# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 3 Calculation Results
### DIPP-CIR in the Option 10 Fronthaul Path  
In fiber-optic communications with optical access, transceivers operating at a wavelength of 1310 nm are very often used. In the case of the Xhaul using optical channels supported by the flexible DWDM system, we move to the S, C and L optical bands. In this case, a transponder should be used in the place of connection to the optical fronthaul path, which will convert the wavelength in accordance with the DWDM grid. In the middle of the C band there is an optimal wavelength of 1550 nm for which preliminary calculations were made to show the influence of the DIPP phenomenon. Figures 6-8 show that RF dropouts due to direct detection are periodic both as a function of the optical path length and as a function of the radio frequency, which is the modulating signal. This is a well-known phenomenon but has always been considered in the context of a G.652D-based optical path and a wavelength of 1550 nm as the primary optical carrier. In Figure 6, we can see the significant effect of G.652D fiber dispersion. In this case, decay occurs quickly, and their periodicity is highly densified. The G.655D/E fiber (Figures 7 and 8) has a much smaller impact on the short periodicity of the interference signal fading. Additionally, in the areas of constructive interference, there are quite flat characteristics, which ensure good transmission of broadband radio signals. This issue is discussed later in the paper. Figure 8: Dispersion-induced power penalty—carrier-to-interference ratio obtained after propagation over G.655E single-mode non-zero dispersion shifted fiber and direct detection in photodetector: (**a**) calculated values for the five selected RF carriers as a function of the optical fronthaul path length; (**b**) calculated values for the five selected optical fronthaul path lengths as a function of the RF carrier. Figure 6: Dispersion induced power penalty—carrier-to-interference ratio obtained after propagation over G.652D or G.657A single-mode fiber and direct detection in photodetector: (**a**) calculated values for the five selected RF carriers as a function of the optical fronthaul path length; (**b**) calculated values for the five selected optical fronthaul path lengths as a function of the RF carrier. Figure 7: Dispersion induced power penalty—carrier-to-interference ratio obtained after propagation over G.655D single-mode non-zero dispersion-shifted fiber and direct detection in photodetector: (**a**) calculated values for the five selected RF carriers as a function of the optical fronthaul path length; (**b**) calculated values for the five selected optical fronthaul path lengths as a function of the RF carrier. On the basis of the obtained results, it can also be concluded that the influence of the DSB signal interference phenomenon during direct detection is significant at higher radio frequencies. This mainly applies to the FR2 band [48] intended for use in 5G/B5G systems, which reaches the range of 60 GHz, as well as higher bands, which are adapted in subsequent releases and generations of wireless access systems. The optical path for signal transmission in the RFoF technique should not exceed 20 km (Figure 1), and thus, can be implemented in a passive network, where chromatic dispersion compensation is usually not practiced. The next stage of calculations is closely related to the dispersion characteristics of single-mode optical fibers of different generations, in relation to the optical channels used in DWDM systems. The single-modality range of G.652D and G.657A fibers is much wider (Figure 3), but due to the limitations of G.655 fibers, the total optical bandwidth (S + C + L) is used for the calculations. This is a practical range of single-mode optical fibers adapted to work with dense wavelength multiplexing systems. Figures 9-11 show the fixed limits of 3 dB and 10 dB that define the areas in the optical wavelength and path length domains.