1.2.1.1 Heterogeneous C-RAN (H-CRAN) 5G promises to achieve more than 1000 Gbit/s/km2 area spectral capacity with stringent requirements on the E2E latency and energy consumptions [32]. In order 1 A Comprehensive Study on 5G BBU pool Fronthaul ((1))) Backhaul Front- 2G/3G/LTE (((o))) X2/S1 Smart-home (((.))) ((.)) ((.))) D2D: Device-to-device communication Side-link UDN: Ultra-dense network HPN: High-power node SC: Small-cell Fig. 1.3 5G network heterogeneity with H-CRAN design to provide connectivity for the massive number of devices with high capacity and QoS, network densification3 has been proposed in the past years [58]. However, the ever increasing of devices and applications leads the macro-cells to reach the limit of theoretical capacity [29]. In order to overcome this challenge, a dense heterogeneous network (HetNet) was proposed by deploying low power nodes (LPN) or small cells (SCs), such as pico- and femto-cells, with high power nodes (HPN) such as micro- and macro-cells [30]. Figure 1.3 shows the heterogeneity of the 5G network with several applications, such as device-to-device (D2D) and vehicular communication, and wireless technologies, such as 2G/3G/LTE. However, several challenges have been raised for the HetNets such as resource allocation, intercell coordination, inter-tire interference management, network capacity, coverage, energy efficiency, and CAPEX/OPEX [31, 59]. In order to cope with these challenges, H-CRAN architecture with centralized cloud and virtualization has been proposed for the 5G networks and beyond to manage the intercell coordination and facilitate the deployment of macro and SCs [27, 30, 31]. H-CRAN enhances the performance of the HetNets by connecting the SCs into the BBU via high-speed optical fibers and deploying massive MIMO with the macro-cells [30]. In [3, 29] and [30], architectural frameworks for H-CRAN have been presented and depicted in Fig. 1.3. In the architectural framework of H-CRAN in Fig. 1.3, the SCs have been deployed beside the macro-cell or HPN in order