ation (macroBS). However, more macroBSs in an already dense network would imply higher inter-cell interference [29]. Also, deploying additional macroBSs can cost immense expense due to site acquisition in dense areas. An alternate strategy to deal with the ongoing issue is deploying smaller base stations, i.e., base station (BS) with lower power (250 mW to approximately 2 W) and smaller transmission range [30], which also poses lower deployment cost. Such small base stations include pico base stations, femto base station (femtoBS), etc. The small base stations can be overlaid with the macroBS, creating a heterogeneous network (HetNet), which is an accepted design for the LTE-A wireless networks [31]. Figure 5.2 shows a HetNet comprising of macroBS, with transmit power of up to 40 W, overlaid with femtoBSs, which transmit at substantially lower power and are typically deployed in an unplanned manner. Installing small base stations improves coverage and provides capacity gain via higher spatial reuse. Unlike the pico base stations, which are meant for outdoor use, the femtoBSs are designed for indoor users. Since the femtoBSs are particularly user-installed, thereby, they can select the mode of their operation, i.e., which users in the network can access the femtoBSs. The femtoBSs can work in two access modes, open access and closed access [32, 33]. Under Femto Base Station Femto Base Station (((()) Femto Base Station Femto Base Station mmWave Base station Macro Base mmWave Station Base station Femto Base Station Femto Base Station D2D Transmitter D2D Receiver Fig. 5.2 Illustration of NOMA enhanced hybrid HetNet utilizing a mix of macro base stations, femto base stations, D2D transmitters, and mmWave-BSs. CU denote the cell center user and EU represents the cell edge user P. Swami and V. Bhatia closed access, only a closed/fixed group of subscribers (users) have access to the service. However, in open access, any user can connect to the femtoBS, i.e., a user is allowed to connect to any tier without any restriction