## 3 Performance Simulation and Analysis
### ACM Transmission Efficiency  
ACM system considers the comprehensive effect of simultaneous reducing of free space loss and atmospheric attenuation when the elevation angle increases. Combined with the data in Table 1, when the elevation angle is \(5^{\circ}\), the theoretical range of link availability interval corresponding to different MCS in ACM system is shown in Table 7. The maximum MCS 28 is always used, and the link availability is determined by the selected minimum MCS. With the increase of the used minimum MCS, statistically, the link availability decreases gradually for a specific G/S, but the changes of each G/S are different. When performing ACM transmission efficiency analysis, the largest MCS 28 is preferred (the link availability is the lowest, but the useful data rate is the highest), and then the MCS is gradually reduced until the selected minimum MCS. When the transmission rate is weighted average, the MCS 28 is weighted by its corresponding link availability, while other MCSs are weighted by the gaps of adjacent link availability. As the elevation angle increases, the atmospheric attenuation decreases and the link availability of each MCS increases. The reliable data transmission capacity is assessed for the full transmission arc segment, so the link availability for CCM is determined only at \(5^{\circ}\) elevation, and it is not the weighted average for different elevation angles, which will be larger. If we recomputed the link budget with the larger weighted average link availability, the atmospheric attenuation at \(5^{\circ}\) elevation angle will be larger correspondingly, which would result in dissatisfied inserted system margin (less than 3 dB or even negative), and this is unacceptable. ACM is similar with CCM. In order to unify the comparison standard for CCM, VCM and ACM, the maximum link availability of ACM system is fixed as the link availability of the selected minimum MCS at \(5^{\circ}\) elevation angle. In this way, with the increase of elevation \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**MCS**} & \multicolumn{4}{c}{**Range of Link Availability**} & \multicolumn{4}{c}{**Weighted Probability**} \\ \cline{2-7}  & **Kashgar** & **Beijing** & **Sanya** & **Kashgar** & **Beijing** & **Sanya** \\ \hline
1 & (99.982, 99.985) & (98.986, 99.216) & (92.751, 93.599) & 0.003 & 0.230 & 0.848 \\
3 & (99.976, 99.982) & (98.735, 98.986) & (91.034, 92.751) & 0.006 & 0.251 & 1.717 \\
6 & (99.966, 99.76) & (98.406, 98.735) & (88.982, 91.034) & 0.010 & 0.329 & 2.106 \\
12 & (99.957, 99.966) & (98.108, 98.406) & (87.179, 89.828) & 0.009 & 0.298 & 1.749 \\
14 & (99.929, 99.957) & (97.405, 98.108) & (83.412, 87.179) & 0.028 & 0.703 & 3.767 \\
20 & (99.881, 99.929) & (96.452, 97.405) & (78.839, 83.412) & 0.048 & 0.953 & 4.573 \\
25 & (99.82, 99.881) & (95.527, 96.452) & (74.207, 78.839) & 0.061 & 0.925 & 4.632 \\
28 & [0, 99.82] & [0, 95.527] & [0, 74.207] & 99.82 & 95.527 & 74.207 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Theoretical range of link availability and weighted probability of each MCS in ACM system at \(5^{\circ}\) elevation angle (%). Figure 6: Switching elevation angle of adjacent MCS (initial minimum MCS 1). angle, the link availability of larger MCS is increasing. Starting from the selected minimum MCS, it is gradually not used, and finally only the largest MCS 28 needs to be selected. Taking the minimum MCS 1 as an example with the parameters in Table 2, the link availabilities of each MCS at different elevation angles are illustrated in Figure 7. The relationship between link availability and elevation angle is derived from the link budget computation widely used in satellite-to-ground data transmission systems. Figure 7: Link availability of each MCS at different elevation angles. (**a**) Kashgar G/S; (**b**) Beijing G/S; (**c**) Sanya G/S. For different minimum MCS, the simulation results of ACM TEF of Kashgar G/S, Beijing G/S and Sanya G/S are shown in Table 5. It can be seen that:  
1.