Channel Modeling for Quasi Static Multistage Optical Intelligent Reflecting Surfaces
Free-space optical (FSO) communication has emerged as a promising technology, but its performance is highly dependent on line-of-sight (LOS) conditions. To address this limitation, we propose a multistage optical intelligent reflecting surface (OIRS)-assisted FSO communication system, which utilizes multiple OIRS nodes to dynamically reflect and redirect optical signals toward the receiver, even in the absence of direct LOS paths. In this work, we first establish a generalized geometric misalignment loss (GML) model for multistage OIRS systems under both two-dimensional (2D) and three-dimensional (3D) deployment scenarios. Building on the 3D GML model, we construct a comprehensive statistical channel model by incorporating Gamma-Gamma (GG) atmospheric turbulence and attenuation losses. Furthermore, we propose a multi-OIRS-assisted unmanned aerial vehicle (UAV)-based non-terrestrial network (NTN) system and analyze its performance. We derive semi closed-form expressions for the outage probability (OP), average bit error rate (BER), channel capacity, and moments of the signal-to-noise ratio (SNR). Additionally, we present asymptotic expressions for OP and BER in the high-SNR regime and determine the diversity order of the system.