Abstract—Wireless communications have become an integral part of global convergence as global connectedness has gradually become dependent on its efficient deployment.The need for “more-broadband” techniques in relation to the ever increasing growth rate of the data hungry society now necessitates novel techniques for the high-speed data transmission.While advancements have been made in this regard,the projection of having an eventual Internet of everything (IoE) deployment will result in an unimaginable transmission data rate requirement as huge data traffic will be conveyed per time within the communications network,which will require a capacity upgrade of the existing infrastructure.Visible light communications (VLCs),as an integral part of optical wireless communications (OWCs),have been reviewed in this article,having the capacity to extend the achievable data rate requirement of the wireless communications network.The technologies,techniques,and best practices have been presented alongside technology integration for the seamless high capacity wireless broadband deployment.

1.Introduction

Over the years,wireless communications have evolved from the discovery of the electromagnetic (EM)spectrum to the invention of the first radio and are still evolving.The EM spectrum is made up of the radio wave,microwave,infrared (IR),visible light (VL),ultraviolet (UV),X-ray,and gamma ray.The wavelength decreases along the EM spectrum while the frequency increases.VL occupies the frequency range of 430 THz to 790 THz,while in comparison,the radio wave occupies the frequency band from 3 kHz to 300 GHz[1],[2].Due to its broad area coverage and small interference,the radio frequency (RF) has been the most commonly used component of the EM spectrum for communications purposes.However,a number of latest variables,including the quickly reducing accessible RF spectrum and the increased bandwidth and information rates requirements,have all pointed to the need for an upgrade or alternative to this media.Visible light communications (VLCs) are emerging as a solution to overcome these stated factors.Since data traffic is growing rapidly,VLC is considered as the complementary technology for high data traffic backhaul.In 1880,Bell introduced the concept of using light as a communications medium with the development of photo-phone,a device used to transmit sound signals through a beam of light,but his experiment failed as a result of the difficulty in generating a suitable carrier frequency[3].After light emitting diodes (LEDs) were invented,the concept of VLC came into limelight with white LEDs deployed for the data transmission by modulating light at speeds invisible to human eyes.VLC is based on the communications in the perceptible part of the EM spectrum with the wavelength ranging from 380 nm to 750 nm[4].

VLC has always been an area of concern;however,because of the inability of traditional light sources to handle fast modulation,its potential was never fully exploited[5].The emergence of LEDs,especially the first white LED,opened up diversities of possibilities and research considerations in VLC.Invariably,data is transmitted in VLC by optical sources,such as LEDs and laser diodes (LDs).Massive attention has been paid to VLCs due to the disposition of LEDs for energy efficiency and developments in the LEDs technology through high-speed nanosecond switching time[6].LEDs are energy saving,environmental conservative,longevity prone,and small in size.Compared with the traditional light sources like incandescent bulbs,LEDs have high modulation performance,highly responsive,and sensitive along with other characteristics that make them suitable for the data modulation and transmission.The possibilities of usage are numerous;the transfer of data from one mobile device to another could be done simply by pointing and aligning the beam of light from one device with the other.Vehicular detection systems can make use of VLC by receiving the light data emitted from other vehicles in the surroundings and making decisions or warnings based on the received data.Other uses of VLC include power line communications,indoor localization,under-water communications,and down-hole communications in the oil and gas field among others.

VLC also offers interesting alternatives to traditional wireless systems such as RF[7],[8].Some of them are its available spectrum which can help in resolving the RF spectrum crunch[7],its inability to penetrate solid objects,making it suitable for communications in a constrained environment.The radiations are free of any health concerns,reduced cost of implementation due to the utilization of LED,and the secured wireless channel,therefore,intercepting data is difficult as you will have to be within the constrained coverage domain and there is no interference like in RF signals,so it can be used in airplanes,medical centers,and any other RF restricted domain[8].VLC systems can be divided into non-real-time (offline) and real-time VLC systems[9].Real-time VLC systems are used when real-time data processing and retrieval is required.While offline VLC systems are the systems that do not require instantaneous processing of the transmitted data.The highest off-line speeds of VLC systems were 4.22 Gbps and 5.6 Gbps[10]-[12].These non-real-time systems make use of relatively complex modulation techniques that include orthogonal frequency division multiplexing (OFDM),discrete multi-tone (DMT)modulation technology,and other modulation techniques with high spectral efficiency and wavelength division to attain high data rates.In an actual use (real-time usage),some of these modulation techniques are saddled with high design complexity because of the insufficient hardware support.Real-time VLC systems usually use the simplest on-off keying (OOK) modulation techniques which are not hardware and resources intensive due to the high signal and data processing requirements.Although these techniques have a relatively low bandwidth and data rates as compared with the more complex method,they can be optimized to satisfactorily perform data communications.

Consequently,there are several modulation and signal processing techniques used to boost system performance in VLC.In VLC,easy and inexpensive intensity modulation (IM) and direct detection (DD) methods are used to convey data.Single carrier techniques such as pulse modulation schemes,the likes of pulse position modulation (PPM),pulse width modulation (PWM),and OOK modulation,are used in some VLC systems[13].Also,multiple-subcarrier modulation such as OFDM exhibits high spectral efficiency.It is robust against co-channel and inter-symbol interference (ISI) caused by multipath propagation[14].Despite the numerous benefits of VLC,its technology integration has its own limitation.Performance degradation in the presence of external light sources other than its own is a major concern.This generates a low signal-to-noise ratio (SNR) which causes distortion in the transmitted data.The RF spectrum has become congested/crowded due to the increasing demand for the high-speed data transmission which has raised the research concern in investigating the“more-broadband” alternative to cater for the virtually unlimited communications capacity that was previously available.VLC provides a way out of this congestion problem,as it has a frequency bandwidth of about 300 THz.

2.Optical Wireless Communications

The wireless technology has been the enabling technology for diverse concerns globally for quite some time.Optical wireless communications (OWCs) are the system of communications where IR,UV,and VL are used for propagation.These optical bands i.e.IR,VL,and UV have exceptional benefits and numerous wireless technologies are based on them[15].IR is imperceptible to people and can be used when lighting is not necessary.The optical wireless (OW) spectral regions have 750 nm to 1 mm for IR,380 nm to 750 nm for VL,and 10 nm to 400 nm for UV[16].For over thirty years,OWC systems have been studied as a substitute for high-speed communications in place of RF systems;as the latter suffers channel capacity limitations and lower rates of transmission.Table 1 depicts an overview of some selected radio waves and OWCs spectra previously deployed and currently investigated for possible applications in the wireless communications field.

