## 3 Simple Model of Bandwidth Consumption Calculation in 5G Fronthaul Interfaces
### Bandwidth Consumption in A-RoF Interfaces
In this case, there is a need to move all system functions to the CU cloud, where radio resource allocation will take place to individual antenna matrices mounted on the mast. The question arises, whether it is possible to coordinate all spatial streams using only one optical channel? Of course, this is not physically feasible, therefore there is a need to use the optical wavelength division multiplexing (WDM) technique or space division multiplexing (SDM) on the optical side (a larger number of optical fibers in the cable, or the use of multi-core optical fibers [50]). Optical resources are presented in the wavelength domain, because the optical fiber influences the change of the wavelength as a function of the optical frequency of the carrier. In order to evaluate the frequency bandwidth of the modulating signal, a basic relationship can be used  
\[\Delta f=2\cdot f_{b}+2\cdot f_{k} \tag{4}\]  
where \(f_{b}\)--radio frequency (RF) carrier (identifying the RF channel number), \(f_{k}\)--the extreme right frequency of the CP-OFDM sub-carrier of the baseband channel. The transition to the wavelength domain can be accomplished using the following approximate dependence  
\[\Delta\lambda=\lambda_{2}-\lambda_{1}=\frac{c\cdot\Delta f}{(f_{2}-\Delta f) \cdot f_{2}}=\frac{c\cdot\Delta f}{(f_{1}+\Delta f)\cdot f_{1}}\cong\frac{c \cdot\Delta f}{f_{LD}^{2}} \tag{5}\]  
Figure 8: Proposition of new Options (Splits) for realization of analog interfaces working in A-RoF techniques. Splits mainly used for fronthaul/midhaul to connect DU and AAU (as a gNB-DU)—simple A-DAS. where \(f_{LD}=\lambda_{LD}/c\)--the frequency of the optical carrier emitted by the laser radiation source. By substituting relation (4) into (5), we get  
\[\Delta\lambda=\frac{\Delta f\cdot\lambda_{LD}^{2}}{c}=\frac{2(f_{b}+f_{k})\cdot \lambda_{LD}^{2}}{c} \tag{6}\]  
On this basis, it can be seen that the width of the optical channel is primarily dependent on the component that constitutes the radio frequency (RF). Regarding the relation (1), it can be seen that the efficiency of the analog solution is higher if the radio frequency used is higher, too. The simple design of the RRH/AAU connected via the A-RoF interface to the fronthaul/midhaul, enforces the use of optical multiplexing techniques, which will ensure full service of the MIMO radio technology. In case of the D-RoF interface, the use of CPRI/eCPRI enables the introduction of multiplexing of temporary TDM, which limits the need for multiple laser sources, but creates a very broadband optical channel. For 5G mobile systems, even at the level of a few-kilometer fiber-optic link, the D-RoF interface will need WDM/SDM multiplexing. Another issue is related to the multi-sectority of the RRH, which makes it necessary to use RF channel separating systems and direct them to the appropriate array of antennas in the sector. We can solve this by using a flexible grid of DWDM channels [25], where the minimum frequency bandwidth of the optical channel is 12.5 GHz. The grid for the optical carrier is 6.25 GHz, but for the simplification resulting from the analog DSB modulation in the A-RoF interface, we assume that the carrier will change the frequency at least every 12.5 GHz.