## 3 Numerical Results
### Effect of Radial Shift  
The drone VLC is anticipated to provide continuous wireless coverage for the terrestrial receiver under different radial shift situations. In this subsection, the effect of terrestrial radial displacement on the achievable energy efficiency for drone VLC employing distinct emission beam configurations is evaluated. Figure 4 compares the energy efficiency performance of drone VLC under different light beam configurations with various radial shifts for the terrestrial user receiver. Since the initial position of the terrestrial receiver is just directly underneath the transmitter drone, the shortest transmission distance and the lowest path loss are faced by the terrestrial receiver. However, with the lateral shift along the radial direction of this terrestrial receiver from the initial center position on the working plane, the link distance is increased, and the relevant path loss is more challenging as well. Accordingly, the key performance metric for drone VLC, i.e., energy efficiency under distinct LED beam configurations, is significantly affected by the radial shift separately. Numerically, for the case of Lambertian beam configuration-based drone VLC, with the radial shift increased to 2.5 m from the initial center position for the terrestrial receiver, \begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameters [33]** & & **Key Application Domains** \\ \hline \multirow{6}{*}{\begin{tabular}{} \end{tabular} } & \(g_{11}^{Z-Power}=0.542\), & \multirow{6}{*}{\begin{tabular}{} \end{tabular} } \\  & \(g_{21}^{Z-Power}=22.75^{\circ}\), & \\  & \(g_{31}^{Z-Power}=49.96^{\circ}\), & \\  & \(g_{12}^{Z-Power}=0.573\), & \multirow{6}{*}{\
\begin{tabular}{} \end{tabular} } \\  & & \\ \(R_{Z-Power}(\phi)=\sum\limits_{k=1}^{K}g_{1k}^{Z-Power}\exp[-\ln 2(\frac{|\phi|-g_{3k}^{Z- Power}}{g_{3k}^{Z-Power}})^{2}]\) & \multirow{6}{*}{
\begin{tabular}{} \end{tabular} } \\  & & \\ \(g_{22}^{Z-Power}=77.84^{\circ}\), & & \\  & \(g_{32}^{Z-Power}=23.7^{\circ}\), & \\  & \(g_{13}^{Z-Power}=0.279\), & \\  & \(g_{22}^{Z-Power}=86.67^{\circ}\), & \\  & \(g_{33}^{Z-Power}=8.43^{\circ}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Spatial radiation characteristic parameters and application domains of Z-Power and NSPW LEDs. the relevant energy efficiency is reduced to 11.69 Bits/J/Hz from the initial 16.63 Bits/J/Hz. As for the case of non-Lambertian Z-Power light beam configuration-based drone VLC, the relevant energy efficiency is reduced to 10.51 Bits/J/Hz from the initial 13.40 Bits/J/Hz accordingly. Moreover, as for the left case of non-Lambertian NSPW light beam-based drone VLC, since the rotational asymmetric radiation characteristics, both the radial shift along the wide cross section and the narrow cross section of the NSPW light beam should be studied separately. For the mentioned case of radial shift along the wide cross section, the relevant energy efficiency is slowly reduced to the 12.77 Bits/J/Hz from the initial 18.87 Bits/J/Hz when the radial shift is increased to 2.5 m from the initial center position for the terrestrial receiver, thanks to that the more optical signal power could be emitted by the wide radiation cross section to the adverse receiver positions with longer radial shift. However, for the mentioned case of radial shift along the narrow cross section, the relevant energy efficiency is sharply reduced to the 9.80 Bits/J/Hz from the initial 18.87 Bits/J/Hz when the radial shift is increased to 2.5 m from the initial position, due to that the much reduced optical signal power is emitted by the narrow radiation cross section to the adverse receiver positions with longer radial shift. Objectively, for the radial shift in the wide cross section, the non-Lambertian NSPW light beam-based drone VLC presents superior performance, i.e., 1.08 Bits/J/Hz energy efficiency gain compared with the counterpart of the Lambertian beam-based drone VLC.