# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 3 Calculation Results
### Optical Channel Selection Algorithm  
The proposed algorithm (Figure 19) assumes that the routing protocol working in the Xhaul will find available optical paths connecting the DU and RU. Thus, a set of possible connections on the optical layer should be available. It should be assumed that these paths are all-optical and transparent to signals modulated in the RFoF format. The first criterion excluding the path is its length \(L\), which must be smaller than \(L\)max, because the permissible delay in the fronthaul link is of key importance. Further criteria are based on scanning free optical resources for the \(D\)max, \(Th1\) and \(Th2\) thresholds. A first decision threshold, \(Th1\), relates to the determination of an optical subband range that can be used for carrying the signal in the RFoF format. Two values were adopted here, i.e., 3 dB and 10 dB. In extreme cases, especially in the presence of a high radio frequency and a wideband frequency channel, an even stricter 1 dB criterion can be adopted. The second threshold, \(Th2\), relates to the slope of the DIPP-CIR characteristic, which indicates how much they differ in the blanking level of the two outside subcarriers contained in the OFDM radio signal or physical resource block (PRB). The second threshold is especially important in cases where a wide frequency channel is used on a high carrier frequency with a high coefficient of chromatic dispersion present in the optical path. The analysis of the phenomena occurring in the optical fronthaul path allowed for the construction of an algorithm (Figure 19) for the effective determination of the optimal optical channel for signal transmission in the DSB-RFoF format. Before the application created on the basis of the algorithm is launched, a lot of initial data must appear, which indicate not only the parameters of the radio signal, but also the properties of the optical path selected for transmission. The properties of the optical path include the chromatic dispersion coefficient \(D\) (or cumulative chromatic dispersion), path length \(L\) and its attenuation \(A_{pp}\). Due to the DIPP phenomenon occurring in a receiver operating in the DD mode, the most important parameter is the chromatic dispersion coefficient. This measurement can be performed with the use of the built-in OTDR system with the CD measurement function or with the use of an algorithm using optical AM modulation [51]. The introduction of the option to select an optical channel that meets the condition of minimum dispersion \(D_{\min}\) greatly reduces the occurrence of cross-phase modulation (XPM) and four-wave mixing (FWM) phenomena. The value of the minimum chromatic dispersion coefficient is variable, as it depends on the type of optical signals multiplexed in the DWDM Xhaul link. In the case of a constant wavelength spacing between the channels in which radio signals with the same RF radio frequency are carried, the requirement for \(D_{\min}\) is much higher. The paths/links of the DWDM Xhaul can transport the streams from the core network, backhaul, midhaul and fronthaul. Only on the fronthaul links can the RFoF signal be used. In other cases, it is a digital transmission in the OOK or coherent  
Figure 18: Calculation results of relative sideband delay in the optical channel (length of optical path/link is equal to 20 km and the radio channel frequency bandwidth for CP-OFDM modulation format is equal to 100 MHz). format. Interleaving in the wavelength domain of RFoF and OOK signals, with a flexible DWDM grid, further reduces the mutual influence of signals transported in adjacent optical channels. Please note that in the case of OOK modulation, the signal is concentrated close to the optical carrier, while in the RFoF signal, the sidebands containing the information are located at the periphery of the channel. The capabilities of the transmitting laser indicate a jump in the tuning of the optical carrier and the maximum range of wavelength change.