## 1 Introduction  
### Preliminaries  
Over the past few years, communication technology and related innovations have taken a huge leap in transforming the meaning of connectivity. Today we are surrounded by a range of diverse communication technologies like smartwatches, smartphones and smart T.V., etc., to name a few, and all such devices demand seamless data connectivity for an optimized and rich user experience. With the exponential growth of data demand and an anticipated similar aggressive trend in the near future, it is believed that existing radio-frequency (RF) infrastructure will face an acute spectrum shortage in coping with the exponential increase in user data demands [1; 2; 3; 4]. The advent of optical fiber communication or light-wave communication as it may be commonly known is, although a promising contender in delivering high-speed data networks, then it falls short on key features like end-user mobility. Furthermore, installation and maintenance costs of optical fiber links make them economically unviable for the majority of the end-users, and it is for the same reason that despite being within a mile distance from fiber networks, approximately 75% of households in North America do not have access to fiber bandwidth [4; 5; 6]. As shown in Figure 1, free-space optical communication, which essentially a subclass of optical wireless communication (OWC) technology, uses the atmosphere as an unguided medium for the propagation of optically modulated information signal. Using a set of electrical-to-optical and optical-to-electrical circuits, FSO systems are capable of transmitting user information onto an optical carrier and then convert back to the electrical domain at the receiver. Since the underlying hardware and transmission mechanism used in FSO systems is very much the same as in light-wave networks, hence the FSO links are capable of delivering high-speed access of the order of Gbps to the end-users [1; 4; 6]. Miscellaneous advantages of FSO link include: (a) plug-and-play installation behavior, (b) installations costs approximately 1/10 th of the fiber links, (c) ultra-wide bandwidth, (d)unmatched user data security, and (e) license-free spectrum [1; 7; 8]. Rising urbanization, population expansion, elevated living costs and desire for better living standards has created an urgent need for optimum usage of resources through the creation of smart cities. Smart cities are designed to look after the welfare of their citizens through the optimum distribution of benefits while ensuring sustained development. An intelligent traffic system is one such example wherein the traffic lights are managed dynamically to ensure a smooth and safe flow of traffic, thus reducing commuting time and carbon emissions. Figure 2 illustrates a smart city framework in which FSO links have been proposed to serve citizens with high-speed data access in different possible ways. Case A, for example, shown in Figure 2, illustrates end-to-end optical bandwidth connectivity to far-flung and remote locations through the use of FSO relay nodes. Since FSO links can be used as plug and play devices with line-of-sight (LOS) connectivity hence, in adverse geographical conditions where installing RF links or digging earth for laying fibers is an arduous task, FSO links can serve as an ideal solution. The use of amplify-and-forward relay nodes to connect end-users with fiber backbone has also been proposed in [7]. Another possible application of FSO systems towards smart cities can be developing intelligent traffic mechanisms. The hybrid-FSO links have been previously shown to be interoperable with RF and visible light communication (VLC) standards [9]. Case B, shown in Figure 2, proposes the application of FSO to serve fast-moving traffic and pedestrians with data connectivity through RF and VLC links. The data aggregation of traffic intensity through technology feature such as the Internet of things (IoT) can be used to manage service lanes, traffic lights and related infrastructure to ensure smooth traffic flow [9; 10; 11].