Advanced Progress of Optical Wireless Technologies for Power Industry: an Overview

applied sciences

Review Advanced Progress of Optical Wireless Technologies for Power Industry: An Overview

Jupeng Ding 1,* , Wenwen Liu 1 , Chih-Lin I 2, Hui Zhang 3 and Hongye Mei 1

1 Key Laboratory of Signal Detection and Processing in Xinjiang Uygur Autonomous Region, School of Information Science and Engineering, Xinjiang University, Urumqi 830046, Xinjiang, China; [email protected] (W.L.); [email protected] (H.M.) 2 China Mobile Research Institute, Beijing 100053, China; [email protected] 3 Xinjiang Vocational & Technical College of Communications, Urumqi 831401, Xinjiang, China; [email protected] * Correspondence: [email protected]

Received: 12 August 2020; Accepted: 9 September 2020; Published: 16 September 2020

Abstract: Optical wireless communications have attracted widespread attention in the traditional power industry because of the advantages of large spectrum resources, strong confidentiality, and freedom from traditional electromagnetic interference. This paper mainly summarizes the major classification and frontier development of power industry optical wireless technologies, including the indoor and outdoor channel characteristics of power industry optical wireless communication system, modulation scheme, the performance of hybrid power line, and indoor wireless optical communications system. Furthermore, this article compares domestic and foreign experiments, analyzes parameters for instance transmission rate, and reviews different application scenarios such as power wireless optical positioning and monitoring. In addition, in view of the shortcomings of traditional power technology, optical wireless power transfer technology is proposed and combined with unmanned aerial vehicles to achieve remote communication. At last, the main challenges and possible solutions faced by power industry wireless optical technologies are proposed.

Keywords: optical wireless technologies; power industry; power line communications; indoor wireless optical communications; optical wireless power transfer

1. Introduction Optical wireless communications (OWC), have many advantages such as large spectrum resources, strong confidentiality, and freedom from traditional electromagnetic interference. Therefore, it has been widely concerned by the habitual industry represented by the electric power field. The further deepening of the integration of various emerging information technologies and the power grid, various optical wireless technology solutions that rely on and serve smart power have been proposed successively and formed a preliminary scale. Optical wireless technologies are mainly classified into two categories: outdoor wireless optical communication, which is often called free space optical (FSO) technology, and indoor wireless optical communications (IWOC), which is regarded as visible light communications (VLC) because of using visible light as the emitter light [1–6]. Specifically, Table1 provides specific parameter indicators to help better understand the differences between the two types of technologies [7–10]. It can be seen from Table1 that the most obvious disadvantage of IWOC is that it is not suitable for outdoor applications because of its short communication distance [7], while outdoor OWC is suitable for long-distance transmission due to its transmission distance up to tens of thousands of kilometers despite environmental interference [7].

Appl. Sci. 2020, 10, 6463; doi:10.3390/app10186463 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 6463 2 of 44

Table 1. Classiﬁcation parameters of optical wireless communications [7–9].

Types Transmitter Receiver Distance Data Rate Spectrum Main Limits Ref 1. Short distance communication; 10 Gbps Indoor 2. Not suitable LED/LD 1 PD 2/camera 10 m (LED)/100Gbps VL [7,8] (VLC) outdoor; (LD) 3. No guaranteed of mobility support Outdoor More than Environment LD PD 40 Gbps IR/VL/UV 3 [7,9] (FSO) 10,000 km dependent 1 LD stands for laser diode. 2 PD is for Photodetector. 3 IR, VL, and UV represent for infrared, visible light and ultraviolet, which their wavelength ranges are 760 nm–1 mm, 360–760 nm and 10–40 nm [7].

Optical wireless technologies with the traditional power of the intersection of information technology can be traced back to 2003. Researchers at Japan’s Keio university took into account the characteristics of white light-emitting diode (LED): high brightness, low power consumption, and long service life etc., and based on the superior properties, first proposed the white LED IWOC and traditional power line communications (PLC) integrated system solutions [1]. This scheme has the inherent advantage that IWOC is free from the traditional radio frequency (RF) electromagnetic interference and the LED light source has high radiation power. Furthermore, the scheme can reduce the construction cost to a greater extent by reusing existing power line infrastructure and avoiding the introduction of new communications cables. At the same time, the above team also presented the prototype system implementation of the proposed scheme, which uses a desk lamp equipped with 120 LED as the IWOC light source and adopts sub-carrier binary shift keying (SC-BPSK). In addition, in the system of compact design, the team could apply to existing ball steep light screw is given and at the same time meet the requirements of the proposed scheme of bulb type IWOC transmitter components. This groundbreaking work has aroused the enthusiasm of many domestic and international research teams. So far, researchers from the University of Johannesburg in South Africa, the University of Texas in the United States, the University of Southampton in the United Kingdom, the University of British Columbia in Canada, Tsinghua University and the Information Engineering University of the People’s Liberation Army have participated in the research work in this interdisciplinary field, and the research results have begun to take shape [10–17]. At present, the research directions in this field mainly include integrated channel characteristics and transmission scheme of PLC and IWOC, modulation schemes, the performances of hybrid PLC and IWOC system, and optical wireless monitoring of high-voltage lines and inspection. This paper will sort out the development of optical wireless technologies for power industry in recent years about the above directions, and demonstrate the frontier progress in this field by combining with a number of experimental prototypes. The main challenges and possible solutions for power industry OWC are summarized to provide useful guidance for the further development of the technology. With the development of science and technology, power industry optical wireless technologies are widely used in positioning, monitoring, and people’s daily life. The application scenarios of power industry optical wireless technologies are shown in Figure1. Figure1a gives the indoor scenario that PLC and IWOC combine for smart home. The tunnel inspection is as shown in Figure1b. The inspector communicates with the LED through a handheld device and transmits detection signals to the data center through the PLC link. The outdoor street lamp monitoring is as shown in Figure1c, which is used to prevent traffic accidents. In addition, Figure1d shows the hospital monitoring system, which could monitor the patient’s physical health in real time. Furthermore, Figure2 lists the block diagram of the power industry optical wireless technologies, including technical classifications, modulation methods, and application scenarios. Appl. Sci. 2020, 10, 6463 3 of 44

Appl.Appl. Sci. Sci. 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 33 of of 45 45

FigureFigureFigure 1. The 1. 1. The The application application application scenarios scenarios scenarios ofof of powerpower power industry industry optical optical optical wireless wireless wireless systems: systems: systems: ( a(a) )indoor (indoora) indoor scenario; scenario; scenario; (b) tunnel(b(b) )tunnel tunnel scenario; scenario; scenario; (c) ( outdoorc(c) )outdoor outdoor scenario; scenario; scenario; ( d( d()d) )hospitalhospital hospital scenario. scenario.scenario.

