ped by different vendors and therefore there is a benefit in having a standardized interface between these. To cater to these deployment needs, many vendors have implemented various base sta- tion split architectures, in which an eNB or a gNB is further split into a number of logical or physical nodes. Many of these implementations are based on the CPRI specification, which however does not provide true multi-vendor interoperability as it severely under- specifies the functionality required to implement a fronthaul interface. While in 4G all these architectures have been proprietary, in 5G 3GPP (and in fact other Standards Developing Organizations [SDOs] and industry fora) considered and eventually standardized a number of split gNB architectures. During the 5G study, split options illustrated in Figure 4.1.9 have been considered by 3GPP. A brief description of each functional split option is provided below. In this preliminary study, an architecture in which a gNB is split into two logical nodes, referred to as central unit (CU) and distributed unit (DU), has been considered, as illustrated in Figure 4.1.10. Note that here and in the rest of the book, we omit the antenna interface for brevity. Option 1 In this option, the radio resource control (RRC) layer resides in the CU. The rest of the protocol stack layers, that is, Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), MAC, PHY, and RF are in the DU. In other words, this option separates the control 4 NG-RAN Architecture Figure 4.1.10 gNB architecture with centralized unit and multiple distributed units. plane (i.e. RRC) from the user plane, allowing centralized control-plane and distributed user-plane deployments. The main benefit of this option is the centralized RRM, allowing a certain level of opti- mization mainly in terms of UE mobility, as these decisions are taken by a CU (potentially connected to a large number of DUs) having a higher level of visibility into potentially many cells it can control. Furthermore, this option is the least