## 1 Introduction
### NG-RAN Concept Description  
The distributed architecture of the gNB base station makes it possible to surround the user equipment (UE) and thus more efficient management of spatial resources. This task is accomplished through the use of wideband and flexible all-optical networks based on the Dense Wavelength Division Multiplexing (DWDM) system with a flexible grid of optical channels [25] or new more flexible Elastic Optical Networks (EON) [26] working with the Ultra-Dense Wavelength Division Multiplexing (UDWDM) format and extra flexible reconfigurable transponders. The optical network is so versatile that it can be used to transmit information/signals both in the area of the backhaul (BH) as well as the fronthaul (FH)/midaul (MH) of the future next generation mobile systems. The purpose of the backhaul (inside the cloud in Figure 1) is to connect the next generation core (NGC/5 GC) with the NG-RAN control units through the gNB-central units (gNB-CUs). The fronthaul is used in creation of fast and often synchronous links between the gNB-CU and components of the distributed 5G base stations, i.e., gNB-distributed unit (gNB-DU)--Figure 1[27]. The architecture of the distributed NG-RAN occurs in two concepts, i.e., based on the fronhaul network (Figure 1a) and with the division of the distribution network on the fronthaul and midhaul--Figure 1b. The second concept indicates the possibility of using a distribution point/unit (DU), whose task will be to perform some activities related to local signal processing and their distribution to functionally limited network termination units such as active antenna unit (AAU)/remote radio unit (RRU)/remote radio head (RRH). Figure 1: NG-RAN architecture with distributed gNB: (**a**) concept of C-RAN controlled by gNB-CU connected to NGC (backhaul side) and gNB-DU (fronthaul side)—forced by 3 GPP; (**b**) concept of C-RAN controlled by gNB-CU/BBU connected to NGC (backhaul side) and gNB-DU/AAU/RRH (fronthaul or/and midhaul side)—forced by 5 GPP. Inside the gray cloud in Figure 1, there are a 5G-NGC network and a set of gNB-CU/ baseband unit (BBU) cooperating with each other. The method of cooperation between central units processing the radio signals symbolizes additional terms such as 'cloud' and 'hotel'. The term 'cloud' means that individual units can be located at a greater distance in separate buildings, which forces them to use quick connections with small delays. The term 'hotel' (alternatively often used as 'pool') symbolizes the placement of units in the same building, which is equivalent to a physical signal processing center, while BBUs/CUs are usually separated in distributed logical structures, which are managed by the so-called virtual machines. From now, in order to simplify further description of the functioning of the network components, we will call the control units generally gNB-CU/BBU, and devices controlled by abbreviation gNB-DU/RRH. The name of the DU nodes that define the boundary between the F1 and F2 interfaces (Figure 1) will remain unchanged. The fiber-optic FH can be a passive component (FTTA or PON) of the C-RAN, which is applicable in mobile networks of current generations (2G/3G/4G). The demand for more and more bandwidths in the backhaul networks forces the optimization of fiber network resources, which is why it is necessary to pay attention to the previously mentioned active flexible optical networks [25; 26] using WDM technique. The WDM technique enables the simultaneous transmission of several optical signals (different frequency channels) in one fiber-optic link or one optical path. Fiber-optic link is understood as a connection between nodes of the network, and the optical path is the path on which the optical signal travels passing through the optical nodes. Fiber-optic links are implemented using single-mode silica fibers, where single-modality is maintained for wavelengths in the range of 1260-1675 nm. An optical node determines whether a fiber-optic network can be called all-optical.