Figure 2. Block diagram of the power industry optical wireless technologies. FigureFigure 2. Block2. Block diagram diagram of of thethe power industry industry optical optical wireless wireless technologies. technologies.

2. Classiﬁcation2.2. Classification Classification and and and Progress Progress Progress of of of Power Powe Power Industryr Industry Industry Optical Optical Wireless Wireless Wireless Communications Communications Communications

2.1. Channel2.1.2.1. Channel Channel Characteristics Characteristics Characteristics between between between Power Power Power LineLi Linene and and Indoor IndoorIndoor Wireless Wireless Wireless Optical Optical Optical Communications Communications Communications In recent years, researchers have carried out a series of theoretical modeling and measurement In recentIn recent years, years, researchers researchers have have carried carried out a a series series of of theoretical theoretical modeling modeling and andmeasurement measurement activitiesactivities forfor thethe channelchannel characteristicscharacteristics ofof thethe powerpower lineline andand IWOCIWOC systemsystem inin [18–22].[18–22]. TheThe blockblock activities for the channel characteristics of the power line and IWOC system in [18–22]. The block diagramdiagram for for an an integrated integrated system system is is illustrated illustrated in in Figure Figure 3. 3. In In this this system, system, the the information information signal signal is is diagram for an integrated system is illustrated in Figure3. In this system, the information signal influencedinfluenced byby thethe inherentinherent noisenoise ofof PLCPLC andand IWOCIWOC channelchannel successively.successively. TheThe modulemodule afterafter is inﬂuencedpre-compensationpre-compensation by the and inherentand signal signal noiseamplification, amplification, of PLC and and and then then IWOC the the data data channel signal signal successively.to to the the LED LED on on the Thethe DC DC module bias bias to to after pre-compensationdrivedrive LEDLED light,light, and whichwhich signal isis ampliﬁcation, asas lightlight transmittertransmitter and then cocoverageverage the data indoorindoor signal areas.areas. to the FromFrom LED an onan integration theintegration DC bias andand to drive LED light, which is as light transmitter coverage indoor areas. From an integration and commercial point of view, the module is very simple to install on the LED light and could be transformed into a communication device immediately. The light illuminated by the LED is received by the avalanche Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 45 Appl.Appl. Sci. Sci.2020 2020, 10, ,10 6463, x FOR PEER REVIEW 4 of4 45 of 44

commercial point of view, the module is very simple to install on the LED light and could be photodiodetransformed (APD), into a and communication then the APD de ampliﬁesvice immediately. and demodulates The light illuminated the signal according by the LED to is the received received lightby signalthe avalanche level. Takingphotodiode into account(APD), and the then structure the APD of the amplifies entire system,and demodulates IWOC is the used signal as the downlink.according Consider to the received an infrared light adaptersignal level. or PLC Taking uplink, into whichaccount will the bestructure very suitable of the entire for multimedia system, dataIWOC services is used [19]. as the downlink. Consider an infrared adapter or PLC uplink, which will be very suitable for multimedia data services [19].

FigureFigure 3. 3.The The block block diagram diagram of of the the integrated integrated indoorindoor wireless optical optical communications-power communications-power line line communicationscommunications (IWOC-PLC) (IWOC-PLC) system system [ 19[19].].

InIn [19 [19],], the the signal signal is ﬁrst is modulatedfirst modulated into aninto analog an analog signal bysignal the PLCby the modulator. PLC modulator. The modulated The signalmodulated is transmitted signal tois thetransmitted power line to throughthe power the line coupler. through The the system coupler. uses The “PLC system to IWOC uses conversion” “PLC to module,IWOCas conversion” is shown in module, Figure4 ,as which is shown is in thein Figure LED lamp 4, which to receive is in thethe dataLED signal lamp fromto receive the power the data cord. signal from the power cord.

Figure 4. PLC to IWOC module [19]. Figure 4. PLC to IWOC module [19].

2.1.1.2.1.1. Channel Channel Characteristics Characteristics of of Indoor Indoor WirelessWireless OpticalOptical Communications TheThe channel channel frequency frequency response response (CFR) (CFR) of of PLCPLC isis asas shown in the the following following [20,23]: [20,23]: N −()α +α x τ v − j2πxτ Hx=ge() N 01 lplp ej2πxτll Hx=gePLC()X l e (1)  (α0+α1x)τlvp j2πxτl HPLC(x) = l=l=1gle− e− (1) l=1 where g is for the weighting factor of the lth path, which includes the reflection factor and ll g l α α wheretransmissionl is for the factor weighting along factorthe path, of the attenuationth path, which parameters includes are the represented reﬂection factor by and0 and transmission 1 , the factor along the path, attenuation parameters are represented by α and α , the exponent of the exponent of the attenuation factor is expressed in terms of x (usually 00.2 to 1),1 vp is for the group attenuation factorτ is expressed in terms of x (usually 0.2 to 1), vp is for the group velocity and τl stands velocity and ll stands for the lth path delay, and the N denotes path number. for the lth path delay, and the N denotes path number. The received signal in the IWOC part can be expressed as: The received signal in the IWOC part can be expressed as: -j2πxt = -j2πxtLOS HxSheVLC() LOS (2) j2πxtLOS HVLC(x) = ShLOSe− (2) Appl. Sci. 2020, 10, 6463 5 of 44

where the discrete Fourier transform of the hVLC(x) is HVLC(x), S is the photosensitivity, the gain is expressed in terms of hLOS, and tLOS represents the delay of the LOS signal, which is line of sight signal. In other words, there are no obstructions in the signal transmission process from the transmitter to the receiver.

