1 Classification Framework for Free Space Optical Communication Links and Systems Abdelbaset S. Hamza, Member, IEEE, Jitender S. Deogun, Member, IEEE, Dennis R. Alexander Abstract—Free Space Optical (FSO) communication technol- performance-sensitive. The IoT market is expected to grow ogy, also known as Optical Wireless Communications (OWC), has from 9.1 billion devices and objects connected to the In- regained a great interest over the last few years. In some cases, ternet in 2013 to 28.1 billion by 2020 [1], that is more FSO is seen as an alternative to existing technologies, such as radio frequency. In other cases, FSO is considered as a strong than three times the global population expected by 2020. candidate to complement and integrate with next-generation As such applications and systems rapidly move closer to technologies, such as 5G wireless networks. Accordingly, FSO end users, wireless communication systems, are the favored technology is being widely deployed in various indoor (e.g., data communication technologies as they allow for user mobility. centers), terrestrial (e.g., mobile networks), space (e.g., inter- Moreover, wireless technologies avoid most of the inherent satellite and deep space communication), and underwater systems (e.g., underwater sensing). As the application portfolio of FSO complexity that wired technologies suffer from, such as, long technology grows, so does the need for a clear classification for setup time, right of the way for digging, and the sunk cost once FSO link configurations. Most existing surveys and classifications the cables are laid [2]. It is expected that two-thirds of total are single-level classifications, and thus not inclusive enough to IP traffic by 2020 will be generated by wireless and mobile accommodate recent and emerging changes and developments of devices [3]. different FSO link configurations and systems. In this paper, we propose a multi-level classification framework to classify Figure 1 depicts part of the electromagnetic (EM) spectrum existing and future indoor, terrestrial, space, underwater, and and the frequency (and wavelength) ranges for each band of heterogenous FSO links and systems using common and simple the spectrum. As the frequency increases, the wavelength and unified notation. We use the proposed classification to review effective area of an antenna decrease. The carrier frequency and summarize major experimental work and systems in the is selected based on the application. For example, ground- area until 2017. Using the proposed classification and survey, we aim to give researchers a jump-start to tap into the grow- to-submarine communications utilize audio waves due to its ing and expanding realm of the FSO technology in different very long wavelengths (i.e., very low frequency and very environments. The proposed classification can also help organize large antenna) and the limited propagation capability of RF and systematically present the progress in the research on FSO signals in electrical conductors such as salt water due to technology. This makes the identification of the market needs for absorption. On the other hand, radio frequencies in the Ultra standards an easier task. Moreover, different entities involved in the standardization process including academic, industry, and High Frequency (UHF) and Super High Frequency (SHF) band regulatory organizations can use the proposed classification as a range are capable of penetrating windows, walls, and ceils. unified language to communicate during the early stages of stan- Therefore, the IEEE 802.11b/g/n (WiFi) networks utilize the dard development which require ambiguity-free discussions and unlicensed 2.4 GHz UHF and 5 GHz SHF radio bands. exchange of ideas between different standardization entities. We RF is a mature technology and is being widely deployed in use the proposed classification to review existing standards and recommendations in the field of FSO. It is also envisioned that many indoor, terrestrial, and space communication systems. the proposed classification can be used as a unified framework However, the propagation nature of the RF communication to define different FSO channel models for simulation tools. systems raises a problem of interference, which in turn af- Index Terms—Classification, Free Space Optical (FSO), In- fects the usability of frequencies, and hence, the capacity. door, Optical Wireless Communications (OWC), Space, Survey, Therefore, the RF spectrum is regulated by the local and Terrestrial, Underwater, Wireless Communications. international authorities to limit the interference, and guarantee proper operation and coexistence of systems relying on RF. I. INTRODUCTION As the applications of RF communication are progressively increasing, the RF spectrum becomes more congested, scarce MERGING Big Data applications and systems found and thus expensive to acquire. Several efforts are put from E in disciplines like social media and Internet-of-Things research and industry to stretch the capability of existing (IoT), are characterized by being bandwidth-intensive and wireless technologies (e.g., alleviating