OWC can be dated back to the era of sending signals through smoke,fire,and ship flags.Africans used drums/fire to send signals,the mostly war signals in time past.Early Greeks and Romans also used sunlight to send messages during battles.The earliest demonstration of using light as a medium for communications called photo-phone was first demonstrated by Bellet al.in 1980[3].The device made use of a light beam to transmit voice data and the methodology involved using a mirror to focus sunlight when talking into a mirror vibrating mechanism in accordance with the speech which in turn vibrated the focused light beam accordingly.On the receiving end,a detector picked up the vibrating beam and decoded the voice data encoded into it[3].The photo-phone encountered problems that prevented the further investigation by the then scientists and inventors,some of which included obstacles in nature,such as fog and rain that could interfere with the transmitted light beam,point to point transmission issues,and other hindrances.

Following the invention of LED,the idea of using the unguided light as a means of communications was revitalized.By switching between on and off at speeds undetectable by the human eye,the use of white LEDs to send data was at the forefront of VLC.In 2003,the word VLC was first used by the researchers in Keio University,Japan’s Nakagawa Laboratory.The Nakagawa Laboratory displayed Keio University’s first VLC system in 2000.For data transmission,LEDs were used[17].As a derivative of OWC,VLC is a type of wireless communications that uses the unguided and unrestricted light to convey a signal.The data signal is encoded and modulated with the modulated signal used to regulate the LED transmitter,which can be pulsed at unnoticeable high speeds for the human eye.The signal transmitted by LED is detected by a photo-detector,usually a photodiode.The prospects of VLC are very wide ranging from the outdoor use including vehicular networks to indoor personal area networks (PANs)[18].Due to the unguided nature of VLC,a point to point link must be established for communications to take place.The advantage of VLC over other media of communications is the access it has to the high frequency spectrum used by the VL spectrum (430 THz to 790 THz),coupled with the minimal cost of LED based system implementation.

In recent times,LEDs and also laser are used for lighting in OWC systems.In July 1960,signals were transmitted at a distance of 40 km using a ruby laser at Bell Labs[19].Transmission of television (TV) signals basedon the OWC technology was tested in 1963 by North American Aviation (NAA).An extensive list of OWC experiments from 1960 to 1970 was documented in [20].In 1979,Gfeller and Bapst introduced a wireless communications system based on IR;the system for short-range indoor environments could transmit data at rates within hundreds of Mbps which sparked a lot of interest[21].In VLC based OWC systems,LEDs are used for communications and fast-switching as suggested in [22].VLC Consortium (VLCC) was later established to regulate the VLC system[23].With the rise of VLC,the OWC market is showing future promise.LEDs are predicted to be a fundamental provider of light in years to come.LEDs will be broadly used for advertising displays,lamps,car lights,and home appliances,making use of VLC for short and medium distance communications promising.VLC-OWC based systems working in the VL band or spectrum are usually categorized as VLC.This system of communications has been reviewed to be the next frontier for fast connection due to its exclusive features,such as the low power,large bandwidth,cost effectiveness,ease of distribution,and improved channel security.The use of LEDs in VLC makes its deployment easy.VLC has applications in indoor localization,communications networks,intelligent systems,RF sensitive environments,etc[24].In free space optics (FSO),data is transmitted wirelessly by using light that propagates in an open space.FSO communications operate in VL,IR,plus UV spectra[25],[26].FSO uses a very large frequency range when compared with the RF spectrum,therefore very high data rates can be achieved.FSO links have limitations;the links are not so reliable for a lengthy communications range due to weather situations and physical obstructions like trees and buildings when they are in the line of sight of transmission[21].Optical camera communications (OCCs) are the OWC based technology used mainly in flashlights and cameras of smartphones[21].OCC uses either the IR or VL spectrum.Mostly,LEDs are used as transmitters and cameras as the receivers;as a result of the poor frame rates of cameras,this communications technique suffers from the low data rate[21].

Table 1:Capacity capability of existing and emerging technologies

3.Insight into VLC

The application of RF as a means of communicating signals marked a turning point in the field of communications.Being at the forefront of wireless communications made it the most implemented means of wireless transmission both by individuals and network service providers.Over the years,with the increase in population and the demand for higher data rates and the larger bandwidth,the need for an upgrade has become necessary.VLC provides this much needed upgrade.The first is its license-free operation since OWC systems function in the unregulated spectrum,making it free from any licensing limitations.The next is its unregulated huge bandwidth.The progress in the development of LEDs has driven VLC systems to evolve over the years.The VLC system in [27]reported data rates of up to 400 Mbps and 513 Mbps,beyond that in [28]and [29]using over shelf LEDs.These values eclipse the 70 Mbps bottleneck proposed for radio frequency communications (RFCs) in [30].Hence,it offers a huge unregulated bandwidth to accommodate future bandwidth requirements.The low cost associated with VLC implementation is another factor to consider.Compared with the relatively expensive RF units,the use of LEDs which are cheap,low power consumption,low heat dissipation,small in size,and long life span for both transmitters and receivers in VLC makes the overall production and maintenance costs cheaper when compared with RFC.In considering safety,while the research on the effects of radio waves in communications to humans is inconclusive,there are reasons to believe that it poses some adverse health issues to the human anatomy in relation to body exposure.In comparison,VLC poses no threat to human’s physiology since VL is its primary propagated energy.In view of the communications network security,RF waves have a long wavelength which gives them the ability to penetrate walls even unintentionally.This semiuncontrollable feature makes data transmitted prone to interception when transmitted.Unlike RFC,VLC has a relatively short wavelength making it almost impossible to penetrate most surfaces.They are most likely to be reflected within a localized domain which makes containing transmitted data easier in VLC[31].Another point is its illumination capability.As the main transmitting component in the VLC transceiver is LED or any suitable light source,VLC systems can perform the dual function of illumination and communications,“Illumication”.This feature enables the use of existing wiring to transmit data through interconnected light sources.The promise of high energy efficiency is another rewarding benefit of the VLC deployment.According to [13],the use of LEDs could lead to a reduction in the energy use of old light sources by 80% and a 50% reduction in global electricity consumption if the sources of light are substituted with LEDs.Since there is no known EM interference caused by light at the moment,the VLC technique will be appropriate for communications in hospitals,airplanes,and any other RF restricted domains.Since LEDs are mostly used for lighting,its properties such as non-linearity and chromaticity have to be considered in OWC systems[13].Also,the responsivity and absorption coefficients of photodiodes used as light detectors are considered in the system model.Hence,there is a big difference in the signal processing techniques applied in traditional wireless communications systems like RF and that of OWC.

