, all subscribers of a given operator can access the node, while in closed access mode, the access is restricted to a closed subscriber group. In hybrid mode, all subscribers can connect to the femtocell with the priority always given to the designated subscribers. A network comprising small cells and 5G Network Architecture Uplink cell Downlink Downlink received signal power by macro-eNB cell edge Downlink received signal ((p)) Macro-cell eNB power by pico-eNB Pico-cell eNB Uplink received signal Uplink received signal power by pico-eNB power by macro-eNB UE location Figure 1.21 Uplink/downlink imbalance issue in HetNet deployments [12]. macro-cells is referred to as HetNet in the literature. HetNets, in general, are considered as a paradigm shift from the classic homogeneous networks. A number of features were added to the later releases of LTE that can be used to mitigate the inter-cell interference issue in the heterogeneous networks. Inter-cell interference cancel- ation (ICIC) was introduced in LTE Rel-8, in which the eNBs can coordinate over the X2 interface in order to mitigate inter-cell interference for UEs at the cell edge. Frequency- domain ICIC scheme evolved to enhanced ICIC (eICIC) in LTE Rel-10 where the time- domain ICIC was added through the use of almost blank subframes. Those subframes included only LTE control channels and cell-specific reference signals and no user data, transmitted with reduced power. In that case, the macro-eNB would transmit the almost blank subframes according to a semistatic pattern and the UEs in the extended range of the small cells could better receive downlink control and data channels from the small cell. Further enhancement of ICIC focused on interference handling by the UE through ICIC for control signals, enabling even further cell range extension. Carrier aggregation (CA) was introduced in LTE Rel-10 to increase the total system band- width and the maximum user throughputs. In this scheme, the component carriers (CCs) are aggregated and any CA-capable UE can be