB would be transmitted to the PCell and then routed to the SeNodeB via backhaul cannot be done as the feedback would arrive to the SeNodeB too late. Cells in the dual connectivity are divided into two groups as shown in figure 4.6: The cells that are controlled by MeNodeB are called as Master Cell Group (MCG) and uplink control information (UCI) related to transmissions in this cell group is sent via PUCCH on the PCell Radio bearer Header compression and ciphering/integrity protection Split in PDCP Scheduling/priority handling Scheduling/priority handling Logical channels Scheduling/priority handling Scheduling/priority handling Multiplexing Multiplexing Transport channels Carriers with carrier aggregation Carriers with carrier aggregation Figure 4.5 Layer 2 architecture with bearer split in the downlink direction LTE Small Cell Optimization PCell PSCell Freq. Figure 4.6 PUCCH on PCell and PSCell or via PUSCH on one of the cells in the MCG. The cells that are controlled by SeNodeB are called as Secondary Cell Group (SCG) and PUCCH related to SCG transmissions is sent on the Primary SCell (PSCell). Because of this there is no need to transmit UCI in the backhaul. As the role of PSCell being similar to that of the PCell, the UE also does Radio Link Monitoring (RLM) for the PSCell 