Emerging Optical Wireless Communications

IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS 2015

Emerging Optical Wireless CommunicationsAdvances and Challenges Zabih Ghassemlooy, Senior Member, IEEE, Shlomi Arnon, Senior Member, IEEE, Murat Uysal, Senior Member, IEEE, Zhengyuan Xu, Senior Member, IEEE, and Julian Cheng, Senior Member, IEEEAbstract— New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such a technology that could help to address the problem would be the optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC and optical scattering communications. Index Terms— Optical wireless communications, visible light communications, free space optics, transcutaneous OWC, underwater OWC, optical scattering communications.

I. INTRODUCTION

W

ITH the evolution of the wireless communications standards into the fourth generation (4G) and fifth generation (5G) networks, we are witnessing rapid progress and development in information and communication technologies, such as ultra-broadband internet access, internet protocol telephony, gaming services, streamed multimedia applications, and high-definition television services [1]. The exponential surge in the commercial demand to pursue Manuscript received January 26, 2015; revised April 29, 2015; accepted May 20, 2015. The work of Z. Ghassemlooy, A. Arnon, and M. Uysal was supported by the EU Cost Action IC1101. The work of Z. Xu was supported in part by the National Key Basic Research Program of China under Grant 2013CB329201, by the National Natural Science Foundation of China under Grants 61171066, and by Shenzhen Peacock Plan under Grant 1108170036003286. Z. Ghassemlooy is with the Optical Communications Research Group, NCRLab, Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne NE1 8ST, U.K. (e-mail: [email protected]). S. Arnon is with the Satellite and Wireless Communication Laboratory, Electrical and Computer Engineering Department, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel (e-mail: [email protected]). M. Uysal is with the Department of Electrical and Electronics Engineering, Ozyegin University, Istanbul 34794, Turkey (e-mail: [email protected]). Z. Xu is with the Key Laboratory of Wireless-Optical Communications, Chinese Academy of Sciences, School of Information Science and Technology, University of Science and Technology of China, Hefei 230026, China (e-mail:[email protected]). J. Cheng is with the School of Engineering, The University of British Columbia, Kelowna, BC V1V 1V7, Canada (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSAC.2015.2458511

unlimited high-speed and ubiquitous broadband wireless access, to accommodate the ever-increasing utilization of internet and multimedia services among individual mobile users as well as residential and enterprise clusters, has spurred prodigious growth in internet traffic demand in the recent decade, scaling much faster than the prediction by Moore’s law [2]. As a result, the data traffic have been rising by a factor of ten every five years, which corresponds to a tremendous growth in the overall traffic volume by several hundred-fold within the next decade [3]. To be able to transport an exponentially increasing amount of (available) data to the end users within an acceptable delay, the current system is experiencing severe congestion of the radio frequency (RF) spectrum and wireless traffic bottleneck [4]. These figures assume approximately a ten times increase in broadband mobile subscribers, and 50-100 times higher mobile data traffic per user. New devices, such as smartphones and tablets with powerful multimedia and sensing capabilities, are launched in the market at a high pace, creating new types of demand and constraints on broadband wireless access. As a result, new data services and applications are emerging continuously, which enhance the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the future network infrastructure. Complementing the existing wireless RF solutions, optical wireless communications (OWC), 350-1550 nm wavelength band, is an affordable and attractive high-data rate (30 Gb/s data rate [5]) technology and is poised to become a promising candidate for the next generation broadband wireless access to resolve the existing “last mile” and “last-leg” access network problems [6], [7]. This is mainly due to the vastly attractive features of OWC technology including (i) no licensing requirements or tariffs for its utilization; (ii) virtually unlimited bandwidth for providing near-optimal capacity and supporting high-speed applications (e.g., visible light communications (VLC) with a bandwidth much higher than the RF bandwidth); (iii) extensive link range in excess of a few meters to "5 km" ; (iv) a green technology with high energy efficiency due to low power consumption, reduced interference and fading immunity; (v) high scalability and reconfigurability; (vi) a high degree of security and privacy against eavesdropping (optical beam confined within a room); (vii) cost-effective in terms of the price per bit; and (viii) reduced time-to-market [8]. Fig. 1 shows the data rate versus the transmission distance for a range of wireless technologies.


