Abstract-New high-data-rate multimedia services and applications are evolving continuously and exponentially increasing the demand for wireless capacity of fifth-generation (5G) and beyond. The existing radio frequency (RF) communication spectrum is insufficient to meet the demands of future high-data-rate 5G services. Optical wireless communication (OWC), which uses an ultra-wide range of unregulated spectrum, has emerged as a promising solution to overcome the RF spectrum crisis. It has attracted growing research interest worldwide in the last decade for indoor and outdoor applications. OWC offloads huge data traffic applications from RF networks. A 100 Gbps data rate has already been demonstrated through OWC. It offers services indoors as well as outdoors, and communication distances range from several nm to more than 10,000 km. This paper provides a technology overview and a review on optical wireless technologies such as visible light communication, light fidelity, optical camera communication, free space optical communication, and light detection and ranging. We survey the key technologies for understanding OWC and present state-of-the-art criteria in aspects such as classification, spectrum use, architecture, and applications. The key contribution of this paper is to clarify the differences among different promising optical wireless technologies and between these technologies and their corresponding similar existing RF technologies. Index Terms-Free space optical communication, infrared, light detection and ranging, light fidelity, optical camera communication, optical wireless communication, radio frequency, ultraviolet, visible light, visible light communication. Fifth-generation (5G) communication is the next step in mobile telecommunication standards. It will offer new services with ultra-high system capacity, massive device connectivity, ultra-low latency, ultra-high security, ultra-low energy consumption, and extremely high quality of experience [1]- [6] . It is expected that 5G communication will comprise ultra-dense heterogeneous networks for which mobile data volume per area will be 1000 times and the number of connected wireless devices will be 100 times higher compared to those in existing wireless networks [1] . Therefore, upcoming networks must Manuscript submitted: (Date) Mostafa Zaman Chowdhury is with Department of Electronics Engineering, Kookmin University, Korea (e-mail: mzaman@kookmin.ac.kr). Md. Tanvir Hossan is with Department of Electronics Engineering, Kookmin University, Korea (e-mail: tanvir.hossan.shaon@gmail.com). Amirul Islam is with Department of Electronics Engineering, Kookmin University, . Yeong Min Jang is with Department of Electronics Engineering, Kookmin University, Korea (e-mail: yjang@kookmin.ac.kr). have the capability to support high user data rates, low power consumption, and negligible end-to-end delays [1]- [6] . High-capacity backhaul connectivity is essential for 5G and beyond communications in order to support hyper-dense ultra-fast access networks [3] . Furthermore, with the development of the Internet of Things (IoT) concept, the rate at which physical devices are connected to the internet is increasing exponentially [7]-[11]. Currently, radio frequency (RF) is widely used across various wireless applications. However, in relation to meeting the growing demand for 5G wireless capacity and serving the IoT paradigm, the currently used RF spectrum is insufficient. The electromagnetic spectrum, with favorable communication properties below 10 GHz, is widely used by existing wireless technologies and has almost been exhausted; therefore, it is predicted that the massive connectivity demand of future mobile data traffic will not be met by existing wireless technologies [12] . Moreover, this band (below 10 GHz) has limitations such as a small spectrum band, regulations related to spectrum use, and severe interference among nearby RF access points. Therefore, researchers are looking for new complementary spectra, such as millimeter and nanometer waves, for wireless communication connectivity [13] . At the World Radio Conference 2015 (WRC-15), the International Telecommunication Union (ITU) proposed 11 new candidate bands for International Mobile Telecommunication-2020 (IMT-2020. The RF band lies between 3 kHz and 300 GHz of the electromagnetic spectrum. The use of this band is strictly regulated by local and international authorities. In most cases, RF sub-bands are entirely licensed to certain operators, e.g., cellular phone operators, television broadcasters, and point-to-point microwave links [14] . The optical spectrum is considered as a promising solution for the development of future high-density and high-capacity networks. Wireless connectivity based on the optical spectrum is termed as optical wireless communication (OWC). In comparison with RF-based networks, OWC-based network technologies offer unique advantages. OWC systems can provide high-data-rate services for communication distances ranging from a few nanometers to more than 10,000 km. It can perform well both for indoor and outdoor services. However, OWC systems suffer owing to their sensitivity to blocking by obstacles and to their limited