Combined Control of Coverage Area and HAPS Deployment in Hybrid FSO/RF SAGIN

Space–air–ground integrated networks (SAGIN) have attracted considerable attention owing to their capability extending telecommunication coverage via satellite communication. They enhance the capacity and coverage of satellite communication systems using amplifying relays deployed in high-altitude platform stations (HAPS) in the air. Free-space optical (FSO) communication, which offers larger capacity compared to conventional radiofrequency (RF) communication, can enhance the capacity of SAGIN to meet the requirements of sixth-generation (6G) systems. This study assumes a hybrid FSO/RF SAGIN, where high-capacity but atmospherically unstable FSO communications and low-capacity but atmospherically stable RF communications are used in a complementary manner in the satellite-HAPS-ground link. For the SAGIN coverage area to expand, the transmission capacity of the system needs to be further improved. Therefore, efficient use of the limited RF link frequency bandwidth is crucial, and the satellite and HAPS must cooperate to share and meet ground traffic requirements by controlling the areas they each cover. In addition, HAPS placement significantly impacts the spectral efficiency of the system because the positional relationship between the satellite and HAPS affects the transmission capacity. Therefore, this study proposes a combined control method for optimizing HAPS placement and coverage area of the satellite and HAPS. Furthermore, a solution space reduction method is incorporated into the proposed method for efficient exploration of the optimal control parameters. The solution exploration efficiency and data transmission performance using the proposed method are evaluated through simulations.