Short range broadband wireless communications currently resides within the well established and matured RF technologies [9]. To enhance the coverage area of existing cellular networks and improve its capacity very dense re-use of resources will be the key requirement. Additionally future networks with heterogeneous architectures would be expected to offer multiple access points within each cell. The emerging VLC technology somehow meets these requirements where the reduced spacing between the nodes will lead to increased spatial reuse of resources, thus providing higher data density as well as increased network capacity. OWC covering ultra-violet (UV), infra-red (IR), VLC, freespace optics (FSO) communications [10], can be used in both the indoor and outdoor environments including underwater [11]. Figure 2 illustrates the system block diagram of a typical OWC system composed of transmitter, channel, and receiver, whereas Figure 3 shows the possible OWC link configuration as well as the type of receivers. The line-of-sight (LOS) link is the best option for OWC systems in terms of very high data rates, low bit error rate performance connection and less complex protocol and is mostly used in outdoor application. However, the LOS link lacks mobility and is susceptible to blocking, particularly in indoor environment. The diffuse and scattering configurations offer improved mobility and are more robust to shading, but for high-speed links the path loss, noise and multi-path induced dispersion limit the maximum data rate compared to the LOS link. OWC systems are being installed for urban networks [12], high speed ground to train networks [13], last-mile FSO links [14], high speed indoor links [15] and indoor VLC systems [16]. Most of OWC systems reported are based on the intensity modulation/direct detection (IM/DD) scheme. Adopting the coherent scheme in OWCs will lead to improved channel usage but at the cost of increased system complexity compared to IM/DD, where accurate wave-front matching between the incoming signal and the local oscillator is needed to ensure effective coherent reception. Similar to optical fibre communications, at the receiver side a photodetector (PD) is used to convert the

Figure 1. Data rate vs. the link range for wireless technologies

optical signals into electrical signals, followed by the standard demodulation/decode of the information. For DD the implementation is simple and uses low-cost transceiver devices without the need for complex high-frequency circuit

Figure 2. OWC system block diagram. R: reponsivity of the photodetector; Pt: transmit power

designs compared to coherent systems. In this paper an overview of OWC systems will be given focusing on a number of specific configurations and technologies including VLC, FSO, transdermal or transcutaneous OWC, underwater OWC, and UV (also known as optical scattering communications (OSC)). The rest of the paper is organised as follows. Indoor OWC based on VLC is covered in Section II, whereas outdoor OWC better known as FSO is outlined in Section III. Transdermal or transcutaneous and underwater OWC systems are presented in Sections IV, and V, respectively. In Section VI optical scattering communications is outlined followed by the conclusion section. II. VISIBLE LIGHT COMMUNICATIONS (VLC) Indoor VLC (380 – 780 nm) is a relatively new technology proposed as an alternative to indoor IR (780-950 nm) access technologies [17, 18] offering a number of functionalities. In addition to illumination VLC offers data communication and indoor localisation (where current RF based global positioning systems (GPS) offers limited or no coverage in indoor and underground environment) using the existing white light emitting diodes (LED) based lighting fixtures [19]. In VLC systems both visible light and IR links could be used for the uplink [20]. LEDs are more efficient light source (> 400 lux, which is high enough to transmit data at high speed) than their incandescent and fluorescent counterparts and have a longer lifespan, thus providing both ecological and financial benefits [21]. These features have made the emerging field of indoor short range VLCs very attractive to the worldwide research community, through bodies such as the VLC consortium (more than 20 organizations) in Japan in 2003 [22], the Wireless World Research Forum [23], the European OMEGA project [24], IEEE standardization body [25, 26], and UK research council funded ultra-parallel VLC [27]. The current IEEE VLC standard approved in 2011 supports up to 96 Mbps. A Task Group recently revived the work in IEEE 802.15.7 and is working on an enhanced VLC physical layer based on OFDM to enable peak data rates at Gbps. It is also planned that this standard will support optical camera communication (OCC) where smartphone cameras are used for reception in low data rate applications [26]. Standardization is expected to be finalized by the end of 2017. With the emergence of Indium Gallium Nitride (InGaN) technology it has been possible to manufacture efficient white LEDs in a number ways: (a) combining red, green, blue light