4.VLC Transmission Process

A typical OWC transmission link is illustrated in Fig.1[21].It comprises of the transmitter and receiver circuits.

Fig.1.OWC transmitter/receiver components.

OWC which encompasses VLC uses LEDs or LDs for the conversion of electrical signals into optical signals.LEDs and LDs have exclusive benefits and restrictions.LDs yield coherent light;therefore the transmission of light waves is in the same direction and the same phase[32].A major advantage is that co-channel interference in LDs is less and they travel longer distances.Also compared with LEDs,the modulation bandwidth of LDs is large[33].There is complexity in designing LD-based illumination devices,while trying to produce the white light,and the levels of radiation of LDs have to be adjusted because they are highly directional and can cause optical damage[34].LEDs are the most suitable for OWC transmitters because of the properties of energy efficient,low cost,lower heat generation,longer life span,and improved color-interpreting[7]resulting from the fact that LEDs are non-directional optical sources;to compensate for the low levels of output power,an array of LEDs can be used[34].A VLC communications system consists of two essential components:The transmitter and receiver.The incorporation of these two components unto one device gives rise to a transceiver.Since VLC is a wireless means of communications,only a VLC channel connects the transmitter and receiver to each other.As light travels in a straight line,a line of sight is required in VLC.In VLC,due to the vast developments in the LED designs and constructions,LEDs have proven to be the best choice for transmitters.LED is a solid-state semiconductor device that can convert electrical energy to light energy directly.There are numerous advantages of LEDs compared with other artificial sources of lighting.These include energy efficiency,light density,reliability,and longevity.Current LEDs can achieve about 100 lm/W efficiency[35].LEDs are capable of emitting light covering the entire VL spectrum(ROYGBIV),making them widely applicable in our daily lives ranging from street lights to general home lighting.The major types of LEDs used in lighting are red-green-blue (RGB) multichip LED and blue-LED on the yellow light radiating phosphorus layer.The latter is the most popular because it is energy efficient and has lower complexity.For communications purposes,the class of LEDs deployed could be phosphor coated LED (PC-LED),multi-chip LED,micro LED,and organic LED (OLED)[36].

5.Principles of Operation of LED

LEDs are constructed based on the principle of operation of a p-n junction.The p-type region has holes as the main carriers and electrons as the minority carriers,while the n-type region has holes as the minority carriers and electrons as the majority carriers.The holes in the p-junction are transmitted to the reverse side when a biasing voltage is applied to the p-n junction.The electrons in the n-junction are also driven towards the p-junction.These minority carriers recombine in the depletion layer,also called band-gap.In order to recombine a high-energy electron with a photon of lower energy levels,the excess energy is released.That is,the electron moves from the conduction band to the valence band.This excess energy is released as photons,which is equivalent to the energy of the band gap.The magnitude of the photon’s energy determines its wavelength that can be adjusted by the material type of the semiconductor.The relationship between the band-gap energy and the emitted wavelength is given as follows[37]:

wherehis the planks constant andfexpresses the frequency of the radiated photon[37]:

wherecis the speed of light in vaccum andλis the wavelength of the photon that was emitted.

6.LED Non-Linearity

LEDs are non-linear in the practical use.The Shockley ideal diode equation as stated in [38]used to describe its non-linearity is given as

whereqis the magnitude of the electronic charge,Vis the voltage drop across the diode,kis the Boltzmann constant,andTdenotes the absolute temperature of the p-n junction.Isrepresents the current when saturation is achieved and can be expressed as

whereArepresents the junction area;DpandDnrepresent the constants for electron and hole diffusion,respectively;τpandτnrepresent the carrier lifetimes of the minority electrons and holes,respectively;niis the intrinsic carrier concentration in the solid state device;NAis the acceptor concentration at the p-side whileNDis the donor concentration at the n-side.

The saturation current is a portion of the reverse current in a semiconductor diode induced by the movement(diffusion) of minority carriers from the neutral areas to the region of depletion.This current is nearly independent of the reverse voltage.

The Shockley equation can be approximated as

It is also important to note that the current-optical power (I-P) conversion is also non-linear.

7.LED Lighting Constraints

These parameters are important in signal processing and modulation of VLC systems;they include dimming control,chromaticity control,and flicker-free communications.

7.1.Dimming Control

The International Commission on Illumination (CIE) uses the spectral luminous efficiency functionV(λ) to describe the light brightness that humans can perceive.It states that people are more sensitive to light in the middle wavelengths than either short or long wavelengths.The power of this luminous flux is a quantity that can be measured as given by [39].However,the power consumption and lighting quality should not be overlooked as they are also crucial aspects of the field.The way through which LEDs brightness can be controlled is by altering its forward current.The process of doing this is grouped into three categories:Analog,digital,and hybrid dimming control.The analog dimming control functions by adjusting,in a linear order,the current amplitude with regard to the optical flux that is radiated.The digital dimming technique employs the PWM scheme.The PWM signal’s period (T) is fixed and the cycle is varied in proportion to the dimming level that is required.The hybrid dimming technique is a combination of both analog dimming and digital dimming as indicated in [40].

7.2.Chromaticity Control

Chromaticity describes the quality of the color distribution irrespective of its particular brightness.The color space chromaticity diagram provided by CIE[41]presented the color maps as can be perceived by the human eye into two defined parametersxandy.In line with the foregoing assertion,there are seven defined bands in the entire spectrum of the human visible wavelength.And to effect chromaticity control,the RGB constellation,mapping of data bits to the chroma-values,and the estimation of the RGB-LEDs intensity are adopted[34].

7.3.Flicker-Free Communications

Flickers are the output power in the form of the brightness fluctuation which can be perceived by the human eye.The regular exposure of the human eye to the fluctuation can cause eye fatigue or even damage to the eyesight.As a result,efforts are being made to create fast switching LED drivers of a good quality design with the purpose of reducing the negative flicker effects.In order to mitigate flickers in VLC systems,the Manchester binary code is a commonly used modulation code.Mitigation of intra-frame and inter-frame flickers is the class of the flicker mitigation technology[6].Intra-frame flicker mitigation is used to remove flickers within the data transmission frame for OOK modulation and variable pulse position modulation (VPPM).

