Fibre optic-VLC versus laser-VLC: a review study

The exponential increase in the demand for a fast and reliable wireless mobile network motivates the need for researchers to engage more actively in visible light communication (VLC) technology. This increase happens in parallel with the augmentation of the population to be accommodated. As a result, the radio frequency spectrum is overcrowded. VLCs, which are part of optical wireless communications, are unregulated ultra-wideband spectrums that offer an alternative to solve RF bandwidth exhaustion. The OWC has demonstrated, among others, a throughput of 100 Gbps and is capable of offloading large data traffic from RF networks. One of the VLC main backdrops is the distance for which light signals can be transmitted, which is generally short in nature. However, this is overcome by its combination with other technologies, such as fibre optics (FO) and laser technologies, to mention only a few, which form a cascaded system with VLC. The data offloaded is fed to the cascaded system of FO-VLC and laser-VLC. In this paper, we provide a comparative review of cascaded FO-VLC and laser-VLC systems. We survey the technology that can provide optimized communication and high bandwidth. The choice of cascaded system to be used or considered is dependent on the area of application. A comparison is conducted from the simulated results of both cascaded systems considering parameters such as the gain and throughput. The FO-VLC cascaded system shows that system gain always decreases as distance increases for a range of reflection coefficient, ρ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\rho$$\end{document}, and slightly decreases with attenuation coefficient, α\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha$$\end{document}. The gain of the laser-VLC cascaded system decreases with an increase in transmission distance for a given range of floor and ceiling reflection coefficient and decreases slightly with attenuation intensity, γ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma$$\end{document}. From our investigation, it is clear that FO-VLC and laser-VLC are outstanding optical cascaded systems, which can be exploited in applications with a harsh environment. This research also highlights standardization efforts, advanced modulation schemes and coding techniques so far for VLC, FO and laser system and their promising applications.


Introduction
Radio frequency (RF) is a technology which uses electromagnetic signals to transfer data. RF is made up of waves of electromagnetic radiation with frequencies ranging from 3 kHz to 300 GHz. It usually occurs at the speed of light and propagates through air and many other materials; it has been widely used in many industries such as televisions, broadcasting systems, radars, computers and most mobile platform networks, just to mention a few. Communication using RF is affected by the limited channel capacity and transmission rate. To achieve a high data rate beyond 10 Gbps, it is hard using the band-limited RF spectrum. This is because of the exponential increase in the demand for fast and reliable mobile network that happens in parallel with the augmentation of the population to be accommodated.