sources (RGB) offer typically 20 MHz of modulation bandwidth; and (b) using a blue light source with yellowish cerium doped yttrium aluminium garnet phosphor, known as white phosphor LEDs (WPLEDs) [28]. WPLEDs have a limited modulation bandwidth of 2-3 MHz that can be extended to 20 MHz by placing a blue optical filter in front of the receiver [29]. Encompassing the visible spectrum, these LEDs are not only capable of OWC, but also have the ability to provide illumination at the same time [30]. In sensitive environments such as hospitals, airplanes and industrial gas production plants where the use of RF technology is not allowed, VLC can be seen as a replacement technology. For VLCs to be fully integrated into the current wireless topologies there are a number of challenges that need addressing including: (i) increasing the data throughput, as WPLEDs are fundamentally limited to only a few MHz [31,32]; (ii) high path losses, artificial light induced interference, multipath induced intersymbol interference (ISI), mobility and blocking [15]; (iii) the nonlinearity of the LED electro-optic response; (iv) dimming of the light. In the IEEE 802.15.7 task group, organised towards the global VLC standardisation, several dimming schemes have been proposed [33], and a number of modulation schemes (on on-off keying, pulse position modulation, and discrete multi-tone (DMT)) have been developed to accommodate dimming [34]. Pulse width modulation (PWM) is the most widely adopted scheme to support dimming control [35]. It is known that pulse position modulation (PPM) is sensitive to the synchronization jitter. In [36] the effect of clock time shift and jitter on the inverse PPM VLC system bit-error-rate performance have been investigated by deriving models that could be used to define the minimum requirements of the synchronization system. Existing off-the-shelf LEDs have a limited bandwidth (up to 50 MHz for incoherent IR, 45 dB/km of loss [29]. In [54] it was reported that the atmospheric attenuation of the laser beam is a random function of the weather, which can vary from 0.2 dB/km in exceptionally clear weather to 350 dB/km in very dense fog. Laser beams propagating through the atmospheric turbulent channel are highly susceptible to the adverse effects of scintillation and beam wander. This is due to the refractive index variations along the transmission paths caused by inhomogeneities in both temperature and pressure of the atmosphere [44]. Correspondingly, in IM/DD based systems this leads to the random fluctuations in both temporal and spatial domains of the received irradiance, known as the channel fading [6], [45]-[54]. There are a number of empirical models which estimate the outdoor fog attenuation based on the measured visibility V from the visible – near IR range of the spectrum. Some of the existing fog models are based on the experimental data while others are obtained using the theoretical considerations. The fundamental law to measure attenuation of optical signal based on V (in km) is Koschmieder law, which defines V as the distance to an object at which the visual contrast/transmittance of an object drops to certain value of the visual/ transmittance threshold of the original visual contrast (100 %) along the propagation path [29]. Kruse model takes into account the effect of wavelength on the fog induced attenuation [55], whereas Kim model estimates the fog attenuation, which indicates that the atmospheric fog attenuation is wavelength independent for V < 0.5 km [56]. Naboulsi model is an empirical model that estimates the attenuation caused by radiation and advection fog for wavelengths from 0.69 µm to 1.55 µm for V from 0.050 km to 1 km [57]. In FSO systems the most widely adopted models to assess the link performance in a turbulence channel are the Lognormal and the Gamma-Gamma (GG), negative exponential, Weibull, and K-distributed [43]. The Log-normal is widely used for weak turbulence, K-distributed for strong turbulence conditions, GG for all turbulence regimes [58], exponentiated Weibull (EW) for the weak-to-moderate turbulence regime, and negative exponential for the saturated regimes. To mitigate the degrading effect of the turbulence-induced fading, a number of schemes have been proposed including the diversity and aperture averaging [58], [59], error-correcting codes [60] and multi-hope relay based transmission [61]. Of the three types of diversity, wavelength diversity requires composite transmitter and with wide spacing between the carrier signals which results in complex optical system design [62]; time diversity involving coding and interleaving results in long delay latencies, thus the need for large size memories

