## 4 Calculation Results and Partial Discussion  
In order to perform exemplary calculations indicating the bandwidth of fronthaul/midhaul links, using the previously presented interfaces, the initial parameters of radio signals that will occur in the 5G-NR Rel-15 wireless interface and subsequent releases should be set first. In 5G mobile systems, a 4096-point FFT and a minimum 15 kHz spacing between the CP-OFDM subcarriers was introduced [51]. Thanks to such arrangements, 5G-NR Rel-15 can cooperate with 4G (LTE Rel-8 and newer). The subcarrier spacing can be increased in a coherent proportion [51], allowing the creation of broader combined frequency baseband channels--Table 1.  
The 3 GPP Rel-15 of mobile systems has defined a physical layer and higher layers, but there are no restrictions that would indicate the number of antenna modules per sector. The extensive arrays of antennas make it possible to create systems for mass communication of the base station with many UE devices. Thanks to the usage of a higher number of antenna modules, the more accurately spatial beams can be determined. However, this entails the need to create a large number of data transmission channels and control channels. The demand for bit-rate is strictly dependent on the type of fronthaul/midhaul interface (type of Split/Option) that is between the mast with radio modules and the device preparing the radio signal for emission. The study assumes that the number of antenna modules per sector in the near future (5G Rel-15/16) should not exceed 256--Table 1.  
If a well-known CPRI interface is used in the fronthaul/midhaul, then the required split speed (Option 8/Split E) can be calculated using (1). Here, each antenna module has its own dedicated stream with a digitized signal from the BB range. This gives the possibility of convenient scaling with spatial resources (MIMO and beamforming) but forces the use of an optical network with very high bandwidth links. Figure 9a shows the results of calculations that visualize the total bit rate that will be generated by the heavily loaded, on the radio side, 4-sector gNB-DU/RRH, and Figure 9b shows the number of optical DWDM-OOK-50 GHz-grid channels (for CPRI stream/Option 10 [31]--24.33 Gbps), which will have to be created in the FH/MH link to allow communication with the signal processing center. The calculations were made for 7 cases of BB channels (Table 1), where only the last one is not compatible with the 5G-NR Rel-15. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**BB Channel** & **Layers \(N_{L}\)** & **Subcarrier** & **Active** & **Symbols/** & **Code Rate** & **Modulation** & **Antennas/** \\
**Size** & **Layers \(N_{L}\)** & **Spacing** & **Subcarriers \(N_{SC}\)** & **Frame \(N_{SY}\)** & \(R_{C}\)** & **Order \(M\)** & **Sector \(A\)** \\ \hline (MHz) & - & (kHz) & - & - & - & - & - \\ \hline
20 & 16 & 15 & 1272/3300 & 140 & 0.85 & 256 & 96 \\
50 & 24 & 15 & 3240/3300 & 140 & 0.85 & 256 & 128 \\
80 & 24 & 30 & 2604/3300 & 280 & 0.85 & 256 & 128 \\
100 & 16 & 30 & 3276/3300 & 280 & 0.85 & 1024 [1] & 256 \\
200 & 12 & 60 & 3168/3300 & 560 & 0.85 & 256 & 256 \\
400 & 10 & 120 & 3168/3300 & 1120 & 0.85 & 256 & 256 \\
800 [1] & 10 & 240 & 3168/3300 & 2240 & 0.85 & 256 & 256 \\ \hline \hline \end{tabular}  
* The option is expected to be introduced in subsequent 5G releases.  
\end{table} 
Table 1: Main parameters of NR-PHY Rel-15 interface and radio-antenna modules needed to calculate FH/MH link bandwidth consumption [2; 29; 51]. Figure 9: Calculation results of optical resources needed to connect 4-sector gNB-DU/RRH to optical FH/MH as a function of antenna elements number of massive-MIMO array: (**a**) total bit-rate for radio BB channels digitized to CPRI format (LC = 66/64)—3 GPP—Option 8/CPRI Forum—Split E (Figure 7a,c); (**b**) number of DWDM-OOK 50 GHz-grid channels for radio BB channels digitized to CPRI format (LC = 66/64) –3 GPP—Option 8/CPRI Forum—Split E (Figure 7a,c).