# Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators
## 5 Numerical Results of the Network  
Here, the analytical formalism developed in the previous section is applied to the performance evaluation of the FSO-coded fronthaul network supporting the creation of dense antenna cells via the OCDMA multiplexing technique. The BER and SINR (signal-to-interference-plus-noise-ratio) performances of the network where each of 32 simultaneous signals is assigned a unique 1D-OCDMA code is evaluated. The 32 different codes were generated on a single wavelength beforehand with a code length \(L=1000\) and weight \(W=10\). However, one can also aim to achieve different data rate transmissions via different configurations of the graphene modulator. This is possible because the different amounts of changes in graphene directly affect both the extinction ratio and modulator's speed, consequently affecting the bandwidth of the modulated signals. As already mentioned, four configurations holding amounts of 25%, 50%, 75%, and 100% of graphene are considered. These structures achieve extinction ratios of the order of 34.27 dB, 39.13 dB, 51.52 dB, and 32.26 dB with bandwidth values of 42.6 GHz, 63.81 GHz, 78.41 GHz, and 88.45 GHz, respectively. Consequently, the data rate transmissions performed for such configurations are 60.2 Gb/s, 90.2 Gb/s, 110.9 Gb/s, and 125.1 Gb/s, respectively. The main values of the parameters of the fronthaul network are summarized in Table 2. With all common configurations of the graphene modulators properly described, the performance of the coded-FSO fronthaul network can now be addressed. For the sake of simplicity and conciseness, the network segment from the RU to the user equipment (see Figure 1) is not considered in the performance analysis. It is noteworthy that (14) has been successfully validated against the BER equation given by (5) in Ref. [118]. Initially, the BER is investigated as a function of the threshold level to be adjusted at the receiver side of the coded-FSO fronthaul network for all the graphene modulators (i.e., 25%, 50%, 75%, and 100% graphene). The final stage at the receiver side is responsible for choosing between the bit transmission \"1\" or \"0\" if the decision variable (electrical power in the device input) is greater or lower than the threshold, respectively. Hence, it is necessary to estimate an optimum threshold that minimizes the BER once the bit levels are dependent on the cross-talk among signals and photodetector noise levels in the coded-FSO fronthaul network evaluated. In this scenario, the optical receiver power is set to 0 dBm and the number of simultaneous antennas is fixed at 32. Accordingly, the BER versus the threshold level is plotted in Figure 6. Initially, it can be observed that the optimum threshold to minimize the BER increases as the percentage of graphene decreases. This is simply because more photons (or optical power) arrive at the photodetector as both the extinction ratio and bit rate transmission related to the signals are not too high. Results suggest that the optimum thresholds are \(-2.66\) dBm, \(-3.27\) dBm, \(-3.58\) dBm, and \(-3.70\) dBm for modulators with 25%, 50%, 75%, and 100% graphene, respectively. In addition, it can be observed that the modulator with 25% graphene has the lowest BER at the optimum threshold among all the configurations. It occurs because the cross-talk is the main deleterious source in the coded-FSO fronthaul network once the photodetector noises are negligible at high levels of received optical power [49; 116]. Lastly, it is possible to further note that the bit error probability approaches \(1/2\) as the threshold decreases or increases from the optimum point. The low levels of thresholds result in erroneous decisions of the bits \"0\" to \"1\" even as low levels of cross-talk and photodetector noises upon construction interfere with the signals. In contrast, high thresholds are harmful because most of the bits \"1\" are interpreted as \"0\", even when low levels of photodetectors noises perform on the signals.