## 2 Radio and Photonic Components in Optical Massive-MIMO gNB-DU/RRH
The combination of these two techniques makes it possible to use hybrid solutions [42; 43], which balances the load on the components of the fronthaul network and increase the precision of beam control on the free space side. A single radio-antenna module must be equipped with a duplexer whose task is to separate and combine signals from different directions. The diplexers allow us to direct a specific radio band to the appropriate antenna dipole. Currently, diplexers are used as passive devices, but in the case of multi-band work (radio interfaces of next generations) of antenna array, we may need an active system, which will also be susceptible to control from the radio module or even gNB-CU/BBU. The coupling of components shown in Figures 3 and 4, through fast and multi-track buses, gives the possibility to build a through or path terminating the optical gNB-DU/RRH. The optical channels will mostly be scaled at the gNB-CU/BBU level, but these functions may be partially moved to the gNB-DU/RRH. The control method of optical massive-MIMO gNB-DU/RRH depends on the method of controlling the entire domain of devices operating in the FH or MH network. The presented proposal assumes C-RAN control with the possibility of distributing the signal preparation centers gNB-CU/BBU Hotel/Cloud (Figure 1). From the radio termination point of the optical gNB-DU/RRH, despite the partially distributed network devices processing BB, IF, of RF signals (gNB-CU/BBU Cloud), gNB-CU/BBU nodes are seen as centrally located devices and communicate in parallel regarding the so-called gNB-CU/BBU clouds. Figure 5 shows an example of a network where a cloud is located in the middle representing an evolved packet core (EPC)/next generation core (NGC), a network of software defined radio (SDR) devices (gNB-CU/BBU) and a central Open Flow (OF) controller [44; 45] as a resource manager in the NG-RAN domain. The example network is based on the optical layer in the ring architecture, however, the logical structure of connections between NGC and gNB as well as gNB-CU/BBU and gNB-DU/RRH has tree architecture. Access to the optical network takes place through the so-called optical concentrators, i.e., optical multiplexers. The proposed gNB-DU/RRH have the built-in microROADM or microOM/microOD, which gives the possibility of their direct connection to the optical ring structure (microROADM case). If there is a need to connect the outside gNB-DU/RRH in the access mode (the so-called south direction), then it should have at least an optical multiplexer and demultiplexer (microOM/microOD case). The assumption that the network is all-optical requires the use of PXC or ROADM in nodes, which guarantees that there will be no conversion of the photonic to electric signal in these nodes and vice versa. Of course, this only applies to the transport of signals carrying user data (user plane). The control layer using OF [44; 45] must be organized so that all optical nodes together with gNB-DU/RRH and O-gNB remain under full control. As seen in the upper part of Figure 5, the optical network can connect gNB-DU/RRH and O-gNB to the core of the packet network and the surrounding cloud of the gNB-CU/BBU modules. All that remains is to design a mechanism for flexible resource management, which will decide how the optical resources will look like when simultaneously (hybrid) serving all streams from NG-RAN. PXC nodes equipped with DU functions will deal with the distribution of optical streams that will transport radio signals with high time requirements (physical layer) to the appropriate gNB-DU/RRH, whose functions can be dynamically reduced to AAU (depending on the load of links in network and signal processing centers in the cloud). In the situation that the midhaul network (serving traffic with lower time requirements) does not exist (Figure 1a), all the DU functions are taken over by gNB-DU. In this case, high time requirements are taken over by the network at the level of interface F1.