## 2 Radio and Photonic Components in Optical Massive-MIMO gNB-DU/RRH  
The NG-RAN built on the basis of all-optical solutions enforces the introduction of the network termination in the form of an integrated optical gNB-DU/RRH. In this way, the created active-distributed-antenna-system (A-DAS) network (Figure 1) will enable solutions based on the spatial surrounding of the terminal by the so-called distributed gNB base station. The distribution of radio signals to/from gNB-DU/RRH via optical links will guarantee very high delay constancy, which in the situation of using digital beamforming (DBF) or hybrid beamforming (HBF) of radio beams directed to/from the UE is superior. An exception will occur in the case when the eCPRI interface is used, in which the synchronous ethernet (SyncE) technology is applied [33; 34; 35], creating a RoE link [36; 37]. Of course, the stream of Ethernet frames can be transported through optical transport network (OTN) links [38], similarly to CPRI [39], but in case of the need to enter on the layer 2 (L2), in order to switch frame streams, the optical path will be terminated. Here, the guarantee of low variance of delay may not be possible, unless the Ethernet switch is equipped with optical ports that are connected through the linear clock on the layer 0 (L0) (the optical layer determines the transferred clock step). The Integrated Optical gNB-DU/RRH can exist in several configurations, what depends on the construction of the radio massive-multiple-input-multiple-output (massive-MIMO) head and the photonic module. Figure 2 presents three types of optical gNB-DU/RRH, which are characterized by a four-sectors (a), one-sector (b) and six-sectors (c) radio head. Construction concepts (a) and (c) are very similar to each other, because they require the use of stepwise transfer of connections between sectors. In case (b), only one sector was used, which is equipped with, in proportion to other solutions, a large number of radio and aerial modules. The single-unit gNB-DU/RRH device will allow for smooth tracking through the UE radio beam around the mast. In this construction, it is required to smoothly switch off individual radio modules when the angle at which the UE terminal lies is exceeded. In each case shown (Figure 2), gNB-DU/RRH is directly connected to a fiber-optic link (at least one pair of optical fibers) in which the xWDM technique is applied (signals in accordance with the DWDM system or OTN/EON type development). In solutions (a) and (c) in the photonic part, the microROADM was used, which indicates the possibility of using the device in a double optical ring, where we will have a reserve route and the possibility of dynamic management of the add/drop band. The single-sector solution gNB-DU/RRH is based on the equipping of the photonic side in microOM/microDM [40; 41], which indicates that this type of device will be able to be placed at the physical terminal of the optical path/link. The optical mux (OM)/optical demux (OD) terminating the optical path/link may be also applied in the cases (a) and (c)--Figure 2. This will depend only on the place where the gNB-DU/RRH device is connected to the optical network. The two downstream devices (modules) in optical a gNB-DU/RRH are described in more detail in Figure 3. The photonic switch (Figure 3a) allows a local optical channel redirection, depending on the need to direct concentrated radio traffic to a specific part of the antenna module assembly (Figure 4). A very important component is the array of media-converters (transceivers), whose task will be to shift from the photonic to the radio domain and vice versa. These systems will have to support signals modulated in D-RoF and A-RoF [30] formats, so they will have to work in a flexible mode. The lasers built into these converters should be longitudinally single-mode and have the possibility of dynamic tuning.