## 1 Introduction  
In recent years, optical wireless communication (OWC) technologies have attracted extensive research interest because of some of their excellent features [1; 2; 3; 4; 5]. Wireless connectivity based on the optical spectrum is termed \"OWC\". OWC has become a favorable complementary technology to radio frequency (RF)-based wireless technologies for future communication networks, including fifth- and sixth-generation (5G and 6G, respectively) communication systems. OWC technologies possess a number of prominent features such as wide spectrum, high-data-rate, low latency, high security, low cost, and low energy consumption, addressing the highly demanding requirements of 5G-and-beyond (5 GB) (e.g., 5G and 6G) communications. Aside from this, the Internet of Things (IoT) network is becoming increasingly important. A large number of end-user devices or sensors are connected in IoT. Moreover, tactile internet will be the essential feature of the future IoT. It will enable real-time communicating systems with a range of societal, industrial, and business use cases. To envision the idea of IoT, the number of end-user physical devices connected to the internet is exponentially growing [6]. Therefore, the IoT generates a large volume of data. The OWC technologies can play an important role of sensing, monitoring, and resource sharing in massive device connectivity of IoTnetworks [2; 6]. Moreover, the OWC can also meet the low-power consumption and high security requirements of the IoT. The 5G communication system specification is already completed, and 5G is expected to be fully deployed by 2020 [7]. The upcoming 5G communication will offer new services with a very high quality of service (QoS). The main features of the 5G communication services will include ultra-high system capacity, ultra-low latency, ultra-high security, massive device connectivity, ultra-low-energy consumption, and extremely high quality of experience (QoE) [7; 8; 9; 10; 11]. The launch of the 6G communication system is anticipated to be between 2027 and 2030. The 6G specification has not yet been exactly identified, but many researchers are working on it [12; 13; 14; 15; 16]. Among the many research issues are capacity improvement, increase in the number of connectivities, latency reduction, security improvement, energy efficiency improvement, user QoE level enhancement, and reliability improvement, which will be addressed by both 5G and 6G communication systems. The 6G communication system is expected to be a global communication facility, with the service level being several folds better compared to 5G. RF is currently widely used for different wireless connectivities. RF-based wireless communication faces several limitations, such as limited spectrum, great interference effect, and strict regulation. Only RF-based wireless communication technologies are insufficient in meeting the demand of 5 GB and IoT networks. Therefore, researchers are working hard to determine a new spectrum that would fulfill the exponentially growing demands. A very large optical band is considered to be a promising solution for the development of 5 GB and IoT networks with high-density and capacity. In comparison to RF-based networks, OWC-based network technologies offer unique advantages, such as high data rate, low latency, high security, and low-energy consumption [1; 2; 3; 6]. Communication distances ranging from a few nanometers to more than 10,000 km are possible through the deployment of different OWC systems [2]. The main technologies of OWC networks include visible light communication (VLC) [17; 18; 19; 6], light fidelity (LiFi) [20; 21; 22], optical camera communication (OCC) [23; 24; 25; 26; 27], and free space optics (FSO) [28; 29; 30] communication. The differences and similarities among these technologies are briefly discussed in another section. Each of these technologies has individual excellent features and some limitations. The verities of services are offered by different OWC technologies in indoor, outdoor, and space communications.