## 1 Introduction
The performance of the communication systems that integrated space-air-ground FSO relay links and single-hop hybrid FSO/RF links was investigated in [15], and the authors revealed that the integrated system could achieve higher throughput and reliability compared to existing solutions. Considering the combination of a hybrid FSO/RF system and space-air-ground FSO system, the authors of [16] have proven that the integrated system can achieve about a 10 dB performance gain compared to existing systems. In [7], the performance of very high-throughput satellite feeder links was studied, where the effect of atmospheric turbulence and weather conditions was greatly reduced by performing site diversity. Considering the effect of various weather conditions, the authors of [17] proposed three AF-based relay schemes under hybrid FSO/RF systems in reconfigurable intelligent surface (RIS)-assisted SAGIN, and a weather-dependent hard-switching scheme was adopted among different relay schemes. However, due to the required link alignment time and setup time, the frequent hard-switching between FSO and RF links may result in service interruptions, reducing the system's reliability. Motivated by this, the adaptive combining scheme was recently studied by researchers [18; 19; 20]. The hybrid FSO/RF system with an adaptive combining scheme was first studied in [18], in which the FSO link had a higher priority, and when the received SNR of the FSO link was lower than the threshold SNR, both the FSO and RF links would be activated simultaneously. The authors of [19] studied the ergodic capacity performance of an adaptive combining hybrid FSO/RF system, where the authors revealed that the adaptive combining scheme was superior to the maximal-ratio combining (MRC) scheme and hard-switching scheme. In [20], the authors have derived the closed-form expression for the optimum beam waist of the FSO link and also obtained the optimum switching threshold SNR by using a numerical optimization technique. Considering the frequent switching between FSO and RF links caused by fixed-rate design, a rate adaptation design scheme was proposed in [21; 22]. The authors of [21] proposed a rate adaptation scheme based on the instantaneous received SNR, revealing that it was superior to traditional fixed rate design. Considering the impact of weather conditions and atmospheric turbulence in SAGIN, a RIS-assisted unmanned aerial vehicle (UAV) was adopted in [22] to further improve the performance of hybrid FSO/RF systems. In order to fully utilize the resources of FSO and RF links, the soft-switching strategy was studied in [23], which was superior to traditional design but with a higher decoding complexity. In practical scenarios, due to the limited coverage of the FSO link, it usually needs to be combined with access points or RF links to provide services for ground users. The above literature has not considered the problem of how the FSO link can access the widely distributed ground users. Actually, it is of great necessity to combine the large capacity of FSO links with the wide coverage of RF links. Moreover, when considering the limited power of the relay node and also for the purpose of matching the energy efficiency of terrestrial networks, the power optimization problem of hybrid FSO/RF systems has also attracted the attention of researchers [9; 24; 25; 26; 27]. The authors of [24; 25] conducted research on power distribution from the perspectives of materials and technology, respectively. In our previous work [26], the power allocation between FSO and RF links with a certain proportion was preliminarily investigated. Considering the influence of weather conditions and the limitations of the satellite transmit power, the authors of [27] studied the problem of power allocation and link switching between FSO and RF links. In [9], the authors have studied the parallel FSO-RF transmissions, in which the satellite can adaptively allocate power between FSO and RF links according to the weather conditions.