## 4 Comparison and Scheme Design  
### Comparison and Analysis of Satellite-to-Ground Transmission Efficiency  
According to the analysis in Section 3, the ACM, VCM and CCM TEF of each G/S are illustrated on one figure for comparison, as shown in Figure 8. For any G/S, it can be seen that:  
1. For a specific MCS (initial minimum MCS for VCM and minimum MCS for ACM), the TEF increases in the order of CCM, VCM and ACM, because of the possibility to vary the MCS according to the available \(E_{\text{S}}/N_{0}\) with VCM and ACM.
2. The TEF increases with the increase of CCM's MCS or VCM's initial minimum MCS. As shown in Table 1; Table 4, for CCM, the useful data rate increases rapidly with the increase of MCS, but the link availability decreases relatively slowly. So, the CCM TEF, which is the product of the link availability and the useful data rate, still increases. VCM is similar with CCM.
3. The TEF decreases with the increase of ACM's minimum MCS. This is because the available number of MCSs decreases when the minimum MCS increases, and it is not always possible to find a suitable MCS to cope with the bigger atmospheric attenuation. For ACM, when encountering bad weather, such as rainstorms, the atmospheric attenuation is big, and the receiving \(E_{\text{S}}/N_{0}\) at the G/S is small. If the smallest MCS is dropped, the bit error rate for other MCSs will be large. Additionally, the remote sensing data cannot be recovered correctly. So, if the smaller MCSs in ACM system were used, the probability of recovering data correctly would become larger, and the ACM TEF should also be larger.  
It is important to note that the maximum transmission efficiency is obtained for ACM used at its best, which is with all the MCSs. So, if ACM is adopted, all the MCSs should be chosen, where the smallest MCS is used to cope with the larger atmospheric attenuation  
Figure 8: TEF comparison of different G/S adopting different transmission schemes.  
when encountering bad weather, such as a rainstorm, while the largest MCS is used to obtain a larger useful data rate and transmit as much data as possible during clear sky.  
To analyze the transmission efficiency improvement quantitatively, the TEF of VCM, ACM and CCM are compared, respectively. The improvement percentages are obtained, as shown in Table 8 and Figure 9. For any G/S, it can be seen that:  
1. Compared with CCM, VCM can obtain a maximum efficiency improvement of about 70%. The efficiency improvement proportion has little to do with the G/S position and rainfall, and gradually decreases with the increase of MCS.
2. Compared with CCM, ACM can obtain a maximum efficiency improvement of more than 796%, and the efficiency improvement proportion gradually decreases with the increase of MCS.
3. Compared with VCM, ACM can obtain a maximum efficiency improvement of more than 426%, and the efficiency improvement proportion gradually decreases with the increase of MCS.  
In Ref. [21], for Svulbard G/S, whose annual probability of rain is 1.44%, the average data rate in ACM system is 1490 Mbps if the minimum link availability is 99.5%. However, if the considering approach in this paper is adopted, for Beijing and Sanya G/S, whose annual probability of rain is 1.94% and 2.46%, respectively (all bigger than the one of Svulbard G/S), the maximum average data rate in ACM system is 2644.63 Mbps and 2468.56 Mbps, respectively, and they are all much bigger.