# Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators
## 5 Numerical Results of the Network
Next, the BER versus the optical power for different modulator's graphene percentage was plotted in Figure 7. In general, the BER improves as the optical power levels increase. In such analyses, we considered 32 simultaneous antennas in the coded-FSO fronthaul network, and also the optimum threshold values as previously obtained. As can be seen, the curves nearly overlap as low optical power reaches the receiver. Such behavior is due to the coded-FSO fronthaul network performance becoming limited to the photodetector noises. In contrast, a considerable difference among the graphene modulators' performance can be realized in higher optical powers once the cross-talk among the transmitted signals becomes most relevant degradation mechanism. One can still notice that the 25% graphene modulator has the best BER among the different graphene percentages due to its higher modulation extinction ratio. One can further note both 75% and 100% have similar levels of BER. At higher levels of graphene, the modulators have similar higher modulation extinction ratios and bandwidths, consequently achieving similar low BER performances. Bear in mind that the 100% graphene modulator supports a higher bandwidth than does 25% graphene. Nonetheless, both the lower modulation extinction ratio and bandwidth of the 25% graphene modulator leads to better BER performance due to the increased signal power that reaches the photodetector. Figure 6: Optimum detection receiver threshold. The BER is minimized by choosing a threshold value of \(-2.5\) dBm for the 25% graphene modulator. Subsequently, the BER versus the number of simultaneous antennas was plotted in Figure 8. The number of simultaneous antennas was varied, considering that each one receives 0 dBm of optical power. Additionally, the optimum thresholds were estimated dynamically and employed at the receivers for each group of simultaneous antennas. It needs to be noted that the 25% graphene modulator has the best overall performance. The latter can accommodate up to 11 simultaneous antennas at the mobile cell under BER \(\leq 10^{-7}\), and up to 6 under BER \(\leq 10^{-9}\). Finally, if someone is interested in accommodating 32 simultaneous antennas under an error-free regime, where all data bits are transmitted with success, FEC techniques at higher levels of BER (i.e., BER \(\leq 10^{-3}\)) can be deployed [58]. For example, under FEC usage (BER \(\leq 10^{-3}\)), all investigated graphene modulators can achieve error-free transmissions, supporting the creation of ultra-dense small cells, or cell expansion. Next, the SINR as a function of the optimum receiver threshold is plotted in Figure 9 for four different percentages of the modulator's graphene. To do so, the optical receiver is set to 0 dBm and the number of simultaneous antennas is fixed at 32, for the sake of consistency of the analysis. Bear in mind that this is the SINR at the receiver side after signal detection. One can note from Figure 9 that similar threshold levels are obtained for the four different amounts of graphene in the system's modulator. Results suggest that the optimum threshold is nearly \(-2.66\) dBm, \(-3.27\) dBm, \(-3.58\) dBm, and \(-3.70\) dBm for  
Figure 8: Bit error rate versus the number of simultaneous antennas at the remote unit side. Figure 7: Bit error rate versus the optical power received in the photodetector for different amounts of graphene in the modulator. the modulator's configuration with 25%, 50%, 75%, and 100%, respectively. Finally, it can be further observed that the 25% graphene modulator has a slightly higher SINR than the remaining modulator configurations. Afterwards, the SINR versus the number of simultaneous antennas was plotted in Figure 10 for four different amounts of the modulator's graphene. It can be seen that the modulator upon 25% graphene stands out and shows the best performance among the four configurations of graphene.