# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 5 Discussion
In order to verify the possibility of transmitting the modulated RF signal over the optical path and the correct selection of optical subbands, a simulation was performed. Two types of single-mode optical fibers (G.652D/G.657A and G.655D) were used in the simulation and two radio frequency carriers were selected, i.e., 28 and 60 GHz. The simulation results showed that the optical subbands were calculated correctly (Figures 26 and 27). Optical transmission of the DSB signal in the RFoF technique is burdened with many distortions, which was partially shown in the computational part of this study. The first limitation is the significant loss in the places where the signal is converted from electrical-to-optical (E/O) and optical-to-electrical (O/E). The results of the passive path loss can be seen in the pictured sample spectrograms (Figures 22 and 24). It is a key component determining the overall level of the signal reaching the radio demodulator. The noise floor of the radio receiver and the photo-detector constitute the basic signal-to-noise ratio, which determines the quality of the received useful signal. It should be noted that there is no optical amplifier on the optical side, which is typical for short-range links. It should also be taken into account that, at the location of the RF signal receiver, before the signal is delivered to the antenna module, a radio amplifier should be used. However, the signal-to-noise ratio is crucial here. The simulation did not use linear and redundant information coding. The Monte Carlo (MC) method was used to assess the SER parameter. However, the results obtained are relative, as the SER value is strongly dependent on the parameters of optoelectronic systems and the cumulative loss of optical components in the optical path. Such components may include, for example, OADMs/ROADMs and splitters (Table 2). The simulation assumed that the total additional attenuation of optical components is 5 dB. An increase in this value proportionally increases the SER value over the entire range, as it lowers the signal-to-noise ratio, which is not due to the DIPP effect. This assumption is due to the fact that the classic optical components used in the optical path are not dispersive, with the exception of dedicated chromatic dispersion compensators. The constellations of the signals modulated in the 256-QAM and 64-QAM formats are shown in Figures 23 and 25, respectively. Figures 23a and 25a show constellations of the signal transported over the optical fiber in the most optimal optical channel in the designated optical subband. These constellations show the presence of phase noise that results from phase shifts due to optical sideband interference. This phenomenon intensifies with an increase in the chromatic dispersion coefficient and an increase in the radio carrier frequency. In this case, the increase in the radio frequency causes the frequency sidebands to recede, which in turn causes rapid changes in the resultant phase. The depicted constellations also show that, with low loss of the optical path (together with the optical components), the effect of the thermal noise of the photodetector is minimal. Figures 23b and 25b show the constellations of the modulated signal carried in the optical channel at the edge of the 10 dB subband. In this case, the differences in the levels of adjacent subcarriers start to differ substantially. We observe this in the form of increasing amplitude noise in the presence of constant phase noise. External symbols (of greater amplitude) are most exposed to the increasing qualitative SER parameter, which is easy to observe. Figures 26 and 27 show the simulation results as a compendium. In the case of the G.652D/G.657A fiber, the simulation of the modulated signal in the 256-QAM format was abandoned, as phase noise causes too much reduction in the transmission quality. It is possible but with a significant reduction in the radio carrier or the bandwidth of the frequency channel.