3
The request for data rate has increased exponentially with the augmentation of the number of users. To contain this deficiency of RF spectrum extinction [1], there are series of techniques adopted which include multiple-input and multiple-output (MIMO), cell splitting with the handover and advanced modulation technique [2]. From the statistics provided by Corps Information Systems Control Officer (CISCO) mobile, the rate at which data traffic will be affected is estimated to 27 times increase within this half decade (2016 to 2021) [3]. However, in the 1990 s, optical wireless communication (OWC) technologies were not yet massively exploited. They came into play later when transmitting and receiving devices were readily available at very low-cost [4]. Then, it was important to adopt ultra-fast OWC such as visible light communication (VLC) [5][6][7].
For the past three decades, research has been done on OWC systems, which are capable of supporting data traffic at a higher rate when compared to its counterpart RF systems [5,8].
VLC is an OWC, which is intensively investigated nowadays. As an antenna, it uses light emitting diodes (LEDs) [9] and laser diodes (LDs) [10], which are also main sources of light. Comparatively, LDs illuminate with higher lighting range than LEDs.
First, a well-implemented VLC system is capable of delivering a sufficient data rate that can support audio and video multimedia services. However, VLC technology is utilized in networks that requires a transmission of small distance between the LEDs/LDs, transmitter and photo-diode (PD), receiver in indoor environments. The VLC applications in such environment are based on standardized local network known as optical wireless personal area network (OWPAN) [11,12]. Second, the frequency range of a laser light goes from far infra-red to ultra-violet on the electromagnetic spectrum. When comparing LEDs and LDs with regard to their priorities in OWC, LDs are more enhanced and may be used for a medium range data transmission [13]. Their illumination coverage is in kilometres, which by far exceeds that of the LEDs, which ranges within hundreds of metres. Laser light communication provides a medium range access system with the help of LDs when combined with VLC. Some recent work done on a hybrid communication system of VLC over the past years which are presented by authors of the following publications [14][15][16][17][18].
In telecommunications, Gallium Arsenide Phosphide (GaAsp) laser is mostly used. Recently, research has shown that LD can light up to a range of more than 2000 m at 180 lumen, while their LEDs counterpart can barely reach 200 m at 100 lumens [19]. For this reason, LDs are preferably exploited in front light bulbs of vehicles. For the same reason, LDs are utilized in access network using VLC technology [20]. Third, fibre optics (FO) is exploited in numerous applications. Recently, there has been an exponential increase in the need for information traffic due to data utilization in technologies such as internet of things (IoT), multimedia services, voice over internet, data, and video communication. It is important to make available a huge data bandwidth medium which will be capable of handling this huge amount of information. FO, which is capable of simultaneously transmitting a large amount of data, is a saviour for such bandwidth predicament and deficiency. Communication with FO occurs at optical frequencies using a light source which is modulated and transmitted in a dielectric wave-guide at a very high speed. As in the case of VLC, both LEDs and LDs are used in fibre optics and the communication can extend to thousands of kilometres as in the case of long-haul transmission to access networks and equally directed to the users (fibre to the home).
It will be interesting to assess what have been done in this exciting field when fibre and laser technologies are combined with VLC for efficient data transmission. In most applications, VLC-based OWPAN may be fed by a light beam or FO systems using amplify-and-forward (AF), decode-and-forward (DF), and selective DF (SDF), to mention only a few [11]. Due to the short range transmission of light, some cooperative communication techniques, which have been widely used in RF systems, are also applicable in VLC. Most common relays scenarios are DF and AF. While DF relay decodes the information, re-modulates and retransmits it to the receiver, AF amplifies and retransmits the received signal with no processing. In RF systems, the signal over the channel is symmetrical (positive and negative), while in optical the signal is strictly positive. As such, traditional relays such AF are accepted in VLC as it improves reliability. Also, modern spectral relay techniques could be adopted, usually in coded systems such as incremental DF and incremental selective DF [11]. Decisively, considering cost effectiveness and reliability, we confidently consider using AF over other relays scenarios in our uncoded cascaded VLC systems due to the fact that it has no signal processing between sub-channels.
The remainder of the paper is arranged as follows: Section II presents the literature review with diagrammatic and explanatory system models, modulation schemes and coding techniques exploited in these systems. Section III presents the cascaded system comprising of FO-VLC and laser-VLC. Section IV is based on comparison of some simulated results. Sections V depicts the standardization efforts so far in VLC, fibre and laser and VI gives critics and concluding remarks on the different cascaded systems. This study provides a comparative review of FO-VLC and laser-VLC. We analyse parameters like the gain, power, bit error rate (BER) and cost-effectiveness, just to mention only a few of both systems.
The VLC system is an emerging and promising technology to solve the RF spectrum shortage. Based upon LEDs and LDs, VLC can be a prominent and available solution to accommodate the upcoming high data traffic of wireless and wired networks. VLC is an emerging communication technology which involves a light-source that is being modulated at high frequency to transmit data. As shown on the electromagnetic spectrum, it is depicted that visible light wavelength ranges from 350 to 800 nm, which corresponds to frequencies between 4.3 × 10 14 Hz and 7.5 × 10 14 Hz. In [30], Qi Tang et al. proposed a similar contribution which demonstrated in a hybrid fibre-visible laser light communication (fibre-VLLC) system. They used an adaptive modulation scheme and discrete Fourier transformspread was used to reduce the peak-to-average power ratio and the impact of imbalanced SNR distribution VLC with its great features such as greater bandwidth, limited health threats, limited power consumption, with a non-licensed bandwidth. It has attracted a lot of research that has led to the realization of numerous practical applications of VLC, including those listed in 2.1.1. Note that FO and laser technologies have also attracted a lots of researchers. Some of their applications are given in 2.1.2 and 2.1.3, respectively.
In the following sections, we provide a quick view of VLC, FO and laser applications.