2.1.2. Channel Characteristics of Outdoor Wireless Optical Communications Outdoor transmission channel is atmospheric channel, which contains a large amount of gases and scattered particles, which will produce deviation in the transmission process of light. Some components and particles in the atmosphere will also absorb the energy of light, so that the transmitting power of the transmitted signal will be attenuated. These factors have a great negative effect on outdoor wireless optical communications system. After the OWC system equipment receives the signal through the power line, it is received through the optical interface, and connected to the receiving end through the optical cable, and is generally not affected by lightning strikes. The power supply voltage of wireless optical communication equipment is generally 48 V DC or 220 V AC, and then converted to 12 V − working voltage through the internal power module. The interference of the background light has a remarkable influence on the optical receiver. However, under the interference of intense natural light, especially under direct sunlight, the photodetector may enter a saturated state, causing the photodetector to fail to receive weak communication light. Atmospheric turbulence effect is due to the existence of air vortices with different temperature, pressure, density, flow velocity, and size in the atmosphere. These vortices collide with each other and superpose to form irregular atmospheric turbulence motion in the continuous movement, which results in the change of atmospheric refractive index, and makes visible light signal produce light intensity scintillation and central beam drift. Compared with the atmospheric attenuation effect, the atmospheric turbulence effect will randomly lead to the intensity fluctuation of the optical signal, so the influence of atmospheric turbulence is mainly considered in the analysis of outdoor visible optical communication channel. Weather is crucial to communications when it comes to outdoor communications. The transmittance of visible light decreases in rain, snow, fog, and haze. Snowflakes, rain drops, and fog drops scatter and absorb visible light, etc. The optical signal transmitted through the atmosphere attenuates and the attenuation amplitude varies greatly, which seriously affects the transmission of light and the communication quality of the system. Therefore, channel conditions should be fully considered in LED outdoor visible light communication system to reduce the influence of external interference and improve system performance. Optical wireless (OW) transmission quality is very sensitive to weather. Sunny days have the least influence on OW transmission quality, while rainy days, snow days, and fog days have the greatest influence on wireless optical transmission quality. According to the test, the empirical values of optical wireless attenuation affected by weather are as follows: sunny days is 5–15 dB/km, rainy days is 20–50 dB/km, snow day is 50–150 dB/km, and foggy days is 50–300 dB/km [24]. OWC technologies need to overcome the influence of weather on communications quality in order to meet the requirement of all-weather operation of power system communications. Different from PLC, the reasons affected the quality of wireless communications are that the wireless penetration is weak and they propagate in the process of various obstacles. Therefore, the signals will be reflected, diffracted, and scattered or directly absorbed during the transmission process, and then the wireless signals are weakness. The wireless channel model can be divided into large-scale fading and small-scale fading [10]. Large-scale fading is usually divided into two categories: one is the fading of signal caused by the influence of other objects in the environment, which is called shadow fading, and the other is the transmission loss caused by signal communication over long distances, which is called path loss. Small-scale fading is mainly due to the multipath signal at the receiving end passing through different paths during transmission, which leads to different amplitudes, delays, and phases. When the signal arriving at the receiving end has no LOS transmission, it is regarded as to obey the Rayleigh distribution. Appl. Sci. 2020, 10, 6463 6 of 44 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 45

the signal arriving at the receiving end has no LOS transmission, it is regarded as to obey the InRayleigh the part distribution. of PLC, researchers usually follow the adaptive echo model put forward by ZimmermannIn the [ 10part]. This of PLC, model researchers comprehensively usually fo reflectsllow the the adaptive attenuation echo characteristicsmodel put forward of multipath by propagation,Zimmermann dependent [10]. This frequency, model comprehensively and line length reflects in a dynamic the attenuation line network. characteristics In the IWOC of multipath part, for the conveniencepropagation, of modeling dependent and frequency, analysis, and researchers line length in usually a dynamic use theline wirelessnetwork. opticalIn the IWOC integrator part, for model to representthe convenience the frequency of modeling response and analysis, of IWOC resear channel.chers usually Unlike use the the traditional wireless optical OW non-parametric integrator modelingmodel scheme to represent that requires the frequency data estimation response to obtainof IWOC the densitychannel. distribution Unlike the form, traditional the channel OW gain of channelnon-parametric irradiance modeling component scheme can that be obtained requires directlydata estimation only by to the obtain surface the areadensity of thedistribution light receiver form, the channel gain of channel irradiance component can be obtained directly only by the surface and the room and the average reflective of the room, and the modeling complexity is significantly area of the light receiver and the room and the average reflective of the room, and the modeling reduced. By changing PLC channel parameters, the researchers simulated different PLC environments complexity is significantly reduced. By changing PLC channel parameters, the researchers simulated and frequencydifferent PLC responses. environments With the and increase frequency in theresponse numbers. With of PLC the channel increase taps, in the the number frequency of PLC response grooveschannel of PLC taps, and the IWOC frequency joint response channels grooves will need of PLC to be and further IWOC increased. joint channels will need to be Thefurther results increased. of diff erent PLC network with IWOC-PLC channel are shown in Figure5[ 11]. Figure5a indicatesThe the frequencyresults of different response PLC to IWOC-PLC network with channel. IWOC-PLC It evaluates channel a are bad shown PLC network in Figure environment 5 [11]. withFigure six taps 5a and indicates bandwidth the frequency in excess response of 10 MHz.to IWOC-PLC The parameters channel. It of evaluates IWOC environment a bad PLC network are a room withenvironment an area of about with 25 six m 2,taps the eandffective bandwidth surface in area excess of IWOC of 10 receiver MHz. isThe 3 cmparameters2, and the of distance IWOC from 2 the LEDenvironment transmitter are a is room about with 3 m an [11 area]. of about 25 m , the effective surface area of IWOC receiver is 3 cm2, and the distance from the LED transmitter is about 3 m [11].

(a)

(b)

FigureFigure 5. The 5. The results results of di ofﬀ erentdifferent PLC PLC network network with with their their resulting resulting IWOC-PLC IWOC-PLC channel:channel: ( a)) 6 PLC taps networktaps [network11]; (b) 12[11]; PLC (b) 12 taps PLC network taps network [11]. [11].

Figure5 shows the PLC channel and the resulting IWOC-PLC channel. From the results, the PLC-IWOC channel has a notch characterized which varies throughout the entire frequency band. We have likewise observed the eﬀect of the IWOC channel, which weakens frequency response Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 45

Figure 5 shows the PLC channel and the resulting IWOC-PLC channel. From the results, the