8.Driving the Transmitter Front-End

Since it is necessary for data to be modulated unto LED for transmission to be achieved,before modulation,there is a need for the incoming electrical signal to be converted to the current needed to drive the transmitting LED.This is achieved by the LED driver or LED driver circuit.For digital modulation,to design an appropriate driver circuit for the VLC system,some basic factors should be considered.The current requirement of LED is an important factor that should be considered.If the driving current falls below the threshold value,LED would fail to turn on.If the value is exceeded,LED might be damaged.The rise and fall time of LED and components simply means how responsive LED and components are to the applied current or voltage.The higher the rise and fall time,the slower the response and the lower the bit rate.The power efficiency should also be considered when designing the appropriate LED driver as the lower power efficiency of components would lead to slower response and lower bit rates.For analog modulation,an extra constraint should be considered,LED non-linearity.This nonlinearity generates the distortion of the intermodulation that requires a compromise between the modulation depth,channel spacing,and modulation system type.The modulation depth is described by how much the modulated carrier signal differs from its unmodulated counterpart.The higher the modulation index,the lower the BER.There are two basic LED-driving topologies,single-ended and differential modes.The single-ended mode is mainly used for the accoupled solutions with discrete components,while the differential mode is compatible with the high-speed differential amplifier.In the differential mode,the LED engine has a differential current steering switch and two current branches.One of the branches is with real LED and the other is with a dummy resistor load equal to the LED resistor.The symmetry of the output stage is improved in this mode.

9.VLC Receiver

In the OWC receiver system,incoming light is collected and concentrated on a photo-detecting element (photodetector) via an optical concentrator.The optical signal is converted to an electrical signal after detection;this signal is then amplified by an amplifier before the analog-to-digital conversion and digital signal processing[15],[21].Image sensors,solar panels,and even LEDs can also be used as photo-detectors;in VLC,photodiodes are mostly used.Receivers are dependent on the bandwidth of photo-detectors as well as its sensitivity and area[21].This implies that the receivers need a detector with the high sensitivity and area to accumulate the maximum signal.The PN (p-type and n-type) photodiode,PIN (p-type,intrinsic,and n-type) photodiode,and avalanche photodiode (APD) are the photo-detectors used in VLC receivers.Compared with PIN photodiodes,APD has higher sensitivity but PIN is favored over APD due to its low temperature tolerance and lower shot noise[13].In VLC systems,photo-detectors are used as the receivers for the transmitted light signals.Some of the requirements of an ideal receiver,photo-detector,are small in size,efficiency,sensitivity,temperature fluctuations resilience,and cost effectiveness.In most VLC systems,a photodiode performs the role of a receiver to convert optical light to the electrical signal.If the energy of a photon is sufficient for the photodiode to absorb,an electron is transferred to the conduction band from the valence band.This leads to an electron-hole pair being generated.The photo’s energy should not be less than the energy gap that exists between the valance and conduction bands[42].Some of the specific parameters that determine the choice of the photo-detector are the absorption coefficient,quantum efficiency,and responsivity.

9.1.Absorption Coefficient

The received radiant fluxΦrin the photodiodes is given by the Beer-Lambert equation:

whereΦtis the radiant flux of the light that passes into the surface of the photodiode andτis the optical depth,given as

wheredrepresents the photodiode’s thickness that light has to penetrate or the width of the absorption region andα(·) represents the coefficient of attenuation which determines the distance the light travels through the photodiode before being absorbed totally.The coefficient is relative to the quantum efficiency of the photodiode.The higher the quantum efficiency,the more electrical sensitive the photo-detector is to light(more electrons generated from incident photons).

For uniformly distributed attenuation,

whereαis also known as the linear attenuation coefficient and it depends on the photodiode material as well as the wavelength of the light.

9.2.Quantum Efficiency

The sensitivity to light of the photodiode is described by its quantum efficiency.It is described as the amount of the incident photons absorbed and the electrons collected at the terminals of the photo-detector.It is also the electron-hole pair ratio that contributes to the current of the detector,the photon flux incident,and is calculated as

whereRsuris the surface reflectance;ξis the fraction of electron-hole pairs;arepresents the absorption coefficient.

9.3.Responsivity

Responsivity is the amperage unit per watt (A/W) of the radiant power.It depends on the incident light wavelength.It is a function of external efficiency that describes the photon energy and is denoted by[43]

whereηerepresents the external efficiency of the photodiode.

10.VLC Channel

There are three common wireless channel capacity models and they are the discrete-time Poisson channel,free-space optical channel,and improved free-space channel[13].Based on these channels,several modulation and demodulation schemes have been established.There are various modulation schemes in OWC,such as single carrier modulation,carrier-less modulation,multicarrier modulation,and multicolor modulation[14].There are several signal processing techniques and estimation techniques used to maximize or increase the transmission performance and they have been discussed extensively in [13].The schematic in Fig.2 depicts a simplified OWC system channel which is a typical example of the VLC channel.

Fig.2.OWC transceiver system[21].

In order to understand the VLC system,a subset of the OWC system,the channel description is important.IM and DD are two techniques that define the operational principle of an OWC (VLC) system.From the block diagram,x(t) is considered as the waveform that carries data and modulates the illumination of the transmitter,i.e.the instantaneous transmitted power.Withω,which is the electro-optical change factor in W/A,x(t) is transformed into an optical signal asu(t)=ωx(t).The channel impulse response (CIR) is represented ash(t),the signal detected is expressed asg(t)=Ru(t).Without loss of generality,ω=R=1,therefore,g(t) equalsu(t) andx(t).After channel filtering,the received signal is tainted by a noisev(t),and is written as[21]

wherev(t) is the background noise (BN) and ? denotes the convolutional operator[21].

11.Propagation Links

There are two categories of propagation links:Line-of-sight (LOS) and non-line-of-sight (NLOS) links.The NLOS links have far less power and are much less efficient compared with LOS.LOS is,however,highly vulnerable to the hindrance.NLOS links,on the contrary,overcome barriers between the transmitter and receiver,allowing for the detection of signals via the bounce or reflection from wall and roof surfaces.If the directionality of the transmitter and receiver is taken into consideration,the VLC propagation can be categorized into three groups:Directed links,non-directed links,and hybrid links.The transmitter in the directed link category has to point directly at the receiver using a small semi-angle and a field of view (FOV) that is adequately small.Therefore,this links power conversion efficiency is high.In the links that are non-directed based,the transmitters as well as the receivers have larger semi-angles,which makes them easy to use.The transmitter and receiver in the hybrid links have different directionality[13].

12.Noise in VLC

Despite the potential in VLC,noise is a major inhibitor.There are two major types of noise in VLC:Shot noise and thermal noise[44].Shot noise is mainly generated by LED and ambient light.Filters are unable to remove the modulated light noise.Ambient light from the sun and other light sources will inevitably be detected by the photodiode.This kind of noise is referred to as additive white noise.Thermal noise is usually generated in the receiver and it causes electrons to move.When there is no external field,the movement is random and there is no current generated.However,in practice,the electrons moving in one direction is not equal to the electrons moving in the opposite direction.This causes rising or falling in the voltages of the semiconductors and conductors.

