n open plat- form and base station virtualization enables processing, aggregation, and dynamic resource allocation, reducing the power consumption and increasing the infrastructure utilization rate. The processing resources on CU can be managed and allocated dynamically, resulting in more efficient utilization of radio and network resources and improved energy efficiency (EE). Moreover, network performance is also improved significantly because collaborative techniques can be supported in C-RAN. The concept of C-RAN has been evolving in the past few years as the technology matures. For example, CU/DU functional split and next- generation fronthaul interface (NGFI) have been introduced in C-RAN to better meet the 5G requirements, for example, higher frequency, larger bandwidth, increased number of antennas, and lower latency. [23,24]. It is observed that in a typical 5G C-RAN architecture, the baseband unit often comprises a CU and one or more DUs. The principle of CU/DU functional split lies in real-time proces- sing requirements of different functions. As shown in Fig. 10, the CU functions mainly Introduction and Background xliii Evolved packet 5G core (5GC) core (EPC) Separation Backhaul Backhaul Central unit Baseband processing unit (BBU) CU-DU interface (F1) Distributed unit (DU) Fronthaul (c()) Remote radio Remote radio head (RRH) Radio head (RRH) Figure 10 Evolution of single-node 4G BBU to separated CU/DU architecture in 5G [23]. include non-real-time higher layer RAN protocol processing as well as core network func- tions that have been moved to the edge of the network to enable mobile edge computing (MEC) services and tactile Internet. Accordingly, a DU is mainly responsible for physical layer processing and real-time processing of some or all layer 2 protocols. In order to mini- mize the transport capacity and latency requirements between RRU and DU(s), part of phys- ical layer processing can be moved from DU to RRU(s). From hardware implementation point of view, the CU equipment can be developed based 