## 1 Introduction
### NG-RAN Concept Description
If a node does not go to the electronic level (on the user's layer), to regenerate the signal or to reorganize the digital data, then this node can be called all-optical. Such nodes include optical add-drop multiplexer (OADM) or reconfigurable OADM (ROADM), as well as photonic cross-connects (PXCs). Of course, the process of signal regeneration can take place in all-optical nodes, but it must be implemented on the optical layer, which is currently extremely expensive (the need for precise recognition of the signal modulation format). ROADM and PXC are usually adapted to the so-called optical grids according to the standards defining transport systems. Optical channel grids, on the other hand, are adapted to the optimal parameters of optical fibers connecting nodes. In this case, it concerns the optical ranges of the S, C, L, and U bands [1]. If we release from these nodal devices the permanent set of optical filters and internal constraints to xWDM grids, then we will get full flexibility. Unfortunately, this is done at the expense of a high increase in the demand for effective photonic resources management in each of these nodes. If an optical node is able to switch any channel (in a technologically limited range--any optical carrier wavelength and any optical channel width), then it is sensible to use different modulation formats, in addition to the classic on-off keying (OOK). As the optical channels are spectrally independent with appropriately selected optical path or link parameters, they can carry completely different signals, i.e., modulated in various formats. Therefore, the flexibility of the optical network using the WDM technique enables an effective combination of traffic, in one fiber-optic link, coming from functionally different networks. Separation of different streams in an optical link can be accomplished by assigning separate optical carriers. An active network, in which there are various signals, must also be equipped with flexible nodes and transponders [26; 28], which will be able to recognize signals and place them in the appropriate space of the grid of a given xWDM system (elastic or static). This assumption forces, on the side of gNB-DU/RRH, the use of all-optical nodes and optical terminations of the C-RAN. Of course, this does not exclude the possibility of mutual communication of C-RAN radio devices via radio interfaces operating in the \"sub-6\" (FRI) and \"mmWave\" (FR2) bands [29]. The following description presents the concept of integrated optical gNB-DU/RRH, that was presented by the author for the first time at the photonic conference in Prague [23] as the optical RRH (O-RRH). The material concerning this concept is also included in this work for the sake of completeness as well as due to the introduction of minor changes regarding the adaptation of the radio-photonic unit to flexible optical networks. Optical gNB-DU/RRH enables communication with gNB-CU/BBU by using non-standardized interfaces and modulations. Integration of an optical multiplexer with a radio module enables the ingress to the optical fronthaul, connecting gNB-CU/BBU and gNB-DU/RRH, signals originating not only from a digitized radio-over-fiber (D-RoF) interfaces [30], i.e., common public radio interface (CPRI) [31] or evolved CPRI (eCPRI) [32], but also an analog radio-over-fiber (A-RoF) [30]. On the side of the signal processing center gNB-CU, the situation looks a little bit different, because here we have a lot of power in terms of signal processing. The task of the central unit will therefore be to collect all physical resource blocks (PRBs) (in terms of time and frequency) directed to specific UE on the wireless side, combining into one or several channels from the baseband and inserting these channels into the IF or RF band using the direct digital synthesis (DDS). In case of the D-RoF format, the process will be terminated on the digitization of the channel created in the baseband. In order to create an A-RoF signal, the other signal processing steps mentioned above must be implemented.