13.Channel Equalization

The equalizers’ main function is to restore the transmitted information and decrease or eliminate channel interference.In a typical wireless communications system channel,the received signal over the dispersive channel is of the form shown in (11).After the required signal processing of analog to digital conversion(ADC) and Fourier transformation (FT),the discrete sample block is recovered so that the received output symbol is represented as[45]

wherey(z) represents the symbol received at timez;hrepresents the fading channel coefficient;x(z)represents the transmitted symbol at timez;v(z) represents the noise sample at timez.

In the above system model,the received symbol depends only on the current input signalx(z).

However,more frequently,the received symbol is given as

whereh(0) andh(1) represent the fading channel coefficients.

Compared with the first model,the output of this system depends on the input symbolsx(z) andx(z?1).Essentially,x(z?1) is “interfering” withx(z).This is termed ISI,which causes degradation.The removal of such ISI is called equalization or channel equalization.

The general representation of an output symbol in wireless communications is

whereh(0),h(1),…,h(L?1) are theLchannel taps.AsLincreases,the severity of ISI also increases.

For equalization ofx(z),withL=2 for simplicity,the following output symbols are employed:y(z+2),y(z+1),andy(z).This is anrtap equalizer,wherer=3 symbols.

Computing this system would give[45]

To obtain the system model,the above system of equation is written as a vector matrix.

whereY(z) is anr×1 matrix,His anr×(r+L?1) matrix,X(z) is an (r+L?1)×1 matrix,andV(z) is anr×1 matrix.The equalizer would be designed based on the 3 output symbols linearly combined together.Therefore,taking the weights of the equalizer (combining weights) to bec0,c1,andc2:

The equalizer is therefore given as[45]

The weightsc0,c1,andc2are therefore designed to eliminate the effects of ISI.While the circuitry is simple at a low cost for single carrier VLC systems,the data rate these systems provide is inherently low.This is due to the low LED modulation bandwidth (typically 1 MHz) and multi-path fading resulting in ISI[46]-[48],which limits VLC systems’capacity and transmission distance.Using advanced modulation techniques such as OFDM,these disadvantages in VLC systems can normally be mitigated.These techniques,however,set a limit on the data rate,thus negating VLC’s basic advantage.In this regard,different equalization techniques have been used on the receiver to mitigate ISI’s influence.

13.1.Zero-Forcing Equalizer

A zero-forcing equalizer (ZFE) is a linear equalizer type that performs reverse channel response to remove ISI effects.It is called zero forcing because of its attribute of forcing ISI to zero in a noise-free environment.It has been suggested in [49]for IR communications.Despite its simplicity,its disadvantages have been stated by [50].Although omitting the effects of ISI,a significant increase in noise requiring high SNR could also be achieved.Although the response to impulse of the channel is limited,ZFE response to the impulse must be very long.

13.2.Fractionally Spaced Equalizer

This equalizer,fractionally spaced equalizer (FSE),consists of decay elements and each of these elements is less than the symbol duration.It removes the outcome of ISI and also,the effect of noise may be improved depending on the input sample.

13.3.Maximum-Likelihood Equalizer

For removing multi-path fading channel effects,a maximum likelihood equalizer (MLE) is an optimal equalizer.It works by calculating the Euclidean distance with the channel state between the received symbols and transmitted symbol product.That is,it is a function of the distance between the received symbols and the product of over-sent symbols,evaluated as

For pulse modulation techniques,MLE was presented in [51]but its complexity rises exponentially as the number of data symbols increases.

13.4.Decision Feedback Equalizer

For removing ISI,the decision feedback equalizer (DFE) utilizes the identified symbol feedback to generate a channel output estimate.DFE is supplied with identified symbols and generates an output that is typically subtracted from the linear equalizer output.As with linear equalizers,DFE comprises of a true finite impulse response (FIR) filter,if the transmitted symbols are true,or if the symbols are complicated,a complicated FIR filter is used.During the steady-state operation,if a linear equalizer is also used,DFE includes an assessment of the channel’s impulse response or the channel’s convolution with the linear equalizer.Since DFE copies the channel output and subtracts the DFE output from the incoming signal,it can compensate for serious distortion of amplitude without raising noise in the extremely distorted frequency bands.DFE therefore utilizes the feedback of sensed symbols to generate an evaluation of the channel output,i.e.,it uses previous symbol decisions to eradicate ISI resulting from earlier detected symbols on the present symbol being detected.The carrier-less amplitude and phase (CAP) modulation uses a linear equalizer and a decision feedback equalizer to improve system performance[52].

14.VLC Signal Processing

Signal processing consists of all actions on the signal from transmission to reception.It involves the modulation and detection of the intended signal.The advent of digital communications theory has given birth to numerous techniques and principles with which data being transmitted can be compressed and transmitted at a high bit rate with little loss of data.Among these techniques and theory are the source encoding and channel encoding.The source coding process reduces the number of redundant bits of information to reduce the bandwidth used.Redundant bits are determined probabilistically.If the probability of the message is higher,then the number of bits required to represent that information is less.

The channel coding is used to protect data transmitted from error.This is achieved by adding extra bits to the transmitted data.They are necessary for the error detection and correction at the expense of the bandwidth.The data,series of bits obtained from the universal serial bus (USB) port of a personal computer (PC),have to be represented by using voltage or current levels in the binary digit (1s and 0s) format.This process is known as encoding.Depending on the encoding technique used,the data can be encoded for secure transmission and error correctible transmission or encoded for easy transmission and circuit simplicity.

15.Modulation Techniques

Modulation is a method of superimposing the message signal on a high frequency signal to change the characteristics of the wave to be transmitted.Digital modulation techniques are used for better quality and effective communications.Digital modulation primarily has more advantages over analog modulation techniques,some of which are permissible power and high noise immunity.A message signal is transformed from the analog to digital message in digital modulation and then modulated using a carrier wave.The carrier wave is keyed between two values representing on and off to create pulses such that the signal to be transmitted is modulated.The popular single carrier modulation techniques in VLC are the pulse amplitude modulation (PAM),PWM,and PPM.As stated in [53],one of the biggest differences between VLC and RF is that some of the well-researched and developed modulation schemes from the field of RF cannot be used in VLC as that in RF at high modulation speeds.This is because of the peculiarities of the VLC processed signal such that IM and DD are preferred.The modulation techniques employed in OWC systems can be baseband modulation,multicarrier modulation,or multicolor modulation[13].The baseband modulation schemes in use are PAM,PPM,and CAP modulation.

