A Contemporary Survey on Free Space Optical Communication: Potential, Technical Challenges, Recent Advances and Research Direction Abu Jahid, Mohammed H. Alsharif and Trevor J. Hall Abstract Due to the unprecedented growth of high speed multimedia services and diversified applications initiated from the massive connectivity of IoT devices, 5G and beyond 5G (B5G) cellular communication systems, the existing electromagnetic spectrum under RF ranges incapable to tackle the enormous future data rate demands. Optical wireless communication (OWC) covering an ultra-wide range of unlicensed spectrum has emerged as an extent efficient solution to mitigate conventional RF spectrum scarcity ranging from communication distances from nm to several kilometers. Free space optical (FSO) systems operating near IR (NIR) band in OWC links has received substantial attention for enormous data transmission between fixed transceivers covering few kilometers path distance due to high optical bandwidth and higher bit rate as well. FSO offers a broad range of applications in outdoor and indoor services for instance, backhaul for mobile cellular networks, local area network (LAN) and metropolitan area network (MAN) connectivity as well as extensions, wireless video surveillances, data centers, terres- trial, mobile networks, military demands, last mile solution, backup of fiber connectivity, medical usage, space communications, and so on. Despite the potential benefits of FSO technology, its widespread link reliability suffers especially in the long-range deployment due to atmospheric turbulence, cloud induced fading, some other environmental factors such as fog, aerosol, temperature variations, storms, heavy rain, cloud, pointing error, and scintillation. FSO has the potential to offloading massive traffic demands from RF networks, consequently the combined application of FSO/RF and radio over FSO (RoFSO) systems is regarded as an excellent solution to support 5G and beyond for improving the limitations of an individual system. The aggregate deployment of wireless scheme can attain significant system performance in the form of spectral efficiency, reliability, and energy efficiency of distinct networks. Over the last decade, numerous FSO research have been carried out in the context of optical wireless communication to address the prominent technical challenges focusing on channel modeling, adaptive transmission and modulation, cooperative diversity, mitigation techniques of channel induced factors, novel design of FSO transmitter and receiver architectures. This survey presents the overview of several key technologies and implications of state-of-the-art criteria in terms of spectrum reuse, classification, architecture and applications are described for understanding FSO. This paper provides principle, significance, demonstration, and recent technological development of FSO technology among different appealing optical wireless technologies. The opportunities in the near future, the potential challenges that need to be addressed to realize the successful deployment of FSO schemes are outlined. Keywords— Optical wireless communication, 5G/B5G, IoT, free space optical communi- cation, Radio over FSO, MIMO FSO. I. Introduction In recent years, diverse types of multimedia applications are expanding enormously, generating a mass volume of mobile data together with high data-rate wireless connectivity. The forthcoming 5G technology arXiv:2012.00155v1 [cs.IT] 30 Nov 2020 offers various attractive services such as massive system capacity, huge device connectivity, high-level se- curity, ultra-low latency, extremely low power consumption with tremendous quality of experience (QoE) [1]–[4]. Notably, 5G communication contemplated with ultra-dense heterogeneous networks allowing hundred times additional wireless device connectivity as well as transmission rate compared to existing wireless networks [5]. Therefore, 5G and beyond networks required high-capacity backhaul connectivity in order to support hyper-dense fast access network, minimal power consumption and little end-to-end delays [6]–[8]. It becomes challenging to handle unprecedented high volume of information for the 5G connectivity and hence, a robust technical solutions are required in order to guaranteed quality of service (QoS) for the end users. It is widely accepted that radio frequency (RF) is commonly used in wireless communications which are more limited due to the shortage of spectrum resources [9], [10]. Envisioning the concept of IoT, it enable real-time communications, sensing, monitoring and resource sharing in huge smart device connectivity ranging from social, industrial and business purpose. With the rapid growth of Internet of Things (IoT)/ Internet of Everything (IoE) technologies, a massive