## 6 Conclusions and Final Discussion  
The proposed construction of the optical gNB-DU/RRH as O-RRH in [23] should meet the set requirements, which mainly consist in handling optical signals occurring in the D-RoF and A-RoF formats. It is unlikely that such devices could be installed as part of the first release of 5G mobile systems, i.e., Rel-15/16, but in the future mass production of technologically advanced devices will certainly significantly reduce their unit price. The proposed radio-photonic device can be connected to the optical network as a node or termination of the path, which means that it is simply a radio access node whose operation can be implemented in the majority of software (SDN/SDR). Available transmission optical network resources and the available signal and data processing power of the gNB-CU/BBU cloud will be a decisive factor in the introduction of the type of Split/Option at a given moment. The results of calculations presented in this paper show what capacity will be needed in fronthaul/midhaul networks, so that communication with such extended radio terminations will be effective. The first versions of the CPRI interface were not characterized by excessive bit rate, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Interface** & **BB** & \
\begin{tabular}{c} **Optical** \\ **Channel BW** \\ \end{tabular} & **OWS** &
\begin{tabular}{c} **Number of** \\ **Optical Channels** \\ \end{tabular} & **IF Carrier** & **Format/Split** \\ \hline - & (MHz) & (GHz) & (mm) & - & (GHz) & - \\ \hline IFoF & 100 & 3.125 & 14.8408 & 512 & 1 & A-RoF/Option 9 \\ CPRI & 100 & 50 & 48.6018 & 107 & - & D-RoF/Option 8 \\ CPRI & 400 & 50 & 178.4642 & 427 & - & D-RoF/Option 8 \\ IFoF & 400 & 6.25 & 29.4204 & 512 & 2 & A-RoF/Option 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Calculated comparative parameters for detailed scenario No. 5 to switch from D-RoF to A-RoF format (the optical channel count-down is starting from wavelength equal to 1675 nm)—Figures 9 and 12 (128-element antenna-array and 4-sector unit). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Interface** & **BB** & \
\begin{tabular}{c} **Optical** \\ **Channel BW** \\ \end{tabular} & **OWS** &
\begin{tabular}{c} **Number of** \\ **Optical Channels** \\ \end{tabular} & **IF Carrier** & **Layers** & **Format/Split** \\ \hline - & (MHz) & (GHz) & (mm) & - & (GHz) & - & - \\ \hline IFoF & 100 & 3.125 & 0.4678 & 16 & 1 & 16 & A-RoF/Option 9 \\ eCPRI & 100 & 50 & 1.4022 & 3 & - & 16 & D-RoF/Option 6 \\ eCPRI & 400 & 50 & 2.8021 & 6 & - & 10 & D-RoF/Option 6 \\ IFoF & 400 & 6.25 & 0.9353 & 10 & 2 & 10 & A-RoF/Option 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Calculated comparative parameters for detailed scenario No. 6 to switch from D-RoF to A-RoF format (the optical channel count-down is starting from wavelength equal to 1675 nm)—Figures 10 and 12 (32-element antenna-array and 4-sector unit). but they were to apply to 3G (UMTS) and 2G (GSM) mobile systems. Subsequent versions had to be adapted to 4G (LTE), where MIMO techniques and spatial beamforming were used more and more. Massive-MIMO technology, which is to be widely introduced in 5G, in order to start IoT communication, forces the use of very fast optical networks already in the local domain and access. This is caused by the need for surrounding the UE devices by antenna arrays that must be connected using very fast, synchronous, and flexible fronthaul/midhaul networks. Therefore, the CPRI splits will only apply where new techniques do not have too high requirements. Figure 8(a) shows that with a number of more than 100 antenna modules, a 4-sector device supporting 400 MHz BB channels will need about 10 Tbps, which is equivalent to about 400 optical OOK-DSB-50 GHz-grid channels--Figure 8(b). Of course, we can use coherent interfaces with high-order modulation and polarization multiplexing, which will reduce the demand for frequency bandwidth in the optical link several times, but the numbers will still be very large.