 allocated resources on all or some component carrier combinations. Cross-carrier scheduling is an important feature in heterogeneous net- works supporting CA where the downlink control channels are mapped to different compo- nent carriers in the large and small cells (as shown in Fig. 1.22). As an example, when LTE downlink control channel which carries downlink control information along with scheduling 54 Chapter 1 Frequency Frequency SCC PCC SCC PCC PCC: Primary component carrier SCC: Secondary component carrier Control channels Pico-cell eNB Pico-cell UE (secondary cell) Extended range Macro-cell Macro-cell eNB (primary cell) Figure 1.22 Illustration of cross-carrier scheduling and multi-cell operation in LTE [69]. information must be received by the UEs at the cell edge, it may be transmitted with a high- er power than the traffic channels. Therefore, using different carriers for the downlink con- trol channels in the large and small cells reduces the risk of inter-cell interference. The latest releases of LTE allow multi-carrier operation with different timing advances, enabling combination of component carriers from macro-eNBs with those of small cells. The CoMP transmission and reception feature, introduced in LTE Rel-11, is an inter-cell and inter-user interference mitigation scheme that allows coordinated eNBs, relay nodes, or RRHs to simultaneously transmit to or receive from a UE. When CoMP is used in a hetero- geneous network, a number of macro-cells and small cells can participate in data transmis- sion to and from a UE. However, this would require that the macro-eNBs and the small cells to be synchronized and coordinated via backhaul links and the content be available almost at the same time at multiple transmission points, which is practically challenging. In the next section, we will see how cloud-RANs with distributed RRHs have made multi- point transmission/reception and data processing practically feasible. In CoMP scheme, a dynamic muting mechanism can be exploited to mute some (radio) resour