transmitted power. Therefore, the coexistence of OWC and RF systems may provide an effective solution for the huge demands of upcoming 5G and beyond communication systems. Fig. 1 shows several areas where OWC can be applied, such as industry, healthcare, railway stations, transportation, homes, offices, shopping malls, underwater, and space. For these application platforms, all types of communication, such as device-to-device (D2D); machine-to-machine (M2M); chip-to-chip; device/machine-to-user; user-to-device/machine; vehicle-to-infrastructure, vehicle-to-vehicle, and infrastructure-to-vehicle (V2X); point-to-point; multipoint-to-point; and point-to-multipoint, can be performed using OWC technologies. Based on the application type, required data speed, and platform, various OWC technologies can be applied. OWC has become a promising technology for supporting high-data-rate 5G communication and the massive connectivity of IoT. 5G and beyond communication systems must possess the necessary features for integrating ultra-dense heterogeneous networks. A straightforward but extremely effective way to increase network capacity is to make the cells smaller [6] . Networks are now rapidly being developed to include nested small cells such as picocells and femtocells for 5G and beyond communications. VLC, LiFi, and OCC can provide ultra-dense small cell hotspot services to meet the demands of 5G. Furthermore, FSO, LiFi, and VLC can effectively provide high-capacity backhaul support for 5G and beyond communication systems. OWC technologies have very low power consumption, which is a key requirement of 5G [2] . A reliable connection, which is the main priority for 5G communication, can also be provided using OWC technology. It can provide secure communications, as required by 5G, and can also connect a large and diverse set of 5G devices for indoor and outdoor communications. It will offer extreme densification and offloading to improve the area spectral efficiency. To support the IoT paradigm, OWC technology is able to provide massive connectivity through low power LED technologies. The use of optical bands complementary to RF will relieve the problems caused by spectrum shortage in RF-based wireless communication.  In this survey, we present a general overview and comparison of emerging OWC technologies. The main focus of this paper is to make clear the differences among different optical wireless technologies and between promising optical wireless technologies and their respective similar existing RF technologies. Here we discuss OWC systems from various viewpoints, e.g., the types of optical sub-bands used (IR/VL/UV); the types of technology applied (LiFi/VLC/OCC/FSO/LiDAR); the application scenarios (range of communication distance, data rate/accuracy/reliability, LOS/NLOS, indoor/outdoor, and underwater/vehicular/space); the types of transmitter/receiver (LED/LD and PD/camera); the network architecture; the principle of operation; and the types of application (illumination/localization/communication/high-speed link/ imaging/mapping), to highlight the differences among the technologies. The rest of the paper is organized as follows. Section II provides an overview of OWC systems; the classification of OWC based on technology, communication distance, and communication media are presented in this section. Different OWC technologies, their applications, and principles of operation are presented in Section III. Section IV summarizes the review and discusses the future direction of research issues. The survey is concluded in Section V. For ease of reference, the various abbreviations used in this paper are summarized in Table I . Fig. 2 shows the classification of OWC applications based on transmission range. OWC can be used for WBAN, WPAN, wireless metropolitan area network (WMAN), wide area network (WAN), etc. Fig. 3 shows the applications for OWC systems for different ranges of communication. It shows that a wide range of communications is possible using OWC, i.e., from ultra-short range inter-chip up to ultra-long range inter-satellite communications. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 4 Based on the transmission spectrum, OWC can be studied in three categories, namely, infrared, VL, and ultraviolet. The VL spectrum is widely used in VLC, LiFi, and OCC > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <  Short and medium ranges in LiFi and OCC  Ultra-short, short, medium, long, and ultra-long ranges in FSOC  Short, medium, long, and ultra-long ranges in LiDAR  Short and medium ranges in VLC, LiFi, and OCC  Ultra-short, short, medium, long, and ultra-long ranges in FSOC  Short , medium, long, and ultra-long ranges in LiDAR  Short and medium ranges in LiFi  Ultra-short, short, medium, long, and ultra-long ranges in FSOC  OWC is widely used for LOS communications. However, NLOS communication is also possible using OWC technologies. High-data-rate NLOS communication can be achieved using UV spectrum. Fig. 4 shows NLOS communications using IR, VL, and UV. Fig. 4(a) shows a low-data-rate NLOS communication system achieved using the reflection of IR and VL. A high-data-rate NLOS communication system is shown in Fig. 4 (b) . To perform NLOS UV