## 3 Numerical Results
### Effect of Drone Altitude  
The drone VLC is anticipated to provide wireless transmission for the aerial receiver at various altitudes. In this subsection, the effect of the altitude of aerial users on the achievable energy efficiency for distinct emission beams-based drone VLC is studied. When the aerial user is located at the side position, as shown in Figure 5a, it can be observed that the energy efficiency at the receiver end under distinct beam configurations is determined by the altitude of the aerial receiver to a different extent. In detail, when the altitude of aerial receiver is increased to 1.95 m from the side position on the ground, the energy efficiency of drone VLC is reduced to 11.92 Bits/J/Hz, 8.80 Bits/J/Hz, and 12.05 Bits/J/Hz from the initial 13.04 Bits/J/Hz, 11.53 Bits/J/Hz, and 14.40 Bits/J/Hz for the Lambertian LED beam, NSPW LED beam (side position), and NSPW LED beam (side position 2), accordingly. On the other hand, the counterpart of the non-Lambertian Z-Power light beam configuration is increased to 12.21 Bits/J/Hz from the initial 11.31 Bits/J/Hz. The above phenomena indicate that a non-Lambertian NSPW light beam (side position 2) provides better performance for near-terrestrial space with an altitude less than 1.95 m, by utilizing its asymmetric spatial radiation capability to cover the more challenging receiver position. 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 0.002 Bits/J/Hz, 0.00 Bits/J/Hz, and 0.046 Bits/J/Hz from the 11.92 Bits/J/Hz, 8.80 Bits/J/Hz, and 12.05 Bits/J/Hz for the Lambertian light beam configuration, non-Lambertian NSPW light beam configuration (side position), and non-Lambertian NSPW light beam configuration (side position 2), accordingly. And the counterpart of the non-Lambertian Z-Power light beam configuration is gradually reduced to 4.99 Bits/J/Hz from the initial 12.21 Bits/J/Hz, which indicates that Z-Power light beam configuration provides the better performance for non-terrestrial space with altitude higher than 1.95 m, by utilizing its maximum radiation intensity to light the almost horizontal direction, but not the ground as other concerned LED beam configurations in this article. Objectively, at the side position with upward orientation, for the considered maximum altitude of aerial receiver, the non-Lambertian Z-Power light beam-based drone VLC provides 4.988 Bits/J/Hz energy efficiency gain compared with that of the baseline Lambertian beam-based drone VLC. Similar results could be found when the light receiver with horizontal orientation is adopted for the side position, as shown in Figure 5b. Specifically, when the altitude of aerial receiver is increased to 1.95 m from the side position on the ground, the energy efficiency of drone VLC is changed to 13.78 Bits/J/Hz, 14.07 Bits/J/Hz, 10.66 Bits/J/Hz, and 13.91 Bits/J/Hz from the initial 12.82 Bits/J/Hz, 11.11 Bits/J/Hz, 11.32 Bits/J/Hz, and 14.19 Bits/J/Hz for the Lambertian drone VLC, Z-Power drone VLC, NSPW drone VLC (side position 2), accordingly, which verifies that NSPW drone VLC (side position 2) still provides the relative more acceptable performance for near terrestrial space with altitude less than 1.95 m. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Radial Shift** \\ **Amplitude (m)** \\ \end{tabular} } & \begin{tabular}{c} **Energy Efficiency of** \\ **Lambertian Drone** \\ **VLC (Bits/J/Hz)** \\ \end{tabular} & \
\begin{tabular}{c} **Energy Efficiency of** \\ **Z-Power Non-Lambertian** \\ **Drone VLC (Bits/J/Hz)** \\ \end{tabular} &
\begin{tabular}{c} **Energy Efficiency of NSPW** \\ **Non-Lambertian Drone VLC** \\ **Wide Cross** \\ **Section (Bits/J/Hz)** \\ \end{tabular} \\ \hline \(-\)2.5 & 11.69 & 10.51 & 12.77 & 9.80 \\ \hline
0 & 16.63 & 13.40 & 18.87 & 18.87 \\ \hline
2.5 & 11.69 & 10.51 & 12.77 & 9.80 \\ \hline \hline \end{tabular}
\end{table}