Visible light communication
A few concepts, technologies and applications proposed in VLC, with the most prominent listed below: • Light fidelity H. Haas in 2011 first coin the word light fidelity (Li-Fi). This technology is a high data traffic network with bi-directional functionality and a wellconnected VLC systems. Li-Fi is similar to wireless fidelity, that utilizes RF for data communication. Li-Fi is a leading technology in highly sensitive electromagnetic radiation areas such as aircraft and operation theatre. A Li-Fi also lends support in IoTs. • Vehicle-to-vehicle communications VLC may be applicable in communication between vehicles because of the ability to use already existing vehicle light and traffic infrastructure. Most important application indicated by vehicle safety communication project such as cooperate forward collision warning prior to accidents, pre-crash sensing emergency which triggers an electronic brake, lane changing warning, left and right turn assistant, and curve speed warning [31]. With the high speed of VLC, this low latency communication is applicable. • Aqua communication VLC is suitable for such communication as RF is not able to travel perfectly under water. Therefore, VLC may be an ideal technology to be used in aqua communication networks [32]. • Hospitals Most hospitals are sensitive to electromagnetic waves and measures should be taken to move from RF with electromagnetic radiation interference to VLC as it does not interfere with radio waves and the hospital equipment [33]. • Billboard Information displayed on billboards usually comprises of an array of LEDs which are modulated to display or convey a message in train and park stations, airports, high ways and other locations where such information is vital. • Visible light identification systems Visible light is exploited in identification system such as building and subways. For instance, by simply identify the room numbers in a building. • A sound communication system LEDs is used for transmission of music signals. As such, sound-activated lights are design to change its LEDs' colour and orientation based on sounds picked up by an internal microphone. The result is a light that reacts to the sounds around it. • Indoor positioning system Recently, there has been an impediment in positioning and tracking as global positioning system suffer effects of attenuation. Indoor positioning VLC has become a promising and attractive technique for indoor positioning. Indoor positioning VLC uses location information with respect to a set of positions of reference of LED for a given space to give accurate information of that location.

Fibre Optics
Most prominent applications of FO are listed as follows • Medical In medical applications, FO is used as light guides, imaging tools. Also, FO is very important in carrying out some surgical operations. • Data Storage FO is exploited as hydrophones for seismic waves and sound navigation and ranging; it light nature makes it very important in wiring aircraft, submarines and other vehicles.
• Telecommunications In general, fibre laid provides a communication medium for transmitting and receiving information. • Industrial/Commercial In industrial and commercial fields, FO is used for imaging in areas where electromagnetic interference poses a challenge, FO is also used for sensory devices to make temperature, pressure and other measurements. • Broadcast/community access television Companies use FO for broadcasting and also for wiring community access television, high definition television, internet services, video services and other applications.

Laser technology
We list below some applications of laser technology.
• Laser is use in computer devices like laser mouse, laser presentation, and other accessories; • Lasers are also used by astronauts and other communication networks; • Lasers serve a great tool in medical field, such as surgery and health; • Most sophisticated weapons are made using laser such as war machines, guns and tanks; • Lasers are used in cutting heavy matters in metallurgy companies and other related industries; • Most industries use laser in devising modern robotics, especially in image processing.

Modulation schemes and coding techniques in WOC
We present an overview of the modulation and coding techniques that will be used in the next generation of high-speed wireless optical communications to deliver multi-terabit bandwidth. We provide modulation and coding techniques for VLC, FO, and laser systems. When such an approach is correctly implemented, we will have a maximum throughput at the cascaded system's access point.