Appl. Sci.PLC-IWOC2020, 10, 6463 channel has a notch characterized which varies throughout the entire frequency7 of 44 band. We have likewise observed the effect of the IWOC channel, which weakens frequency response of the PLC channel. IWOC-PLC channel also simulates and approximates the low-pass frequency of thecharacteristics PLC channel. IWOC-PLCof the PLC channel. channel Specifically, also simulates Figure and approximates5b shows the joint the low-pass channel characteristics frequency of characteristicsPLC and of IWOC the PLC with channel. 12 PLC Specifically, taps. In particular, Figure5b the shows VLC the in jointFigure channel 5a,b represented characteristics by ofIWOC PLC in this and IWOCpaper. with 12 PLC taps. In particular, the VLC in Figure5a,b represented by IWOC in this paper. Gao etGao al. studied et al. studied the random the random channel ch generatorannel generator of the hybridof the hybrid PLC and PLC IWOC and IWOC system, system, and a setand a set of variableof variable parameters parameters in [25] in is used[25] is to used describe to describe the complex the complex transfer transfer function function of the generalof the general typical typical apartmentapartment complex complex system system network. network. The power The power line (with line branch)(with branch) is the backboneis the backbone network. network. IWOC IWOC LED isLED used is as used a visible as a visible light modulator light modulator for electrical for electr signalical scheduling,signal scheduling, as shown as shown in Figure in 6Figurea. 6a. In [26],In the [26], CFR the of theCFR indoor of the power indoor line power channel line is channel presented is presented as the sum as of allthe propagation sum of all propagation paths underpaths low-voltage under conditions.low-voltage APDconditions. is a photoelectric APD is a phot converteroelectric that converter demodulates that demodulates an optical signal an optical into ansignal electrical into signalan electrical and is generallysignal and considered is generally to be considered an all-pass to filter be an because all-pass of itsfilter broadband because of its properties.broadband Figure 6properties.b shows the Figure measured 6b pureshows power the measured line channels pure and power integrated line channels.channels Itand can integrated be seen fromchannels. the figure It can that, be comparedseen from tothe pure figure PLC that, channel, comp theared attenuation to pure PLC of thechannel, integrated the attenuation channel is of the large andintegrated the channel channel performance is large and is poorthe channel [25]. performance is poor [25].

(a)

(b)

FigureFigure 6. The 6. integrated The integrated PLC and PLC IWOC: and IWOC: (a) Channel (a) Channel structure structure of the of integratedthe integrated PLC PLC and and IWOC; IWOC; (b) (b) measuredmeasured pure pure PLC PLC channel channel and and integrated integrated channel channel [25]. [25].

To characterize the inﬂuence of LED modulation bandwidth on the frequency response of integrated PLC and IWOC channel, the researchers measured part of the frequency response of IWOC Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 45 Appl. Sci. 2020, 10, 6463 8 of 44 To characterize the influence of LED modulation bandwidth on the frequency response of integrated PLC and IWOC channel, the researchers measured part of the frequency response of IWOCand the and overall the frequencyoverall frequency response characteristicsresponse characte of theristics mixed of systemthe mixed in the system laboratory in the environment, laboratory environment,as shown in Figure as shown7. in Figure 7.

Figure 7. The measured channel frequency response (CFR) of а a typical integrated PLC and IWOC system [[19].19].

The measurement results are demonstrated that the attenuation introduced by the LED light source near 50 MHz is only 10 dB. However, once the IWOC system is intended to be integrated into the PLC system, the attenuation of the overall system at high frequencies will significantlysignificantly reduce the available bandwidth of the system.system. Specifically, Specifically, PLC PLC channel us useses a typical 6-diameter model, while thethe IWOCIWOC channelchannel filters filters out out stray stray light light with with blue blue light light filters. filters. Quantitative Quantitative results results show show that that the theattenuation attenuation of the of hybrid the hybrid system system is about is 40about dB at 40 30 dB MHz, at indicating30 MHz, thatindicating the available that the modulation available modulationbandwidth ofbandwidth the system of will the besystem severely will limited.be severely limited.

2.2. Modulation Methods of Hybr Hybridid Optical Wireless Communications Orthogonal frequency division mu multiplexingltiplexing (OFDM) is a kind of modulation technology with high spectral efficiency, efficiency, which can provide a simplesimple solution for channel dispersion and can be implemented in the digital domain. OFDM OFDM can can divi dividede the available bandwidth of the channel into several sub-channels, and realize frequency division multiplexing through the orthogonality among sub-channels. It It can can also also realize realize signal transmission and and adapt to the adjustment conditions of the channel byby using using the the allocation allocation of bit of and bit power and onpowe ther subcarrier.on the subcarrier. It reduces theIt reduces subcarrier the transmission subcarrier transmissionrate and prolongs rate theand period prolongs of the the code period element, of the so code it has element, excellent so anti-multipath it has excellent effect anti-multipath performance effectand realizes performance the high-speed and realizes transmission the high-speed rate under transmission the condition rate of under limited the bandwidth. condition In of addition, limited bandwidth.combined with In addition, multi-input combined and multi-output with multi-input (MIMO) and technology, multi-output it is required(MIMO) thattechnology, the receiver it is requiredcan receive that the the optical receiver signal can receive from the the LED optical lamp signal array from in orderthe LED to resolvelamp array multiple in order independent to resolve multiplecommunication independent channels. communication The parallel transmission channels. The of multipleparallel transmission transmitting andof multiple receiving transmitting units could andimprove receiving the performance units could of improve IWOC and the make performance different usersof IWOC occupy and non-overlapping make different subcarrierusers occupy sets, non-overlappingtherefore realizing subcarrier downlink sets, multi-user therefore transmission. realizing downlink multi-user transmission.

2.2.1. Hybrid Wireless Optical System in OFDM Modulation 2.2.1. Hybrid Wireless Optical System in OFDM Modulation In view of the fact that multiple light sources with visible light communication functions overlap In view of the fact that multiple light sources with visible light communication functions the service area, S. Hranilovic research team from Canada proposed collaborating with and provide overlap the service area, S. Hranilovic research team from Canada proposed collaborating with and data for IWOC emitters through power lines. Speciﬁcally, they proposed a system architecture named provide data for IWOC emitters through power lines. Specifically, they proposed a system hybrid IWOC-PLC, and gave the corresponding analytical framework [14,27,28]. In this scheme, architecture named hybrid IWOC-PLC, and gave the corresponding analytical framework [14,27,28]. Appl. Sci. 2020, 10, 6463 9 of 44

Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 45 researchers apply spatial optical-orthogonal frequency division multiplexing (SO-OFDM) to multiple In this scheme, researchers apply spatial optical-orthogonal frequency division multiplexing sources, and propose an OFDM scheme to explore frequency selectivity of IWOC and PLC channels, (SO-OFDM) to multiple sources, and propose an OFDM scheme to explore frequency selectivity of as shownIWOC and in Figure PLC channels,8[ 14,23]. as Compared shown in withFigure the 8 integration[14,23]. Compared of traditional with the IWOC integration and PLC, of traditional this scheme improvesIWOC and the PLC, peak this to average scheme improves power ratio the (PAPR)peak to problemaverage power caused ratio by the(PAPR) application problem ofcaused OFDM by to LEDthe emitter, application and overcomesof OFDM to the LED interference emitter, and between overcomes diﬀerent the lightinterference sources between through different the cooperation light betweensources spatial through distributed the cooperation light sources. between In thespatial meantime, distributed the robustnesslight sources. of theIn the system meantime, against the local linkrobustness occlusion of is the enhanced. system against local link occlusion is enhanced.

