A self-organizing network engine for optimizing a telecommunications network is described. The telecommunications network may be a heterogeneous network with different hierarchical layers, such as macro cells and small cells. For example, the telecommunications network may include a macro cell with a small cell located inside the macro cell. The self-organizing network engine is configured to obtain traffic data describing traffic in the macro cell and in the small cell. The traffic data provides an indication of the load on each of the macro cell and the small cell and can be used to determine, for example, if the macro cell is overloaded and user devices need to be offloaded to the small cell. The self-organizing network engine is configured to determine one or more almost blank sub-frames to be reserved in a transmission schedule depending on the traffic data, and to command the base stations of the macro cell and small cell to transmit data to and from user devices at the edge of the small cell using the almost blank sub-frames, and to transmit data to and from user devices in the macro cell and in the center of the small cell using sub-frames other than the almost blank sub-frames. By reserving almost blank sub-frames for the small cell edge users, interference between the macro cell and the small cell is mitigagted. The extent to which interference between the macro cell and the small cell is mitigated is thereby controlled by a self-organizing network engine in dependence on the load on the macro cell and small cell.
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