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Optical Systems, A Survey

Osama Alsulami1, Ahmed Taha Hussein1, Mohammed T. Alresheedi2 and Jaafar M. H. Elmirghani1 1School of Electronic and Electrical , University of Leeds, LS2 9JT, United Kingdom 2Department of , King Saud University, Riyadh, Kingdom of Saudi Arabia [email protected], [email protected], [email protected], [email protected]

Different candidates have entered the race to Abstract—In the past few years, the demand for high data rate provide ultra-fast wireless communication systems for users, services has increased dramatically. The congestion in the such as optical wireless communication (OWC) systems for frequency (RF) spectrum (3 kHz ~ 300 GHz) is expected to limit indoor usage [4]–[6]. the growth of future wireless systems unless new parts of the OWC systems are candidates for high data rates in the last spectrum are opened. Even with the use of advanced engineering, mile of the access network. They have been investigated for such as signal processing and advanced schemes, it will be very challenging to meet the demands of the users in the next more than three decades as an alternative solution to support decades using the existing carrier frequencies. On the other hand, high speed data instead of (RF) systems [4]. there is a potential band of the spectrum available that can provide However, this technology was not deployed until the 1990s tens of Gbps to Tbps for users in the near future. Optical wireless when the and the receiver components became communication (OWC) systems are among the promising available at low cost [7]. Their rate of adoption is expected to solutions to the limitation problem faced by radio grow now in view of the use of sources in systems. In this paper, we give a tutorial survey of the most applications indoor and in view of the significant significant issues in OWC systems that operate at short ranges growth witnessed in data rates. In OWC systems, a modulated such as indoor systems. We consider the challenging issues facing beam of , visible or ultra violet light is transmitted, these systems such as (i) link design and system requirements, (ii) transmitter structures, (iii) receiver structures, (iv) challenges and typically through the atmosphere, by an OW transmitter. OWC possible techniques to mitigate the impairments in these systems, systems use light emitting diodes (LEDs) or diodes (LDs) (v) the main applications and (vi) open research issues. In indoor to transmit light similar to what is done in fibre optic systems. OWC systems we describe channel modelling, mobility and Indoor OWC systems include systems such as those mitigation techniques. Infrared communication (IRC) standardized by the infrared data association (IrDA), and and visible light communication (VLC) are presented as potential systems developed to support infrared optical wireless implementation approaches for OWC systems and are communication (IRC) and visible light communication (VLC) comprehensively discussed. Moreover, open research issues in [8]. OWC systems are discussed.

Index Terms— Optical wireless communication (OWC) system, indoor optical wireless systems, IRC , VLC.

I. INTRODUCTION RADITIONAL radio communication systems suffer from T limited channel capacity and transmission rate due to the Fig. 1: The . limited available, while the data rates requested OWC systems have been shown in many studies to be able by the users continue to increase exponentially. Different to transmit video, data and voice, at data rates up to 25 Gbps in techniques have been proposed to use the radio spectrum in indoor systems [9], [10], [11], [12], [13]. more efficient ways including advanced modulation, coding, OWC systems offer many favourable features when smart antennas and multiple input and multiple output (MIMO) compared to RF systems, such as rapid deployment, low start- systems [1], [2]. up and operational costs and higher bandwidth, similar to fibre Achieving very high data rates (beyond 10 Gbps and into the . The available OW spectral regions offer virtually Tbps regime) using the bandwidth of radio systems is unlimited bandwidth, such as 1 mm – 750 nm for infrared, 780 challenging. According to Cisco, mobile traffic over nm – 380 nm for visible light and 10 nm – 400 nm for UV as this half of the decade (2016-2021) is expected to increase by shown in Fig. 1. However, there are certain that 27 times [3]. Given this expectation of dramatically growing are suitable for communication in OW spectrum such as the demand for data rates, the quest is already underway for range between 780 – 950 nm which is the optimal range alternative spectrum bands beyond radio waves. The latter are considered in IRC and the -C band, which is the best bands currently used and planned for near future systems, such range for wide field of view (FOV) communication [14]. as cellular systems. 2

Since, the OW spectrum is unregulated and is licence free, systems, while , receiver noise and the cost of the OW systems can be reduced compared to RF interference tend to limit the capacity of indoor OWC systems. systems. The main features and drawbacks of OWC systems are presented here. The nature of light gives OWC systems immunity against interference caused by adjacent channels in neighbouring rooms, with the possibility of frequency reuse in different parts of the same building, which means increased capacity. OWC systems also offer better security at the . This is due to the fact that light does not penetrate opaque barriers, which means the potential for eavesdropping is reduced unlike in radio systems, and this reduces the need for data encryption [5], [15]. In addition, the () has a very large area, typically tens of thousands of wavelengths, and this leads to efficient spatial diversity at the receiver [15]. These combined favourable features make OWC systems suitable replacements for conventional radio systems. However, OWC systems have several drawbacks. In indoor Fig. 2: Data rates and coverage area of different wireless [9], systems, optical wireless access points, in different rooms, have [11], [12], [13], [20]. to be connected via a wired backbone as light cannot penetrate walls. Also, because of multipath propagation, reflections and The rest of the paper is organized as follows: Section II signal spread, optical signals suffer from and presents a discussion of link design, modulation techniques and dispersion leading to inter symbol interference (ISI). the channel characteristics of OWC systems. Section III Furthermore, the signal at the receiver can include shot noise discusses the available transmitter architectures. Section IV induced by intense ambient light sources (sunlight, summarizes the OW receiver structures and hence the design of incandescent lighting and fluorescent light), and this leads to OWC systems. In Section V, the main challenges faced in OWC signal corruption by this background noise [16], [17]. systems are described. Due to the importance of VLC systems, Moreover, the transmitter power is limited by eye and skin we provide a comprehensive review of VLC systems in a safety regulations [18], [19]. separate Section VI. Section VII summarizes the main OWC systems require a receiver with a photo sensitive applications of OWC (IRC and VLC) systems. Section VIII detector with large area to collect the maximum optical signal gives an overview of the open research issues in IRC and VLC possible, and this leads to an increase in the capacitance of the systems. Finally, we conclude the paper in Section IX. photodetector, consequently the available receiver bandwidth decreases. II. LINK DESIGN, CHANNEL CHARACTERISTICS AND Table 1 compares Radio and OW systems. To place OWC in MODULATION TECHNIQUES IN OW SYSTEMS the scene, Fig. 2 summarizes the coverage area and data rates of the available systems in traditional RF and In this section, we discuss various link designs that can be OWC systems. employed in OWC systems. A brief description of the Table 1: Comparison of radio and OW systems modulation techniques and channel characteristics of indoor Property Radio System OW System OWC system is also provided. Bandwidth Regulated YES NO A. Link design Security Low High There are two criteria to classify OWC links: (a) the level of RF Electromagnetic YES NO directionality of the receiver and the transmitter and (b) if there interference Passes through walls YES NO is a direct path between the receiver and the transmitter. These Technology Cost High Low classifications are based on the pattern of the transmitter and field of view (FOV) of the receiver. Link Beam Directionality Low Medium classification schemes are illustrated in Fig. 3. The two most Available bandwidth Low Very high Restricted Restricted (Eye safety common link configurations for indoor systems are line of sight Transmitted Power (Interference) and interference) (LOS) and non-LOS (NLOS) [5], [9], [21], [22]. LOS links Other Users and Sun light and Ambient provide a direct path between the transmitter and the receiver Noise Sources Systems Light which minimizes multipath dispersion and enhances the power Power consumption Medium Relatively low efficiency of the communication system. However, LOS links Multipath Fading YES NO suffer from shadowing. On the other hand, NLOS links rely on reflections from the walls, ceiling and other objects. They offer OWC systems can provide high data rates beyond 10 Gbps robust links and protection against shadowing but are severely [9], [11], [12], [13], however, there are many challenges to affected by multipath dispersion, which results in ISI and pulse implement OW systems. For instance, fog, rain and dust reduce spread [22]. data rates and coverage area of free space optics (FSO) outdoor Both LOS and NLOS schemes can be classified into directed, hybrid and non-directed links [22]. The NLOS non-directed 3 scenario is considered to be the most desirable method for transmitter faces up and produces a number of spots on the indoor systems from the point of view of mobility, however it ceiling [23]. In addition, the impulse response when using suffers dispersion which can limit the achievable data rate and different types of receiver configurations in VLC systems are is also limited by the low power collected. A very popular shown. A -tracing algorithm simulation package was diffuse system is the conventional diffuse system (CDS) [22]. developed using the same room configuration and parameters CDS employs a wide FOV receiver and a transmitter with wide used in [21], [23], [24], [25] and the simulation scheme was . The transmitter and receiver point towards the based on [26]. The room has length × width × height of ceiling and the received signal reaches the receiver through 8푚 × 4푚 × 3푚. The devices used for communication are multiple reflections. The CDS impairments can be reduced by placed on a “communication floor”, ie typical desk surfaces that using specific receivers, such as the triangular pyramidal fly- are one meter above the floor. In the IRC system, the transmitter eye diversity receiver which uses an optimum value of the FOV and receiver were located at the center of the room on the to mitigate ISI in indoor OWC systems [21]. However, non- communication floor, and the receiver specifications for the directed links do not have good power efficiency. Thus, IRC example were based on [27]. In the VLC system 8 light employing different degrees of directionality at the transmitter units were used as and each light unit has 9 LEDs and receiver (hybrid link) can give good performance [22]. where the light units are located in different positions on the LOS communication using directed links makes use of a ceiling and the receiver is located at the room center on the narrow radiation pattern between the transmitter and the communication floor. The locations of the light units, room receiver (FOV), see Fig. 3 [22]. This scenario reduces the dimensions (same as above) and receiver specifications of the impact of ambient noise and minimizes the path loss, while also VLC system are based on [24], [25]. Fig. 4A illustrates the improving the power efficiency. impulse responses of the most well-known indoor IRC systems Directed links are however not suitable for mobile (CDS, LSMS), Fig. 4B shows the impulse response of a VLC applications especially for indoor OWC usage. In contrast, non- system with a single wide field of view (WFOV) receiver, Fig. directed links use a wide transmitter radiation pattern and wide 4C shows the impulse response of a VLC system with three FOV at the receiver. Therefore this configuration can provide branch angle diversity receiver (ADR) and Fig. 4D shows the mobility for users (see Fig. 3). impulse response of the VLC system using a 50 pixel imaging receiver.

