xxxviii Introduction and Background In order to understand 5G deployment scenarios, we start our analysis with the study of high-level RAN architectures and network topologies supported by 5G. The design princi- ples of 5G RAN architecture are based on disaggregation of the base stations and separation of the control plane (CP) and user plane (UP) entities. The architecture further supports con- nectivity between 3GPP LTE and 3GPP NR access and core network entities. The RAN architecture scenarios shown in Fig. 5 support co-located deployment of LTE eNBs and NR gNBs connected to either LTE core network (EPC) or 5G core (5GC) network in a non- standalone (NSA) mode as well as standalone (SA) deployment of NR disaggregated gNBs connected to 5GC. In the early deployments of 5G systems and before 5GC equipment is available, the NSA scenarios are going to be predominant and later SA deployments of 5G systems will prevail [16]. In the deployment scenarios shown in Fig. 5, gNB-CU and gNB-DU denote the central unit (CU) and distributed unit (DU) of a logical gNB, respectively, which may or may not be co-located. The logical interfaces F1 and Xn are new network reference points that com- prise control plane and user plane paths between the CU and the DU(s) that will be dis- cussed in more detail in Chapter 1. There are two gNB deployment options when considering C-RAN architectures, as shown in Fig. 6 [16,17]: Collapsed gNB deployment: In this scenario, all RAN protocols and functions are co-located within the same site. This deployment option corresponds to the current deployments of LTE systems and as such it ensures backward compatibility with the existing LTE deployments. Disaggregated gNB deployment: In this scenario, RAN protocols and functions are distributed across different sites; that is, in a C-RAN architecture, the DU and the CU may be physically apart. The CU may be further divided into CP and UP entities. The DU hosts the RLC, MAC, and PHY protocols, the CU-CP entity hosts the NG-RAN NB-CU gNB-CU LTE eNB LTE