## 4 Standardization of 5G Network Slicing Energy Efficiency KPIs
### Improving Energy Efficiency in the RAN Part of 5G Network Slices
#### 5.4.2 Possibilities for Energy Savings of Sliced 5G RAN in the Spatial Domain  
Spatial energy savings in 5G sliced networks can be obtained through the implementation of the Multiple-Input-Multiple-Output (MIMO) technique. MIMO, as a technique, is based on wireless transmission over arrays of multiple antennas, which results in reduced signal fading, improved signal-to-noise ratio at the wireless communication ends, and the ability to create multiple communication streams that boost channel capacity over a finite electromagnetic spectrum. The implementation of multi-user MIMO (MU-MIMO) and massive MIMO (mMIMO) enable the further enhancement of channel capacities. The densification of antenna arrays in the MIMO system enables the implementation of energy-saving techniques through the spatial allocation of wireless streams over arrays of MIMO antenna elements. The basic idea of creating energy savings in the spatial domain is related to the switching on and off of MIMO antenna arrays according to traffic intensity. During time  
Figure 14: Allocation of BS PAs to specific NSs: (**a**) permanently; (**b**) based on the scheduling of PAs among NSs. Figure 13: The activity of the BS power amplifier according to data transmission variations [44]. periods when the activity of all MIMO antenna transceivers is not necessary as there is low or no traffic intensity, a certain number of MIMO antenna arrays can be turned off (Figure 15) if switching off antenna elements will not affect the overall user experience. Considering that 5G generally uses more antenna arrays, especially when utilizing MU-MIMO and mMIMO, possible energy savings in the spatial domain can be significant. After turning off certain transceivers representing channel transmitters, there will be a reduction in the antenna total transmission power and antenna gain. This can be compensated by increasing the spectral power density of the remaining active transceivers of the MIMO antenna array [45]. To enable further energy savings through the implementation of the 5G sliced network, there is the possibility of allocating different MIMO antenna arrays to the NSs. The possible configuration of the MIMO transmission in the 5G sliced network can be seen in Figure 16.  
It can be noted that two approaches dedicated to the allocation of NSs to MIMO antenna arrays can be implemented in practice. The first one is based on a dedicated configuration of a specific set of MIMO antenna arrays for specific NSs (Figure 16a), while the second approach encompasses the possibility of the dynamic scheduling of different MIMO antenna arrays among NSs (Figure 16b). Every option includes turning off some MIMO antenna array elements that are allocated to specific NS, in order to reduce the energy required by a specific NS (Figure 16). The practical implementation of the dynamic allocation of NSs among different antenna array elements with the ability to switch off some antenna array elements to save energy will require new approaches and algorithms that can support such implementations. However, the use of massive MIMO antennas in 5G sliced networks according to the proposed approaches can lead to a reduction in energy consumption. The conventional  
Figure 16: Allocation of BS antenna array elements to specific NSs: (**a**) permanently; (**b**) based on the scheduling of antenna array elements to be shared among NSs. Figure 15: MIMO spatial energy saving based on the scheduling of antenna array activity states [44]. use of antennas in mobile networks has been characterized by signal emissions in specific sectors covering a vast area. This results in a significant percentage of the emitted signal energy being unused. With the proposed approach, based on the scheduling of NSs among massive MIMO antenna elements, electromagnetic wave emissions are directed to the users of specific NSs, and there is no energy wastage as occurs with conventional MIMO transmission with sector antennas.