## 5 Numerical Results
In a nutshell, we should flexibly set the value of \(\rho\) according to different weather conditions to satisfy the QoS of users and achieve the best energy efficiency. The system energy efficiency under both IM/DD and HD techniques is plotted in Figure 12, in which three different weather conditions are considered. We assume the average SNR for OGS-to-satellite and satellite-to-HAP FSO links as 40 dB, and \(\rho=0.75\) is assumed. As we can see, system energy efficiency decreases with the increase in total transmit power, which is due to the fact that the logarithmic increase in capacity is less than the linear increase in total transmit power. In addition, the HD technique can achieve a higher energy efficiency than the IM/DD technique due to its better spectral efficiency. Considering the clear weather conditions, the HD scheme can achieve an energy efficiency of 593.2 Mbit/J, while the IM/DD scheme can only achieve an energy efficiency of 460.9 Mbit/J at a total transmit power of 10 dBW. Moreover, the rainy weather and foggy weather will reduce system energy efficiency. Considering the HD technique and a total transmit power of 15 dBW, the energy  
Figure 3.1 Capacity outage probability of users outside the hotspot area. Figure 3.2 System energy efficiency under different weather conditions. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Weather Conditions** & **Value Range** & **Optimal Value** \\ \hline Clear weather & 0.05 \(\leq\rho\leq\) 0.94 & 0.80 \\ \hline Light fog & 0.35 \(\leq\rho\leq\) 0.94 & 0.70 \\ \hline Moderate rain & 0.20 \(\leq\rho\leq\) 0.77 & 0.77 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The optimal value of \(\rho\) in different weather conditions. efficiency is 238.8 Mbit/J in clear weather, while it is 179.5 Mbit/J and 126.1 Mbit/J in moderate rain weather and light fog weather, respectively. Furthermore, the energy efficiency in moderate rainy weather is larger than that in light fog, especially at a small total transmit power. In Figure 13, we plot the system energy efficiency under different transmission schemes using the IM/DD technique, and both clear weather and moderate rain weather conditions are considered. The average SNR for OGS-to-satellite and satellite-to-HAP FSO links is assumed to be 40 dB and \(\rho=0.75\) is also assumed. It can be observed that the parallel FSO/RF scheme can achieve the best energy efficiency in different weather conditions. Due to the limited bandwidth of the RF link, the only RF scheme owns the worst energy efficiency performance. Considering clear weather with a total transmit power of 15 dBW, the parallel FSO/RF scheme can achieve an energy efficiency of 194.7 Mbit/J, while the only FSO scheme and only RF scheme can achieve an energy efficiency of 148.7 Mbit/J and 82.3 Mbit/J. Therefore, the parallel FSO/RF scheme can improve energy efficiency by 30.9% compared to only the FSO scheme. The parallel FSO/RF scheme has significant advantages in energy efficiency at low total transmission power regions. Furthermore, the parallel FSO/RF scheme can achieve the balance between enhanced coverage and extensive coverage. Figure 12: System energy efficiency under different detection schemes. Figure 13: System energy efficiency with different transmission schemes under different weather conditions.