[63]; the spatial diversity requiring multiple transmitters and receivers [64]; and different multi-branch diversity receivers with arbitrarily correlated Rician channels [65]. Aperture averaging can be regarded as a simple form of spatial diversity when the fading correlation length is smaller than the receiver aperture [64]. Misalignment-induced fading is another nonnegligible effect in FSO systems as optical terminals, typically installed on high-rise buildings require LOS, are susceptible to building sway. In such links continuous precise pointing is required to establish link connectivity for successful data transmission with minimum error probability, particularly when narrow beam divergence angle and the receiver field-ofview (FOV) are employed [17]. In addition, pointing errors can arise due to mechanical misalignment, errors in tracking systems, or presence of mechanical vibrations within the system [47]. In recent studies it has been demonstrated that laser-based FSO communications do still suffer from security problems, especially when the laser beam main lobe is considerably wider than the receiver size (e.g., in IM air-to-ground FSO links) [66]. Conventional security strategies including private key-based cryptosystems have been used to improve FSO security; however the use of a pre-shared secret key for data encryption can be an issue. A private secret-key-based cryptosystem with the key management, based on statistics of the random atmospheric turbulence induced fading channel measurements, can be employed to enhance FSO security [67]. In [68] a technique to secure orthogonal space-time block codes (STBC) using a shared key obtained from received signal strength indicator without the need for the transmit channel state information or increased transmit signal’s peak-to-average ratio was reported. The proposed method preserves the linear detection complexity of the orthogonal STBC while guaranteeing full diversity order for the intended recipient and zero diversity order for the eavesdropper. Table I summarises the differences between FSO, VLC and

Figure 3. OWC link configurations: (a) directed LOS, (b) non-directed LOS, (c) diffuse, (d) quasi diffuse, (e) multi-spot LOS, and (f) receiver configurations


RF communications.

IV. TRANSDERMAL OR TRANSCUTANEOUS OWC There is a snowballing demand for transdermal or transcutaneous high data rate wireless communications. Device and application such as glucose monitoring, pacemakers, prostheses, neural signals at the brain interface, and imager (e.g. for an artificial eye) are just a few examples of the devices that may benefit from improved transdermal communications (TC) [18], [69]-[72]. In these applications recording signals from in-body devices and activating these devices from outer-body signals could be done. Functional electrical simulation (FES), in which the neural signals are sampled processed and used to actuate artificial limbs, is one representative example. The technique is used for rehabilitation human affected by paralysis resulting from stroke, head injury, spinal cord injury (SCI), and other neurological disorders by actuate his or her own artificial limbs [71]. This is a great example to the importance of high speed bidirectional transdermal communication. The present FES implementation is granting a quality of life and wellbeing to amputees that was only a fantasy in the past. This technology could in some cases to relieve pain, to improve functional neurological disorder or even to regain functionality of damaged limbs using electrical stimulation. The emerging applications define the requirement for the next generation of transdermal or TC. The requirements are (a) high speed wireless modality (no transcutaneous wires that could cause infection injury and discomfort), (b) low-power consumption of the transplant device to extend its longevity, (c) high level of cyber security and (d) high signal-to-noise ratio to ensure functional independence of the ambient environment [72]. Nowadays, the common wireless communication modalities used for transcutaneous communication are based on RF, acoustic wave, conductive, or electrical induction. An altered modality to these methods is the OWC technology, see Figure 4 [73]. The 1st OWC configuration is a direct communication link that includes modulator, light source (laser or LED), optics and at the other side of the cutaneous link, optics, optical filter (OF), detector, trans-impedance amplifier (TIA), decision device (DD) connected to a control unit. The IM light propagates through the cutaneous link and is collected at the receiver by a PD via an OF. The 2nd OWC is the modulated retro reflector link that uses the same optical and electrical components as in the 1st configuration. Light propagates through the cutaneous link to the modulated retro reflector, which reflects it to the cutaneous link. The reflected light propagates again through the cutaneous link and is collected at the receiver. In both configurations the amplified current is applied to the DD module to make decision of the signal. It is hoped that future generation of modulated retro reflector will make the energy consumption inside the body minimal and therefore increase the longevity of the device. It is very important to choose the appropriate wavelength of the OWC system in order to minimize the link attenuation. The medical window or the therapeutic window is in the near-

infrared spectrum between 650 to 950 nm where light has its maximum depth of penetration through the skin [18], [69], [74]. At lower and upper wavelengths scattering and absorption by water and blood components is higher. Scattering is the dominant light-tissue interaction mechanism that limits the penetration, which increases the distance travelled by photons within tissue, which also increases the absorption probability. Research in transcutaneous OWC is carried out in several organizations around the world. In [69], [70] the mathematical models and techniques to design a TC link considering parameters such as the thickness of the skin, size of the integration area of optics, degree of transmitterreceiver misalignment were presented. In [70] experimental results as well as the mathematical model for the direct and retroreflection link configurations were reported. In [18] a design methodology for an optimization of the optical receiver in a neural recording system was described. The 125 neural channels are simultaneously sampled at 30 Kbps with a resolution of 10-bit and Manchester encoding, thus resulting in a data rate of 75 Mbps. However the transcutaneous optical telemetric link suffers from significant scattering of the transmitted photons as a result of the propagation of light through tissue. Therefore, to increase the received signal level one should use a larger size photodiode. However larger size photodiodes lead to a limited bandwidth. In [75] implantable sensors for continuous glucose monitoring with OW

Figure 4. Medical doctor check neurological disorder patient

transceiver was presented that uses the complementary metaloxide-semiconductor technology with a combined area of 0.665 mm2.

