E receives data from both macro and small cells. Transmits corresponding CSI and HARQ Ack/Nacks to macro and small cells. Figure 8.6 High-level sketch of assumptions for a user in DC between a macro and a small cell LTE Small Cell Optimization UE buffer UE buffer no. bits no. bits Initial data forward X2 latency Flow ctrl periodicity forward request forward grant Figure 8.7 Schematic illustration of the X2 flow control mechanism The performance of DC with split bearers depends on multiple factors, of which flow control between the MeNB and SeNB over the X2 interface is of great importance. The flow control mechanism is presented schematically in Figure 8.7. It is a simple request-and-forward-based scheme, where the SeNB (in accordance with 3GPP specifications) is in charge of requesting data from the MeNB. Received data from the MeNB are buffered in the SeNB until they have been successfully transmitted over the air interface to the UE via the SCell. The data requests from SeNB to MeNB are sent periodically on a per-user basis. As by definition, the design target of the flow control algorithm is to avoid data overflow and underflow in the SeNB. More specifically, the goal is to always have data available for transmission in the SeNB (so that users can benefit from the additional resources in the small cell layer) while limiting the additional delay introduced by transmission via the SeNB. The performance of DC with split bearers is evaluated in a system-level simulator in line with the 3GPP Release 12 Scenario 2a as defined in Reference 5. The small cells are randomly deployed in condensed clusters with 4 cells within a circular area with 50 m radius. The transmission powers for the macro eNB and small cell eNB are 46 and 30 dBm, respectively. Macros and small cells are deployed at 2 and 3.5 GHz, respectively, assuming 10 MHz carrier bandwidth. Closed loop 2 X 2 single-user MIMO with pre-coding and rank adaptation is assumed for each link and the UE receiver type is interference rejection combining (IRC). A dynam