Figure 8. Block diagram of the hybrid IWOC and PLC for indoor multiuser downlink [14,23]. Figure 8. Block diagram of the hybrid IWOC and PLC for indoor multiuser downlink [14,23].

Specifically,Specifically, in in the the aboveabove scheme, the the basic basic mathem mathematicalatical model model of the of proposed the proposed system systemis to be is submitted, specifically considering static continuous noise, periodic static continuous noise, and to be submitted, specifically considering static continuous noise, periodic static continuous noise, periodic pulse noise. In the visible light link, only the contribution of line of sight (LOS) component and periodic pulse noise. In the visible light link, only the contribution of line of sight (LOS) component and the influence of stray light scatter noise and receiver preamplifier noise are considered. At the and the influence of stray light scatter noise and receiver preamplifier noise are considered. At the same time, the effect of multipath fading on the performance of the system is ignored since the same time, the effect of multipath fading on the performance of the system is ignored since the detector detector size is much larger than the optical wavelength. Through the quantitative analysis results, sizethe is team much further larger confirmed than the optical that the wavelength. IWOC link can Through provide the SNR quantitative up to 22 dB analysis to meet results, the functional the team furtherrequirements confirmed of that IWOC the IWOCand basic link lighting. can provide Theref SNRore, up the to 22overall dB to meetperformance the functional of the requirementsintegrated of IWOCsystem andis to basica great lighting. extent affected Therefore, by the the PLC overall link. performance of the integrated system is to a great extent aResearchersffected by the also PLC try link. to introduce OFDM and color shift keying (CSK) technologies into the IWOCResearchers channel, alsoand trystudy to introducethe behavior OFDM of these and series color channels shift keying under (CSK) the condition technologies of full into link the IWOCtransmission channel, [29]. and Specifically, study the behaviorin the PLC of link, these researchers series channels sought underto apply the the condition quadrature of fullphase link transmissionshift keying [ 29(QPSK)]. Specifically, and OFDM in themodulation PLC link, scheme researcherss. In IWOC sought link, to applyCSK scheme the quadrature is utilized phase to shiftimprove keying system (QPSK) performance. and OFDM The modulation researchers schemes. are determined In IWOC to the link, mapping CSK scheme scheme is design utilized of to improveOFDM system modulation performance. constellation The researchersmap of PLC arelink determined to CSK constellation to the mapping map in scheme IWOC designlink. of OFDM modulationFacing constellation the communications map of PLC scene link between to CSK constellationoutdoor buildings, map in researchers IWOC link. in South Africa proposedFacing thethe communications integrated scheme scene of betweenPLC and outdoor IWOC buildings, to conduct researchers targeted inexploration. South Africa Similarly, proposed thespecific integrated system scheme schemes of PLC introduce and IWOC phase to shift conduct keying targeted (PSK) exploration.and CSK modulation Similarly, respectively specific system in schemesPLC and introduce IWOC links phase [30]. shift The keying mapping (PSK) method and CSK between modulation PSK and respectively CSK modulation in PLC symbols and IWOC is linksgiven [30 ].and The verified mapping by simu methodlation between and experiment. PSK and CSKAt the modulation same time, symbols the interference is given effect and verified of solar by simulationrays on the and red, experiment. green, and Atblue the symbols same time, of CSK the is interference analyzed and eff ectemphasized. of solar rays Based on on the the red, above green, research work, investigators further try a different modulation scheme combines to obtain the and blue symbols of CSK is analyzed and emphasized. Based on the above research work, investigators further improvement of the system performance. Specifically, spread frequency shift keying (S-FSK) further try a different modulation scheme combines to obtain the further improvement of the system is processed in PLC link, and an on off keying (OOK) modulation is tried in IWOC. The significant performance. Specifically, spread frequency shift keying (S-FSK) is processed in PLC link, and an on advantages of the scheme depended on its low cost and low complexity. Correspondingly, the off keying (OOK) modulation is tried in IWOC. The significant advantages of the scheme depended on disadvantage is the case that the transmission rate is extremely limited. Through the prototype itsexperiment low cost and based low on complexity. this scheme, Correspondingly, the researchers measured the disadvantage the impact is the of sunlight case that exposure the transmission on the ratesystem is extremely performance limited. at four Through different the points prototype in the experiment day, and determined based on this that scheme, sunlight the irradiance researchers is measuredthe main the source impact of system of sunlight noise exposure[31,32]. on the system performance at four different points in the day, and determined that sunlight irradiance is the main source of system noise [31,32]. Appl. Sci. 2020, 10, 6463 10 of 44

The various applications of hybrid PLC and IWOC technologies have been studied in [1,33–36]. The first integrated scheme using OOK for data modulation is discussed in [35]. On this basis, OFDM technology was used in hybrid IWOC/PLC system to decrease inter-symbol interference [33]. In [34], an integrated PLC and IWOC communication system based on OFDM modulation was proposed to provide electronic medical services inside the hospital. For cascaded BPLC and IWOC communication systems, the performance of binary phase shift keying (BPSK) OFDM for subcarrier modulation is studied to reduce the impact of impulse noise and ISI. In [36], the opportunity of PLC and IWOC channel integration was investigated. Literatures [37,38] analyzed the capacity of hybrid IWOC/PLC systems based on AF and DF. First, the source node passes the information to the relay node through the PLC link, and then the relay node amplifies [39] or decodes [20] the transmitted information, and finally forwards the information to the destination node through the IWOC link. A hybrid system of IWOC and RF is considered in the indoor environment [40] to improve the average throughput and interrupt throughput of each user. In [1], a cascaded IWOC/PLC system is considered, which is compatible to provide a variety of services with different service quality requirements.

