## 4 Standardization of 5G Network Slicing Energy Efficiency KPIs  
Although the first standardization of 5G mobile networks occurred in 3 GPP Release 15 [30], the KPIs related to the energy efficiency of 5G network slicing were initially addressed in 3 GPP Release 17 [31]. While it is reasonable to assume that KPIs addressing the energy efficiency of 5G network slicing will be further extended in the ongoing 3 GPP Release 18 and the upcoming 3 GPP Release 19, the currently standardized KPIs in [31] provide a general definition of 5G NS energy efficiency. More specifically, standardized energy efficiency KPI for 5G networks is a metric that is expressed as the ratio of data volume (DV) traversing through NS and the energy consumption (EC) of a specific network part or the NS [32]. Data volume, used as a parameter for the calculation of 5G NS energy efficiency KPI, is measured on 5G network interfaces serving a specific NS. Figure 6 shows standardized interfaces in the 5G network architecture that are relevant for defining the energy efficiency KPIs of 5G network slicing. The 5G network architecture is composed of a core network and base stations (BS), defined as next-generation Node B (gNB), in the 5G RAN part of the network (Figure 6). As shown in Figure 6, the NG is the physical interface between the 5G core network and gNBs, i.e., the RAN part. E1 is the logical interface between the control plane and the user plane in the central unit (baseband processing unit) of 5G gNB. Its function is to support the exchange of signaling information between the user and the control plane (endpoints). The F1-C interface defines the interconnection between the central unit (CU) and the distributed unit (DU) of the gNB. The DU is (an active) antenna element (AAE) or remote radio head (RRH) of gNB, which can belong to the same vendor or a different vendor than the CU. F1-U is the interface related to communication between the user plane of CU and the user plane of DU. The Xn is the interface through which communication occurs between gNBs. As in the previous case, the labels U and C in the case of this interface (Xn-U and Xn-C) refer to interfaces that enable communication between the user planes and control planes of gNB, respectively. The N9 interface is used to connect two UPFs in the core part of the network. The X2 interface connects the 4G RAN and the 5G RAN parts of the network. Also, in this interface, there is a control part and a user part, labeled as X2-C and X2-U, that are used for the transfer of control and user information between BSs in 5G and 4G networks, respectively. In Table 3, the KPIs expressing the energy efficiency of various standardized NS SSTs are presented, with the description of key NS performance indicators and corresponding measurement units. As a metric of 5G network energy efficiency, the 3 GPP standard [24] relies on the EE KPI defined in [34]. According to the standard, Mobile Network Data Energy Efficiency (\(EE_{MN,DV}\)) is determined by calculating the ratio of the transferred data volume (\(DV_{MN}\)) to the consumed energy (\(EC_{MN}\)) of the network, with both metrics measured concurrently. \[EE_{MN,DV}=\frac{DV_{MN}}{EC_{MN}}\qquad(\text{bit}/J) \tag{1}\]  
Also, in [34], the Mobile Network Coverage Energy Efficiency (\(EE_{MN,CoA}\)) is an additional KPI, defined as the ratio of the area covered by the observed mobile network (\(CoA\_des_{MN}\)) and the consumed energy for the time period in which the efficiency assessment is performed. \[EE_{MN,CoA}=\frac{CoA\_des_{MN}}{EC_{MN}}\qquad(\text{m}^{2}/J) \tag{2}\]  
where \(EC_{MN}\) is the annual consumed energy and \(CoA\_des_{MN}\) represents the network coverage achieved by the analyzed network. Clearly, \(EE_{MN,CoA}\) can be applied only to the RAN part of the network, while the \(EE_{MN,DV}\) metric is applicable across the different entrieties (RAN, transport, or core) of the 5G network.