Enhancing FSO Links Using Multi-HAPS and Opportunistic Relay Selection Over IGGG Channels
High-altitude platform station (HAPS)-assisted freespace optics (FSO) communication has emerged as a promising solution to enhance the coverage, flexibility, and resilience of existing network infrastructures through aerial relaying. By employing HAPS as relay nodes, the strict line-of-sight (LoS) constraints of conventional FSO links can be effectively alleviated. In this work, we investigate a multi-HAPS-based FSO communication system with decode-and-forward (DF) relaying, where opportunistic relay selection (ORS) is utilized to select the most suitable HAPS for data forwarding. The optical wireless channel is modeled using the doubly inverted Gamma-Gamma (IGGG) distribution, which jointly accounts for atmospheric turbulence, attenuation, and pointing errors. Closed-form analytical expressions are derived for outage probability (OP), while the average symbol error rate (ASER) and ergodic capacity (EC) are evaluated using efficient Gauss-Laguerre (GL) quadrature approximations. In addition, asymptotic analysis in the high signal-to-noise ratio (SNR) regime is performed to characterize the diversity gain of the proposed system. The analytical results are validated through Monte-Carlo simulations. Numerical results demonstrate the impact of relay selection strategies, turbulence conditions, and system parameters on system performance, providing useful insights into the reliability and diversity benefits of multi-HAPSassisted FSO systems under realistic atmospheric environments.