15.1.PAM

One of the simplest methods of modulation is PAM[54],[55].PAM is a modulating system in which the pulse carrier amplitude varies proportionally with respect to the message signal’s instantaneous amplitude.The modulated signal of the pulse amplitude will match the amplitude of the initial signal as the signal traces the entire wave’s route.A signal sampled at the Nyquist rate is recreated in natural PAM by passing it through an effective low-pass filter/frequency (LPF) with a precise cutoff frequency.LPF only allows signals with frequencies below a given threshold to pass.With the use of PAM,considerable data rates of VLC transmission have been achieved[56]-[58].OOK modulation is a common technique in OWC;it is a type of PAM with only two states,i.e.,the intensity of LEDs or LDs used can be changed between 1 (bit 1) and 0 (bit 2) to modulate data.However,OOK is not as efficient as other modulation schemes[13].

15.2.PPM

The PPM technique is based on the pulse position.In PPM,the duration is divided intoLequal slots of time,L-PPM,in which a pulse is transmitted.This implies that the symbol period is segmented intottime slots of equivalent periods in this technique and in one time slot,the pulse is transmitted[59].It is relatively simple to implement as compared with some other modulation techniques as the earliest designs are implemented with this scheme[60].It is widely used in wireless optical communications systems as shown in [61].When the pulse is transmitted,the pulse’s position identifies the symbol that is transmitted.The authors in [62]proposed the use of the adaptive rate transmission scheme in which repeat coding is used to reduce the throughput when the channel conditions are poor.In [63],the author designed a rate-variable PPM system punctured convolutional codes for communications with IR.In [64],it was proposed that in case of bad channel conditions,the rate adaptive PPM transmission would be used for both repeated and punctured codes in order to achieve higher data rates.

Other variants of the PPM-based modulation have been proposed over time since the spectral efficiency and data rates are relatively low (one pulse per symbol duration).Overlap pulse position modulation (OPPM) is a variant that allows the transmission of more than one pulse during the duration of the symbol.Reference [65]showed that not only can OPPM achieve higher spectral efficiency when compared with PPM and OOK,but it also has a wide range of dimming levels along with a higher bit rate.Another variant of PPM was proposed in [66]called multiple pulse position modulation (MPPM).Like in OPPM,it allows the transmission of multiple pulses during the duration of the symbol.The pulses within the duration of the symbol do not need to be continuous.In[60],MPPM could achieve higher spectral efficiency than OPPM.In [67],the authors proposed a PPM variation combining OPPM and MPPM.It is called overlapping multiple pulse position modulation (OMPPM).For each optical pulse,more than one pulse position is permitted in OMPPM.This proves that with the implementation of OMPPM,spectral efficiency can be improved without the increased use of the bandwidth.The authors in [68]and[69]studied the trellis-coded OMPPM to demonstrate how efficient it is in DD channels with background noise.The IEEE standard[70]proposed a pulse modulation scheme which is a hybrid of PWM and PPM.It was called VPPM.Choosing different positions of the pulse as that in PPM,the bits are encoded.However,when the need arises,the width of the pulse can be modified.VPPM retains PPM simplicity while adding the different dimming levels feature.

15.3.CAP Modulation

The CAP modulation scheme is a single carrier VLC system scheme utilized in OWC systems to attain higher data rates.It is called “carrier-less” because before transmission,the carrier is suppressed and then reconstructed at the receiver.Two unlike data streams are usually modulated by orthogonal filters in this technique.Beneath its limited bandwidth,it has high spectral efficiency and increased VLC connection capacity.CAP is,in essence,much simpler than OFDM,characterized by much lower peak to average power ratio (PAPR).The use of orthogonal signals eliminates the need for fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) blocks in its implementation,which further reduces its design complexity.

15.4.Multicarrier Modulation

The single carrier experiences high ISI as a result of non-linearity in frequency response.OFDM is a multicarrier technique to lessen the conflict of symbol interference and multipath fading.In this technique,data is modulated over subcarriers;these subcarriers transmit parallel data streams instantaneously.Asymmetrically clipped optical OFDM (ACO-OFDM),DC-biased optical OFDM (DCO-OFDM),and flip OFDM are the typical types of OFDM.ACO-OFDM modulates only odd subcarriers and this gives a uniform time domain signal.DCO-OFDM modulates all subcarriers and adds a positive direct current for signal unipolarity while flip OFDM flips the negative half-cylce of the OFDM symbol so that both the positive and negative half-cycles are transmitted on a single OFDM symbol[71].

15.5.Multicolor Modulation

This technique has lately been considered as one that provides high data rates.Single-chips LEDs and RGB LEDs are the major types of white LEDs used.The latter joins the light of colors red,green,and blue and is superior to the single-chips LEDs because the transmission rate is enhanced due to their quicker response periods and the wavelengths from these colors can be applied to modulate numerous data streams separately and offers an opportunity for wavelength division multiplexing (WDM)[13].Due to the limited bandwidth of simple LEDs,it is difficult to attain high-rate transmission;multiple input multiple output (MIMO) uses multiple units of LEDs to boost the data rate.MIMO has two optical approaches,imaging MIMO and non-imaging MIMO.To battle inter-user interference under the lighting limitations in OWC,MIMO-OFDM is introduced and it offers high spectral efficiency with excellent reception[13].

16.Deployment of VLC Technology

VLC is being investigated in academia and industry with increasing interest in developing high-performance systems.Investigations into the possibilities of using white LEDs to provide illumination and communications for VLC at a low data rate had been previously proposed in [14],as the concern for higher rates is currently being researched.The system proposed in [72]was a PWM system,a single carrier system specifically designed to transfer texts between two PCs.The transmission was achieved using IM while the reception was achieved using the DD method.The transmitter side had an analog front end and consisted of high-power white phosphorus LED.While the receiver side consists of a photodiode and circuit for signal conditioning,the entire signal processing was implemented on a microcontroller.LED used was low power (5 W) with a luminous intensity of about 300 lx,while the photodiode used had the spectral response of about 700 nm.At the receiving end,an analog circuit was used to condition the output signal to mitigate signal attenuation and other related distortions.The system was found to maintain a level of 70% accuracy for a distance of about 65 cm.As a point to point system was in view,an increase in the receiver’s angle reduced the modulation accuracy.As the data rate increased,the BER value increased.For different modulation scaling,1 bit PWM,2 bit PWM,and 3 bit PWM,the maximum achieved data transfer rate was 1.5 kbps.