Fig. 3: Optical wireless link classification

B. Channel characteristics and modulation techniques This section discusses the indoor OW channel characteristics and the popular modulation schemes used.

1. Channel characteristics Fig. 4: (A) Impulse responses of CDS and LSMS systems with single element wide FOV receiver (퐹푂푉 = 90°) and LSMS systems with three Channel characterization in OWC is vital to better branches ADR; transmitter and receiver are located at the room center, (B) understand the system performance. The channel impulse Impulse responses of VLC system with single wide FOV receiver, (C) response ℎ(푡), which can fully describe the multipath Impulse responses of VLC system with three branch ADR, (D) Impulse propagation in the OWC system, is given by [23]: responses of VLC system with 50 pixels imaging receiver. 퐾 푦(푡, 퐴푧, 퐸푙) = ∑ 푅푥(푡) ⊗ ℎ ( 푡, 퐴푧, 퐸푙) 푘=1 푘 Fig. 5 (A) shows that the CDS has the highest delay spread 퐾 + 푅 ∑푘=1 푁푘 (푡, 퐴푧, 퐸푙) (2) and hence the smallest channel bandwidth compared to the other systems. The LSMS system results in a lower delay spread where 퐾 is the total number of reflecting elements in the due to the presence of the diffusing spots which act as room, 퐴푧 and 퐸푙 are the directions of arrival in the azimuth and secondary transmitters close to the receiver. Also as Fig. 5 (A) elevation angles, respectively. By estimating the impulse shows, reducing the receiver FOV, results in the collection of a response, many parameters can be determined, such as the root smaller range of rays and hence a larger channel bandwidth. mean square, delay spread, 3 dB channel bandwidth, spatial Table 2 provides a comparison of the OW channel power distribution and signal to noise ratio (SNR), [23]. bandwidths achievable using different OW systems[24]. The results are shown at x=2m and along the y axis. The minimum To give examples of the OW channel impulse response, we channel bandwidth is observed in the case of the CDS with wide show the impulse responses of CDS and of line strip multi beam FOV receiver, and is 41 MHz in the 8푚 × 4푚 × 3푚 room system (LSMS). The latter is a NLOS hybrid system where the using the parameters in [24]. The use of an imaging receiver in 4 conjunction with the CDS increases the minimum channel bandwidth to 160 MHz due to the use of narrow FOV pixels in the imaging receiver which reduce the range of rays collected. The imaging receiver however provides wide coverage through its many pixels which also result in increased collected power. The use of an LSMS transmitter results in multiple diffusing spots cast on the ceiling, which act as secondary emitters close to wherever the receiver is located. This increases the channel (A) (B) bandwidth. The use of power adaptation, where the spot closest to the receiver is allocated more power, reduces the delay spread further, resulting in a minimum channel bandwidth of 7.5 GHz as Table 2 shows.

Table 2: Comparison of the 3dB channel bandwidth of different OW systems [28] (C) Fig. 5: (A) Delay Spread of CDS and LSMS systems with single element wide FOV receiver (90°) and LSMS system with three branch ADR; transmitter and receiver are located at the room center, (B) Delay Spread of VLC system with single wide FOV receiver and three branch ADR, (C) Delay Spread of VLC system with single wide FOV receiver and 50 pixels imaging receiver.

Figs. 5 (B) and (C) show the delay spread of VLC systems with wide FOV receiver, angle diversity receiver (ADR) and imaging receiver. Both the ADR and imaging receiver employ (A) (B) multiple receivers with smaller FOV which reduces the delay spread. Full coverage of the room is achieved through the use of the multiple receivers in ADR and the many pixels in the imaging receiver. The smaller FOV of each receiver element in the imaging receiver used, results in reduced delay spread compared to ADR, as Figs 5 (B) and (C) show. (C) (D) Fig. 6: (A) SNR of CDS and LSMS systems with single element wide FOV The VLC system with an imaging receiver provides low receiver (90°) and LSMS system with three branch ADR; transmitter and delay spread as shown in Fig. 5. It also provides good SNR at receiver are located at the room center, (B) SNR of VLC system with single high data rates compared to the other systems as shown in Fig. wide FOV receiver and three branch ADR, (C) SNR of VLC system with single wide FOV receiver and 50 pixels imaging receiver at low data rate, 30 6. The oscillation in delay spread is due to the proximity (or Mbps, (D) SNR of 50 pixels imaging receiver at high data rate, 5 Gbps. otherwise) from the transmitters (lighting sources in VLC) as the receiver moves along the y axis. The oscillations in the SNR 2. Modulation techniques are due to similar effects and can be more significant in infrared OWC channels are different from traditional RF channels. systems where the room lighting can act as a noise source. The This has resulted in different methods of modulation being use of optimum receiver element selection or receiver element used. Modulation schemes that fit well in RF channels do not signal combining techniques in ADR [24] and imaging necessarily perform well in the optical domain. There are four receivers [24], [25] can result in more uniform SNR in the room criteria that guide the choice of a specific modulation technique where the best receiver element that avoids noise and/or for OWC systems. Of particular importance as a criterion to be interference for example is selected. applied, is the average power used (power efficiency) of a given modulation format. This is important in view of eye hazards and power consumption in mobile terminals. The second criterion is the available channel bandwidth and receiver bandwidth requirements. The third factor is the complexity of the modulation format (and power consumption in portable devices). The last set of factors relate to the physical limitations in the transmitter (i.e. LD or LED) which the modulation format may have to take into account. Modulation in OWC systems consists of two steps: in the first step the information is coded as a waveform(s) and then (second step) these waveforms are modulated onto the instantaneous power of the carrier [22]. 5

Intensity modulation (IM) and direct detection (DD) is the OOK and PPM are the most popular modulation schemes preferred transmission technique in OWC systems [9], [22]. IM applied in OWC systems [32]. is achieved by varying the bias current of the LD or the LED. b. Multicarrier modulation In OWC systems, the transmitted signal must always be More advanced techniques can be used in OWC systems to positive in . Direct detection is the simplest method transmit multiple carriers, such as subcarrier modulation that can be used to detect an intensity modulated signal. The (SCM). This can provide multiple access for simultaneous users photodetector generates a current that is proportional to the and a high data rate. However, SCM is not power efficient like incident optical power intensity. A simple description for the single carrier schemes [33]. For instance, each quadrature phase IM/DD channel is given as [22]: shift keying (QPSK) or binary PSK subcarrier requires about

1.5 dB more power than OOK. In [34] a high data rate was 푦(푡) = 푅푥(푡) ⊗ ℎ(푡) + 푅푛(푡) (1) achieved while reducing the average power requirements in