Modulation schemes
Modulation techniques can be classified into single-carrier, and multiple-carrier. Schemes exploited in VLC technology include orthogonal frequency division multiplexing (OFDM), colour shift keying (CSK), on-off keying (OOK), variable on-off keying (VOOK), pulse width modulation (PWM), pulse position modulation (PPM), variable pulse position modulation (VPPM), MIMO and M-ary pulse position modulation. In [34], two types of modulation schemes were chosen for comparison. These schemes were OOK and frequency shift keying (FSK), and it was found that OOK was cost-effective with a considerable BER. Rana Shaaban et al. proposed a CSK-code division multiple access (CDMA) VLC system where CDMA data1 and data2 added up to form the CSK signals and sent by an array of red-green-blue RGB-LEDS, to two users, with two distinct mobile-phone cameras [35]. Each user is capable of decoding its own data with specific CDMA spreading code. The simulation results in [35] show that the CSK-CDMA VLC network is capable of providing many users with simultaneous communication using many orthogonal spreading codes when compared with its counterpart OOK, while maintaining interference avoidance. Yuxian Gong et al. in their research [36] proposed a space shift keying (SSK) modulation scheme for VLC combined with MIMO. Their research was based on several transmitters exploited but only one is active at any given time in a one-to-one mapping. The authors prove that performance of the system is enhanced when the element spacing between the LEDs is increased in the single indoor deployment with a better BER in a low channel path correlation [36]. In [37], sparseactivated sub-carrier OFDM index modulation and interpolated sparse-activated sun-carrier OFDM index modulation are presented to increase the spectrum efficiency. Simulations show both schemes presented an improved spectral efficiency and BER when compared to other conventional index OFDM modulation with a gain of 5 dB. The authors in [38] provide a comprehensive review on different kind of modulation techniques, such as OOK, presenting it in an analytical and simulation to get the BER and signal-tonoise ratio (SNR) for PWM, OFDM, CSK, generalized SSK, optical asymmetric modulation with their respective advantages and disadvantages. Authors in [39] implemented a VLC system by using an alternating current-direct current power factor as a correction converter by using a dimmable property changing from 10% to 90% for OOK-M-FSK, also the system is proven to be robust with VLC systems and it's applications, a data rate of 1.11 Mb/s is obtained over a transmission distance ranging up to 20 m, with the help of other external light sources. Also, in [40], the authors established VLC system for indoor application for nonreturn-to-zero (NRZ)-OOK, with data rate of 10 kbps at a distance ranging up to 10 m. Again, [41] presents a VLC system capable of transmitting data at a speed ranging up to 115.2 kbps for a transmission distance of 1 m utilizing OOK modulation and MATLAB as simulator. The need of choosing the right modulation technique is crucial and the basic key of building a system which is flexible and having a greater optic fibre network capacity.
There are a couple of reviews on FO communication offering some critical features, which includes efficiency and resistance to electromagnetic interference. Sheng Li has done a review on emerging technologies with FO data communication bandwidth for the next-generation broadband networks [42]. Liaw and Shin [43] in their attempt to investigate which fibre laser technology is suitable and resistant for transmission, made an overview of a linear cavity tunable fibre lasers. With a laser source, different wavelength could be chosen by scanning over a wavelength range to achieve wavelength tunable. Each of the designs could achieve 0.1 nm tunable resolution.
Shashi Jawla et al. propose a study on the performance of different modulation format which include amplitude shift keying (ASK), FSK, phase shift keying (PSK) and polarization phase shift keying (PPSK) [44]. Proposed that if the optic fibre communication channel uses a multilevel signalling, one or more bit per symbol will be transmitted using M-ary ASK modulation scheme and when using multiple phases obviously more than one bit per symbol is transmitted when making use of M-ary PSK modulation format. In [45], the authors proposed an overview on experiment for a 10.7 Gb/s polymethyl-methacrylate polymer optical fibre with large-core of 1 mm. The authors proposed that in an optic fibre communication channel, a low latency, low power consumption with low complexity, simple NRZ is the primary choice. They also proposed an overall performance of pulse amplitude modulation added to feed-forward equalization or decision-forward equalization at a better complexity in addition with an added advantage of a small speed and inexpensive components, this makes such modulation schemes a powerful and promising solution for next-generation network of 10 Gb/s power over fibre systems [45].
For any laser that is connected to the current source may be modulated depending on whether it is an analogue or digital application. The modulation is in response to the changes within the cavity of the laser. The output usually produces an amplitude modulation, optical frequency or frequency modulation and pulse modulation. A couple of some advanced modulation techniques are used in laser technology. To obtain a 50 GHz bandwidth, a more energy efficient internal modulated laser sources known as direct modulation laser (DML) is used. During this modulation, there is a 3 dB power loss and some other propagation losses. This always becomes a problem when it requires to transmit higher bit rate. Recently, this modulation has been able to approach that of externally high-speed modulated laser [46].

