# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 5 Discussion
The simulations were performed for frequencies within the FR2 range and above this range, where it was not planned to use a numerological parameter with a value below 2. The simulation results also show that the transmission quality is significantly influenced by the bandwidth of the frequency channel, and more specifically, the numerological parameter of the CP-OFDM signal. For \(\mu=3\), the subcarrier spacing increases to 120 kHz, which greatly increases the distance differences between the levels of the same numbered interfering subcarriers in the optical sidebands. Additionally, with the increasing value of the parameter \(\mu\), the length of the OFDM symbol is shortened, which reduces its energy. The total consequences can be seen in Figure 26, where we compare the SER value for 16-QAM modulation, with the \(\mu\) parameter being 2 and 3. Increasing the \(\mu\) parameter by one degree results in a SER similar to 64-QAM for \(\mu=2\). This applies to both the 60 GHz and 28 GHz radio carriers. The same phenomenon is observed in the case of the G.655D fiber, although in this case, the results are much better, due to the lower chromatic dispersion coefficient. Here, 256-QAM modulation for \(\mu=2\) was introduced, due to its much better dispersion param eters. Confusingly, Figure 27a,b shows swapped values. In the case of the 28 GHz radio carrier, the SER values are slightly higher than in the case of the 60 GHz carrier. However, one should look at the subband in which the simulations were performed. The optical subband for 60 GHz is characterized by higher frequencies, i.e., shorter wavelengths, so here, there is a lower chromatic dispersion coefficient. In the case of 28 GHz, the optical frequencies are much lower, so the optical wave is longer at a higher value of the chromatic dispersion coefficient. It is undisputed that the introduction of redundant coding will significantly increase the quality of the transmitted digital information. However, the strict quality characteristics presented in Figures 26 and 27 clearly show which parameters of the radio signal must be taken into account when we are selecting an optical channel. For lower radio frequencies, the available optical subbands are much wider. This results in a lot of freedom in the selection of optical channels, and in such a channel, the amplitude characteristics are much flatter. The discussion presented above concerned the research performed on the basis of the well-known standards of single-mode optical fibers, which are designed and manufactured with the use of silica. These types of optical fibers have become very entrenched in telecommunications fiber-optic networks, so breaking this barrier will certainly not be easy. A very significant disadvantage of standard single-mode optical fibers is the small diameter of the core, in which optical signals appearing in the adjacent optical channels very quickly increase the power density. This fact causes the appearance of the above-mentioned non-linear effects, which can significantly deteriorate the transmission. The chromatic dispersion occurring in, and in the vicinity of, the core, which is the essential limiter of DSB-RFoF signal transport, is important from the point of view of limiting the Kerr effect, which results in XPM and FWM distortions. In conclusion, these two phenomena should be balanced so that communication is effective. If we want to stick to this technology, we need to popularize multi-core fibers much faster [55]. Fibers based on a greater number of cores are more difficult to couple with the transmitter and receiver systems, but they allow for a significant spatial increase in cable capacity. In the B5G/6G wireless access systems, the massive-MIMO technique will be the norm. Connecting a large number of antenna modules, located on the RU/RRH mast, to the BBU, also via the DU, requires the use of a large number of optical fibers heavily loaded with high optical power density. This is the specificity of the C-RAN architecture described in the introduction.