SCM modulation. In [35], orthogonal frequency division where 푅 is the photodetector responsivity, 푦(푡) is the (OFDM) was applied in indoor OWC systems to instantaneous photo current received, 푡 is the absolute time, ⊗ achieve high data rates over a noisy channel and to reduce ISI. denotes convolution, ℎ(푡) is the channel impulse response, 푥(푡) The system, however, did not achieve a high SNR. The main is the instantaneous transmitted power and 푛(푡) is the disadvantages of OFDM are the sensitivity to frequency offset background noise (BN), which is modelled as white Gaussian and phase noise as well as the high peak to average power ratio noise, and is independent of the received signal. (PAR) [36]. In general, the use of complex modulation leads to Three broad types of modulation schemes can be applied in an improvement in the performance of the OWC systems, such OWC (a) modulation, (b) multicarrier modulation and as mitigating the ISI effect and increasing the data rates. (c) multicolor modulation [14], [29]. However, these modulation techniques require a complex a. Baseband modulation . The main baseband modulation techniques considered in c. Multicolor OWC include (i) pulse modulation (PAM), (ii) pulse Multicolor modulation has recently been considered to position modulation (PPM), (iii) pulse interval (PIM) provide high data rates or multiple access for users [11], [14]. modulation and (iv) carrierless amplitude phase (CAP) White light can be generated from red, green and blue (RGB) modulation. type LEDs which means data can be transferred through each i. PAM color or . Wavelength division multiplexing PAM is a simple modulation technique which is widely (WDM) is the multiplexing used here. In addition, [37] considered in OWC systems. On-off keying (OOK) modulation achieved white light from four color LDs which provide a better is a type of PAM with only two levels. OOK is a suitable OWC result compered to three colors LEDs in term of multiple access modulation approach and is easy to implement in such systems and higher data rates due to the improved modulation due to the ability of LDs and LEDs to switch on and off quickly. capabilities of LDs. However, OOK is not efficient when compared with other modulation formats [14]. III. TRANSMITTER ARCHITECTURES IN OWC SYSTEMS ii. PPM PPM is another type of modulations that is widely considered This section presents a review of the available transmitter in OWC systems [17]. Differential PPM (DPPM) was evaluated architectures in OWC systems. The CDS system, the most in OWC system to increase the achievable data rates. It commonly investigated transmitter - receiver configuration, is consumes less power on average when compared to PPM. widely considered [4], [9], [21], [23]. A number of spot However, the in the DPPM signal is greater than in diffusing transmitter architectures were proposed and the PPM signal [30]. evaluated. A uniform multi-spot diffusing transmitter system iii. PIM was first proposed in [38]. While a diamond multi-spot PIM encodes the information by inserting empty slots between diffusing system configuration can be found in [39]. In addition, two pulses. Its design relies on accurate synchronization which there are two attractive methods for multi spot diffusing can increase its complexity compared to PPM. Digital pulse configurations: the LSMS and the beam clustering method interval modulation (DPIM) is a digital version of PIM. The (BCM) proposed in [23], [39], [40]. The last two methods have performance is improved in DPIM compared to PPM by also been widely investigated by other researchers [41] – [47]. removing the redundant frame space in PPM, however errors The results of these investigations are very promising. Different can propagate from a frame to the next [31]. transmitter structures for IRC are shown in Fig. 7. Table 3 summaries the main advantages and disadvantages of the iv. CAP different transmitter structures in OWC indoor systems. CAP modulation is a promising modulation format that can be used in OWC systems to achieve higher data rates. It is a bandwidth-efficient two-dimensional transmission scheme [14]. In CAP modulation, two orthogonal filters are chosen to modulate two different data streams.

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Reduced delay spread Single line of spots, hence and increased SNR poorer coverage in some areas Multi beam Improved coverage Increased complexity as transmitter through beam steering the optimum beam with Power and and optimum allocation direction has to be Angle of power to spots determined and the Adaptation optimum power allocation System Further reduction in has to used (MBPAAS) [44] delay spread and increase in SNR Beam Delay Adapting the Complex technique, but and Power differential delay simple implementation Adaptation between the beams algorithms given LSMS leads to transmitter pre- transmitter distortion and hence Fig. 7: Different spot diffusing transmitter structures in IRC systems; (A) (BDAPA- improved data rates Spot diffusing transmitter [4], (B) uniform multi-spot diffusing transmitter LSMS) [41] [47], (C) diamond multi-spot diffusing transmitter [38], (D) line strip multi- Beam steering Improved LOS User mobility is difficult beam transmitter [36], (E) multi-beam clustering transmitter [39]. transmitters component, hence [12] reduced delay spread Table 3: Comparison of different transmitter structures in IRC indoor and improved SNR systems

IV. RECEIVER STRUCTURES USED IN OWC SYSTEMS Type of Advantages Disadvantages Transmitter There are many types of receiver configurations for use in Spot Diffusing Provides link LOS links do not exist. OWC systems. The single element receiver with a wide FOV transmitter for robustness against (Signal is received due to (90o) is the most basic configuration. The use of a wide FOV CDS system [4]. shadowing and reflections from walls and receiver enables improved collection of the optical signal. On blockage. ceiling). the other hand, huge amounts of undesirable ambient light are High path loss. also received with the desired optical signal and this reduces the SNR. An angle diversity receiver (ADR) that employs narrow High delay spread. FOV elements aimed in different directions is an attractive

ISI. receiver architecture. Such ADRs were proposed in [49], [50] Uniform multi- Enhanced received High delay spread due to however the same FOV was used for all branches in the ADR. spot diffusing power, due to existence large numbers of beams. The authors in [21], [23] developed and optimized ADR [38]. of LOS link between structures using an optimum FOV in each receiver branch. They spots and receiver. ISI. also considered several diversity schemes, such as selection Good coverage area. combining (SC), maximum ratio combining (MRC) and equal Diamond multi- Provides good Signal power received is gain combining (EGC). The MRC approach can achieve better spot coverage of the sides very low in the room performance compared to the other methods [39]. An ADR has transmitter and corners of the center. several advantages: reduced effect of ambient light noise, [39]. room. High delay spread. reduced signal spread and improved SNR. These advantages are Line strip Significant LSMS suffers from delay realized as MRC does not ignore the unselected branches (as multi-beam improvement in the spread when a wide FOV SC does) where these branches can make a useful contribution. transmitter received power receiver is employed (see It also does not simply give an equal weight to all receiver (LSMS) [23]. compared to the Fig. 4A). techniques above. branches (EGC case) with no regard to their SNR. Its main Power received in the drawbacks are design complexity, high cost (separate Reduces ISI when sides and corners of the concentrator used for each detector) and bulky size. combined with angle room is very low. Significant performance improvements can be achieved diversity receivers. Does not provide full through an imaging receiver, which is one of the most attractive mobility in the room. receivers in OWC systems due to its reduced complexity Multi-beam Provides full mobility The delay spread is still compared to ADR (only one image concentrator (eg. ) is clustering for the optical receiver. high when using wide shared among all pixels), reduced transmitter power needed, transmitter FOV receiver (BCM) [39]. Increased power (see Fig. 4A.). reduced signal spread hence reduced ISI and therefore support received at the sides for high data rates. It also allows space division multiple access and the corners of the (SDMA) to be used as the image formed resolves the signals room (good coverage received from different directions. An imaging receiver was of the surroundings and room center). originally proposed in [51]. In [51] the imaging receiver Adaptive Line Improved allocation of Increased complexity to improved the SNR by 20 dB. Development and modification of strip multi- power to spots achieve optimum the imaging receiver can be found in [41], [28], [43]–[45], beam according to receiver transmitter power [52],[10]. where the impact of the number of pixels was transmitter location allocation (ALSMS)[28], considered together with change in the shape of pixels, use of [48], [43] multiple imaging receivers mounted in different directions and 7 power adaptation of transmitter power to fit the imaging lights produce most of their optical power outside the pass band receiver. These changes enhanced the performance of the of commonly used filters [16], [55]. overall system. Fig. 8 illustrates the main types of receivers. An Various techniques have been proposed and investigated to adaptive receiver was presented in [53] for reducing ISI. eliminate the effects of directional ambient light noise. For instance directional ambient noise can be reduced through the use of the fly eye receiver proposed in [38], which can optimally select a reception direction where ambient noise (for example from a window or spot light) is minimum [7], [58]. An imaging receiver can introduce better spatial resolution of signals and directional noise [41], [28], [43]–[45], [52],[10]. The authors in [21] proposed a pyramidal fly-eye diversity to reduce the impact of background noise. However, even with its great advantages, this design still needs more optimization in terms of azimuth and elevation angles, triangular base dimensions as well as number of sides. In [59], an ADR was used to mitigate the background noise. The main problem with this receiver is the size of the optical elements. In addition, the imaging receiver was investigated widely, and one of its main characteristics is reduced background noise [41], [43], [51], [60].

Fig. 8: Types of receivers used in IRC systems; (A) single element wide FOV (900) receiver, (B) angle diversity receiver employing narrow FOV elements and (C) imaging receiver using single imaging concentrator.

V. MAIN IMPAIRMENTS IN OW SYSTEMS OWC systems including IRC and VLC face many challenges. In this section we will cover the main impairments (A) in IRC systems. While, the VLC system impairments will be covered in Section VI. The major design limitations encountered in IRC systems include: background noise, multipath dispersion, transmission power restrictions and the large capacitance associated with large area photodetectors.