Coding techniques
Indoor VLC system is characterized by transmission using LEDs in small distances, which are free from atmospheric degradation. The performance of a VLC system is affected by interference due to multipath propagation. The overall system can be enhanced with increased capacity performance without an increase in the transmission power, if a careful signal processing is done and advanced coding techniques are exploited. Researchers have in the past, proposed techniques with multiple LEDs, MIMO [47] techniques for VLC systems, exploited space-timeblock coding (STBC) [48,49] and repetitive coding (RC) techniques [47,50,51]. In [52], a comprehensive lineof-sight (LoS) and non-light-of-sight (NLoS) numerical performance analysis conducted with a STBC and RC to investigate the coding technique that is suitable for VLC system. Abdulmalik Alwarafy et al. in their work pointed out that RC outperforms STBC in a single photo-detector reception scenario with a maximum likelihood detection. However, when a MIMO technique was exploited, STBC performs better than RC having an extra complexity at the receiver for a LoS scenario. The authors in [50] proposed a binary system using Alamouti STBC having one carrier communication receiver with an outdoor image-sensorbased VLC network. The work in [51] demonstrated a multiple-input single-output (MISO) VLC network using RC technique by adding the pilot bit which permits reliability in blind approximations of channel coefficients. The authors in [47] presented linear space codes used in an indoor VLC MIMO having two transmitters and multiple receivers. In [53], the authors proposed an RC and spatial modulation (SM) coding scheme by optimizing the way LEDs are placed and the power of the LEDs for a 4x4 MIMO configuration while obtaining a unified SNR with respect to the BER and bits per symbol. The data rate in  each polarization per symbol period for FO communication channel can be improved by a joint design of modern coding techniques coupled with a modern modulation, for instance, coded modulation and forward error correction [54]. All of them are already a crucial factor in FO communication network standards. Table 1 shows some very crucial modulations and coding techniques offered in the field of VLC, FO, and laser technology. Error correction coding is an essential signal processing technique to improve the channel bandwidth of optical communication systems. The ITU-T G975 recommended Reed Solomon (RS) (255, 239) code, which are mostly applicable in long distance or long-haul fibre optical communication networks. The proposed code offered a net gain of about 4.3 dB with an output BER of 10 −8 after the coding correction. Monte-Carlo also proposed an analysis for RS (255, 239) code with 6.7% redundancy simulation and theoretical performance over a white Gaussian noise (AWGN) communication channel having a hard decision decoding and BPSK signalling. Mohammed Abdul Alwahab et al. in [55] performed a comparison between the candidate of RS codes in terms of coding gain. The RS (255, 235) code performs almost twice with the redundancy compared to the RS (255, 247) code which offers about 1 dB coding gain. RS (255, 223) offers a 2 dB coding gain with 4.5 times more redundancy in coding than RS (255, 247) code (Fig. 1).

The architecture of VLC systems
There are two major integral parts of a VLC system, we have the transmitter and the receiver. These parts are made of three common layers: the physical, MAC and the application layer. For the purpose of this paper, only two integral parts will be discussed for simplicity.
The transmitter part of the VLC is usually the source of light. The development of LED lighting makes the technology much more interesting using solid-state lighting. The efficiency of LEDs, as well as the white light they emit and wavelength, make LEDs the best choice for a VLC light source. There are many spectra for which light is produced by LED light. The commonly used ones for producing white light is the trichromatic commonly known as RGB [14,56].
The receiver of VLC systems usually consists of optical filter, optical concentrators and amplification circuits. The light generator from the transmitter is usually weak due to beam divergence. The signals are then picked up by the optical concentrator and amplifiers the signal. The signal is then picked up by the photo-diode and converts it to current value. This model is depicted in Figs. 2, 3, 4.

Cascaded systems with VLC
As mentioned earlier, several technologies can be combined with VLC to form a cascaded system: Power line communication (PLC)-VLC, FO-VLC, RF-VLC, for example. The reason of this combination is multiple. The most significant reasons are related to the fact that VLC stands alone only for short-distance transmissions. However, reasons related to security are also accounted. In this section, we focus only on FO-VLC and laser-VLC, which are the main targets of this paper. The mathematical modelling will be derived from Fig. 5.

FO-VLC system model
In [21], authors proposed a system that is composed of two main subsystems, Fig. 1, with each having its transmitter and receiver. The initial sub-system, FO, comprising of FO-Tx and FO-Rx has access to data traffic from the "cloud", while the second subsystem VLC, comprising of FO-transmitter (FO-Tx), FO-receiver (FO-Rx), VLCtransmitter (VLC-Tx), VLC-receiver (VLC-Rx). VLC-Tx is made of one or several LEDs, which produce light rays. The latter are detected by the mobile terminals (MTs), which constitute the VLC-Rx. As shown in Fig. 1, data collected from RF by FO-Rx are re-transmitted through a 5 km distance to a VLC access point (AP) down to the indoors MTs. Considering the fact that, the link between FO (outdoor) and VLC (indoor) has no data processing, this system is said to be based on an AF strategy. The amplification gain (Fig. 3) reduces the attenuation that is experienced in the optical fibre link. The channel is hybrid with the transfer function of the bridge considered as unity [21]. Communication in the FO sub-channel is governed by where X FO is the alphabet at the FO-Tx, Y FO is the detected alphabet, H FO is the FO channel transfer function and N FO is the noise introduced at the bridge.
The gain of a FO channel is given by [21] where k is known as the propagation constant, = 2 f is called the angular frequency, n is the refractive index, is the absorption coefficient of the fibre, and D is known as the dispersion coefficient. H FO is as a function of the transmission distance L, depicted in Fig. 6, for different values of attenuation.

