using different frequency re- sources at the edge of the cell. eNBs that support this feature can generate interference information for each used frequency and exchange that information (frequency, transmit power, and SO on) with neighboring eNBs over the X2 (inter-cell signaling) interface defined by 3GPP. As shown in Figure 10-16, if the UEs (UEA and UEB) at the edge of cell A are using the same fre- quency f2, one of the cells, for example, cell B, will switch to a different frequency, f3, to commu- nicate with UEB at the edge of the cell while continuing to use f1 or f2 with low transmission power at the center of the cell to avoid interference. Low power Low power High power High power Low power interference interference Cell A Cell B '={{1,f2.....fn} : fx = Sub-carrier Figure 10-16 Inter-cell Interference Coordination 10.3.1 Enhanced Inter-cell Interference Coordination Traditionally, cell sites were mostly homogenous, which means that each cell site was much the same as the rest (usually large, "macro" cells). So, homogeneous networks are ones where all the cell sites are of the same type and function. With more recent developments, there is a trend toward the heterogeneous network (sometimes called the HetNet), which has a mix of low-powered small cells and large macro cells. The ICIC technique is used to manage the resource requirements, such as frequency (bandwidth) and power, at each eNB and the interactions between eNBs. It does this by allocating frequency re- sources to UEs using data channels, while other resource allocations (for example, power) are man- aged through the control channels. Unlike data channels, which can use different frequency ranges, the control channel uses the entire carrier bandwidth. In homogeneous networks, this is not a big problem because there is not much difference in the transmit power values from adjacent eNBs, and hence no significant inter-channel interference is caused by the control channel. In heterogenous deployments, the UE receives the same frequency signal fro