## 1 Introduction  
The increasing requirement for high-speed, cost-effective data streaming owing to end users' exponential growth has been appearing as a big issue for microwave and radio frequency (RF) transmission networks. It occurs owing to last-mile challenges, lack of infrastructure, and regulatory issues. Because of the growth of throughput, it is considered that the RF spectrum will be depleted in the incoming years. The utilization of the optical spectrum will have a considerable potential to fulfill the constantly expanding requirements for this high-speed throughput [1]. Moreover, the demand for high-speed internet traffic, which is contemporarily attributed to the increasing popularity of 5G applications, big data, and cloud computing, is in high demand. To withstand this challenge, optical communications are widely developed, where the current deployments are primarily based on optics [2]. Free space optics (FSO) communication is a widespread technology that incorporates laser beams to transmit information via air. It is a cost-effective, fast-deploying fiber alternative technology for when fiber cables are impossible to install [3]. In FSOsystems, data-carrying beams can be affected by distinct impairments, like limited-size receiver aperture, atmospheric turbulence, ink misalignment, or obstructions [4].  
During the last few years, the formation of new market needs for Internet and telecom data services has driven research in fiber optics to develop through multiple stages. Indeed, the transmission capacity of fibers has seen an exponential rise in multiple dimensions. Enhancements have been achieved through multi-wavelength, polarization, and modulation techniques [5], although single-mode fiber (SMF) channel capacity has verged on the nonlinear Shannon limit. The emergence of mode division multiplexing (MDM) has been influential in overcoming the issues of traditional fiber transmission. MDM incorporating multimode fibers (MMFs) has been shown to significantly enhance transmission capacity, but mode coupling-generated crosstalk remains a primary issue in MDM transmission using MMFs, necessitating a costly as well as complex digital signal processing system for valuable mitigation [6].  
Furthermore, passive optical networks (PONs) have been appraised as the most economical technology in the deployment of fiber access networks. PON provides extensive benefits upon fiber-to-the-x architecture deployment. It offers internet connectivity from an optical line terminal (OLT) in a downlink (DN) transmission to optical network terminals (ONTs) at the consumer's side as a point-to-multipoint architecture. Other advantages incorporate high-quality broadband access, energy-efficient deployment, high-speed internet services, and data services in an economical manner [7]. The current Ethernet PON and Gigabit PON usually offer a 1.25 or 2.5 Gbps data rate. Additionally, current next-generation PONs (NG-PONs) based on 10G Ethernet PON and 10G Gigabit PON networks upgrade the data rate to up to 10 Gbps per wavelength [8]. Despite the listed benefits, PON architectures tend to be influenced by network issues, mainly in the channel capacity of PON networks in optical distribution networks (ODNs), which is regarded as more conquerable [7].  
Also, comparing the data transmission over multi-SMFs, PON-FSO permits the minimization of power consumption and the upgrade to an existing single-mode scenario, which provides cost-effectiveness in PON applications [9].