Communication in the VLC sub-channel is governed by
where Y VLC is the detected alphabet at MT, X VLC is the alphabet at the VLC AP, H VLC is the VLC channel transfer function, and N VLC is the noise received by MT. In the VLC sub-channel, the transfer function is made up of (i) the LoS and (ii) and the NLoS (diffuse link) parts [21]. (1) The equations describing LoS and NLoS are well elaborated in Eq. 4 and Eq. 5.
, respectively, where A r and A room are the effective PD area and the area of the room, respectively, is the irradiance angle, is the average walls reflectivity, is the Lambertian order given by = − log 2 (2)∕ log 2 ( 1∕2) and the incidence angle. Note that f o is the 3 dB cut-off frequency of a pure diffuse channel. H VLC is the VLC channel frequency response shown in Fig. 7, for different values of , of the indoor walls, floor and ceiling reflection. The transfer function of the VLC subsystem is the summation of the LoS and NLoS links, given by The overall communication channel is governed by By combining Eqs. 1, 3, and Eq. 7 is given by Comparing Eq. 7 and Eq. 8, we can deduce that  Finally, the overall transfer function for the FO-VLC cascaded system can be deduced and is given by In Fig. 8, the gain of the cascaded system of FO-VLC is such for a reflection coefficient, , ranging from 0.2 to 0.95, the transfer gain decrease slightly with frequency up to 1 GHz over a distance (L) of 5 Km.

Laser-VLC system model
Here, the end-to-end system is also made up of two main subsystems, composing of a transmitter and receiver. Again, the first subsystem serves as a backbone network to the second subsystem. The laser subsystem uses LDs, and the VLC subsystem uses LEDs. The backbone is exploited in a peer-to-peer (P2P) topography. A couple of possibilities are exploited which include (i) single colour LD-LED system, (ii) multiple colour LD-LED system, and (iii) hybrid combination. By considering a single colour LD-LED system, the original data from the RF cloud is then transmitted using the laser light beam which uses a single colour light source. The light is detected by the laser light detector (LLD) included in the Laser-Tx. The detected light is then amplified and re-transmitted over LED light beam and finally detected by the PD. Various modulation techniques such as OOK [57,58], VPPM [57,58] and OFDM may be exploited [57][58][59].
In this system, Fig. 4, we have two transfer functions, H LL and H VLC , which all correspond to the outdoor and (10)  [21] indoor channels, respectively. It is assumed that there is no processing between the sub-channels, the transfer function of the bridge is unity. The outdoor sub-channel is governed by where X LL is the message from the source, Y LL is the message at the laser side, and N LL the AWGN. The outdoor part is made up of a single mode Gaussian beam stochastic channel governed by [60] (11) where A l is the effective area of laser receiver, is the small angle beam divergence, L is the range of transmission of the fibre and is the intensity of the attenuation coefficient (Fig. 9).
The laser channel gain of the outdoor environment, as shown in Fig. 10, decreases with an increase in transmission distance. The parameters used in the analysis were as follow: 0 ≥ L ≤ 5 km, A l = 4.8 m 2 , = 3.∕1000 , = 0.5.∕1000.
For the outdoor-indoor channel transition the messagesignal gotten by the LLD is then convoluted by channel frequency response of the system. Therefore, the message received by the LED follows Eq. 11.
For the indoor channel, the signal and noise that comes from the outdoor environment of the system are convoluted by the indoor impulse response. The transition in the overall system is the same as that of FO, governed by Eq. 8, where H = H VLC H LL and N = H LL N LL + N VLC . Assuming that no attenuation is observed at the bridge, the transfer function of the bridge is unity. It is seen that both LD and the LED light transmit data in a similar model, as they both obey the optical propagating principle. As such they have a common transfer function. Again, for the outdoor laser P2P model, the transmitter and receiver are both fixed and there exists no path loss. Thus, eliminating the diffuse part of the channel. Then, the outdoor of the P2P transfer function is governed by Contrary to outdoor P2P model, the indoor LED channel takes into account LoS and diffuse (NLoS) links parts. Hence, the indoor channel model is governed by Here, LoS and Diff represent the LoS and NLoS links, respectively. The overall cascaded LL-LED model is gotten by doing a convolution of the outdoor channel with its indoor counterpart. Hence, the transfer function H is governed by Finally, the overall transfer function of the LL-VLC cascaded system is given by

