## 1 Introduction  
The demands of the fifth generation (5G) network are becoming challenging due to a significant increase in complexity. The 5G network requires ultra-low deterministic latency to operate normally, and must be kept running and maintained at a relatively low cost from a commercial perspective. From 2010 to 2030, the explosive growth of data traffic (expected to be approximately 20,000-fold) will require communication network technology to provide higher coverage and availability, in addition to denser networks [1]. Future networks will also need to support the continuous appearance of new services, such as machine-to-machine (M2M) applications and Internet of Things (IoT) applications. Due to their high bandwidth, low latency, accurate synchronization, high reliability, and flexible application, optical networks are among the promising technologies that can meet the increasing demands of the upcoming 5G technology. However, in several applications, particularly in the last mile, it is costly to lay optical fiber cables for users. Free-space optical (FSO) communication has become one of the preferable methods with comparable data rates and very low error rates compared to optical fiber cables, although the biggest challenge is power attenuation due to atmospheric turbulence under various weather conditions. FSO communication is a line-of-sight wireless communication scheme designed with short optical links, typically in the range of 200-2000 m [2]. Regarding communications, FSO technology is considered to be a part of wireless optical communications applications. To fulfill the 5G or higher network technology requirements, the cross-spectrum and cross-medium of optical fibers, millimeter waves (mmWave), and optical wireless communication schemes were proposed in [3]. The proposed concept can achieve satisfactory performance by transmitting a 40 Gb/s radio signal over the hybrid system in the 100 GHz band. Different hybrid methods were investigated in [4] with a hybrid K-band (25 GHz) microwave photonic link (MPL) configuration, including optical fibers, FSO, and radio frequency (RF) wireless channels. The experimental and simulation results reported in [4] proved the feasibility of the proposed hybrid system for 5G wireless networks with the advantages of lower demand for high-frequency equipment. Furthermore, a substantial component of FSO communication to support dense networks is the multiplexing technology. The most commonly used multiplexing technology in optical systems is wavelength division multiplexing (WDM), which enables the use of different light wavelengths to send data over the same channel. The disadvantage of WDM is spectral dispersion. With the limitation of spectral bandwidth resources, many studies have focused on improving WDM performance. In 2020, [PERSON] et al. verified a novel optical transmission model with the split-step Fourier and transfer matrix methods to improve the performance of WDM for long-haul optical transmission systems [5]. Another study implemented a novel high-speed long-haul WDM-based inter-satellite optical wireless communication system. In the proposed scheme, differential quadrature phase-shift keying-polarization-shift keying was employed as an optical orthogonal modulation scheme to improve the spectral efficiency of the WDM [6]. The other multiplexing technique that implements effective spectrum sharing for multi-user schemes in optical networks is time-division multiplexing (TDM). In this technique, a single channel or band is divided into time slots, which is especially useful for compressed video and other high-speed data. The performance of the TDM is significantly affected by the optical clock distribution and synchronization [7]. In 2020, [PERSON] et al. proposed the delay-controlled and energy-efficient clustered (DEC) time-division multiple access (TDMA) to reduce the delay and increase the energy efficiency in fiber wireless access networks (FiWi) by utilizing time stamping for synchronization.