2.2.2. Hybrid Optical Wireless System in NOMA Mode Non-orthogonal multiple access (NOMA) is deemed to be another promising technology that can significantly improve spectral efficiency and connectivity. Different from orthogonal multiple access (OMA), multiple users in NOMA could transmit on the same frequency resources at the same time by superimposing the signals of multiple users in the air. At the receiving terminal, successive interference cancellation (SIC) could be applied to sequentially decode each individual signal from the received composite signal. It has been proven that NOMA could increase spectral efficiency and promote large-scale connectivity. Meanwhile, NOMA signaling can reduce the cost of communications systems to help provide low-latency services. In [41–43], researchers analyzed the signal-to-noise (SNR) about different OFDM modulation methods of PLC and IWOC system. It is anticipated that the application of NOMA to the IWOC systems will further increase the system capacity and user connectivity [44,45]. In [45], it was shown that the spectrum efficiency of IWOC system with NOMA function is superior to that of IWOC system with conventional orthogonal frequency division multiple access (OFDMA). In [45], the authors analyze the performance of the coverage probability and the ergodic rate. The results show that using NOMA could enhance these performance metrics compared to OMA. Though there have been some pioneering works in the NOMA-enabled IWOC systems, these studies mainly focus on low order modulation schemes. Furthermore, Lin and his researchers in [46] proposed a NOMA scheme based on VLC based on optical power domain to address the deficiencies of the traditional PD-NOMA scheme. The maximum drive current of the scheme is significantly reduced, and the gain bandwidth product of the drive circuit is also obviously reduced. Experimental results show that the OPD-NOMA scheme provides better bit error rate performance. The experiments are as shown in Figures9 and 10. Where Figure9 shows the experimental setup of the proposed IWOC-OPD-NOMA for two users [46]. Use arbitrary waveform generator (AWG, RIGOLDG5352) to convert digital OFDM signal to an analog format. Note that the AWG is also used as a driver amplifier. The detail block diagram is shown in Figure 10. In TX, two 1.7 Mbaud-based band OFDM signals are mapped to QPSK and converted to an RF subcarrier whose frequency is 1.25 MHz. When the symbol rate is 1.7 m, the maximum output peak voltage (Vpp) is 20 V. For the driver amplifier, the gain-bandwidth product is a constant [46]. As the symbol rate increases, the maximum output value decreases. The LEDs in the LED array are divided into two groups, and each group is assigned to a user. The OFDM electrical signals of different Vpp use the offset tee module to perform DC level shift before the IM of the grouped LEDs. The modulation bandwidth of the LED is 2.5 MHz [46]. Appl. Sci. 2020, 10, 6463 11 of 44

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 45

FigureFigure 9. 9. ExperimentExperiment setup setup for for IWOC-OPD-NOMA IWOC-OPD-NOMA [46]. [46].

FigureFigure 10. 10. BlockBlock diagram diagram for for the the downlink downlink IWOC-OPD-NOMA IWOC-OPD-NOMA [46]. [46].

InIn addition, addition, the the bit error ratioratio (BER)(BER) comparisoncomparison between between NOMA NOMA and and OMA OMA is is shown shown in in Figure Figure 11 . 11.As canAs can be seen, be seen, it could it could be used be inused the OMAin the low OMA SNR low regime SNR [ 47regime]. On the[47]. contrary, On the thecontrary, simulation the simulationresults of NOMA results show of NOMA that compared show that with compared OMA, the BERwith is OMA, derived the under BER the is conditionderived ofunder low SNRthe conditionregime more of low accurately. SNR regime Furthermore, more accurately. it should Furt behermore, noted that it NOMAshould hasbe noted a worse that BER NOMA than has OMA a worseowing BER to the than multi-user OMA owin interferenceg to the multi-user in NOMA. interference in NOMA. Specifically,Specifically, forfor the the user user that that is decoded is decoded in NOMA in NOMA first, the first, influence the ofinfluence this mutual of this interferences mutual interferencesis more obvious. is more For obvious. the user thatFor the is decoded user that last is indecoded NOMA, last if thein NOMA, SIC performance if the SIC isperformance good enough, is goodits bit enough, error rate its performance bit error rate is performance similar to that is ofsimila OMA.r to In that addition, of OMA. for In users addition, that are for decoded users that first are in decodedNOMA, first when in a NOMA, higher modulation when a higher scheme modulation is utilized, scheme the BER is utilized, gap between the BER OMA gap and between NOMA OMA will and NOMA will increase. This is because under the same transmission SNR, the constellation of high-order modulation schemes is more susceptible to noise [47]. Appl. Sci. 2020, 10, 6463 12 of 44 increase. This is because under the same transmission SNR, the constellation of high-order modulation schemes is more susceptible to noise [47]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 45

(a)

(b)

FigureFigure 11. ( a11.) The(a) The comparison comparison of non-orthogonalof non-orthogonal multiple multiple access access (NOMA) (NOMA) withwith OMAOMA in terms of of BER BER with QPSK; (b) the comparison of NOMA with OMA in terms of bit error ratio (BER) with with QPSK; (b) the comparison of NOMA with OMA in terms of bit error ratio (BER) with 16PSK [47]. 16PSK [47]. In [48], cellular respiration has been applied to traﬃc transmission between devices in cellular In [48], cellular respiration has been applied to traffic transmission between devices in cellular networks and access point. In [49], a distributed algorithm based on pricing has been used, networks and access point. In [49], a distributed algorithm based on pricing has been used, which whichregards regards the thecongestion congestion degree degree of the base of the station base (BS) station and the (BS) transmission and the transmission environment of environment mobile of mobileterminals terminals as parameters as parameters of load ba oflancing load balancing in heterogeneous in heterogeneous networks. networks. Appl. Sci. 2020, 10, 6463 13 of 44

