Performance study of hybrid space-air-ground-free space optical/single hop-free space optical/radio frequency satellite-ground links based on dual-threshold switching

We propose an integrated transmission system model with dual threshold switching, combining space-air-ground (SAG)-free space optical (FSO) communication and single hop (SH) FSO/radio frequency (RF) transmission. Considering the effects of scintillation, beam wander, and pointing errors, the FSO and RF links are modeled using the Málaga distribution and the Nakagami-m distribution, respectively. A dual-threshold mechanism is implemented between the SAG-FSO and SH-FSO/RF links; by introducing varying hysteresis widths, frequent switching caused by minor SNR fluctuations due to atmospheric turbulence and pointing errors is effectively suppressed. Specifically, we derive closed form expressions for the bit error rate of the hybrid SAG-FSO/SH-FSO/RF system. Simulation analyses examine the impacts of different link switching thresholds, zenith angles, modulation schemes, and weather conditions on system performance. Finally, Monte Carlo simulations validate the analytical results. Numerical results demonstrate that the hybrid scheme yields significant performance improvements. The dual FSO threshold approach yields superior performance compared with a single threshold, while also extending the lifetime of the FSO link and enhancing system stability, thereby providing a theoretical basis for uplink experiments in satellite-ground communications.