Comparison
Generally, the gain of FO-VLC is such that the cascaded system always decreases as the distance of the signal being transmitted increases for a range of reflection coefficient, , and slightly decreases with attenuation coefficient, , up to gigahertz, for some values of , as shown in Fig. 6, with the gain of the channel being 10 −5 dB for a 5 km of the FO transmission length. Similarly, the gain of the LL-VLC cascaded system decreases with an increase in transmission distance for a given range of floor and ceiling reflection coefficient, . The gain also decreases slightly with attenuation intensity, , for frequency up to Giga Hertz. For some values of , in the range 0 < < 1 , as shown in Fig. 9. The gain of the channel is −304 dB for a 2 km of the transmission range (Fig. 11). Comparing FO-VLC and laser-VLC cascaded models, both systems present a clear and concise features ( Table 2).
In Fig. 12, for some values of , in the range 0 < < 1 , a transmission range of 0.5 km. The channel gain -150 dB shows the best scenario for = 0.5 dB/km and = 0.8 with clear air.
It is known that the relationship of the optical power which is transmitted using LED and that received by the LED where the transmitted and received visible light is having a flat frequency response, denoted by a direct current (DC) gain ( H d ). Thus, the relationship between optical (16) received power ( P r ) in watts and optical transmitted power ( P t ) can be denoted as where is responsivity of the PD. In both channels, we assume a value of 0.9, domineering power transmitted to be 10 watts and gain 10 −5 . Therefore, for a cascaded system of FO-VLC with various light sources, we assume Eq. (17) for a system where there is no data processing between the sub-channels then, P r FO−VLC = 10 −5 × 10 × 0.9 , P r FO−VLC = 10 −9 watts. Again, for a cascaded system of LL-VLC with similar light sources, P r FO−VLC = 10 −151 × 10 × 0.9 and P r FO−VLC = 9 −151 watts. The most important factor of VLC is controlling dimming, since it uses lights expected for illumination, as such a good quality signal for communication. In this paper, the FO-VLC is limited in the fact that, most components and setup of a fibre link are expensive. Although, expensive in terms of implementation, Fo-VLC communication is free from electromagnetic interference. On the other hand, LL-VLC has a limited range of transmission with an increased complexity of implementation and cost effective.

Standardization efforts
Standards simply define the technical requirement and specifications for a particular technology or product to ensure it is in conformity or compatible with other international products to maintain a minimum level of safety. It is important to enhance the physical challenges in light communication systems in order to overcome its numerous challenges. Having  1 3 in mind here that various domain of visible light communication could have a wide benefit from a well standardization structure. A comprehensive widely accepted standardization will greatly mitigate the gap between academia and the industry while improving development, and fasten up the deployment towards the market. In this standard, a number of ways are proposed to mitigate dimming and flickering in OWPAN [12]. Dimming which is defined as the method of controlling the brightness perceived by a user from a light source, according to the user specifications, dimming can be accomplished by adjusting the pulse width, pulse amplitude and by adding compensation symbols. This standard also proposed methods to mitigate flicker for intra-frame and interframes.

Standardization efforts in VLC
The initial standardization of VLC began at a very early age by the Japan Electronics and Information Industries Asso Due to the fact that IEEE 802.15.7 provided very low data rate there was need for revising the standards by task group TG 7 r1 [35]. Nevertheless, applications for indoor and outdoor VLC face diverse challenges in terms of speed, robustness and data rates. It is important that subsequent and future efforts should focus on providing a VLC standard for outdoor VLC applications. In this context, we can say one can not independently talk on a global accepted standardization point of view based on VLC standardization in general and outdoor VLC in particular. Since they exist the Japanese VLCA standard, and European standards, likewise different research groups are demanding different specification standardization.