In [28],a point to point system (LOS link) was designed.The highest recorded data rate was 513 Mbps with an average of 450 Mbps.The authors performed experimental measurement of the electro-optical-electrical (EOE)channel frequency response,followed by the measurement of the DMT-based transmission performance.The modulation technique used was the quadrature amplitude modulation (QAM) scheme,a variant of the PAM technique.The transmitter made use of a high-power white phosphorescent LED module (OSTAR LE CW E2B),primarily used for lighting.The training sequence was used for channel estimation and channel equalization.The ultimate BER limit was chosen to be 1×10?3,which was reduced to 10?16by the use of forward-error correction algorithms,thereby increasing the achievable data rate.The system of [73]was centered on DMT modulation with an offline digital signal processor.It used solo RGB LED (that generates a bright flux of 105 lx with a radiation angle of 120°) for transmission and silicon APD (3 mm diameter,80 MHz bandwidth) joined with a lens for detection.Gross transmission rates of 293.7 Mbps,223.4 Mbps,and 286 Mbps were achieved for the RGB channels correspondingly,leading to a cumulative rate of nearly 803 Mbps.

The experiment in [56]provided the first confirmed maximum bit rate for OOK-non-return-to-zero (OOK-NRZ)modulation.The authors ensured that the solid-state devices used were commercially available.The aim of the paper was to provide high-speed VLC systems without having to use complex or specific components.The photodiode used was a low-cost PIN photodiode.The practical LED with a simple pre-emphasis circuit was proposed.BER recorded was suitably low that the data transmission was near error free of 10?9.The design was scalable,providing the possibility of increasing the number of LEDs thereby reducing the receiver sensitivity degradation when the distance between the transmitter and receiver is increased.The maximum operating speed recorded was 456 Mbps.The distance limit for allowable BER was 2.5 m.

Reference [58]proposed a system that had a promising balance between the data transfer rate and BER.This feat was achieved with the implementation of feed-forward pre-equalization in conjunction with a PAM modulation scheme.The transmitter made use of the micro-pixelated LED with a bandwidth of 150 MHz.The modulation scheme used was PAM-4,an improved version of regular PAM,which proved to raise the data rate to about 2 Gbps.The paper further investigated the effects of a PAM-8 modulation scheme and it was found to achieve a data rate of about 3 Gbps but at a higher BER value.The receiver side made use of APD with a bandwidth of 650 MHz and the responsivity of 0.275 A/W.

In [9],the design of a VLC system (transceiver) based on white light LEDs was investigated.The design proposed investigated the direct use of white light for modulation in VLC without the use of a blue filter.The modulation technique used in this system was not specified but it was inferred that the OOK modulation technique was used.The pre-emphasis technology along with the receiver equalization was used in this design.The equalization technique used was the back-end equalization technique.The usable data rate recorded by this design was 100 Mbps.The viable limit to the distance between which communications are possible with this design was recorded to be up to 3 m.It is important to note that this system was designed to be bi-directional (uplink and downlink).The transceiver design was small in size with low complexity and low power consumption.

In [14],the authors analyzed a VLC system using LED lights on its transmission end.An experimental system setup was used to acquire practical values.The specification of the design in the paper included 4 LED lamps with 3600 LED lights in them.The transmission rate was measured taking various parameters into consideration.FOV with tracking achieved a minimum of 300 Mbps when the angle was less than 30° and 10 Gbps when the FOV angle was at 5°.FOV without tracking achieved 200 Mbps when the angle was between 40° and 50°.The modulation technique used was OOK which demonstrated severe communications performance degradation due to ISI.

The authors in [74]proposed a VLC based power line communications PLC system using white LEDs in indoor communications for greater broadband access.The proposition comprised of the design of a high data rate LED-based system capable of providing lighting without leaving any blind spots.The LED system consisted of 9 LED lamps,each lamp consisting of several LED lights with 2 m between them.The findings showed that the coverage of the receiver could reach over 2 m and the reflected signals had lower power.It was also shown that with appropriate impedance matching,data rates up to 1 Gbps can be achieved.

The modulation technique with dimming control was investigated in [75].The authors proposed the MPPM technique to offer not only the function of modulating data signals but also controlling the brightness simultaneously.According to the dimming level,the system could control a number of MPPM pulses in the single symbol duration.The performance of the system was graded in terms of normalized power requirements and spectral efficiency.The other dimming techniques compared were variable on-off keying (VOOK),VPPM,and MPPM.At the end of the experiment,the MPPM technique proved to be more attractive than VOOK and VPPM due to the higher spectral efficiency at less optical power with an increase in the data length.Despite its advantages,the setup lacked any synchronization scheme necessary for PPM schemes.

A two-way VLC system employing LEDs as the transmitter and receiver was proposed in [76].The system comprised of an analog switch to separate the transmitting and receiving circuits while controlling the sending and receiving data flows going through the analog switch with a microcontroller.The system was made up of a transceiver and relay module.The designed transceiver module had low complexity in comparison with other models.The relay modules were to extend the communications between the transceiver modules.The experiment proved that with the use of analog switches with a microcontroller,low-cost transceiver modules that could support two-way data exchange between two devices can be achieved.The data transmitted in this setup was limited to only text transfer.

In [77],the transmission of audio and video media files with systems based on VLC was explored using cheap commercially available LED lamps.The experiment was performed within a 3 m range which allowed the transmission of high-quality multimedia files in real time.The experiment with LED lighting in a room environment showed the tight relationship between the layout of light sources and distribution of illuminance.Compared with[74],the researchers explored the illuminance of two distinct arrangements of light sources:Square and ring arrays which were proposed as possible solutions for future media streaming.

The concept of the feed-forward pre-equalization alongside with PAM-4 modulation was investigated in [58].The authors proposed the feed-forward pre-equalization to be implemented together with a PAM scheme implemented with a VLC wireless system with a target data rate of over 1 Gbps.The simulation results showed that the simple feed-forward pre-equalization is capable of removing ISI caused by the limited bandwidth of a line of the sight VLC link.Micro LEDs were used as the transmitter due to their high modulation bandwidths.The implementation of the pre-equalization in this paper showed up to 5 dB better receiver sensitivity than the post equalization.

The work in [78]demonstrated the importance of the equalization in mitigating errors that arise from LED nonlinearity while increasing the data rate.It was stated that the OFDM modulation technique was used for the demonstration because of its known proneness to signal degradation caused by the non-linearity of LEDs.The equalization technique used was the Volterra equalization.To calibrate the Volterra equalization process,training signals were sent from the LED transmitter to the receiver in order to adapt the proper setting to minimize BER.The results obtained from the demonstration were favorable for VLC as there was a clear difference in the bit/symbol rates of the various carriers with and without using the Volterra equalization.This equalization technique can also be applied to single carrier modulation techniques.The data rate achieved in this demonstration was over 700 Mbps over an average distance of 1 m.Table 2 presents a summary of selected VLC-based signal conditioning schemes[9],[14],[28],[56][58],[72],[74]-[79].

