## 6 Conclusions and Final Discussion
Studies of A-RoF interfaces in this area have been conducted for many years, and their results can be found in many papers and books [34; 35]. The proposals for analog Splits/Options with high ID numbers (Option 8/9/10) may be useful in many solutions, but they cannot be universal, as in the case of CPRI. With the currently used grid of optical DWDM channels, the IFoF technique seems to be the most universal, since the intermediate frequency can be selected systematically, depending on the demand and available photonic resources. In this case, it is also possible to use a multistage BB signal transfer to the intermediate band. According to the calculations, the results of which are shown in Figure 12 and Tables 3 and 4, the use of the BBoF technique will become particularly important when a much denser UDWDM grid is available. The possibility of introducing the O-OFDM format to optical links has been working for many years, and the results of research are more and more promising. This type of format does not differ significantly from the BBoF technique, because we have been hosting the OFDM format for over 25 years in wireless interfaces. In order to indicate the degree of use of the optical channel by interfaces from the D-RoF family and others, used in classic NG-RAN solutions, the author carried out calculations, the results of which are presented in [55]. The bit rates that will occur in the 5G network will undoubtedly be high. When approaching the next generation of mobile systems, i.e., 6G, we need to look for new solutions for optical networks. The issue of future capacity crunch of optical networks has been raised for several years [56; 57; 58]. However, previous analyses concerned solutions for broadband backbone networks. The emergence of heavy IT traffic in the access domain is another new challenge. A serious generator of this traffic in the RAN domain is the widespread use of transporting digitized radio signals to an antenna MIMO modules. The introduction of radio signal transport in its original form will be a must. The flexibility of future all-optical EON networks will enable the introduction of another degree of freedom in optical resources management. As indicated above, depending on the type of Option engaged at a given moment, the occupation of the optical channel will vary within wide limits. In addition to WDM multiplexing, TDM technique can be introduced in digital channels. This will provide us with OTN or its newer generations with EON interfaces, in which optical OFDM may be used [55]. In the case of A-RoF signals we can only use WDM technique. The flexibility of the optical network consisting in the optimal selection of the optical channel width for the A-RoF signal, as well as packing the channels independently of the DWDM grid, significantly increases the use of resources lying in the scope of optical nodes and the range of optical fiber single-modality. The latter is particularly important in network solutions with FTTA or PON architectures. An additional advantage of the optical link with the irregular grid of optical channels (EON feature) is a significant increase in resistance to non-linear distortions resulting from the Kerr effect. Interleaving of A-RoF and D-RoF optical channels will increase this resistance. With a large number of optical channels, the total power can reach a high density in the fiber core, but in fronthaul there is no need for high optical power. It is anticipated that on 5G fronthaul connections (high number Options) the transport of signals will take place over a distance of not more than 20 km. Therefore, problems in access networks will differ from those we see in wide area networks. Another way to avoid capacity crunch of the fiber-optic networks in the NG-RAN access domain is to introduce new fiber technologies, i.e., multicore fibers (MCFs). The author proposed this type of solution for the first time in [50]. Connecting O-RRH/gNB-DU to fiber optic cables based on multi-core structures will enable the implementation of the next SDM stage.