communication, an omnidirectional transmitter is directed at an elevation, while the receiver is required to intersect the transmitted beam. The transmitter comprises a xenon flashtube inside a searchlight dish. By design, the front-end receiver of such a system must have high gain and low noise, and it is built using a photo-multiplier tube and a counter [66] . The atmospheric gases ozone and oxygen are strong absorbers of light in the UV spectral region [67] . A filter in front of the phototube generates a pulse when it detects an > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < incident photon of operation. The knowledge of the transmitted and received beam positions is important to achieve higher data rate, reduced latency, and reduced transmitted power. LEDs and LDs offer unique advantages and limitations from the viewpoint of their use in OWC systems. The advantages and limitations of OWC systems based on LEDs and LDs are briefly listed below: Owing to recent advancements in solid-state lighting, there has been a trend in the last decade to replace incandescent and fluorescent lamps with high-intensity solid-state white LEDs. LEDs have benefits such as extremely high energy efficiency, longer lifespan, compact form factor, lower heat generation, reduced usage of harmful materials in design, and improved color interpreting without using harmful chemicals [26] . Because of these benefits, LED adoption has been rising consistently, and it is expected that nearly 75% of all illumination will be provided by LEDs by the year 2030 [68] . One additional important benefit of LEDs is that they are capable of switching to different light intensities very rapidly. This special functionality of high switching rate creates the opportunity to use LEDs as OWC transmitters for high-speed communication and highly efficient lighting source simultaneously. Therefore, LEDs can serve the dual purposes providing of highly efficient illumination and very-high-speed communication. All light produced by an LED is incoherent in nature. Therefore, all the waves are not in phase and the transmitted optical power produced by an LED is comparatively very low. Also natural and artificial light sources cause interference for an LED source light. Lasers are monochromatic. Therefore, all light produced using a laser is of a single, specific wavelength. Coherent lights mean all individual light waves are precisely lined up with each other, all waves travel in the same direction, in the same manner, and at the same time [69] . Laser lights are concentrated and directed forward because of coherent characteristics. Therefore, laser light can travel long distances. Moreover, given this nature, LDs cause less interference and provide a high-data-rate compared to LED lights. LD has low aperture. Only point-to-point communication is possible using LD. The idea of using LD-based FSO communication has been well known for many years. High-data-rate communication has already been demonstrated for mobile access using IR LDs [70] . Nevertheless, LDs are not popular due to possible health threats (e.g., eye injury, hyperthermia, coagulation, and ablation due to thermal effects of laser radiation [71] , [72]), cost, color mixing complication, and the questionable quality of laser light for illumination purposes [33] . A recent study shows that diffused laser light does not compromise the user experience compared with conventional light luminaires [73] . Each of the optical technologies (VLC, OCC, LiFi, FSO, and LiDAR) has a unique architecture and principle of operation. They may also differ in terms of modulation technique, transmitting system, receiving system, and communication media. Fig. 5 shows the basic architectures of the various OWC technologies. Different OWC technologies use either LEDs or LDs as physical transmitters. PDs or cameras (or ISs) are used as physical receivers, and IR, VL, or UV spectra are used as communication media. This figure explains how the various OWC technologies differ from each other in terms of transmitter, receiver, and communication medium. VLC, a subset of OWC, has emerged as a promising technology in the past decade. VLC based on LEDs or LDs can be a promising solution for upcoming high-density and high-capacity 5G wireless networks. IoT is becoming increasingly important because it allows a large number of > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < devices to be connected for sensing, monitoring, and resource sharing, and VLC could play an important role to this end [74] . VLC technology offers 10,000 times more bandwidth capacity than RF-based technologies [75] . VLC based on the VL spectrum provides significant opportunities for application in homes, offices, cars, airplanes, trains, and roadside [21] . Moreover, it is not harmful to humans. However, this technology suffers from a few limitations: (i) it is not very effective for outdoor applications; and (ii) it cannot perform long-distance communication. LEDs can switch to different light intensity levels at a very fast rate, which allows data to be modulated through LED at a speed that the human eye cannot detect [65] , [76] , [77] . Therefore, these energy-efficient LED devices can be