On the bases of line voltage, the lines in the transmission network can be divided into high voltage,Appl. medium Sci. 2020, 10 voltage,, x FOR PEER and REVIEW low voltage in [50–53]. In general, the power system includes generation,13 of 45 transmission, distribution, and consumption [51]. HV generates the power and transmits it. Then MV and LV regionsOn the bases distribute of line the voltage, power, the andlines finally in the transmission LV completes network the consumption can be divided [51 into]. Typically,high voltage, medium voltage, and low voltage in [50–53]. In general, the power system includes the range of high, medium, and low pressures varies depending on the region, industry, and so on. generation, transmission, distribution, and consumption [51]. HV generates the power and transmits Generallyit. Then speaking, MV and LV the regions standard distribute voltage the classes power, forand HV finally are LV between completes 110 the kV consumption and 380 kV, [51]. such as 132 kVTypically, and 220 the kV, range the of standard high, medium, voltage and classes low pressures for MV varies are from depending 10 kV on to the 33 kV,region, like industry, 13.2 kV and 33 kV,and which so on. are Generally mentioned speaking, in [54 the], andstandard the LV voltage classes classes are belowfor HV 1are kV, between generally 110 0.24kV and/0.4 380 kV kV, [52 –54]. Consideringsuch as 132 the kV rapid and development 220 kV, the standard of power voltage grid classes in large for and MV super-large are from 10 citieskV to in33 recentkV, like years, 13.2 kV the HV is goingand to33 500kV, kV.which are mentioned in [54], and the LV classes are below 1 kV, generally 0.24/0.4 kV Distinct[52–54]. Considering from the low-voltage the rapid development power line of studies powerabove, grid in researcherslarge and super-large from Penn cities State in Universityrecent discussedyears, thethe transmissionHV is going to characteristics500 kV. of medium and low piezoelectric transmission media and Distinct from the low-voltage power line studies above, researchers from Penn State University IWOC system [50]. According to [50], both medium-voltage and low-voltage power networks can discussed the transmission characteristics of medium and low piezoelectric transmission media and provide very high transmission capacity that cannot be achieved by any other cable network except IWOC system [50]. According to [50], both medium-voltage and low-voltage power networks can opticalprovide fiber, very and thehigh reflection transmission caused capacity by the that impedance cannot be achieved mismatch byin any the other above cable networks network will except worsen the performanceoptical fiber, ofand the the system. reflection At the caused same by time, the theimpedance research mismatch results show in the that above the PLC networks subsystem will has the possibilityworsen the to performance realize 1 Gbps of the transmission system. At ratethe same through time, the the reasonable research results network show design that the for PLC instance impedancesubsystem matching. has the possibility In the IWOC to realize subsystem, 1 Gbps tr theansmission different rate influences through the of multipathreasonable network propagation on high-speeddesign for datainstance transmission impedance arematching. considered. In the LEDsIWOCare subsystem, commonly the useddifferent inindoor influences settings of as transmittersmultipath of propagation OWC. Therefore, on high-speed in [50 data], the transm authorsission consider are considered. the IWOC LEDs model, are commonly and conclude used in that distortionindoor are settings related as to transmitters room size andof OWC. system Therefore, configuration, in [50], andthe authors the above consider distortion the IWOC can be model, minimized and conclude that distortion are related to room size and system configuration, and the above through judicious design. This work theoretically preliminarily proves the technical potential of distortion can be minimized through judicious design. This work theoretically preliminarily proves medium and low voltage PLC and IWOC integrated systems to provide high-speed transmission. the technical potential of medium and low voltage PLC and IWOC integrated systems to provide Inhigh-speed order to transmission. verify the actual performance of smart power OWC, research teams at domestic and abroadIn order actively to verify carry the out actual the performance construction of smart and test power of powerOWC, research wireless teams optical at domestic system and [55 –57]. Furthermore,abroad actively to better carry meet out the the need construction of hot scenes and such test asofhospitals, power wireless shopping optical malls, system and gymnasiums,[55–57]. researchersFurthermore, from Tsinghua to better University meet the areneed committed of hot sc toenes applying such integratedas hospitals, PLC shopping and IWOC malls, technologies and to indoorgymnasiums, broadband researchers data broadcasting. from Tsinghua The University proposed are system committed significantly to applying reduces integrated the complexityPLC and of IWOCIWOC network technologies protocol to and indoor the degreebroadband of infrastructure data broadcasting. transformation. The proposed system significantly Specifically,reduces the complexity the researchers of IWOC gave network an experimental protocol and demonstrationthe degree of infrastructure of LED two-light transformation. network and Specifically, the researchers gave an experimental demonstration of LED two-light network and evaluated the performance of the proposed scheme [15], as shown in Figure 12. The high-definition evaluated the performance of the proposed scheme [15], as shown in Figure 12. The high-definition television (HDTV) program is directed to the hybrid system through the time domain OFDM technology. television (HDTV) program is directed to the hybrid system through the time domain OFDM The measurementtechnology. The shows measurement that the bandwidthshows that ofthe the band systemwidth is of 8 MHz,the system specifically is 8 MHz, distributed specifically between 2 MHzdistributed and 10 MHz. between 2 MHz and 10 MHz.

Figure 12. Demonstration setup of the PLC and IWOC broadcasting system in the laboratorial Figure 12. Demonstration setup of the PLC and IWOC broadcasting system in the laboratorial environment [15]. environment [15]. Appl.Appl. Sci. Sci. 20202020, 10, 10, x, FOR 6463 PEER REVIEW 1414 of of 45 44

Under the condition that the visible light path length is extended to 8 m and the indoor Under the condition that the visible light path length is extended to 8 m and the indoor interference interference lighting source is normally turned on, and the proposed hybrid system can provide a lighting source is normally turned on, and the proposed hybrid system can provide a transmission transmission performance of about 48 Mbps. Since the typical ceiling height is 3 m, the receiver performance of about 48 Mbps. Since the typical ceiling height is 3 m, the receiver consisting of an consisting of an avalanche diode is placed in an overlapped area covering both lights at a distance of avalanche diode is placed in an overlapped area covering both lights at a distance of 3 m. To evaluate 3 m. To evaluate the system performance under different display configurations, the researchers the system performance under different display configurations, the researchers kept the transmitting kept the transmitting signal power of left LED constant, and evaluated the system performance signal power of left LED constant, and evaluated the system performance changes under different changes under different multipath propagation conditions by adjusting the power line length multipath propagation conditions by adjusting the power line length connected to right LED and the connected to right LED and the transmitting power level of right LED, as shown in Figure 12 [15]. transmitting power level of right LED, as shown in Figure 12[ 15]. The result shows that the modulation The result shows that the modulation error ratio (MER) is proportional to the power line length and error ratio (MER) is proportional to the power line length and inversely to the power of right LED. inversely to the power of right LED. The above phenomenon can be explained as: when the The above phenomenon can be explained as: when the secondary path increases or the power level secondary path increases or the power level decreases, the interference level will be correspondingly decreases, the interference level will be correspondingly reduced. Therefore, attention must be paid to reduced. Therefore, attention must be paid to the profound influence of inter modulation the profound influence of inter modulation interference on the performance of the receiving end. interference on the performance of the receiving end. To further illustrate the various modules in Figure 12, Song et al. from Tsinghua University To further illustrate the various modules in Figure 12, Song et al. from Tsinghua University illustrated the different hardware devices of PLC and IWOC broadcasting system in the laboratorial illustrated the different hardware devices of PLC and IWOC broadcasting system in the laboratorial environment in Figure 13, including LED temp, PLC to IWOC module, PLC modulator, and demodulator environment in Figure 13, including LED temp, PLC to IWOC module, PLC modulator, and in receiver [15]. demodulator in receiver [15].

