# Performance Analysis of a Vertical FSO Link with Energy Harvesting Strategy
## 4 Results and Discussions
### Optimization for EH  
As described in the previous section, the process of optimizing the modified OOK scheme for EH consists of choosing the optimal \(\zeta\) value that maximizes the average EH while keeping the BER below the target value for each channel condition. Note that, since the increase in EH is achieved at the cost of degrading the link quality, the maximum \(\zeta\)  
Figure 5: Average BER for the OOK and OOK-EH schemes versus signal-to-noise ratio for a vertical FSO link under different channel impairments and different values of the ratio \((w_{z}/r)\). A transmitter with a divergence \(\theta_{T}=\)1 mrad and a \(P_{m}\) up to 20 dBm is assumed. value will depend on the considered channel impairments, i.e., turbulent fading, pointing error, and atmospheric attenuation. Consequently, a lower channel degradation will lead to higher \(\zeta\) values and, thus, to higher harvested energies. The value of the optimal \(\zeta\) has been obtained numerically from expression (26) by setting the link parameters and the target BER. To form a complete picture of the dependence of optimum \(\zeta\) on the FSO link parameters, Figure 6 shows the optimal value of \(\zeta\) as a function of transmitter peak power considering different \(\omega_{z}/r\) ratios, jitters and target BERs. In all cases, the turbulence fading and the path loss corresponding to \"very clear air\" are included. As can be seen in the figure, as the peak power of the transmitter increases, the optimum \(\zeta\) value also increases. In our analysis, realistic power values up to 35 dBm have been considered. Note that, since the UAV is intended to collect as much energy as possible, the power value chosen in the link design will be high. Therefore, the optimal \(\zeta\) values will also be high. For the highest of the powers considered (\(P_{m}=\)35 dBm), the value of \(\zeta\) is always higher than 0.8. In addition, as expected, the optimal \(\zeta\) values obtained for the \(10^{-8}\) target rate (red lines) are lower than for \(5\times 10^{-5}\) (blue lines) and, similarly, higher jitter values lead to lower \(\zeta\) values. From that Figure 6, it can be concluded that values of \(\zeta\) higher than 0.8 one can be selected for realistic propagation scenarios. On a different matter, Figure 7 depicts the average harvested energy for the OOK and OOK-EH schemes as a function of the peak transmitted power. For the OOK-EH scheme, the optimal \(\zeta\) values obtained in Figure 6 have been here employed. Hence, from the AEH results depicted in Figure 7, the following comments can be drawn. First, it is clearly observed that the energy collected with the OOK-EH scheme (black lines) is higher than that of the OOK scheme (red lines). In fact, the energy harvested by the OOK-EH scheme tends to twice that in OOK scheme as the transmitted power increases. Figure 7 also shows that a minimum \(P_{m}\) is required to achieve the previously selected BER target (a power below \(P_{m}\) cannot satisfy the performance required by the BER target and, accordingly, no energy would be harvested). This \(P_{m}\) is higher for the ratio \(\omega_{z}/r=\)3 than for \(\omega_{z}/r=\)10 due to what was explained when introducing Figures 3-6. Figure 6: Optimal \(\zeta\) values for the proposed OOK-EH scheme as a function of the transmitted peak power considering different (\(\omega_{z}/r\)) ratios, jitters and target BER. These \(\zeta\) values maximize the average harvested energy while maintaining the target BER. Turbulence fading (\(\alpha=30\), \(\beta=30\)) and “very clear air” conditions are assumed. Therefore, the choice of the ratio \(\omega_{z}/r\) has a huge impact on the value of the collected energy. The figure shows that the energy obtained with \(\omega_{z}/r=3\) is much higher than that obtained with \(\omega_{z}/r=\)10. In particular, for the highest power considered (Pm=35 dB), the AEH is about 70 mJ/s for \(\omega_{z}/r=\)3, while it hardly reaches 10 mJ/s for \(\omega_{z}/r=\) 10.