# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 3 Calculation Results
### DIPP-CIR in the Option 10 Fronthaul Path
On this basis, it is possible to determine the relationship that classifies the quality of the optical channel (QoOch) prepared to carry the DSB signal over the optical path in the RFoF format:  
\[\text{QoOCh}=\left\{ \begin{array}{l}\text{strong}\quad for\quad\text{DIPP}- \text{CIR}_{IM}<3\;\text{dB}\\ \text{weak}\quad for\quad\text{3 dB}<\text{DIPP}-\text{CIR}_{IM}<10\;\text{dB} \\ \text{unusable}\quad for\quad\text{DIPP}-\text{CIR}_{IM}>10\;\text{dB} \end{array} \right. \tag{16}\]  
Figure 9: The results of the dispersion-induced power penalty calculations, taking into account the 3 dB and 10 dB thresholds, obtained in the range of the S, C and L optical bands for the fronthaul path based on the G.652D or G.657A optical fiber of variable length: (**a**) \(f_{RF}=12\;\text{GHz}\); (**b**) \(f_{RF}=28\;\text{GHz}\); (**c**) \(f_{RF}=60\;\text{GHz}\); (**d**) \(f_{RF}=84\;\text{GHz}\). Figure 11: The results of the dispersion-induced power penalty calculations, taking into account the 3 dB and 10 dB thresholds, obtained in the range of the S, C and L optical bands for the fronthaul path based on the G.655E optical fiber of variable length: **(a)**\(f_{RF}=12\,\)GHz; **(b)**\(f_{RF}=28\,\)GHz; **(c)**\(f_{RF}=60\,\)GHz; **(d)**\(f_{RF}=84\,\)GHz. Figure 10: The results of the dispersion-induced power penalty calculations, taking into account the 3 dB and 10 dB thresholds, obtained in the range of the S, C and L optical bands for the fronthaul path based on the G.655D optical fiber of variable length: **(a)**\(f_{RF}=12\,\)GHz; **(b)**\(f_{RF}=28\,\)GHz; **(c)**\(f_{RF}=60\,\)GHz; **(d)**\(f_{RF}=84\,\)GHz. The white area visible in Figures 9-11 is the best optical carrier range for a particular optical path length. The blue field is a transition region with a fast quenching of the sidebands in the 3-10 dB range. Yellow areas indicate a DIPP-CIR greater than 10 dB, which, combined with the absorption losses, gives ranges of poor transmission quality. It should be noted, however, that in the case of radio carrier modulation with a low order applied, this range may also be partially used. Figure 9 shows the results of the calculations concerning the DIPP phenomenon occurring in the photodetector after the modulated optical signal has passed over the G.652D or G.657A optical fiber. In this type of fiber, there are yellow areas that cover the entire range of the S + C + L bands. This mainly concerns short sections of several kilometers for radio frequencies from 28 GHz (Figure 9b-d). In the case of the G.655D fiber, the situation is much better (Figure 10) because we can always find the optical range that lies in the white or blue field. The G.655E standard optical fiber is characterized by an indirect characteristic of the chromatic dispersion coefficient (Figure 2), which allows the lack of availability for most radio carrier frequencies to be avoided. The exception here is the case of frequencies above 80 GHz, where for an optical path with a length of about 1.5 km, there may be problems with selecting the appropriate optical channel. Figures 12-14 show the results of the DIPP-CIR signals calculated as a function of the optical wavelength. In order to determine the 3 dB and 10 dB optical wavelength range, a cut-off condition was applied. On this basis, we can try to adjust the optical channel for a specific optical path length and for a specific radio carrier frequency. The calculation results presented in Figures 12-14 relate to a path with a length of 20 km. Figure 12 shows the results of the DIPP interaction after passing over the G.652D or G.657A fiber. It is significant that for the case \(f_{RF}=12\) GHz, no optical band range with a threshold below 3 dB is available (Figure 12b--black color). The only thing left to do here is to select the optical channel in the range between the 3 dB and 10 dB thresholds (Figure 12c). The situation looks much better in the case of the optical path built on the basis of G.655D (Figure 13) and G.655E (Figure 14) fibers.