number of physical smart devices are connected to the networks is accelerating exponentially [11]–[13]. Consequently, IoT devices generates a huge volume of data. To meet the ever-increasing demand of 5G networks and serving the huge demands of IoT paradigm, it is predicted that currently available electromagnetic frequency band is insufficient. Meanwhile, this RF frequency band suffers a small spectrum range, also have limitations related to regulation based spectrum use and intense level of interference from the surrounded RF access points. In many cases, RF sub-bands are entirely allocated to mobile cellular operators, TV broadcasting and end-to-end microwave links. From the indication of RF based wireless network drawbacks, researchers are looking alternative approach millimeter and nanometer waves for wireless communication. As a consequence, academia and industries are currently interested in license free optical spectrum covering 1 mm-10 nm as an emerging alternative of RF for future ultra-density and ultra-capacity networks in perspective of optical wireless communication (OWC) [14]–[16]. The term OWC is referred as the wireless connectivity using optical spectrum. OWC technologies offers a number of remarkable features for example broad spectrum, ultra-high data rate, very low latency, low cost and lower power consumption, addressing the massive demand requirements of 5G/B5G communications. In contrary to RF-enabled networks, OWC technology offers some remarkable advantages like as high data rate transmission capacity ranging from nanometers to several kilometers both for indoor and outdoor applications. Moreover, OWC technology has the potential to provide some outstanding communication features such as reliable security, electromagnetic interference free transmission, and high system efficiency due to using broad optical spectrum [17], [18]. Reference [19] demonstrates that OWC technology can attain 100 Gb/s at standard indoor illuminations and capable to provide energy efficient communication. The most important feature of OWC technologies it do not require a comprehensive infrastructure and thereby reducing the installation cost maintaining all the green agenda for high speed communication [20]–[23]. Visible light communication (VLC) and light fidelity (Li-Fi) techniques under OWC technology use the existing illumination structure to realize wireless data transfer [24]–[26]. OWC provides the greater level of data security since the light waves does not penetrate the enclosed walls. Typically, visible light (VL), infrared (IR) or ultraviolet (UV) spectra are used as propagation media. Most promising wireless systems in OWC such as VLC, LiFi, optical camera communication (OCC), and free space optical (FSO) communication are being developed based on those three optical bands. However, VLC, LiFi, OCC, and FSO technologies have some similarities and disparity by means of communication protocol, propagation media, architecture and applications. On the other hand, the aggregate option of OWC and RF refereed as a hybrid RF/OWC systems may establish an effective solution for tremendous upcoming user demands. Despite the remarkable advantages of OWC technologies, some factors for instance limited coverage area, dependence on line-of-sight (LOS), interference incurred by multiple light sources, outdoor atmo- sphere, and low transmit power degrade the system performance. Moreover, the performance OWC suffers from external disturbances and limited their transmit power as well as signal to interference plus noise ratio (SINR). Hence, it is a challenging issue to exploit successful OWC deployment addressing these limitations. The use of RF spectrum across existing wireless applications has almost been exhausted. However, RF-based communication is highly sensitive to interference but provides better performance under non-LOS (NLOS) conditions. This key feature of RF systems capable to overcome some weakness of OWC schemes particularly for the end users. In order to ensure desired QoS, the conjunction of RF and optical wireless based networks in the context of wireless communication play a significant role through integrating a diverse frequency bands. The heterogeneous (endeavoring of two or more network architectures operates simultaneously) operation of access network technologies cooperate each other for traffic offloading, thereby mitigate the QoS constraints. Due to the complementary characteristics of RF and OWC technologies, researchers have proposed some hybrid RF/OWC approaches to sup- port IoT/IoE, 5G/B5G communications systems [27]–[41]. The co-deployment of RF/optical wireless hybrid system solves the last mile problem where end users can be benefited from RF coverage. The hybrid approach incorporates two or more different