# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 2 System, Methods, and Network Components
### Fiber-Optic Fronthaul Path
of G.655 fibers were modeled using the polynomials contained in the appropriate tables D and E of the recommendation [43] (Figure 3):  
\[D_{\text{G.655D}}=\left\{ \begin{array}{l}\frac{4.955}{90}(\lambda-1460)-0.455\; \;\text{for}\;1460\;\;\text{nm}\;\leq\lambda\leq 1550\;\;\text{nm}\\ \frac{4.015}{75}(\lambda-1550)+4.500\;\;\text{for}\;1550\;\;\text{nm}\;<\!\! \lambda\leq 1625\;\;\text{nm}\end{array} \right. \tag{2}\]  
\[D_{\text{G.655E}}=\left\{ \begin{array}{l}\frac{5.035}{90}(\lambda-1460)+2.650 \;\;\text{for}\;1460\;\;\text{nm}\;\leq\lambda\leq 1550\;\;\text{nm}\\ \frac{3.710}{75}(\lambda-1550)+7.685\;\;\text{for}\;1550\;\;\text{nm}\;<\!\! \lambda\leq 1625\;\;\text{nm}\end{array} \right. \tag{3}\]  
Due to the specificity of the operation of flexible DWDM systems, as mentioned before, the average characteristics determined in the wavelength range from 1460 nm to 1625 nm are used for further calculations. Figure 3 shows the characteristics of the chromatic dispersion coefficients (modeled on the basis of (7)-(9)) for three generations of single-mode fibers, which can be found in the optical fiber network working in the Xhaul architecture. The dashed line defines the upper and lower limits of the chromatic dispersion coefficient that can be found in standard SMFs (S-SMFs) and non-zero dispersion-shifted SMFs (NZDS-SMFs). Optical fibers of the G.652D standard are the basic medium in optical access networks, so it can be assumed that they are present in cities and their surroundings, but also in rural areas. The G.655D/E optical fibers, which are adapted to DWDM links, are often found between cities and between larger regions. We assume that these fibers appear only in the links between optical nodes that support DWDM streams and very often constitute all-optical nodes as the reconfigurable/optical add-drop multiplexers (OADM/ROADM). The fibers of the recently emerged G.657A generation have parameters very similar to those of the G.652D standard but are more resistant to macro-bending events. As a result, fiber-optic cables with G.657A fibers can be installed in locations with more difficult installation conditions. This type of cabling is an excellent solution when connecting AAU/RRH/RU to CU/DU in FTTA architecture. Figure 4 shows an exemplary slice of the flexible optical Xhaul with fiber access connections. The flexible Xhaul is based on G.655D/E fibers, which guarantees the possibility of providing broadband services with a long range. It should be added that the Xhaul can also provide services in the backhaul domain, which introduces the need to significantly increase the length of optical links (Figure 1). In the access part of the network, on the DU side, standard G.652D fibers are usually used, and on the RRH side, the G.657A standard fibers are preferred. Each of the proposed optical fibers is single mode, so it can carry  
Figure 3: Modeled characteristics of the chromatic dispersion coefficient of the single-mode optical fibers in range of single-modality limited by the cut-off wavelength [41; 42; 43]. the traffic of optical signals in accordance with the flexible DWDM channel grid, which is represented in Figure 4 by multi-colored lines connecting network nodes. The optical path, created as a result of connecting several different optical links, can be characterized by a specific attenuation depending on the carrier wavelength (Figure 2), as well as the averaged chromatic dispersion coefficient (Figure 3). The path attenuation depends not only on the length of the optical fiber used, but also on the number of splices or the physical contact (PC) connectors, the number of ROADM/OADM or passive optical splitters (Table 2). In a network using the flexible DWDM system, we assume that optical splitters are not applicable, but in the case of the access part, such optical systems can be used. In this case, PON networks, which currently constitute the basis for the operation of cable networks and fiber-optic Internet access in FTTH format, come to the rescue.