## 1 Introduction  
In today's era, there is an enormous demand for fast data rates, bandwidth, and capacity for live streaming, social networking, and video conferencing [1]. Radio frequency technology has been widely used for terrestrial communication, but it has limited bandwidth and suffers from interference that decreases the capacity and security. Radio frequency links offer lower data rates, need large antennas, and are highly susceptible to multipath fading. Free space optics (FSOs) is considered a strong candidate to complement and integrate with next generation technologies and is currently considered as one of the key technologies for solving the last mile access problem resulting from the backbone of optical fibers to user terminals. It may offer high-speed broadband access in regions where fiber optic cabling may be impractical or costly. There is a challenge of getting high speed, stable, and secure communication service from core network infrastructure (e.g., central office, local exchange) to remote sites (e.g., businesses, offices, individual homes, hilly areas, or other end-user locations) [2]. Unrestricted spectrum access, low latency, the absence of interference to radio frequency and electromagnetic induction, and the possibility of quick and easy installation are some of the main features of this technology. As FSOs use air as a transmission medium to transfer the laser pulses, there is no need to dig up the ground, as in the case of optical fiber [3; 4]. Thus, utilizing this technology for last mile communication or difficult terrain can significantly contribute to the connectivity between remote rural areas and cities. It also finds applications in the interconnection of high-rise buildings in urban areas and office blocks or structures on both sides of highways or railways as well as banks of rivers that are difficult to bridge. Technology faces many challenges, like scintillation, turbulence, geometric losses, and attenuation, due to different weather conditions. Random variations in temperature and pressure cause the refractive index of air to deviate, causing atmospheric turbulence. Scintillation may result in rapid fluctuations in the phase and intensity of light as it propagates through the atmosphere [5]. Geometric losses spread the optical beam due to misalignment of the transmitter and receiver, which causes diffraction of the signal during propagation through the atmosphere. Performance estimation could be useful for applications, including FSO link beam tracking and localization. The losses impact the received signal strength and may be considered as constant factors for a fixed link configuration [6]. Weather conditions, like fog, haze, and clouds, may heavily attenuate the signal by absorbing and scattering the information-carrying photons, affecting link availability [7]. Dual polarization comprises two distinct, orthogonally polarized optical signals at the same wavelength. The transmission of signals may require half the data rate by splitting the data across the two polarizations. Channels can be spaced closer together by utilizing dual polarization quadrature phase shift keying (DP-QPSK) as the modulation technique, thus reducing the amount of bandwidth needed to transmit the optical signal [8]. In WDM, different wavelength signals share the same transmission medium, thus improving the capacity of the transmission. Information rates and spectral efficiency can be increased by employing MDM with parallel transmission of information through different modes within the same frequency band [9]. Further capacity enhancement is achieved by incorporating MDM along with WDM in a FSO system by utilizing both spatial modes and multiple wavelengths for free-space channels [10; 11]. DP-QPSK combined with WDM and MDM enables the simultaneous transmission of numerous independent data streams at different wavelengths, polarizations, and modes. This may enable a very high data rate throughput and capacity enhancement of optical networks.