Comprehensive performance analysis of hybrid FSO/RF space-air-ground integrated network

Free space optics (FSO) communication due to its large optical carrier bandwidth offers better data rates close to gigabits per second (GBPS) compared to radio frequency (RF) communication and can support seamless ubiquitous high-speed broadband connectivity, which is a significant requirement for sixth-generation (6G) communication. It is also envisioned that the data transmission in 6G communication will be over space-air-ground integrated network (SAGIN) to provide broadband global mobile connectivity. Thus, our main aim in this paper is to investigate the performance of a dual-hop (DH) system model utilizing high-altitude platform station (HAPS) as a relay station with FSO communication for SAGIN scenario, which is expected to be featured in 6G communication. Despite various advantages, HAPS-based FSO satellite communication (SATCOM) is undermined by several factors such as atmospheric turbulence, pointing errors, and atmospheric attenuation. To improve the system performance in terms of capacity and reliability, hybrid FSO/RF communication with RF link as a backup is utilized between ground station (GS) and HAPS. Further, comprehensive end-to-end system performance is investigated by deriving unified closed-form expressions for outage probability and average symbol error rate (SER). We also obtain approximate upper bound expressions for ergodic capacity and outage capacity. The asymptotic expressions are also derived to obtain the diversity order. Here, FSO and RF links are modelled using generalized Malaga distribution with non-zero boresight pointing errors and shadowed kappa - mu distribution, respectively. Further, the closed-form expressions for optimum switching threshold signal-to-noise ratio (SNR) are derived and the obtained optimal SNR values are verified using a numerical optimization technique. Finally, the results show that the proposed DH SAGIN-based hybrid FSO/RF SATCOM system is more reliable as well as achieves higher capacity compared to the FSO-based SATCOM systems.