Table 2:Summary of VLC-based signal conditioning schemes

17.VLC Integrated Network

With the progress in the study of VLC and the rise in the numbers of mobile,hand held devices,and Internet dependent infrastructures including the Internet of things (IoT),there is a need for networks and Internet infrastructures to evolve to meet the demand of high-speed data connectivity required to sustain the mobile devices.Wireless heterogeneous networks (HetNets) have been the center of this evolutionary transition especially in indoor environments where the data rate is at its peak[80].The use of light as a fresh mobile access medium is regarded as promising which has given birth to a new architecture of wireless communications used in conjunction with wireless-fidelity (Wi-Fi),known as light-fidelity (Li-Fi).

17.1.Li-Fi Architecture

The Li-Fi system is made up of a transmitter and a receiver,and according to [5],the architecture of the Li-Fi system is layered.The layered architecture is shown in Fig.3 consisting of the application layer,medium access control (MAC) layer,and physical layer.

The application layer deals with all the functions relating to the network-user interface.The MAC layer is responsible for the mobility support;dimming support;visibility,security,and flicker mitigation schemes and other functions.The MAC layer supports peer-to-peer,broadcasts,and star topologies.The physical layer provides a form of the physical link or specification between the device and free space medium.The implementation of the general physical layer is described in Fig.4.

The physical layer has the responsibility of transmitting and receiving,activation and deactivation of the optical transceiver and detection according to the transmission channel status being idle or busy.The physical layer has 3 operation modes summarily presented in Table 3.The variations of the operation modes were presented in [81]and [82].

Fig.3.Li-Fi architecture[5].

Fig.4.Physical layer implementation of a VLC link[5].

Table 3:Physical layer implementation

17.2.Wi-Fi/Li-Fi HetNets

As the demand for the bandwidth increases,HetNets have become a necessary part of our communications topology,and the potential seen in VLC (Li-Fi) has caused it to be paired with other wireless network architectures,specifically Wi-Fi.Wi-Fi uses the RF spectrum to provide wireless communications.The authors in [80]proposed a prototype based on the concept of using a variety of spectra to deliver the increased indoor quality of service(QoS).Wireless HetNet uses small-cells in the indoor environment to provide extra wireless capacity.The proposed models are the hybrid model and aggregated model.Wi-Fi is used in the hybrid model to connect to the Internet,but a user’s downlink is linked via a Li-Fi link while the aggregated system is linked in parallel to the users on Wi-Fi and Li-Fi.In the hybrid system,the unidirectional Li-Fi connection is used to complement the downlink connection to Wi-Fi.In [80]and [83],access systems based on Li-Fi and Wi-Fi HetNets were presented.It recommended a two-stage method for the HetNet access point (AP) selection.A fuzzy logic scheme is developed in the first stage to determine the customers who should be linked by Wi-Fi.In the second stage,the remaining users are assigned to a homogeneous Li-Fi network.The authors described a set of compartments each with a number of ceiling LED lamps enabled as confined area Li-Fi AP.Each compartment has single Wi-Fi AP,which encompasses the entire compartment.Carrier sense multiple access with collision avoidance (CSMA/CA) is introduced with Wi-Fi activated with collision avoidance.While at each Li-Fi AP,the time division multiple access(TDMA) technique is adopted to serve multiple users.HetNets consisting of OW and RF can co-exist and with VLC,and the system bandwidth can be enhanced.Any system built on this technology can boast the extra bandwidth that helps to relieve congestion.The desire for uninterrupted data services is the drive for this integration.In transportation systems,Wi-Fi congestion can be dealt with by providing single VLC channels from every overhead light.A couple of systems based on this RF-OWC relationship were discussed in [80].In [84],the authors introduced a hybrid femtocell/Li-Fi system for indoor applications;it was used for users in a vehicle.The system was used to monitor patients in ambulances and communications in cars;the paper also discussed the restrictions of both technologies.In [85],the paper discussed the relationship between Wi-Fi and Li-Fi and how the integration of the two technologies was useful in tackling the issue of data traffic.A Wi-Fi positioning system that used the Li-Fi technology was discussed in [86].In this system,the photo-detector detected the movement around a building,and values of locations were instantly calculated.The hybrid system was preferable in terms of precision to the present Wi-Fi based positioning system.In [87],they investigated two heterogeneous systems that utilized Wi-Fi and VLC.It explained how VLC outperforms Wi-Fi for short range communications and vice versa.A system that lessens Wi-Fi congestion and solves the issue of the uplink transmission in VLC was designed.

18.Future Work

In view of the ever growing demand of the data hungry society as more high-speed data dependent devices are introduced into the wireless communications network,the need for more energy efficient wireless optical signal conditioning modules is expected.These modules will result in minimized energy production and consumption between the user device and communications infrastructure,which will not only impact on the overall system performance in terms of QoS and grade of service (GoS) but will also ensure the longevity of user device batteries,minimize device over-heating,as well as improve device efficiency among others.Another track in research will be the development of highly spectrally efficient signal processing techniques which will maximize the available optical spectrum for a high throughput deployment.Ingenuity in the development of signal modulation/demodulation will revolutionize the field of spectrally efficient signal processing as better ways to improve on data compression and error-free signal reception emerge.Another side to researching spectrally and power-efficient techniques will be to investigate adaptive modulation schemes for OW applications by dynamically manipulating the transmission parameters with respect to the channel gain to achieve high-performance systems.The automation of the communications pathway to dynamically modify the ambient interfering light sources irrespective of the particular application domain (indoor or outdoor) will also revolutionize the VLC field as external noise-resistant systems are built by adopting diverse artificial intelligence techniques among other possible innovations.This will further engender the smart Li-Fi transmitter-receiver alignment/tracking modules to surmount LOS challenges,designs of efficient sources and detectors to combat saturation due to ambient light sources and noise,and the development of high-performance roaming/mobility algorithms for RF-VLC and VLC-VLC mobility management.

19.Conclusion

The potential of VLC has been presented,detailing its principle of operation,techniques,technologies,integration,and future work.Invariably,traffic offloading in the envisaged high data traffic demanding network will be realizable by its seamless integration within the existing wireless communications networks.As VLC systems mature along integration lines,techniques for improving the data rate requirement in the emerging wireless network will revolutionize the communications pathway,as extreme high-speed (in Gbps) data transmission and reception is achieved.