used for illumination, communication, and localization [78] , a unique feature not available in other devices. Potential VLC data rates of over 10 Gbps have already been demonstrated using LEDs [12] . A data rate of 3 Gbps has been achieved using a single color LED [51] . The main disadvantage of LEDs is their inherent tradeoff between bandwidth and optical efficiency. Therefore, LDs can be considered as a promising alternative for better utilization of the VL spectrum for communication purposes. A data rate of 100 Gbps is achievable at standard indoor illumination levels using LD-based VLC [12] . VLC has many indoor and outdoor applications, a few of which are as follows [26] A basic architecture for user equipment (UE)-to-core network (CN) connectivity in VLC system is shown in Fig. 6 . LED lamps or LDs are used as transmitters. Cameras or ISs or PDs are used as receivers. Then, VLC access networks are connected to the Internet, any server, or CN through wired or wireless backhaul connectivity. The backhaul network connectivity can be either internet service provider (ISP) or cable TV or FSO or optical fiber or cellular or even satellite. In indoor environments, it is normally a wired connection. However, in outdoor cases, especially in vehicular environments, wireless backhaul connectivity is used. LiFi and OCC also use similar connectivity. The only differences are the physical transmitter, receiver, and transmission media. LiFi can be classified as a nm-wave communication [29] . The main differences between LiFi and VLC are (i) VLC uses only the VL portion of the light spectrum, whereas LiFi uses VL in the forward path but it is possible to use either IR, VL, or UV in the return path [29] [31] ; (iii) LiFi must provide seamless user mobility, whereas mobility support is not mandatory for VLC [31] ; and (iv) VLC systems include any of point-to-point, point-to-multipoint, and multipoint-to-point communications, whereas LiFi systems must include multiuser communications, i.e., point-to-multipoint and multipoint-to-point communications. Hence, LiFi is a complete wireless networking system and supports seamless user mobility; therefore, it forms a new small attocell layer within existing heterogeneous wireless networks. As such, a VLC system will be treated as LiFi only if it has LiFi features (e.g., multiuser communication, point-to-multipoint and multipoint-to-point communications, and seamless user mobility). On the other hand, a LiFi system can be treated as VLC only when VL is used as the transmission media. The term LiFi is similar to WiFi, with the exception that optical spectrum rather than RF spectrum is used for transmission. Existing LEDs can be used as the transmitter for LiFi cellular deployment [29] . Moreover, LDs combined with optical diffuser can also be used as LiFi transmitters. Speeds of over 3 Gbps from just a single microLED have been demonstrated [33] , and a speed of 56 Gbps was achieved using a vertical-cavity surface-emitting laser (VCSEL) [30] . The LiFi attocells concept can reduce cell size compared to mm-wave communication [85] . Large amounts of data can be offloaded to LiFi attocells from congested RF-based networks. The lower-tier LiFi attocells within existing multi-tier heterogeneous wireless networks have zero interference from and add zero interference to RF-based networks such as femtocell networks [29] . Fig. 7 shows selected application scenarios (both indoor and outdoor) of VLC/LiFi technologies. The application scenarios of VLC and LiFi are similar. In the indoor environment, communication and illumination are performed simultaneously. For certain cases in the outdoor environment, both communication and illumination are performed (e.g., in street lamps and vehicle headlamps), whereas in other cases, only communication is performed (e.g., in traffic signals and the rear lamps of vehicle). The examples shown in Fig. 7(a) apply to VLC as well as LiFi communication, depending on the supported communication media for uplink and the provided features. In Fig. 7(b) , street light-smartphone communication is an example including both VLC and LiFi systems. On the other hand, infrastructure-to-vehicle, vehicle-to-infrastructure, and vehicle-to-vehicle are examples of VLC. The potential advantages of LiFi over WiFi include very fast data rate, lower cost, readily available spectrum capacity, and better security. The security features of LiFi make it promising for use in electromagnetic-sensitive areas such as hospitals, nuclear plants, and aircraft cabins. However, LiFi technology is not very effective for outdoor applications and also it cannot offer long-range communications. A few important differences between LiFi and WiFi are listed in Table IV. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 10 LEDs connected with internet server 110101010101010101 101010101010101010
Specifically, the authors in REF presented the differences among various categories of optical wireless communications (OWC) technologies such as free space optical (FSO) communications, VLC and light fidelity (LiFi).
A Comparative Survey of Optical Wireless Technologies: Architectures and Applications
IEEE Access, vol. 6, Jan. 2018
Computer Science