## 3 Numerical Results
### Effect of Drone Altitude

Table 3: Results summary of the radial shift effect for Lambertian and non-Lambertian drone VLC. In addition, when the altitude of aerial receiver is further increased to 2.95 m from the 1.95 m, the energy efficiency of drone VLC is quickly reduced to 6.40 Bits/J/Hz, 1.76 Bits/J/Hz, and 8.68 Bits/J/Hz from the 13.78 Bits/J/Hz, 10.66 Bits/J/Hz, and 13.91 Bits/J/Hz for the Lambertian light beam configuration, NSPW drone VLC (side position), and NSPW drone VLC (side position 2), accordingly while the counterpart of Z-Power drone VLC is further enhanced to 15.64 Bits/J/Hz, which proves the tremendous superiority of the non-Lambertian Z-Power light beam configuration for the non-terrestrial space with higher altitude, by straightforwardly utilizing its almost maximized omnidirectional horizontal radiation pattern. Particularly, at the side position with horizontal orientation, for the considered maximum altitude of aerial receiver, the non-Lambertian Z-Power light beam-based drone VLC provides 9.24 Bits/J/Hz energy efficiency gain compared with that of the baseline Lambertian beam-based drone VLC. The above numerical results show that compared with the previous upward orientation setting, the horizontal orientation setting of the aerial light receiver could further amplify the unique superiority of the non-Lambertian Z-Power light beam-based drone VLC in higher non-terrestrial space. Figure 5: Energy efficiency performance versus the altitude of receiver drone: (**a**) side position of light receiver with upward orientation; (**b**) side position of light receiver with horizontal orientation; (**c**) corner position of light receiver with upward orientation, and (**d**) corner position of light receiver with horizontal orientation. Actually, the unique superiority of the non-Lambertian Z-Power light beam is robust in harsher corner positions for the aerial receiver, as shown in Figure 5c of upward orientation and Figure 5d of horizontal orientation. As for the case of upward orientation, once the altitude of the aerial receiver is increased from 1.35 m to the maximum height, the energy efficiency of drone VLC is accordingly reduced to 0.00 Bits/J/Hz, 2.00 Bits/J/Hz, and 0.00 Bits/J/Hz from the 10.24 Bits/J/Hz, 10.21 Bits/J/Hz, and 8.21 Bits/J/Hz for the Lambertian drone VLC, Z-Power drone VLC, and NSPW drone VLC, which illustrates that up to 2.00 Bits/J/Hz energy efficiency gain can be achieved by the Z-Power drone VLC compared with the that of the Lambertian drone VLC, for the considered maximum altitude. Moreover, for the case of horizontal orientation, when the aerial receiver altitude is increased from 1.35 m to the maximum height, the energy efficiency of drone VLC is accordingly changed to 3.40 Bits/J/Hz, 13.60 Bits/J/Hz, and 0.55 Bits/J/Hz from the 11.80 Bits/J/Hz, 11.76 Bits/J/Hz, and 9.77 Bits/J/Hz for the Lambertian drone VLC, Z-Power drone VLC, and NSPW drone VLC, which surprisingly illustrates that for the considered maximum altitude of aerial receiver, compared with the baseline Lambertian drone VLC, up to 10.20 Bits/J/Hz energy efficiency gain could still be achieved by the Z-Power light beam configuration. The above figures of drone altitude effect indicate that the case of non-Lambertian Z-Power light beam provides the best performance consistency, especially for high altitude domains, while the cases of Lambertian beam and non-Lambertian NSPW light beam illustrate more obvious performance fluctuations with higher altitude. The main reason for these phenomena is that the non-Lambertian Z-Power light beam emits the maximum radiation power in all directions close to horizontal, which, to a large extent, illuminates the whole space domain of the drone VLC. On the other hand, both Lambertian beam and non-Lambertian NSPW light beam emit the maximum radiation power downwards towards the ground, which objectively limits the provision of sufficient lighting to the space domain of the drone VLC.