interference) and to A.S. Hamza was with the University of Nebraska-Lincoln, Lincoln, develop new ones to fulfill the emerging needs [4], [5]. NE, 68588 USA. He is now with the Department of Computer Sci- Free Space Optical (FSO) communication, also known as ence, Appalachian State University, Boone, NC, 28608 USA (e-mail: Optical Wireless Communication (OWC) as discussed later in [email protected]). J.S. Deogun is with the Department of Computer Science and Computer Section II-A, is being extensively investigated over the last few Engineering, University of Nebraska-Lincoln, Lincoln, NE, 68588 USA (e- decades as an attractive alternative technology to RF. Similar mail: [email protected]). to fiber optics, data are used to modulate a light beam in FSO. D.R. Alexander is with the Department of Electrical and Computer Engi- neering, University of Nebraska-Lincoln, Lincoln, NE, 68588 USA (e-mail: The light beam then propagates from one point to another, [email protected]). however, in a wireless manner. The recent spike in interest 2 Fig. 1: Part of the electromagnetic (EM) spectrum showing the frequency (and wavelength) ranges for each band. in FSO technology stems from the fact that FSO combines RF-sensitive navigation and avionics electronic systems [21]. high-bandwidth of optical communication systems and the Moreover, the next generations of wireless communication flexibility of wireless technologies. systems (e.g., 5G) incorporate several complementary access FSO technology operates in a broad spectrum (see Fig- technologies along with the RF technology, including FSO ure 1) including Near Infrared (NIR), Visible Light (VL), [22], [23]. and Ultraviolet (UV) bands. Conventionally, terrestrial and A preliminary optical communication experiment was space FSO links operate in the NIR band similar to fibre- among the secondary objectives of the mission Gemini 7 optic systems [6]. As will be shown in subsequent sections, conducted by NASA in 1965 [24]. The experiment was terrestrial systems can also operate in the VL [7] and UV only partially completed due to the cloud obscuration and [8] bands. On the other hand, Indoor FSO links commonly the spacecraft altitude restrictions [25]. Three years later, operate in the NIR [9] and VL [10] bands, whereas, underwater Erhard Kube published the original FSO communications OWC systems operate in the NIR [11] and VL [12] bands. white paper ”Information transmission by light beams through The extremely short wavelengths (i.e., high frequencies) at the atmosphere” [26]. In this paper, E. Kube explained the which FSO systems operate make FSO detectors immune to possibility of transmitting data through the atmosphere using multipath fading (i.e., large fluctuations in received signal green (0.6 µm) and red (0.8 µm) laser sources. Continued magnitude and phase) as opposed to RF links, which are development of lasers led to the development of a small and highly susceptible to multipath fading. This can be attributed continuous-beam semiconductor light sources that work at to the spatial diversity resulting from the fact that FSO detector room temperature by Zhores Alferov in 1970. This invention areas are extremely large compared to the wavelengths [13]. opened new horizons for the development of OWC systems. In addition to the unregulated spectrum, most of the optical In 1979, Gfeller and Bapst introduced the first indoor OWC components used in FSO links are cheaper, smaller, lighter system in which the diffuse emissions in the infrared (IR) and have lower power consumption as compared to that of band were used [27]. The continued research and development RF components leading to cost and energy saving [14]–[18]. by academic institutions, industry and military organizations, Although most of the FSO components are cheaper, lighter, enabled the FSO communication to find its place in many and smaller than that of RF links, one must keep in mind that applications, such as, mobile networks backhaul [28], [29], FSO networking solutions are not as mature and commercially space communication [30], underwater (UW) sensing [31], available as their RF counterpart. We believe that this is a [32], wireless sensor networks (WSNs) [8], indoor local area main contributor to the fact that FSO commercial solutions networks [33], [34], data center networks (DCNs) [9] and can be sometimes more expensive and bulkier especially in the many other applications. case of terrestrial FSO links [19]. As the technology becomes more popular and with the expected increase in the market A. Motivation and Contribution competition, the price of FSO solutions is expected to drop. Advantages of the FSO technology have been known for a On the other hand, as the technology matures, designer of long time. However, utilization of these advantages was facil- FSO solutions will be able to develop the best design practices itated by recent development and advances in FSO enabling which will influence the size of the modules used in the FSO technologies. As a result, a large number of research papers systems. on new FSO applications has been published recently.