## 4 Calculation Results and Partial Discussion
Based on Formula (1) and Figure 9, it can be seen that the increase in the bandwidth required for the CPRI link increases very strongly with the increase of the basic bandwidth (proportional increase in the sampling frequency). It should be noted that when the BB width increases, the number of CP-OFDM subcarriers is not significantly increased, therefore, when expanding the BB channel, the efficiency of using the CPRI interface becomes smaller and smaller. It is possible to reduce the bandwidth demand when we lower the number of Split/Option level in the fronthaul/midhaul interface. This will, of course, entail an increase in the functions related to signal processing at the gNB-DU/RRH. In order to compare the degree of reduction in the bandwidth demand, calculations were made, the results of which in the form of diagrams are shown in Figures 10 and 11. At the discretion of the radio device communicating with the UE in the massive-MIMO mode, which will be connected to the gNB-CU cloud via the eCPRI-Split D/3 GPP-Option 6, it cause to process the PHY signal in the UL and DL directions. This contact lies at the border between PHY and MAC layers, which is why it is clearly defined functional boundary (Figure 7). The dependency (2) was used to calculate the bit rate generated at the level of this Split/Option and some assumptions were made, which are included in Tables 1 and 2. The eCPRI interface is prepared for transporting of radio signals via Ethernet (RoE) paths, therefore the total redundancy of the frame headers go to 1.33 (Table 2). At the Split D level, the number of active subcarriers in the specified BB channel is significant (in Table 1 this is the first \(N_{SC}\) value, the second is the permissible number of subcarriers in NR-Rel-15, i.e., 3300). Therefore, the frequency bandwidth of the BB channel is not the reason for the bit rate increase in the Split/Option, but the number of active subcarriers and the number of layers created in order to be able to use MIMO and beamforming techniques at the level of radio-antenna modules. In this case, it was assumed that the number of layers is equal to the number of chains converting the signal into a digital form and vice versa (ADC/DAC)--the values were assumed approximately. The increase in the demand for spatial multiplexing in the wireless link will entail the need to increase these values. The larger bandwidth of the BB channel is applicable in higher millimeter RF bands (FR2) [29], where the devices from the internet-of-things (IoT) group will be communicated, therefore the number of layers is much lower. The TDM technique parameters in the 5G-NR frames were kept the same as in LTE (4G), with the difference that when the frequency distance between CP-OFDM subcarriers increased, the duration of the symbols shortens proportionally, and their number in the time-frame increases. As a result, broader BB channels will be used in the massive machine-type communications (mMTC) and ultra-reliable and low-latency communications (URLLC), where OFDM symbols do not have to be too resistant to multipath effects occurring in the wireless channel. The results of the calculations presented in Figure 10 show only the demand for the bandwidth of fronthaul/midhaul links at the level of the user plane (UP). The final bitrate must be increased by the control stream of the components on the gNB-DU/RRH board (control plane (CP)), but the increase in demand at the level of this Split/Option should not exceed 10% of the calculated value--Figure 10. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**BB Channel Size** & **ADC/DAC Chains \(N_{p}\)** & **Quantizer Resolution \(N_{\text{QF}}\)** & **Frame Duration** & **Headers Factor HF** \\ \hline (MHz) & - & - & (ms) & - \\ \hline
20 & 16 & 9 & 10 & 1.33 \\
50 & 24 & 9 & 10 & 1.33 \\
80 & 24 & 8 & 10 & 1.33 \\
100 & 16 & 8 & 10 & 1.33 \\
200 & 12 & 7 & 10 & 1.33 \\
400 & 10 & 7 & 10 & 1.33 \\
800 1 & 10 & 7 & 10 & 1.33 \\ \hline \hline \end{tabular}  
* The option is expected to be introduced in subsequent 5G releases.  
\end{table}