• Background noise OWC receivers detect the desired signal as well as background noise due to ambient light. The main sources of ambient light in OWC systems are sun light, fluorescent and incandescent bulbs [42]. The substantial amount of power emitted by these sources within these wavelength ranges is detected in the OWC system. This can introduce shot noise at the receiver as well as saturate the photodetector when their intensity is high [16]. The impact of ambient light can however (B) be reduced by using optical and electronic filters. Inexpensive Fig. 9: (A) Power spectra of the main sources of ambient light, (B) daylight filters can eliminate the components of the sunlight responsivity and transmission of a p-i-n photodiode employing below 760 nm [54]. Fig. 9 shows the spectrum of the different daylight filter [22]. sources of ambient light and the responsivity and transmission of a typical silicon p-i-n photodiode that employs a daylight • Multipath dispersion filter. The characteristics and effects of ambient noise on the The signal received through multipath channels may lead to OWC system transmission have been investigated widely in ISI. ISI is normally characterized through the delay spread of [16], [17], [53] – [56]. The work has found that the background the channel. Multipath dispersion and ISI have a huge impact noise contribution from fluorescent light is negligible compared on the performance of the OWC system. In [21], the optimum o to incandescent and sunlight light (see Fig. 9A). Fluorescent value of the FOV (60 ) was determined for an ADR in the presence of ISI and background noise. A considerable decrease 8 in multipath induced dispersion can be achieved through careful and maximize the collected power at the same time. The design of the detecting element’s (orientation and their) photodetector’s effective area can be enhanced by using a directionality. However, the main drawbacks of such a receiver hemispherical lens as suggested in [22]. Bootstrapping was (i.e. ADR) are their bulky size and high cost of fabrication. An proposed by authors in [27] to minimize the effective imaging receiver is a suitable alternative and can be effective in capacitance of the large area photodetector. combating multipath dispersion and ISI [41]. Beam angle and delay adaptation techniques were proposed by the authors in VI. VLC SYSTEMS [42], [61] to improve the link performance and reduce the In the past few years, the green agenda has made a huge mobility induced impairments. impact on the field of communications [70]–[79]. This has pushed many researchers to produce and investigate reliable • Transmission power restriction due to eye safety and energy efficient communication systems [80]–[91]. VLC Transmitted IR beams, if operated incorrectly, can cause systems are energy efficient communication systems as injury to eyes and/or skin. However, the damage to the eye is communications is a useful by-product of the lighting / more significant due to the ability of the eye to concentrate and illumination system. The main factors that fueled the growth in focus optical energy. The eye can focus light to the within VLC systems are the recent developments in solid state lighting the wavelengths 400-1400 nm [62]. The cornea (front part of (SSL), which already provides commercial high brightness eye) absorbs other wavelengths before the energy is focused. LEDs of about 100 lx - 200 lx [92], [93], with a longer lifetime There are a number of factors that determine the level of the (about six years) in comparison to conventional artificial light radiation hazard, for example, beam characteristics, operating sources, such as incandescent light bulbs (whose lifetime is wavelength, exposure level, exposure time and distance from about four months, continuous). In addition, SSL has fast the eye [18]. The optical power generated by LDs and high- response time (modulation rate), smaller size, low power radiance LEDs within the wavelength band 700-1550 nm may consumption and no known health hazards. Fig. 10 shows the cause eye damage if absorbed by the retina. The maximum practical luminous efficacy of different available light sources permissible exposure (MPE) levels are very small in this band [92]. because the incident light can be focused by the human eye onto The dual functionality of the VLC system, i.e. illumination the retina by a factor of 100,000 or more [63]. According to the and communication, makes it a very attractive technology for international electro technical commission (IEC) standards many applications, such as car-to-car communication via LEDs, [64], an IRC system that employs a laser source must not lighting infrastructures in buildings for high speed data transmit power in excess of 0.5 mW (class A) [64]. Different communication, traffic light management in trains and high transmitter technologies are used to meet the IEC standard data rate communication in airplane cabins. The first proposed when using LDs with high transmit power. Safety regulations VLC system was at Keio University in cooperation with have been revised downward by a factor of 100 when using a Nakagawa laboratory in Japan [70] – [72]. hologram [65], (typically computed by computer driven algorithms and known as a computer generated hologram (CGH)). In addition, using an optimum holographic diffuser leads to reduced path loss for the diffused signal [66]. An alternate solution can be found in [67] where an integrating- sphere diffuser for an IRC system, was designed. This diffuser is eye safe up to 0.84 W at 900 nm. Skin safety should be considered in IRC system design. Short term effects, such as skin heating, are considered in eye safety regulations, due to the power level regulations for eyes allowing lower power levels than the power levels allowed for skin [68].

• Photodetector capacitance Many OWC systems employ and direct Fig. 10: Comparison of different light sources in terms of luminous detection. The SNR of the DD receiver is proportional to the efficacy [92]. square of the received optical power. Furthermore, the A. VLC Standards transmitted power is limited by eye safety regulations and power consumption. These constraints mean that a In 2003, VLC system standardization was started in Japan photodetector with a large photosensitive area should be used [97]. The Japan Electronics and Information Technology to gather maximum power, to improve the SNR. Unfortunately, Industries Association (JEITA) proposed two standards based a large photosensitive area leads to high capacitance (as the on visible light communication in 2007. The first one is the photodetector capacitance is proportional to the area of the visible light standard (CP-1221) while photodetector). Large detector capacitance limits the available the second is the visible light ID system standard (CP-1222) receiver bandwidth due to the capacitance at the input of an [98]. Six years later, a new Japanese standard based on CP-1222 amplifier acting as a low pass filter. The authors in [69] was proposed, the visible light system standard (CP- proposed the use of an array of photodetectors instead of a 1223). single photodetector to mitigate the effects of large capacitance In 2007, the Smart Lighting Engineering Research Centre (ERC) was established in the USA funded by the National 9

Science Foundation (NSF) for developing solid state lighting in colour TV. The lower cost and complexity of the first method systems that have an advanced functionality [99]. makes it more popular. However, the modulation bandwidth is In 2011, the IEEE published the physical and medium access limited to tens of MHz due to the slow response of the phosphor control (MAC) layers as a standard for VLC systems, called which limits the communication data rate. Both techniques have IEEE 802.15.7 [100]. This standard can provide a data rate of been demonstrated in conjunction with VLC [107], [108]. Fig. up to 96 Mbps for transmitting video and audio services. The 11 depicts the methods used to generate white light from the standard also considers the mobility of visible light LEDs. communication links as well as noise and other interference sources. The IEEE 802.15.7 standard utilizes only the visible light spectrum and considers optical receiver specifications. In the same year, a technology called light fidelity (Li-Fi), which has features similar to the Wi-Fi technology but uses light as instead of RF, was launched [101]. Three years later, the IEEE 802.15.7 standard revised the standard by releasing an update, the IEEE 802.15.7m. The updated standard utilizes infrared, visible light and near ultraviolet wavelengths as medium, also, considering optical camera communications (OCC) and imaging receivers as Fig. 11: Two approaches used to produce white emissions from LEDs, (A) detectors [102]. In 2017, the IEEE 802.15.13 standard was phosphor LED method, (B) RGB method. published for providing data rates up to 10 Gbps with range up to around 200 m [103]. The standard is designed to support 2. LDs point-to-point and point-to-multipoint communications that can Laser diodes (LDs) can be used as sources for illumination be in both coordinated and non-coordinated topologies. One and communication in VLC systems. The experiment in [37] year later, in 2018, the IEEE announced a new light shows that mixing different LD colors can provide a white light communications task group for devolving a global standard in source. Fig. 12 shows the experimental setup used to generate wireless local area networks based on visible light white color using 4 different LDs colors. The four laser colors communication. The standard, named IEEE 802.11bb, intends are combined using chromatic beam-splitters. The combined to engage with manufacturers, operators and end users to build laser colors reflected by a mirror pass through a ground- the system [102]. diffuser which reduces speckle before the illumination. B. VLC sources The VLC system structure is similar to that of IRC systems. However, the main essential difference between VLC and IRC systems is the function of the transmitter. The transmitter in VLC systems is used as an illumination source as well as a communications transmitter for data users. Therefore, the VLC network designers must consider the following two requirements. Firstly, the visible light transmitter (LEDs or LDs) should work as sources of illumination. The optimum Fig. 12: Generating white light using 4 different LD colors [37] lighting configuration and LED angles have been studied in previous work [104], [105]. According to the international A simple physical layer block diagram of a VLC system is standards organization (ISO) and European standards, the shown in Fig. 13. Digital and analogue components are used in illumination should be 300 lx -1500 lx for an indoor office the transmitter and the receiver. The data stream, baseband [106]. Secondly, the data should be transmitted through LEDs modulator and digital to analogue convertor (DAC) are the without affecting the illumination. digital components in the transmitter. Similarly, the digital An essential advantage of VLC systems is that they can components in the receiver are the analogue to digital converter provide communication and illumination at the same time. (ADC), demodulator and data sink. The trans-conductance White light can be generated by using LEDs or LDs. The amplifier (TCA), bias Tee and LEDs are the analogue following sections describe each source. components in the transmitter. The receiver includes the photodiode (PD), trans-impedance amplifier (TIA) and band 1. LEDs pass filter (BP). The AC signal in the transmitter is added onto At present, there are two different approaches generally used the DC current by a bias Tee (a device used in a high frequency to generate white light from LEDs. In the first method white to produce a DC offset). Since LEDs work in light is generated using a phosphor layer (emitting yellow light) a region with unipolar driving currents, the absolute driving that is coated on blue LEDs. The light emitted by blue LEDs current (AC+DC) has to be larger than zero. The total current is (λ≈470 nm) is absorbed by the phosphor layer, where the blue fed to the LEDs to emit the modulated output power. The power wavelength excites the phosphor causing it to glow white, then received by the PD is converted into a current (I-PD), and this the phosphor emits light at longer wavelengths. The second current consists of two components, the AC and DC parts. The method uses RGB LEDs. With the correct mixing of the three AC component is amplified and then filtered using TIA and BP colours, red, green and blue, white light can be created such as 10 respectively. Finally, the is demodulated after conversion using ADC. where Ω is the spatial angle and ∅ is the luminous flux. From the energy flux (emission spectrum), ∅e and ∅ can be calculated as [96]:

780 ( ) ∅ = 퐹 ∫380 퐴(휆) ∅푒 휆 푑휆 (13)

where 퐴(휆) is the eye sensitivity function and 퐹 is the maximum visibility, which is equal to 683 lumen/Watt (lm/W) at 555 nm wavelength. Assuming that the LED light has a Lambertian radiation pattern, the direct luminance at any given point in an office can be estimated as:

