BBU 60 Chapter 1 Unified core network 2G/3G core LTE core network network (EPC) (GGSN, SGSN) Migration from distributed to centralized network Cloud BBU|pool 2G base 2G RRH station LTE base LTE RRH LTE base station (eNB) LTE RRH station (eNB) 3G base 3G RRH station (NB) Control plane User plane Figure 1.24 Mobility management in multi-RAT scenarios in distributed and centralized RAN architectures [61]. pool, shown in Fig. 1.24, integrates the essential 2G, 3G, and LTE [and later 5G NR (new radio)] network protocols, the inter-RAT handover process can be simplified and the distance between traffic anchor and interface can be reduced, which ultimately reduces the handover interruption time for network services. The inter-RAT handover differs from intra-RAT hand- over, which is a procedure to select a cell with the strongest received signal. The objective of inter-RAT handover is to select a suitable cell, considering user needs (especially mobility prediction), traffic type, and network property and state, by switching the logical interface between different RATs, which is more manageable in a C-RAN architecture. Based on the distributed base station architectural model for the C-RAN, all or some of the baseband functions are performed in the centralized unit (CU). The processing resources on the CU can be dynamically managed and allocated. The C-RAN architecture allows improvement of resource utilization rate and energy efficiency as well as support of collabo- rative techniques. The concept of C-RAN has been evolving in the past decade and numer- ous architectural and deployment schemes for improving the spectral efficiency, latency, and support of advanced interference mitigation techniques have been studied and trialed by the leading operators. As an example, demarcation of the BBU into a CU and DU(s), func- tional split, and the next-generation fronthaul interface (NGFI) were introduced under the C-RAN context to meet the 5G requirements. The principle of CU/DU functional split origi- nated from the real-time 