V. UNDERWATER OWC Demands for underwater communication systems are increasing due to the on-going expansion of human activities in underwater environments such as environmental monitoring, pollution control/tracking, underwater exploration, scientific data collection, maritime archaeology, offshore oil field exploration/monitoring, port security and


tactical surveillance [76]-[82]. Wire-line systems (particularly fibre optical systems) can be used to provide real-time communication in underwater applications, but their high cost, lack of flexibility and operational disadvantages become restrictive for most practical cases. This triggers the growing demand for underwater wireless links. Traditionally, acoustic communication is used covering long ranges up to several kilometres. However, it suffers from a very low data rate (a few hundreds or thousands of kilobits/sec), very low celerity, and large latencies due to the low propagation speed [76], [77]. In underwater environments, light suffers from high absorption rates due to the electron transitions in the far ultraviolet and to different intra/inter molecular motions in the infrared band. On the other hand, water is relatively transparent to light in the visible band of the spectrum. In fact, absorption takes its minimum value in the blue/green spectral range (450 nm - 550 nm). This paves the way for underwater VLC (UVLC) [82]-[85]. UVLC is an attractive complementary solution to long range acoustic communication and is able to achieve much higher data rates, up to hundreds of Megabits/sec and even Gigabits/sec, for short ranges. For example, in [80], a data rate of 1.2 Mb/s over a 30 m range was demonstrated using 6 blue LED chips at 480 nm and discrete pulse interval modulation. In [82], an impressive data rate of 1 Gb/s was achieved over 2 m in a laboratory water pipe at a wavelength of 532 nm using some high-end equipment consisting of a laser diode, lithium Niobate external modulator, and high power ytterbium-doped fibre amplifier. In [85] with a simpler and cost-effective approach, a data rate of 58 Mb/s over 2.5 m distance was achieved in an outdoor water tank using 14 blue (470 nm) LED chips and an avalanche photodiode. Recently, the company Ambalux [86] announced the release of an UVLC modem, which provides up to 10 Mb/s through a watercolumn range of up to 40 meters, depending on water clarity and ambient sunlight. The high-end model of the same product is able to support up to 100 Mb/s. In [87] a UVLC modem operating over 200 m with a data maximum rate of 20 Mb/s was demonstrated. Channel modelling and characterization will lead to a better understanding of underwater optical channel and are the very first step for efficient, reliable and robust UVLC system design. The two main mechanisms, which affect light propagation underwater, are absorption and scattering. The absorption effectively reduces the intensity of the light and results in attenuation of the transmitted signal. On the other hand, scattering refers to the deflection of light from its original path. This effectively results in spatial and temporal dispersion of the transmitted signal. The propagation of light underwater is typically modelled by the radiative transfer equation (RTE) [88], which basically describes the energy conservation of a light wave traversing a scattering medium. RTE involves integro-differential equation of time and space and exact analytical solutions of the RTE can be obtained only under the assumption of no scattering. A closed form solution even for homogeneous water with isotropic scattering does not