푐표푠푛(휃) 푐표푠(훾) Fig. 13: Block diagram of VLC system. 퐸 = 퐼(0) (14) 퐷2 Brightness (light dimming) control for the LEDs is required in VLC systems as these sources are also used for illumination. where 퐼(0) is the centre luminous intensity of the LED Many methods have been proposed to implement dimming measured in candela (cd), 휃 is the irradiance angle, 퐷 is the control [109]–[112]. The simplest method is amplitude distance between the LED and the point of interest, 훾 is the modulation (AM) dimming: the luminous flux is controlled by angle of incidence and 푛 is the Lambertian emission order, controlling the input DC current. However, the chromaticity defined as [22]: coordinates of the emitted light can change [113]. Pulse width 푙푛 (2) modulation (PWM) is another method that controls the width of 푛 = − (15) the current pulse. The main feature of PWM dimming is that the 푙푛 (푐표푠 (훷1 )) 2 amplitude of the pulse is constant. The width of the pulse varies according to the dimming level, thus resulting in the emitted 1 where 훷 is the semi angle at half power of the LED. light spectrum being constant. Binary code modulation or bit 2 angle modulation (BAM) is a good dimming method that was Using the relations above, the illuminance distribution is introduced by artistic licence engineering (ALE). It is a new determined in a room with dimensions 5m×5m×3m, where four LED drive technique that can be used in VLC systems [114]. LED light units with specifications similar to those in [96] were The main advantages in using this technique are used. Fig. 15 illustrates the illumination distribution. Note that implementation simplicity, the potential to reduce flicker the illumination is within the acceptable range of (300 lx -1500 compared to PWM, lower processing power and simple data lx). recovery operations. BAM encoding for LED dimming makes use of the binary data pattern shown in Fig. 14. In the 8 bit BAM system, the pulse width of bit 7 (most significant bit, (MSB)) is equal to 27=128, whereas the pulse width of bit 0 (least significant bit, LSB) is equal to 20=1 (a unit width). This allows continuous brightness levels to be defined where the granularity of the dimming levels depends on the number of bits used in BAM.

Fig. 15: The distribution of illuminance, Min. 315 lx, Max. 1220 lx.

C. Design challenges in VLC systems VLC systems are a potential candidate to achieve multi gigabits per second data rates in indoor wireless systems. There are however several impairments that VLC systems have to deal Fig. 14: BAM signal waveform used in dimming control. with. These impairments include the ISI caused by multipath propagation, the low modulation bandwidth of the transmitter As mentioned earlier, the primary function of LEDs is to (i.e. LEDs), and the provision of an uplink connection for VLC provide illumination, so it is essential to define the luminous systems. intensity, the quantity used to determine the illumination level. To get comfortable office lighting, a certain amount of 1. ISI in VLC systems luminance is required (300 lx -1500 lx) [106]. The brightness In real environments, there is more than one transmitter of the LED is expressed as luminous intensity. Luminous (LEDs), and these transmitters are used for illumination as well intensity is defined as the luminous flux per as [96]: as communication. The VLC receiver therefore collects signals 푑∅ 퐼 = (12) from more than one transmitter, and each signal arrives using 푑Ω 11 potentially a different path with a different delay profile. In scheme used is very important in VLC systems because it addition, due to the reflections from the walls, ceiling and other affects the achievable data rate, BER and luminance. For objects, the received optical signal suffers dispersion due to example, multiple PPM was used for modulation and dimming multipath propagation. This results in ISI, especially at high control [111]. Different types of modulation were evaluated in data rates. In the literature, many techniques have been terms of their combined modulation and dimming control proposed to mitigate ISI in VLC systems. Among the most capabilities. The results showed that overlapping PPM (OPPM) notable solutions is the use of return-to-zero OOK (RZ-OOK) can achieve flexible luminance and good performance [127]. modulation with the space between pulses being used as a guard Higher data rates can be achieved when complex modulation interval to reduce the effect of the delay spread in low data rate approaches are used. For example, Discrete Multi-tone applications [115]; while, in [116] the authors used OFDM to modulation (DMT) can provide data rates of about 1 Gbps reduce the ISI. has also been considered to [108]. This type of modulation requires however a complex combat ISI, however, it reduces the bandwidth efficiency as transceiver. In terms of parallel transmission, VLC systems well [117]. The authors in [118] used zero forcing (ZF) have naturally favorable features for OMIMO as many LEDs equalization with transmitter (i.e. LED) arrangements to reduce (transmitters) are used for illumination and different data the effects of ISI. The bit error rate (BER) achieved using this streams can be sent on every single LED to maximize the technique was similar to the channel without ISI. In [96] it was throughput. At the same time, an array of photodetectors can be found that decreasing the receiver field of view leads to reduced used at the receiver. This setup offers improvements in security, ISI, whereas increasing the data rate leads to an increase in ISI. link range and data rates while the power required is unaltered The use of adaptive equalization using the least mean square [128], [129]. Detailed study of OMIMO revealed that channel algorithm has the ability to reduce the effects of ISI and was cross talk is a critical limiting factor in VLC systems [130]. A investigated at data rates up to 1 Gbps [119]. The authors in significant enhancement in the data rates can be achieved with [120] used MIMO approaches to reduce shadowing effects. RGB LEDs. A 1.25 Gbps system was reported in [131] using Other approaches can be used to reduce ISI in VLC systems. RGB LEDs in a single color transmission mode, and 1.5 Gbps For instance, using pre and post equalization techniques, was achieved using a new micro LED (μLED) array with non- coding, angle diversity receivers, and imaging receivers [10]– return-to-zero OOK (NRZ-OOK) as the modulation scheme. [13]. The 3 dB modulation bandwidth of this LED was 150 MHz [132]. The maximum data rate achieved using commercial RGB 2. Low modulation bandwidth of the LEDs LEDs with low complexity modulation (OOK) was 500 Mbps The modulation bandwidth of the transmitters (i.e. LEDs) is [133]. Recently, a 3 Gbps VLC system based on a single μLED typically less than the VLC channel bandwidth, which means with OFDM modulation was successfully demonstrated [134]. that the former limits the VLC data rates. The highest throughput achieved by LEDs was reported in To achieve high data rates in VLC systems a number of [107], which reported an aggregate throughput of 3.4 Gbps different techniques can be used, for instance, optical filters, pre using DMT, wavelength division multiplexing (WDM) and and post equalization (or both), complex modulation techniques RGB LEDs. The design and implementation complexity are a and parallel communication (optical MIMO, (OMIMO)). A major concern in these systems however. blue at the receiver can be used to filter the slow response yellowish component, and this approach is probably 3. Provision of an uplink connection in VLC systems the simplest and most cost effective approach to increase the Using white illumination LEDs for communication is VLC data rates [121]. However, the achieved bandwidth is naturally one directional communication (downlink). So, insignificant (8 MHz). The drop in the white LED response can providing an uplink connection can be a major challenge. Many be compensated for by using a simple analogue pre-equalizer at techniques have been proposed to provide an uplink for VLC the transmitter, and this technique was shown to offer 40 Mbps systems. In [135], the use of retro reflecting with a without the use of a blue filter [122]. However, the data rate corner cube modulator to provide low data rate uplinks was achieved is still very low compared to the available VLC proposed for visible light communication as shown in Fig. 16A. spectrum. A moderate data rate (80 Mbps) can be achieved In [136], [137] an IR transmitter was proposed to provide an using more complex pre-equalization [123]. Pre-equalization uplink for VLC systems. In [138] the authors suggested that RF however has the drawback that the drive circuit of the LED systems can be employed in conjunction with the VLC system needs to be modified, and this leads to higher costs and lower to provide reliable bidirectional communication as shown in emitter efficiency (i.e. not all the input power is converted to Fig. 16B. However, all the above techniques add complexity to light). By combining simple pre and post equalization, 75 Mbps the VLC systems and do not provide a high data rate uplink can be achieved [124]. Both analogue and digital equalization connection. Recent research demonstrated high data rate bi- techniques can be applied. Analogue equalization techniques directional VLC transmission with 575 Mbps downlink and 225 are suitable for OOK modulation, while digital techniques are Mbps uplink using SCM with WDM [139]. In addition, other preferable for OFDM [125]. Using least mean square (LMS) recent research proposed a solution for the uplink VLC system and decision feedback (DFE) equalizers at data rates above 200 using IRC. As a result, the uplink data rate reached 2.5 Gbps Mbps was shown to be very effective to mitigate ISI [119]. The [140]. use of such equalizers however leads to a computationally complex receiver structure. A data rate of 100 Mbps - 230 Mbps was the maximum data rate achieved using phosphorescent LEDs with simple OOK modulation [126]. The modulation 12

E. Multiplexing techniques OFDM and WDM are two of the most widely considered multiplexing techniques in VLC systems. Their main features are summarised here. 1. OFDM The use of OFDM in VLC systems offers the ability to combat ISI. High data rates can be achieved by dividing the available bandwidth into sets of orthogonal subcarriers [147]. However, OFDM signals suffer in-band and out-band distortion due to the high peak to average power ratio (PAPR). This flicker in the power leads to reduced device lifetime and eye safety problems [148]. Numerous approaches have been proposed to realize optical OFDM (O-OFDM), ie approaches that satisfy the IM/DD constraints. These include direct-current (DC) O-OFDM

Fig. 16: Two techniques used to provide an up-link connection in VLC (DCO-OFDM) [36], asymmetrically clipped (AC) O-OFDM systems, (A) retro reflecting transceivers and (B) bidirectional VLC system (ACO-OFDM) [149], Flip-OFDM [150] and Hermitian combined with an RF system. symmetry (HS) free (HSF)-OFDM (HSF-OFDM) [151]. D. VLC system architectures 2. WDM Popular wireless system architectures used in RF wireless WDM allows the multiplexing of different data sources on systems have also been explored in VLC systems. Fig. 17 different wavelengths and then into a single channel. It illustrates four system architectures: single input single output promises great flexibility and bandwidth efficiency, as it did in (SISO), single input multiple output (SIMO), multiple input the case of fibre optic communication systems. It is feasible to single output (MISO) and multiple input multiple output apply WDM in VLC systems, as white light is composed of a (MIMO). The number of inputs and outputs in RF combination of colors which can be produced using colored communication systems is dictated by the number of antennas LEDs (RGB) or colored LDs (BGYR). Colored LEDs can be at the and the receiver, while, in VLC systems it is based individually modulated and an optical multiplexer then on the number of sources (LEDs, or LDs) and the number of transmits the parallel data. At the receiver the signal is passed optical receivers. through a de-multiplexer (optical filters used as de-multiplexer) and then optical receivers are used (each optical receiver thus detects a certain wavelength) for data recovery. Fig. 18 shows a WDM VLC system. By using LEDs (RGB), a WDM data rate of 3.22 Gbps was reported in [152]. Another demonstration was reported in [153] of a VLC system that employs WDM. The use of LDs (BGYR), in VLC systems was proposed in [11] resulting in a data rate of 10 Gbps.

