## 4 Standardization of 5G Network Slicing Energy Efficiency KPIs
### Spatial Arrangement of Network Elements for Enabling Energy-Efficient Network Slicing in 5G RAN  
The latest 5G RAN architectures are characterized by separating the baseband unit (BBU) and remote radio head/unit (RRH/U) as two main elements of the 5G BS. This means that BBU and RRU (RRH) can be implemented in different physical locations (Figure 19). Furthermore, the 5G new radio introduces split BBU architecture, where the BBU functionality is generally divided into the functions of the DU and CU. In practice, the DU and CU do not have to be implemented at the same physical location (Figure 19). This means that the BBU functionalities of DU and CU can be distributed or implemented in the cloud. While the link known as the fronthaul enables an optical connection between DUand the RRU (RRH), the link known as midhaul is a packet-based optical fiber link that connects the CU and DU in the 5G RAN architecture (Figure 19). With the advent of VNF and SDN as the main enablers of 5G network slicing, the DU and CU, as BBU functionalities, can be shared among different NSs in the 5G sliced network (Figure 19). This can enable the precise allocation of DU and CU resources to specific NSs. Figure 19 illustrates some possible combinations for implementing the RRU (RRH), DU, and CU in the RAN and core network for a 5G BS serving a specific NS. With respect to the energy efficiency of the 5G sliced network, the arrangement of these network components will depend on the type of traffic that will be transmitted through that network. In sliced 5G networks, the types of traffic are generally known in advance, as the traffic of a specific NS is grouped in this NS and transmitted through the respective 5G network resources. For example, for eMBB, mMTC, and V2X service types (Table 2), the recommended 5G network slicing architectures are the ones presented in Figure 19, Figure 19, and Figure 19, respectively. Since the DU and CU in the new BBU architectures presented in Figure 19 can be physically separated from the RRU/H (BS), they can be implemented in the edge or central data centers (DCs). This implementation of BBU functionalities in DCs allows for the virtualization of the DU and CU functionalities per specific NSs on DC virtual machines or containers. This usage of precise virtual DC resources for a specific BBU functionality and the possibility of quickly scheduling these resources to be used by different NSs, opens different possibilities for improving the energy efficiency of 5G BSs BBUs in comparison with legacy BBU realizations based on the fixed allocation of BBU on the BS site. Also, the implementations of DU and CU in the DCs can benefit from the DC-implemented mechanisms and techniques for optimizing energy efficiency. These can even include a power supply of DC from a renewable energy source. Moreover, the modern cooling methods of DC equipment are mostly in use for the entire DC, which reduces the costs of using separate cooling devices for the air conditioning of individual BBUs at specific BS sites that are characteristic of legacy 5G and previous network generations. These possible  
Figure 19: Architectures of 5G RAN and core elements for (**a**) NS with eMBB service type, (**b**) NS with mMTC service type, and (**c**) NS with V2X service type. energy savings are furthered by the realization of BBU elements as VNFs, which enables the faster processing of necessary functions and for BBU elements to be put into a sleep mode of operation when they are not processing data and signalization. The functions of CU and DU in the architecture of the split BBU RAN model presented in Figure 19 are defined by the division of OSI layers. This enables flexible use case performance optimization, scalability, and load management. For example, as presented in Figure 19b, by shifting non-real-time functions that are processor-intensive to more CUs that are centrally located, the routing of NS traffic and RAN resource utilization can be made more efficient.