exist. Some approximate analytical solutions [89] have been proposed under the assumption that only 1storder and 2ndorder scattering are considered. These approximate RTE solutions depend on various simplifying assumptions and the predicted irradiances are typically accurate to a few tens of percent at best, and can be off by an order of magnitude. Alternatively, various numerical methods have been proposed for solving RTE including the invariant embedding solution [90], which is computationally efficient and highly accurate that considers the variation of the water inherent optical properties and the boundary conditions at the bottom and the water-air surface. Another method is the discrete ordinates solution and more specifically the eigenmatrix solution that can only be applied to homogeneous water bodies [91]. An alternative way to find a solution for RTE is Monte Carlo simulations, which employs statistical methods to evaluate the channel characteristics by generating numerous photons and then simulating the interactions of each photon with the medium [92]. In the literature, these two main approaches (numerical vs simulations) have been commonly used to model and characterize the UVLC channel. Particularly, in [93], the spatial and angular effects of scattering on an UVLC link based on the RTE are studied. In [87], the channel time dispersion is quantified, considering different link distances, transmitter beam divergences, and receiver lens aperture sizes. It is shown that except for highly turbid waters, the channel time dispersion can be neglected when working over moderate distances. In [94], the RTE is used with the modified Stokes vector to model light scattering in water. They consider a highly dispersive medium and conclude that ISI is very restrictive over a range of 50 meters and at high data rates (1 Gb/s). Indeed, it is also reported [95] that the channel time dispersion becomes an important factor to take into account in highly turbid waters. Most current UVLC works assume un-coded IM schemes while some recent works also consider coded schemes. For example, Reed-Solomon (RS) codes are used in [13] while a combination of reconfigurable RS code and a Luby transform code was used in [96]. A comparative performance analysis of various intensity modulation techniques including OOK, pulse position modulation, PWM, and digital pulse interval modulation was further presented in the context of UVLC [97]. The use of more advanced modulation techniques for UVLC such as optical OFDM has been proposed [85], [98]. As the literature survey demonstrates, UVLC research on the physical layer (PHY) aspects is still in its infancy and requires the exploration of more advanced PHY layer technologies to further improve the underwater link reliability, throughput and/or coverage. While both underwater acoustic and optical transmission have been studied independently in the literature, there is little work on hybrid network architecture that combines the advantages of the two [99]-[101]. The work in [99] is mainly conceptual and demonstrates the complementary nature of acoustic and optical communication technologies through some computer simulations. In [100], real-life performance measurements of a custom design UVLC modem was outlined and plans for integrating it with an


acoustic link were discussed. In a follow-up work of [95], wireless data harvesting from a subsea node was demonstrated using a “ship of opportunity” [101]. VI. OPTICAL SCATTERING COMMUNICATIONS Traditionally, FSO has been a major player in OWC. To ensure link connectivity in a dynamic system, it typically requires concurrent transceiver pointing, acquisition and tracking (PAT) assisted by adaptive optics (AO). Though theoretically feasible, it is a very challenging task in practice. In order to relax the restrictive PAT requirement, researchers have investigated optical scattering communications (OSC) techniques for non-line of sight (NLOS) communications between terminals whose LOS path might be blocked by obstacles. Scattered intensity is inversely proportional to the fourth power of the wavelength, thus shorter wavelength gets easier to be scattered, particularly for ultraviolet (UV) carrier. In the VL range, for example, a flashlight beam is usually observable at night even being viewed from the direction offset from the beaming direction. Blue sky in a sunny day is also a direct consequence of strong scattering to blue parts in the solar radiation. Most scattering systems have used conventional devices as light sources and detectors [102][104]. In recent decade, semiconductor devices in the UV range have been developed for LEDs sources and avalanche photodiodes (APD) [105], [106]. The first ever LED-based OSC research was reported in [107]-[109]. The system consisted of 10 LEDs at 274 nm wavelength, where each unit has a 24-element array, producing a total optical power of 40 mW and a Perkin-Elmer PMT module combined with a solar blind filter to achieve a high solar noise rejection ratio. The system could deliver a data rate of kb/s using the 4-ary PPM scheme at a range of tens of meters. Upon availability of UV LED sources, OSC systems and models have been extensively investigated in the literature [110]. One of the most important tasks is to understand the channel characteristics, both analytically and experimentally. Channel path loss determines the received signal power and thus the BER performance for any given data modulation format. Channel impulse response describes the pulse broadening effect after scattering. It provides signaling reference to avoid ISI. Following previous works, in [111] an approximate closed form path loss expression was derived for NOLS single scattering geometry, assuming that the overlapped cone volume was small. Considering multiple scattering, a Monte-Carlo ray tracing technique was proposed incorporating probabilistic modeling of the phone migration process from the transmitter to the receiver [112]. Applying beam shaping techniques using optics, the received signal power could be significantly increased by changing the traditional cone beam from a circle footprint to an elliptical or a rectangular footprint [113]. Short-range path loss and impulse response under different geometric settings were experimentally measured in [114], [115]. Corresponding models were proposed and tabulated model parameters were found by curve fitting. When the beam axis and FOV axis are not coplanar, multiple scattering is more crucial to received

