0 and 11 was boosting the performance with heterogenous networks operation using enhanced Inter-cell Interference Coordination (eICIC). The work focused on the co-channel case, where the both macro and small cell are LTE Small Cell Optimization CSI-RS symbols masking the symbols for legacy UE PRB in Antenna 0 PRB in Antenna 1 eNodeB Data with DM-RS Not used for transmission on this antenna port CSI Feedback Reference to eNodeB signals on this antenna port Subframe X Subframe x UE CSI-RS measurement Figure 2.14 Release 10 MIMO operation with CSI-RS on the same frequency. The dedicated frequency case was addressed with dual connectivity in Release 12 as covered in Chapter 4. The key for getting most of the small cells installed in the same carrier as the macro cell is to capture as much traffic as possible via the small cells. This can be achieved with good planning but always it is not trivial to predict exactly where the traffic hotspots are. The improvements done in LTE Advanced enable cell range extension, which allows the UE connect to a small cell even if the signal from the macro cell would be stronger. The solution introduced the following three methods for enabling such operation and capturing thus more traffic for the small cells: Introduction of Almost Blank Sub-frames (ABS). The purpose of ABS is to allow small cell to communicate during those frames with UEs that are actually having stronger signal from macro cell than from the small cell. The capacity is improved when multiple small cells can use the same ABS resources even if one macro cell has reduced total capacity. Use of ABS is illustrated in Figure 2.15. Use of offset values for selection between macro cell and small cell Advanced receivers to cancel the interference from the macro cell CRS. The ABSs are not fully empty as they retain the minimum reference symbol configuration to support measurements of also legacy devices. If everything would be removed, a UE making handover and measurements could suddenly provide wrong results if some of the s