## 4 Comparison and Scheme Design
### Scheme Design of Satellite-to-Ground Data Transmission  
In engineering applications, it is necessary not only to improve the average useful data rate, but also to minimize the outage period (improve link availability) to avoid excessive impact on user data receiving tasks.  
As mentioned above, the maximum CCM TEF of the aforementioned three G/Ss are all achieved when the MCS is 28. For a specific G/S, by comparing the TEF of VCM and ACM with the one of CCM of MCS 28, the data-transmission scheme (CCM, VCM or ACM) could be decided. The comparison results are given in Table 9.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**MCS**} & \multicolumn{3}{c}{**VCM vs. CCM**} & \multicolumn{3}{c}{**ACM vs. CCM**} & \multicolumn{3}{c}{**ACM vs. VCM**} \\ \cline{2-10}  & **Kashgar** & **Beijing** & **Sanya** & **Kashgar** & **Beijing** & **Sanya** & **Kashgar** & **Beijing** & **Sanya** \\ \hline \(1\) & 70.39 & 70.31 & 70.37 & 808.25 & 806.18 & 796.61 & 433.05 & 432.07 & 426.29 \\ \(3\) & 40.84 & 40.74 & 40.82 & 464.05 & 463.04 & 457.80 & 300.50 & 300.05 & 296.11 \\ \(6\) & 24.82 & 24.78 & 24.88 & 236.76 & 236.30 & 234.00 & 169.80 & 169.51 & 167.47 \\ \(12\) & 24.36 & 24.34 & 24.35 & 150.16 & 149.93 & 148.98 & 101.16 & 100.99 & 100.22 \\ \(14\) & 19.38 & 19.35 & 19.44 & 99.85 & 99.72 & 99.12 & 67.41 & 67.34 & 66.71 \\ \(20\) & 12.92 & 12.92 & 12.95 & 40.67 & 40.63 & 40.48 & 24.58 & 24.54 & 24.37 \\ \(25\) & 6.85 & 6.84 & 6.85 & 12.69 & 12.69 & 12.66 & 5.47 & 5.47 & 5.44 \\ \hline \hline \end{tabular} Note: The leftmost column is the initial minimum MCS for VCM and the minimum MCS for ACM.  
\end{table}
Table 8: TEF improvement percentage (%).  
Figure 9: TEF improvement percentage. (**a**) VCM vs. CCM; (**b**) ACM vs. CCM; (**c**) ACM vs. VCM.  
Compared with the CCM of MCS 28, if VCM is adopted:  
1. The VCM TEF of Kashgar G/S and Beijing G/S are even smaller. However, the link availability is only increased from 99.820% to 99.881% for Kashgar G/S and from 95.527% to 96.452% for Beijing G/S. According to the principle of \"maximizing transmission efficiency\", VCM is not recommended.
2. The VCM TEF of Sanya G/S is slightly larger (only increased by 0.74%) and the link availability is increased from 74.207% to 78.839%. The improvement is limited. Therefore, VCM is also not recommended. Compared with the CCM of MCS 28, if the ACM of minimum MCS 1 is adopted:  
1. For Kashgar G/S with little rain, the link unavailability is reduced from 0.180% to 0.015%, the average annual outage period is reduced from 0.66 days to 0.05 days, which can be almost ignored, and the TEF is only increased by 0.16%. Therefore, according to the principle of \"simplification of satellite ground interaction\", the CCM of MCS 28 is more suitable for use.
2. For Beijing G/S with moderate rainfall, the link unavailability is reduced from 4.473% to 0.784%, and the average annual outage period is reduced from 16.33 days to 2.86 days. The reduction proportion is as high as 82.47%, and the TEF is increased by 3.62%, making ACM more suitable for use.
3. For Sanya G/S with abundant rainfall, the link unavailability is reduced from 25.793% to 6.401% and the average annual outage period is reduced from 94.14 days to 23.36 days. The reduction proportion is as high as 75.18%, and the TEF is increased by 24.51%, making ACM more suitable for use.