Fig. 17: VLC system architectures (A) SISO (B) SIMO (C) MISO and (D) MIMO.

MIMO approaches were widely investigated in VLC systems to provide high data rates and to support multiple users [117] – [122]. Imaging and non-imaging receivers are investigated in VLC systems [141]. A hybrid diffusing IR transmitter was Fig. 18: WDM used in VLC systems (A) using LEDs and (B) using LDs. studied to support VLC systems when the light is turned off or dimmed [52]. In addition, the authors in [142] developed The effect of the change in the correlated colour MIMO VLC systems by combining transceivers and dimming due to modulation has not been substantially explored. An control. important feature of the source of light is its apparent colour as a perfectly white source when viewed. This feature can be quantified using chromaticity coordinates on a chromaticity diagram. The International Commission on Illumination (CIE) 13 created in 1931 the RGB colour space diagram shown in Fig. there are four techniques based on FDMA that have been 19. The circumference of the area includes the coordinates of investigated [157]–[159]. all the real colours. A black body radiator at different The first method uses single carrier FDMA (SC-FDMA) produces different colours. The white colour is which is based on frequency division multiplexing. SC-FDMA defined by the region close to the black body radiators locus was proposed in [159]. It reduces the value of the peak-to- (starting at approximately 2,500 K). RGB hues are located average power ratio (PAPR) and that is one of its main within regions that span from the white region towards the advantages [160]. corresponding corners of the figure. Sources that are very close The second technique is orthogonal frequency division to the Planckian locus may be described by the colour multiple access (OFDMA) which was studied in [157]. temperature (CT) [154]. OFDMA is a multi-user approach based on OFDM modulation. The chromaticity coordinates can be defined as [154]: It is an extension of OFDM which means each user can employ a group of subcarriers in each time slot. OFDMA was evaluated 푛 ∑푖=1 푥푖∅푖 in a number of studies [161]–[163]. The data rate achieved 푋 = 푛 (16) ∑푖=1 ∅푖 when using OFDMA can be reduced because of spectrum partitioning [164]. The third technique is orthogonal frequency and division multiplexing interleave division multiple access (OFDM-IDMA), which was proposed in [157]. OFDM-IDMA 푛 is also a multi-user technique based on OFDM modulation. It is ∑푖=1 푦푖∅푖 푌 = 푛 (17) a scheme that combines OFDM and IDMA technologies. Both ∑푖=1 ∅푖 techniques: OFDMA and OFDM-IDMA are asymmetrically clipped at zero level after OFDM modulation. OFDM-IDMA where ∅푖 is the radiant flux and 푥푖 and 푦푖 are the primary sources’ coordinates. From Fig. 19, a white colour can be provides good power efficiency compared to OFDMA. In obtained from two colour sources (blue and yellow, see dotted addition, it was shown in [157] that signal to noise ratios (SNR) line in Fig. 19). In addition, a white colour can be obtained from above 10 dB in a system with modulation size equal to 16, three sources (see solid triangle in Fig. 19). OFDM-IDMA provides better results compared to OFDMA. It is shown in [157] however that the decoding complexity and PAPR are lower in OFDMA compared to OFDM-IDMA. The last technique is interleaved frequency division multiple access (IFDMA) which was proposed in [158]. IFDMA was introduced to reduce PAPR compared to OFDMA. In addition, the effect of the non-linear characteristics of LEDs is lower in IFDMA which can provide a higher power efficiency. The computational complexity needed is lower in IFDMA compared to OFDMA as IFDMA does not include discrete Fourier transform or inverse discrete Fourier transform operations. Moreover, it was shown in [165] that IFDMA is a Fig. 19: 1931 CIE chromaticity diagram [154]. promising multiple access technique in VLC due to its ability to mitigate multi-path distortion and reduce synchronization F. Multiple access schemes errors. Several multiple access (MA) schemes have been studied for use in VLC systems. Some of the MA schemes that have been 3. CDMA used in RF systems are also used in VLC systems. These CDMA is a non-orthogonal multiple access scheme which include time division multiple access (TDMA), frequency can offer high spectral efficiency compared to OFDMA and division multiple access (FDMA), space division multiple TDMA. Each user in CDMA uses a special code to provide access (SDMA), code division multiple access (CDMA) and simultaneous transmission and reception. The special code used non-orthogonal multiple access (NOMA). by each user is typically an optical code; an example was 1. TDMA proposed in [166]. In addition, random optical codes were TDMA for multi-user VLC systems was proposed and proposed in [167], [168] to support a large number of users by investigated in [155]. It is also sometimes used for preventing spreading the signal bandwidth. A synchronization technique collisions where other MA schemes may fail, such as situations was reported in [169] to help prevent the undesired correlation where two transmitters have the same distance to the receiver characteristics over random optical codes. As a result, a good [29]. A controller can share time between the transmitters in a performance was achieved. medium access control (MAC) protocol, which may also allow The authors in [170] proposed color shift keying (CSK) as a uneven time splitting between trasnceivers. The throughput of technique based on CDMA that can increase the multiple access TDMA schemes decreases if the number of users increases. As capacity. Consequently, each transmitter can gain 3dB a result the system cost may rise due to the need for compared to OOK. synchronization between transmitters [156]. Multi-carrier (MC) is another approach investigated in [171], 2. FDMA [172] based on CDMA. MC-CDMA combines features of FDMA is a scheme that can support multiple access based on CDMA and OFDM. MC-CDMA diffuses the data symbols of orthogonal frequency bands for example. In VLC systems, each user in the frequency domain over OFDM subcarriers. 14

