Performance analysis and threshold optimisation of hybrid FSO/RF systems over IGGG turbulence and Nakagami-m fading channels

Hybrid free space optical (FSO) and radio frequency (RF) communication systems leverage the complementary features of both technologies. While FSO links provide ultra high bandwidth and data rates, RF links ensure robust performance under adverse propagation conditions. This paper provides a comprehensive performance evaluation of a hybrid FSO/RF system that employs a single threshold switching mechanism, where the FSO link is modelled over a Doubly Inverted Gamma–Gamma (IGGG) turbulence channel and the RF link experiences Nakagami-m fading. Closed form expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the instantaneous signal to noise ratio (SNR) are derived for both links, accounting for the combined effects of atmospheric turbulence (AT), path loss (PL), and non zero boresight pointing errors (PE) in the optical channel, along with fading severity in the RF channel. Based on these statistical models, analytical expressions for the outage probability (OP), average symbol error rate (ASER) and ergodic capacity of the hybrid system are obtained. Analytical optimisation of the switching threshold with respect to ASER is also investigated. The system performance is analysed for heterodyne detection (HD) and intensity modulation/direct detection (IM/DD) techniques at the optical receiver. Furthermore, an asymptotic analysis in the high SNR regime is conducted to determine the diversity order and coding gain. The analytical findings are verified through extensive Monte Carlo simulations, which demonstrate close agreement under the adopted channel assumptions and considered operating conditions.