signal power. Analytical and approximate closed form path losses under arbitrary non-coplanar geometry were analyzed [116]. Advanced receiver design techniques and communication system performance were further studied [117]. Cooperative relay protocols were suggested for Poisson channel conditions, utilizing intermediate nodes to extend the range from source to destination [118]. Link connectivity and neighbour discovery protocols were designed to maximally explore the degrees of freedom in three dimensional space of a scattering channel [118]. The above studies were focused on short-range point-topoint communications, neglecting the atmospheric turbulence effects. For longer range links the detrimental effects from fully coupled scattering and turbulence have been investigated [120]-[125]. In [120] preliminary models under weak to strong turbulence regimes were proposed showing that under certain geometric conditions the collected signals from rich scattering are averaged resulting in smoothed signal at the receiver output. In addition, multiple scattering interference was evaluated in [121], and multiple scattering channel models were applied to conveniently establish sensor networks in [122]. In [123], [124] efficient coding and advanced receiver design techniques were introduced. In [125] the communication system performance employing a receiver diversity scheme was studied. These studies have significantly advanced the OSC research area and will cast incredible values to practical system design. So far, tremendous work on short-range optical scattering communication links and systems has been reported in the literature. However, easy to use channel and system models suitable for practical application scenarios are still demanding. As communication range increases, joint modeling of turbulence and multiple scattering is undergoing. Corresponding modulation/coding and transceiver design techniques to tackle channel impairments and multiple access interference are necessary. Moreover, there lack practical OSC relay and ad hoc network protocols to coherently connect various mobile nodes in a dynamic environment. Furthermore, extending the optical spectrum beyond the UV band can enjoy a lot of benefits from high power and low cost sources as well as commercial off-the-shelf detectors, and thus OSC in those bands such as infrared and visible light will receive increasing attentions. VII. CONCLUSIONS This paper presented a general overview of the optical wireless communications, which is finding new applications in ever evolving communication networks, where there is a growing demand for higher bandwidth to deal with multimedia type services. The main topic covered in this paper included visible light communications for illumination as well as data communication, free space optical communication for outdoor application, transcutaneous OWC for medical application, underwater OWC for a range of applications and optical scattering communications for scenarios where there is no line of sight path between the transmitter and receiver. In the coming decade the optical


wireless communication technology will be adopted in many applications that traditionally were based on the RF technology. The main reason for this has been the spectrum congestion associated with the existing RF systems as a result of the exponential growth in demand for unlimited high-speed and ubiquitous broadband wireless access anywhere and anytime. The complementary OWC technology to the RF could support part of this increasing demand in certain Parameter Data rate Devices size Bandwidth regulated

ACKNOWLEDGMENT The works of Z. Ghassemlooy, A. Arnon, and M. Uysal was supported by the EU Cost Action IC1101, and Z. Xu’s work was supported by National Key Basic Research Program of China (Grant No. 2013CB329201), National Natural Science Foundation of China (Grants No. 61171066), and Shenzhen Peacock Plan (No. 1108170036003286).

TABLE I COMPARISON BETWEEN FSO AND RF COMMUNICATION SYSTEMS. FSO VLC (Indoor) IR (Indoor) 10’s Mbps – 10’s 4 Gbps (a very short link) >1Gbps Gbps Small Small Small No No No Low-medium High No

Low High No

Low High No

Scalable Multipath

Multipath

Distance Coverage Path loss

Scalable Fog, Atmospheric turbulence misalignment or obstruction Medium-Long Narrow High

Noise

Sun + ambient light

Short Narrow and wide Medium (high for non-line of sight) Sun + ambient light

Fog Attenuation (V = 200 m) Mobility Standard

37 dB/km at 830 nm

None

None Well developed

Services

Communications

Limited In progress (IEEE 802.15.7 TG) Illumination + Communications + localisation

Power consumption Security Electromagnetic interference Network architecture Link performance effects

RF < 100 Mbps Large Yes (not always WiFi for example ) Medium Low Yes Non-scalable Multipath fading, rain, interferences

Short Narrow and wide

Short-Long Mostly wide High

Sun + ambient light

All electrical/electronic appliances 3 dB/km at 58 GHz

Limited Well developed (IrDa) Communications

Good Matured Communications + positioning

applications both indoor and outdoor. As a result we are witnessing the opening of very exciting new era of light based wireless technologies with numerous opportunities.

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