Standardization efforts in laser
In 1994, the standardization of safety products by IEC 825.1 was published, for applications of lasers and its products and eventual installation. In 1993 another standard of IEC 825.1 similar to that of IEC 825.1:1994 was published specifying the various requirements for manufacturing of laser equipment. Also, IEC 825.1 points out actions that should be taken when laser hazards such as radiation to the skin and eyes are concerned. Again, machine safety-based standard type A, known as EN 12626, was published. This standard incorporates the laser processing machines and this is equivalent to ISO 11553. ISO also publish a couple of standards including beam parameter which are only applicable to the performance of the laser. EN 60204 specifies requirement for the electrical safety of laser machines. EN 60825.1 was also published based on laser equipment. Again, 481 and 482 specify issues of measurement of airborne particles and chemical agent at work place. Furthermore, EU 643 and IWG 6 setup norms and standards to obtain recommendations concerning laser safety.
In August 1997, various working groups ranging from WG 1 to WG 11 were also established. Eventually the United State standards, member of the ISO and IEC were also published The international standardization of laser in order to improve optical communication globally and permit the sharing of communication infrastructure is very essential. In [61], a consultative system for space data systems (CCSDS) back then in 1982 stated goals that will go long way to enhance governmental likewise commercial interoperability with cross-support, at the same time reducing risk and development time and cost. The CCSDS developed standards recommendation which are now ISO standards associated with space link communication and ground data networks. Also, CCSDS later formed an optical communication group which was co-chaired by NASA and European space agency in January 2014.

Standardization efforts in Fibre Optics
With the inherent need for high data communication, FO provides a promising technology capable of handling huge amount of data. Fibre Optic Technical Advisory group made some standardization efforts to integrate IEEE 802.8 into traditional IEEE 802 copper-based-network. There are three efforts of standardization, categorize into military, computer, and industrial use [62].

Military
The SAE AE-9C Fibre Optic Subcommittee aided the establishment of the Air Force in implementing MIL-STD-1773, considered version MIL-STD-1553 multiplex data bus. Again, the SAE AE-9C Fibre Optic Subcommittee did a review and upgrade that resulted in the implementation of MIL-STD-1760A.

Computer
This effort was brought forth by Project 802 under the IEEE Computer Society working on FOs networks with speeds up to 20 Mbits/s with length up to 2.5 km.

Industrial
In a bit to standardize industrial FO manufacturing, Manufacturing Automation Protocol came with efforts to control the manufacture of equipment with coaxial-cable backbone. Because this needed to be in compatibility with FO. A 802.4 F/O TAG, with token-passing bus, FO implementation considered a hybrid copper-fibre media system. Later, a group of task force successfully setup under 802.4 working group combined efforts from 802.4, PROWAY-ISA and 802.8

Conclusion
In any communication systems, it is imminent for such system to upload it best possible data traffic in a minimum available bandwidth with cost effectiveness. This is the main goal of such communication network. This review illustrates that in both VLC cascaded systems, FO-VLC has shown without doubts that it is a promising way of providing a good communication networks to support the unprecedented increase in data traffic. There are many various ways where VLC-based networks can be used in real-time applications that is going to renovate the present and future lifestyle. In this article, a review is provided for cascaded OWC which are FO-VLC and laser-VLC. This study takes into consideration a couple of aspects for the state-of-the-art of cascaded OWC research from the point of view of communication engineering: modulation techniques, cascaded OWC channel modelling, coding technologies, quick comparisons, and standardization efforts. This survey will help readers to have a better understanding of cascaded OWC channel characteristics with their various related features that may limit the performance of such cascaded OWC networks.
Besides making available the readers with important and necessary technical theoretical information and a much clearer picture of cascaded OWC networks, we also highlighted some crucial challenges of cascaded VLC networks. Unlike free space optical communication, cascaded VLC systems are not over exploited, and the principal problem is more challenging. The proposed layouts of FO-VLC and LL-VLC improve system optimization, and the simulation results prove the system with FO-VLC provides reliable, optimal power to the MTs, and almost uniform. Moreover, FO-VLC cascaded technology with an M-ary phase shift keying modulation scheme can improve the data rate and boost the overall VLC cascaded throughput. With their numerous advantages, we strongly believe that cascaded FO-VLC and laser-VLC are good candidates for VLC backbone.
Funding Open access funding provided by University of Johannesburg.

Data availability
The data that support the findings of this study are available in IEEE xplore repository, [20,21].
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