Simple and robust transmit diversity based free-space optical communications for 5G and beyond networks

Abstract The free-space optical (FSO) system has been considered as a promising scheme for various communication use cases and deployment scenarios due to the associated advantages and inherent features. In view of the fact that it can effectively support high-capacity fronthaul and backhaul links at super-fast speeds, it has been considered as one of the potential solutions for the fifth-generation (5G) and beyond communication systems. However, it is challenging to be employed as a standalone technology in real-life scenarios due to its susceptibility to atmospheric turbulence-induced fading. There are a number of viable schemes that can be employed for turbulence-induced fading mitigation. In this paper, to enhance the FSO system performance, we focus on spatial diversity based mitigation scheme. In this context, we derive a simple closed-form expression for the ergodic capacity of multiple-input multiple-output (MIMO)-FSO communications over the Gamma–Gamma fading channels using intensity modulation with direct detection. Besides the considered effect of the atmospheric turbulence-induced fading, we also study the influence of the pointing errors, using the ergodic capacity as the performance metric. Also, we present numerical results that are based on practical parameters to study the FSO system performance under different scenarios. We inferred that the diversity of the MIMO-FSO system is drastically degraded by the effect of pointing errors and atmospheric turbulence-induced fading. The accuracy of the proposed closed-form expression is validated by Monte-Carlo simulation under different atmospheric and channel scenarios. An excellent agreement is observed between the simulation and analytical results. The proposed model is envisaged to be effective for the performance evaluation of MIMO-FSO-based 5G use cases.