Adaptive-Combining-Based Hybrid FSO/RF Satellite Communication With and Without HAPS

In recent years, a growing interest has been witnessed in the usage of free space optics (FSO) link for satellite communication (SATCOM) scenarios, as it offers much higher data rates up to gigabits per second (Gbps) compared to existing radio frequency (RF) link. However, FSO links are sensitive to beam scintillation and pointing errors. In this paper, we consider a hybrid FSO/RF communication between ground station (GS) and satellite, where the RF link will act as a backup link to improve the reliability of FSO communication. In addition, we also consider high-altitude platform station (HAPS), which will act as a relay station, between GS and satellite to improve the end-to-end system performance. This has led to the development of space-air-ground integrated hybrid FSO/RF SATCOM networks. We analyse the performance of the proposed hybrid network considering an adaptive-combining-based switching scheme for both uplink and downlink scenarios with and without using HAPS as a relay station. In case of adaptive-combining-based switching scheme, the data is continuously transmitted over the FSO link, while maximal-ratio-combining (MRC) of RF and FSO links is performed when the quality of FSO link deteriorates. The performance analysis of adaptive-combining-based switching scheme in terms of outage and average symbol error rate (SER) is carried out and the same is compared with the single-link FSO SATCOM and single-threshold-based switching scheme proposed in the literature for hybrid FSO/RF SATCOM. In addition, the performance gain obtained by the proposed adaptive combining scheme over single-link FSO system for different channel conditions is also reported. Further, the asymptotic analysis is also carried out to obtain the diversity gain of the proposed system.