# Performance Analysis of a Vertical FSO Link with Energy Harvesting Strategy
## 4 Results and Discussions
### Optimization for EH
Note that the AEH values shown in that figure are consistent with those ones published by other authors in [31] and [47] for peak transmitted powers of 100 and 200 mW, respectively. As explained above, one of the main features affecting energy harvesting is the \(\omega_{z}/r\) ratio. In this respect, it is possible to design transmitters for EH with an accurate control of their beam width since significant gains can be achieved with the appropriate selection of \(\omega_{z}/r\). Thus, when increasing this ratio, the pointing problem is relaxed and, as well, the negative effect inherent to the jitter is reduced, i.e., when increasing \(\omega_{z}/r\) then fading induced by jitter suffered by the receiver is less intense since the received beam waist becomes (much) bigger than the receiver aperture radius. In this respect, although the received spot can wander due to the jitter effect, however, the total amount of caught power in the receiver side is maintained without significant variations since the size of the received beam waist is large compared to the receiver aperture radius. Consequently, the amount of power that can be captured from the section of the beam illuminating the receiver is, more or less, of the same magnitude. On the contrary, for smaller values of \(\omega_{z}/r\), the sway in the received beam footprint caused by misalignment is more critical in the sense that such a sway may make all the received beam spot drop out of the receiver photosensitive area. For that critical situation, the amount of captured power in the receiver would drop to zero. Of course, large \(\omega_{z}/r\) values lead to both severe geometrical losses (since most of the received footprint area is spread out of the physical photosensitive area implemented in the receiver side) and, consequently, low values of harvested energy. Therefore, the design of any FSO link should try to minimize this ratio by using adaptive pointing tracking systems [18]. It is worth noting that all the the results of average EH shown so far are for a single vertical FSO link between a given BS and UAV. Nevertheless, each UAV could receive simultaneous transmissions from a number of BSs as long as their locations fall within the region, \(\mathfrak{R}\subset\mathbb{R}^{2}\), covered by the FOV of the UAV's receiver. The area of such a region,  
Figure 7: Average harvested energy for the OOK and OOK-EH schemes versus peak transmitted power considering different (\(\omega_{z}/r\)) ratios and target BER. Turbulence fading (\(\alpha=\)30, \(\beta=\)30), a jitter (\(\sigma_{s}=0.1\) m) and “very clear air” conditions are assumed. A single transmitter has been considered. \(|\Re|\), can be expressed in terms of the FOV angle and the height of the UAV, \(Z\), as follows: \(|\Re|\)\(=\pi(Z\tan(\text{FOV}/2))^{2}\). Therefore, the average number of BSs, \(N\), that are covered by the FOV of the receiver is written as \(N=|\Re|\lambda_{\text{BS}}\), where \(\lambda_{\text{BS}}\) is the BS density expressed in number of nodes per m\({}^{2}\). Assuming that the macro BSs are spatially distributed according to a hexagonal grid, with a number of RRHs, \(n_{\text{RRHs}}\), randomly distributed within each macro cell as described in [59], that BS density leads to \(\lambda_{\text{BS}}=\frac{(1+n_{\text{RRHs}})}{2\sqrt{3}\left(\frac{|\Re|}{2 }\right)^{2}}\), where the term ISD stands for the inter-site distance (ISD), which is the distance between two neighbouring macro BSs. Finally, according to sections 6.1.2 and 6.1.4 of [59], Thus, for a given FOV, the value of \(N\) can be obtained as a function of the UAV height. In the case of dense urban microcellular deployment scenarios, the value of \(N\) increases from 6 to 62 when the height of the UAV increases from 300 m to 1000 m. However, despite the significant increase in the number of BS, the total collected energy hardly changes due to the increase of optical link loss with UAV height.