Subsequently, the sum of the data symbols from multiple users Table 4 provides a summary of the multiple access schemes are re-modulated in the time domain using OFDM. studied in the context of VLC. Additionally, the authors in [171] showed that the transmitted optical power can be reduced by using sub-carrier selection. Table 2: Summary of VLC multiple access schemes Another multiple access technique based on CDMA was proposed in [173] which provides interference-free links while Scheme Summary transferring information. The technique allows users to decode TDMA can help prevent collisions between users in VLC their signals without pre-confirmation. However, this technique systems. The per user throughput decreases when the number TDMA of users increases and resources can be wasted during user was investigated in a small area less than one square meter inactivity. Accurate synchronization is needed between the which reduces its practical utility. transmitters which increases cost and complexity [156]. The power allocation issue in CDMA was studied in [174]. Four main FDMA techniques have been studied in VLC Two types of distributed power allocation were proposed which systems: SC-FDMA, OFDMA, OFDM-IDMA and IFDMA. SC-FDMA reduces the PAPR. However, it may not be the best are partial and weighted distributed power allocation, which can approach at high data rates and with a large number of users. be optimized. The result of the study showed that the proposed OFDMA and OFDM-IDMA can provide better throughput. techniques offer low computational complexity compared to the OFDMA offers lower decoding complexity and lower PAPR optimized optimal centralized power allocation approach. compared to OFDM-IDMA, however it can introduce spectrum FDMA partitioning which may reduce the data rate. The power Different CDMA schemes were proposed in [175] that utilize efficiency in OFDMA is high compared to OFDM-IDMA. micro-LED arrays. These schemes provide a higher modulation Moreover, at some SNRs and modulation sizes, OFDM-IDMA bandwidth which can also support a large number of users. has better throughput compared to OFDMA. IFDMA can mitigate multi-path distortion and can help reduce the impact of synchronization errors. The computational 4. SDMA complexity and PAPR in IFDMA are lower compared to their SDMA can provide multi-user support in VLC systems. It values in OFDMA. was shown in [176] that increasing the number of transmitters Special and random codes are utilized. Random code were in SDMA VLC systems increases the throughput. Moreover, proposed to increase the number of users. Synchronization techniques were added over Random codes to improve the the system capacity was shown to improve more than ten times system performance. CSK is another technique that was when SDMA is used compared to TDMA VLC systems. In CDMA proposed in conjunction with CDMA. It can provide 3dB gain [177] a low complexity suboptimal algorithm was introduced for each user compared to OOK. Techniques were proposed for coordinated multi-point VLC systems with SDMA based on CDMA to provide interference-free links, but in small areas while other approaches were reported to increase the grouping. The proposed technique offers an improvement in the achievable data rates and number of users. system performance and fairness in throughput. SDMA is not widely studied in VLC systems to date, but is a very promising scheme. One technique based on SDMA was SDMA 5. NOMA evaluated and shown to offer an improvement in the system NOMA was studied in [178], [179], [180] as a promising performance and throughput fairness. NOMA can provide non-orthogonal multiple access in VLC technique that can provide multiple access in VLC systems. It systems. It can outperform OFDMA in terms of data rate and NOMA is also called power domain multiple access. NOMA differs system capacity. MIMO NOMA provides further improvement from other multiple access techniques which provide in data rate, capacity and spectral efficiency [144]. orthogonal access for multiple users either in frequency, time, G. Mobility in VLC systems code or phase. Each user in NOMA can use the same band at the same time. Thus, the power level can be used to help One of the requirements in indoor communication systems is distinguish users. In addition, the transmitter in NOMA applies to provide support for the expected forms of user mobility. To superposition coding to simplify the operation at the receiver. do this with minimal impact on the system performance As a result, channels are separated at the receiver for both adaptation may be needed in the transmitter and receiver. In uplink and downlink users [179]. NOMA was also studied in IRC systems mobility can be supported through the use of some recent work as a means to support multiple users at high adaptive LSMS, adaptive BCM, angle diversity receivers and data rates [164], [179], [181], [182]. imaging receivers [28], [43]–[45], [48] which when used NOMA outperforms OFDMA in terms of the achievable data together can enable the IRC system to adapt to the environment. rate in VLC as was shown in [179]. In addition, it was shown However, adaptive techniques such as LSMS and BCM cannot in [183] that the system capacity is improved when NOMA is be directly applied in VLC systems due to the dual functionality employed. MIMO NOMA VLC systems were studied in [160]. of the VLC source (i.e. illumination and communication). The results indicated that this approach can offer high data Mobility may be supported in VLC systems by using improved rates, high capacity and higher spectral efficiency. An receivers such as angle diversity receivers or imaging receivers experimental study was reported in [164] where a MIMO [10]–[13]. NOMA VLC system was demonstrated. A normalized gain Beam steering approaches are already applied in FSO difference power allocation (NGDPA) method was proposed in systems and can also be utilized in VLC systems. VLC beam the experiment to provide an efficient and low complexity steering can be achieved by using electronically controlled power allocation approach. The experimental results showed mirrors in front of the receiver and receiver tilting. One that NOMA with NGDPA can result in up to 29% sum rate inexpensive approach used to provide good link quality during improvement compared to NOMA with the gain ratio power mobility uses mirrors with piezoelectric actuators in front of the allocation scheme. receiver [184]. This method however, leads to a bulky receiver and cannot be used in mobile devices. Fig. 20 illustrates this 15 technique. Another approach is the tilting of the transmitter and receiver together using piezoelectric actuators that are controlled electronically. As with the mirrors method, the tilting method also needs to be controlled using electronic circuits. The tilting approach has two different structures as shown in Fig. 21A. In the first structure all of the transceiver (LED and photodetector) are placed on actuators and in the second structure, in Fig. 21B only the of the transceiver are placed on the actuators. The advantage of the second structure is the ability to zoom in and out, which means the FOV can be changed [184].

Fig. 22: Hologram and illumination produced.

I. Security in VLC Systems Security in VLC system can be divided into two categories, namely physical layer security and data layer security, ie Fig. 20: Beam steering using a mirror. encryption. 1. Physical Layer Security VLC systems offer physical layer security for the user information, as light signals cannot travel through opaque objects such as walls. Therefore, an attacker cannot get access to the medium unless they are in the visible range for the VLC communication system. This makes VLC systems more secure than an RF communication system [29]. VLC physical layer security was studied in [163.5], [163.6]. 2. Encryption (QKD) can help secure data in OWC systems. QKD can use photons to exchange a secret key between two points. Therefore, it can increase security in OWC Fig. 21: Beam steering using tilting method, (A) all body placed on actuators, (B) only lenses placed on actuators. system by encrypting the transferred data. The secret key can be exchanged for example using a one-time pad (OTP) H. Hologram Design for VLC Systems cryptographic algorithm as well as the characteristics of Holograms were recently studied in VLC systems to provide quantum physics. Eve is restricted by the quantum theory from a high data rate and to improve the SNR [12], [13], [185], [186]. storing or copying transferred photons via the quantum channel A hologram is a transparent or reflective device that can be used because of the changes in the photon characteristics. After with VLC sources. It modulates the amplitude or phase of the exchanging the secret key, the data are encrypted by the secret signals that passes through it. Consequently, the light rays are key and transferred via classical communication. A wireless diffused by splitting the light into beams to cover the required QKD approach was examined in an indoor environment [187]. area, as shown in Fig. 22. The authors in [12] used a CGH in their study to achieve a data rate of up to 20 Gbps. In addition, J. Future trends for VLC fast CGH (FCGH) adaptation was proposed in [13] as a new VLC systems are rapidly developing, however, before VLC method that can support user mobility. The data rate was also becomes widespread, many issues should be resolved, such as increased up to 25 Gbps, and the SNR was improved. the identification of the optimum modulation techniques and

the best dimming control methods, together with the optimum multiple access approaches and standardization. Currently, VLC systems do not fit with most types of LED lighting systems, as there are different drive mechanisms and architectures. So, collaboration between communications organizations and the solid state lighting (SSL) is necessary to establish standards for SSL systems to be used for illumination and communications [188]. 16

VII. MAIN APPLICATIONS OF OWC SYSTEMS imaging (MRI) scanners, therefore VLC systems can be a OW is generally used in three applications area: office potential solution in such scenarios. , access and personal communications. A. Main applications of IRC systems To date, there have been many applications that use infrared OW systems. A key application area is personal communications. OW personal communications (short range applications) are widely studied due to their low complexity, high bandwidth and low cost. Two main factors that make short range OW systems preferable from the users’ point of view are the security features (light is contained in the environment Fig. 23: Indoor VLC navigation system for visually impaired people. where it originates) and the potentially high data rates these Recently, various methods for VLC indoor positioning were systems can achieve. In addition, lower transmission powers studied [195], [196]. VLC systems are strong candidates for can be used and a data rate of 1 Gbps has already been indoor positioning due to several features. Firstly, better standardized in the Giga IR standard with data rates up to 10 positioning accuracy (few millimetres) can be achieved, which Gbps planned [189]. A range of devices incorporated IrDA is better than the accuracy achieved by systems. The ports, however these were short range and relatively low data added accuracy is mainly attributed to the lower interference rate. This led to the development of many subsequent standards, and multipath effects suffered by VLC systems and the short most notably the recent effort by the IEEE to develop an OW wavelength used. Secondly, VLC positioning systems can be physical layer option in IEEE 802.11, namely the planned IEEE used in environments where radio positioning systems are 802.11bb standard [190]. The IrDA standard can be considered restricted, such as industrial settings with large metallic obsolete as it was not recently updated and the maximum data structures [197] or in hospitals [198]. rate is low. The IrDA standardized short range infrared OW for point to point communication (half duplex mode), with direct VIII. VLC experimental demonstrations and products LOS being the preferred configuration for IrDA links. The This section provides a brief overview of recent VLC projects IrDA link architecture does not cater for user mobility. The and products. standard was created in 1993 by IrDA, which includes more A. VLC experimental demonstration than 160 members with the majority of these being manufacturers of hardware, peripherals, components and 1. Ultra-parallel visible light communications (UP-VLC) software [191]. This standard provides interoperability between project different software and hardware devices using wireless IR. The UP-VLC project started in October 2012 and concluded in between 2004 and 2008 more than 600 million devices were February 2017. It involved six research groups at 5 institutions produced based on IrDA technology, and this number used to and was funded by the UK Engineering and Physical increase by 20% every year according to [191], [192]. Personal Research Council (EPSRC). Its vision was built on the unique communication devices that use IrDA transceivers include: capabilities of gallium nitride (GaN) LEDs which can provide printers, PDAs, electronic books, digital cameras and good illumination and high data rate optical communications. notebooks (most kinds of communications and computing New forms of spatial multiplexing were implemented where devices using IR). Other OW indoor systems were developed, independent communications channels were provided by for example those that use Optical Camera Communications individual elements in high-density arrays of GaN based LEDs [193]. [199]. The project combined the strengths of researches in the UK in complementary -oxide- (CMOS), nitrides, organic semiconductors and communications systems B. Main applications of VLC systems engineering to provide a new wireless communication Indoor VLC applications include applications for data technology that transfers data through visible light instead of communications, indoor location estimation, indoor navigation RF (Wi-Fi) [200]. Multi-Gb/s VLC was shown as a possibility for visually impaired people and accurate position for the first time by using GaN LEDs and novel multiplexing measurements. schemes were explored [200]. • Indoor applications Subsequently, color-tunable smart displays were developed Like IRC systems, VLC systems can be used to transfer data using CMOS electronics combined with GaN based micro- in offices and indoor spaces. The user location can be estimated LEDs. The modulation bandwidth of the LED pixels of these in an indoor environment using white LEDs. For example, color-tunable displays is 100 MHz which provides a indoor spaces can be illuminated by LEDs with a unique ID for wavelength-agile source simultaneously for high-speed VLC each LED. White LEDs in conjunction with geometric [199]. integrated in smart phones can help visually impaired people move inside buildings [194]. Fig. 23 shows a prototype 2. Visible light based interoperability and networking navigation system for visually impaired people [194]. RF (VisIoN) project signals can be undesirable inside hospital environments, The VisIoN project started in September 2017 and will especially in operating theatres and magnetic resonance conclude in September 2021 and is led by Centrale Marseille in 17

