# Performance Analysis of a Vertical FSO Link with Energy Harvesting Strategy
## 4 Results and Discussions
Figure 3 shows the typical attenuation of vertical optical links (Figure 3a) together with the potential energies harvested (Figure 3b) for different separation distances between any UAV and the ground BS, ranging between 200 m and 1000 m.  
Figure 3: (**a**) Average optical channel loss as a function of link length. (**b**) Average harvested energy as a function of the link length. Both figures consider different UAV jitters and weather conditions and a beam divergence of 1 mrad. To obtain the AEH, a single transmitter with \(P_{\text{at}}=\)500 mW is assumed. Channel loss depicted in Figure 3a is calculated by \(-10\log(\tilde{h})\), with \(\tilde{h}=(h_{l}A_{\circ}\gamma^{2})/(1+\gamma^{2})\) representing the average attenuation coefficient calculated from Equation (12). It is straightforward to check how the coverage provided by any deployed UAV enhances with increasing its altitude but, as shown in Figure 3a, at the cost of a significant increase in power losses at the receiver. Specifically, that increase is mainly caused by the beam broadening induced by the transmitter divergence. Namely, for \(L=\)300 m (i.e., \((\omega_{z}/r)=\)3, as commented above), a power loss of 8.5 dB is reached for \(\sigma_{\rm s}=0.1\) m. That value grows to 18 dB for \(L=\)1000 m as \((\omega_{z}/r)=\)10. In both cases, a transmission divergence of 1 mrad and a 10 cm aperture radius were considered. In addition, Figure 3b shows that increasing the channel loss dramatically reduces the harvested energy. Thus, for a single transmitter with a \(P_{ap}=500\) mW, the AEH drops from 18 mJ/s for the best case at 200 m; to less than 2 mJ/s at 1000 m. Note that the AEH in the figure has been calculated using Equation (17). Finally, Figure 3 shows how atmospheric losses are not relevant for the amount of energy the UAV can harvest due to the short link lengths considered in the analysis. Furthermore, they can be neglected without loss of accuracy when the UAV is affected by UAV's jitter caused by its motor vibration. Moreover, both figures also show the impact on the link performance of the pointing deviation caused by the jitter inherent to the UAV that acts as receiving side. For example, it can be observed that an increase in the UAV's jitter (higher \(\sigma_{\rm s}\)) is more detrimental when flying at lower altitudes than at higher altitudes. The reason for this is that the \(\omega_{z}/r\) ratio decreases for lower altitudes due to the beam divergence angle and, thus, the random variations inherent to the UAV location cause a more serious channel loss than in higher altitudes, even causing that the communication link may even be interrupted if the UAV moves out of the transmitted beam footprint. Consequently, energy harvesting is less affected by jitter as the UAV is operating at a higher altitude and, accordingly, the \(\omega_{z}/r\) ratio increases. Figure 4 summarizes this discussion. Figure 4: Adverse effect of UAV’s jitter caused by its motor vibration versus altitude in the flight state. It is supposed that the same UAV is flying at two different altitudes and affected by a same value of \(\sigma_{\rm s}\). For the case of the UAV operating at the lower altitude, the communication link may even be interrupted due to the presence of UAV’s jitter. Of course, for a higher altitude, the beam broadening induces more serious power losses at the receiver. Following the analysis of the channel behavior from Figure 3, now, Figure 5 shows the BER performance of the OOK and OOK-EH schemes under different channel impairments as a function of the received SNR. To illustrate the behavior of the OOK-EH scheme, a \(\zeta=0.8\) has been assumed. In addition, two ratios \(\omega_{z}/r\) have been taken as representative values: \((\omega_{z}/r)=3\) and \((\omega_{z}/r)=\)10, corresponding to UAV heights of 300 m and 1000 m, assuming a transmission divergence \(\theta_{T}=1\) mrad. BER curves plotted in black represent the case of \((\omega_{z}/r)=3\); whilst the curves plotted in red depict the performance for \((\omega_{z}/r)=10\).