by X2 signalling messages load balancing which is also integrated in eNodeB Signalling volumes Low signalling volumes since only Potentially higher signalling cell-level metrics exchanged volumes since UE-specific information is carried Coordination area and Liquid cluster enables flexible Fixed cluster size typically assumed scalability coordination area Robustness No single point of failure Higher reliability required for a centralized element The eCoMP gains with ideal backhaul are studied in the system simulations and shown in Figure 7.29 for a 2 x 2 single-user MIMO system. Both full buffer and bursty traffic cases are shown. Three-sector site configuration was assumed in the studies. Release 11 UEs are used for providing multiple CSI feedback, which is assumed ideal in these simulations, corresponding to different hypotheses of CoMP set cells being muted versus transmitting and a MMSE - maximal ratio combining (MRC) receiver is assumed at the UE. The reference case is single-cell scheduling without any multi-cell coordination, and no additional overhead for TM 10 is assumed. The first step is to consider multi-cell scheduling within each eNodeB. The eCoMP gains ideal backhaul Intra-site coordination Inter-site (9-cell fixed cluster) Inter-site (7-cell liquid cluster) Cell edge Average Cell edge Average Full buffer Bursty Figure 7.29 eCoMP gains with ideal backhaul LTE Small Cell Optimization eCoMP gains with non-ideal backhaul Intra-site coordination Inter-site coordination (0 ms) Inter-site coordination (5 ms) Inter-site coordination (10 ms) Cell edge Average Full buffer Figure 7.30 eCoMP gains with non-ideal backhaul intra-site coordination gives 7% cell edge improvement with full buffer and 21% with burst traffic. The gains with bursty traffic are higher than with full buffer because there are typically unused resources available in the adjacent cells in case of bursty traffic. If we also consider inter-eNodeB coordination with nine-cell fixed cluster, the gains are increased to 10% and 23% with full buf