France. The project focuses on synergizing optical 6. Designing Future Optical Wireless Communication communications technologies with wireless communication Networks (DETERMINE) project technologies. It is built on three key technical work packages The project started in June 2013 and concluded in May 2017, (WPs) with a proof-of-concept demonstration for each WP. The and was led by Linkopings University in Sweden. The project first WP focuses on smart cities, offices and homes where focused on developing concepts, algorithms, models and everything in the home or city is connected wirelessly. As such, architectures that exploit the optical spectrum and had a focus the communication network must provide high data rates, high on EU exchange of knowledge between the partners of the reliability, high availability and low . VLC is proposed consortium in the physical layer and the other OW layers [205]. in this project instead of RF communication systems as the project expects VLC to provide improvements in terms of the 7. Wireless Optical/Radio TErabit Communications number of devices that can be supported and the achievable (WORTECS) project capacity. Also, VLC is expected to offer localization and The project, with a time window September 2017 to August diverse control with motion detection for these devices. The 2020, led by Orange, France will develop ultra-high data rate second WP has a focus on smart transportation, with to wireless systems by combining high radio frequency vehicle (V2V) and vehicle to infrastructure (V2I) communications with optical wireless communications in the communication through VLC. The last WP considers regions of infrared and visible optical spectrum by utilizing and medical uses of VLC. In manufacturing, heterogeneous networking concepts. Two Proof-of-Concept VLC can provide unique safety and security features. In demonstrators will be delivered in the project: an ultra-high medical applications, VLC is expected to offer interference density LiFi/ with multi-Gbps data rates, and an immunity compared to RF. In addition, it is expected that the ultra-high data rate system capable of delivering Tbps VLC systems studied will offer , networking [206]. localization and sensing together with high bandwidth, high reliability, robustness and low latency [201]. B. VLC’s products

Light Fidelity (Li-Fi) helped propel OW exploitation bringing 3. Super Receivers for Visible Light Communications project it closer to end users and standards. pureLiFi has a commercial The project was established in August 2017 and will conclude product, LiFi-XC, that can transfer data using indoor lights. The July 2020, led by University of Oxford. It focuses on creating a system is full duplex and bi-directional. It uses visible light for more sensitive new super receiver. This super receiver is downlink communication and infrared for the uplink. It can designed to overcome sensitivity and data rate limitations and offer up to 43 Mbps with fully networked Li-Fi with mobility will offer easy alignment. It can be easily integrated onto any allowing users to move around the environment. The pureLiFi device due to its characteristics (thin and flexible). The project device can be separated from, or integrated into the computer plan is to test the receiver in free space visible light [207]. In addition other products include those from Oledcomm communications links to quantify its performance. The project which provides a receiver, LiFiNET® Dongle. This receiver estimates that the new super receiver will lead to a hundred can connect the computer to the Internet and can offer a data times improvement in the performance compared to the existing rate around 10 Mbps [208]. Signify, a new Philips lighting receiver by offering higher data transmission rates, longer company, offers a Li-Fi system with excellent light quality and transmission distance and lower noise [202]. 30 Mbps data rate for two way communications [10]. VLNComm provides an advanced Li-Fi adapter called LUMISTICK LIFI ADAPTER. This adapter connects devices 4. Multifunctional Polymer Light-Emitting Diodes with to the Li-Fi network offering 108 Mbps for downlink Visible Light Communications (MARVEL) project communication and 53 Mbps for uplink communication [209]. The project started December 2016 and is due to conclude

November 2019. The project will introduce new Polymer Light- Emitting Diode designs with new approaches that can IX. OPEN RESEARCH ISSUES maximize the light efficiency [203]. This section summarizes new areas of research that deserve further attention in IRC and VLC systems. 5. Terabit Bidirectional Multi-user Optical Wireless System (TOWS) for 6G LiFi project A. Open research issues in IRC systems The project will take place between January 2019 and Despite the notable solutions introduced for IRC systems to December 2023, and is led by the University of Leeds working provide mobility, mitigate ISI and achieve high data rates, low with the Universities of Cambridge and Edinburgh. The project complexity and easily implementable techniques are still will develop and experimentally demonstrate multiusers Tbps required. Areas of further investigation in IRC systems are optical wireless systems. The developed systems will provide listed below. capacities at least two orders of magnitude higher than the • Mobility can degrade the performance of CDS and LSMS existing and planned 5G optical and radio wireless systems. In systems. However, using certain techniques, such as beam addition, a roadmap will be developed to achieve capacities up delay adaptation, power adaptation, beam angle adaptation to four orders of magnitude higher than those of existing 5G and imaging receivers, can enhance the system wireless systems [204]. performance. However, experimental verification of these techniques has not been carried out yet. Real time 18

adaptation in these systems may lead to very complex provide an up link connection for users and enable solutions. Therefore, investigation is needed to assess these communication between users. techniques in realistic IRC systems to provide mobility and • Further studies are necessary to answer questions relating to achieve multi-gigabit per second data rates. In addition, to the optimum dimming control and optimum modulation further investigations are needed to find new approaches to for VLC that provide a good balance between support mobility with minimum complexity. communication and illumination. • Power, angle and beam delay adaptation have been • Providing mobility for VLC systems is vital for indoor evaluated for a single user scenario. An open area of users. The ADR and imaging receiver evaluated in IRC research is the use of these techniques in multiuser systems can be used to offer mobility for VLC systems and scenario. Multiuser scenarios are necessary for the wide to mitigate the ISI problem at the same time. Our previous use and adoption of these OW systems. work in this area has shown that significant enhancements • In multi-user OW systems, the optimum allocation of can be achieved when using an imaging receiver [24]. resource such as wavelengths, time slots, beams, imaging • LDs can be used for illumination and communication in receiver pixels, and ADR facets is an open research area. VLC systems instead of LEDs in order to enable higher • In the case of a multiuser scenario, scheduling can also be data rates. Initial results have shown that data rates of tens used with the techniques above to maximise throughput, of gigabits per second per user and more can be achieved reduce interference, reduce power consumption, increase [10], [24], [25]. Further work is needed in this area to reliability, reduce delay or optimize with respect to other optimize the transmitter and receiver devices, their metrics. properties and architecture. • Efficient use of the optical transmitted power is key in IRC • Due to the energy efficiency, scalability and flexibility of systems due to the power transmitted in IRC systems being VLC systems, they can be used in next generation data restricted by eye safety. Increasing the transmit power can centres replacing hundreds of metres of cables or optical however lead to improved SNR if interference is properly fibre. Investigations in this area are required to examine the managed. High SNRs can allow simple receiver structures benefits of applying VLC systems in data centres. to be used reducing the system complexity. Therefore, • The main frontier in VLC and OWC now is networking. effort is required to identify methods that can enable the This includes networking and MAC techniques applied in use of high transmit power in a safe way, such as the optical medium to optimize beams, receiver detectors holographic techniques. fields of view and orientations, and to determine the • The optimization of an imaging receiver can lead to low optimal allocation of resources that include wavelengths, complexity and easier implementation. To the best of our space and time resources. The optimum front haul and knowledge, there are no high-speed imaging receivers to backhaul network architectures are open questions that date that have been specially designed for mobile IRC deserve substantial attention to deliver and collect data systems. At high data rates most of the components will from optical luminaires or from IRC sources and detectors. probably be adopted from the optical fibre domain, which • The use of learning and artificial intelligence in is not ideal for IRC systems; thus, more research is needed VLC and OWC is important and timely to carry out the in this area to design and fabricate new devices for IRC optimum resource allocation described above, to enable the systems. network to self-configure and self-adapt taking into • Applying new techniques, such as OMIMO, in account interference, capacity, power consumption, combination with advanced modulation (Optical OFDM throughput and reliability and availability into account. and other) may further enhance the data rates achieved in IRC systems. X. CONCLUSIONS B. Open research issues in VLC systems In this paper, we have presented an overview of OW systems The main current research focuses on modulation techniques, that emphasized their deployment to provide indoor mobility, uplink connectivity, dimming control and communication. We illustrated how OW technologies can offer illumination, increasing the achievable data rate, improving the a complementary technology for future wireless modulation bandwidth of transmitters and mitigating ISI. communications in indoor applications, co-existing potentially with RF systems. • Uplink channel provision is an important issue. The Two distinct categories of OW systems (IRC and VLC) proposed solutions, such as the use of IRC, corner cube systems were discussed. Link design, modulation techniques, reflectors or RF systems, unfortunately do not fit well with transmitter/receiver structures, possible challenges, the main the VLC system goal to provide high data rate in the uplink applications and open research issues for each category have and down link as these techniques may conflict with been identified. providing mobility and minimizing the size of the VLC IRC systems can provide extremely high data rates and can devices (portable device). Therefore, new schemes to support mobility. These are however still significantly provide an uplink for VLC are required. Intermediate challenging due to the indoor OW channel nature. However, stations (relays) have been studied previously for use in a suitable techniques such as the use of the space dimension variety of networks to increase the coverage area and the through the introduction of suitable beams, beam angle, beam capacity. 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