m multiple cells, and it selects the signal from the cell with the highest power. However, selecting the cell that transmits at the highest power implies that the UE may often select the macro cell over the small cell. But the macro cell signal will have a higher propagation delay and path loss to the attached UE compared to a nearby small cell. And this will obviously not be optimal for the network from an up- link coverage and capacity point of view (implementing small cells is pointless if every UE selects the macro cell instead). Second, because in heterogenous networks the low-powered small cells are deployed along with macro cells in overlapping control channels, the ICIC technique is only partially effective because it cannot mitigate the control channel interference between the small cells and the macro cells. eICIC, defined in 3GPP Release 10, is an advanced version of ICIC to support the heterogenous en- vironment. Whereas ICIC uses different frequency ranges to control interference on the data chan- nels, eICIC instructs the UEs at the edge of the cell to use different time ranges to avoid interfer- ence on the control channel. They do this by introducing the concept of the Almost Blank Subframe (ABS), which allows the UEs at the edge of the cell to use the same radio resources, but in different time ranges. As shown in Figure 10-17, eICIC uses alternate time-slots for the overlapping cells, and for that to work there must be accurate phase/time synchronization between the macro eNB and small cells. Frequency High power Low power Radio frame (10 Subframes) Macro cell Subframe Almost Blank Subframe Frequency Cell B-small cell Small cell Cell A macro cell Subframe Figure 10-17 eICIC with ABS In summary, ICIC handles interference using frequency domain partitioning, whereas eICIC provides its solution with time-domain partitioning. Although ICIC demands accurate frequency synchronization, eICIC in a heterogenous network needs tight and accurate phase alignment. For eICIC to work, you must define the maxim