(a) (b)

FigureFigure 13. 13. The hardwarehardware devices devices of PLCof PLC and IWOCand IWOC broadcasting broadcasting system insystem the laboratorial in the laboratorial environment: environment:(a) LED temp ( [a15) ];LED (b) PLCtemp to [15]; IWOC (b module) PLC to [15 IWOC]; (c) PLC module modulator [15]; [(15c)]; PLC (d) Demodulator modulator [15]; [15]. (d) Demodulator [15]. In addition, they proposed extending the above system to underground tunnel sites in the next year,In as addition, shown inthey Figure proposed 14. They extending implemented the above the system IWOC-PLC to underground integrated duplextunnel sites communications in the next year,network, as shown and equipment in Figure 14. mainly They consisting implemented of the the LED IWOC-PLC ﬁxture with integrated DF module duplex and IWOC-PLC communications coupler, network, and equipment mainly consisting of the LED fixture with DF module and IWOC-PLC Appl. Sci. 2020, 10, 6463 15 of 44 Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 45 overviewcoupler, ofoverview IWOC-PLC of IWOC-PLC access network access network [17], duplex [17], duplex IWOC IWOC handheld handheld terminal, terminal, and and RX R&TX &X TmodulesX inmodules handheld in terminalshandheld [terminals17]. The display[17]. The system display is system based onis based decoding on decoding and forwarding and forwarding technology to realizetechnology duplex to transmission, realize duplex so transmission, as to build aso symmetric as to build duplex a symmetric broadband duplex access broadband network access to support diversiﬁednetwork to duplex support high-speed diversified communication duplex high-speed services. communication In the underground services. In tunnelthe underground scene, a plurality oftunnel light emitting scene, a plurality diodes areof light used emitting to realize diodes the are lighting used to function, realize the and lighting they arefunction, connected and they by power lines.are connected In the experimental by power lines. prototype, In the experimental the length of prototype, the power the line length exceeds of the 100 power m. line exceeds 100 m.

(a) (b)

FigureFigure 14. 14.The The IWOC-PLCIWOC-PLC integrated integrated dupl duplexex communications communications network: network: (a) The (a LED) The fixture LED ﬁxturewith DF with DF modulemodule and and IWOC-PLC IWOC-PLC coupler coupler [17[17];]; (b(b)) overview overview ofof IWOC-PLCIWOC-PLC access network [17]; [17]; ( (cc)) duplex duplex IWOC IWOC handheld terminal [17]; (d) RX & TX modules in handheld terminals [17]. handheld terminal [17]; (d)RX &TX modules in handheld terminals [17].

InIn DF DF module, module, PLCPLC and IWOC IWOC coupler, coupler, uplink uplink photodetector photodetector is encapsulated is encapsulated into LED into device. LED device. In addition, the handheld terminal is embedded in the IWOC receiving and sending modules. In addition, the handheld terminal is embedded in the IWOC receiving and sending modules. Experimental test results show that the distance between LED light source and receiving terminal is Experimental3 m under testthe resultscondition, show the that above the distancesystem can between support LED voice, light video, source and and duplex receiving broadband terminal is 3 m undercommunication the condition, up to the 5 aboveMbps. As system long canas 3 supportdays of field voice, test, video, BER performance and duplex broadbandcan be maintained communication at 5 up10 to−5 5. The Mbps. data As rate long of asthe 3 system days of can field be test, further BER ex performancetended to 30 canMbps. be maintainedIn the multi-user at 10 −scenario,. The data rate ofefficient the system and can cost-effective be further non-interference extended to 30 Mbps.commu Innications the multi-user are realized scenario, between effi cientdifferent and users. cost-e ffective non-interferenceFor the application communications of power tunnel, are the realized research between team of di statefferent grid users. also proposed For the application the design of of power tunnel,IWOC the and research PLC protocol team of for state power grid tunnel also proposed inspection the and design the ofpatrol IWOC scheme and PLCof LiDAR-based protocol for power tunnelnavigation inspection UAV, andproviding the patrol new schemetechnical ofoptions LiDAR-based for the enhancement navigation of UAV, power providing wireless newoptical technical patrol technology [55,56]. options for the enhancement of power wireless optical patrol technology [55,56]. In addition, the research team from the Institute of Semiconductors of the Chinese Academy of Sciences focused on smart home scenarios and carried out home network design and a prototype Appl. Sci. 2020, 10, 6463 16 of 44 Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 45 implementationIn addition, are the based research on IWOC team from and the PLC. Institut Thee proposed of Semiconductors core equipment of the Chinese of the Academy home network of systemSciences mainly focused includes on smart home home gateway, scenarios white and LEDcarried driver out home and networknetwork design appliance. and a prototype Among them, the homeimplementation gateway assumesare based theon IWOC function and of PLC. connecting The proposed with core external equipment network of the or home external network personal consumptionsystem mainly terminal. includes The home controlled gateway, terminal white LED in driver the home and network network appliance. is called Among network them, appliance. the Eachhome communications gateway assumes unit is the interconnected function of connecting by PLC bus with and external gateway network to realize or independentexternal personal two-way consumption terminal. The controlled terminal in the home network is called network appliance. communications. The test results of this system show that 22,953 instructions are sent through the Each communications unit is interconnected by PLC bus and gateway to realize independent network transmitter for testing, and the network receiver can correctly receive 22,829 instructions, two-way communications. The test results of this system show that 22,953 instructions are sent and thethrough correct thetransmission network transmitter rate of for system testing, instructions and the network is 99.46%, receiver meeting can correctly the basic receive requirements 22,829 of actualinstructions, use [50]. and the correct transmission rate of system instructions is 99.46%, meeting the basic requirements of actual use [50]. 2.3. Integrated Transmission Schemes of Power Line and Indoor Wireless Optical Communications PLC2.3. Integrated and RF Transmission integration Schemes into wireless of Power relay Line and systems Indoor hasWireless been Optical proposed Communications for many applications, includingPLC indoor, and RF outdoor, integration and into smart wireless grids relay [29,58 systems–62]. In has [63 been], a cascadedproposed IWOCfor many and applications, outdoor WOC communicationsincluding indoor, system outdoor, is studied and smart and grids DF relay [29,58–62]. is established. In [63], a cascaded The capacity IWOC of and the outdoor system WOC is studied, and thecommunications results show system that theis studied hybrid and system DF relay has is highestablished. data transmission The capacity of e ffitheciency. system Someis studied, different integratedand the scheme results ofshow power that line the andhybrid IWOC system is shown has high in Figuredata transmission 15. DF relay efficiency. hybrid IWOC-PLCSome different scheme, integrated scheme of power line and IWOC is shown in Figure 15. DF relay hybrid IWOC-PLC hybrid IWOC-PLC scheme, multichannel hybrid IWOC-PLC system and full duplex DF IWOC-PLC scheme, hybrid IWOC-PLC scheme, multichannel hybrid IWOC-PLC system and full duplex DF schemeIWOC-PLC are proposed scheme [15 are–17 proposed,19]. Where [15–17,19] the PLC. Where represents the PLC forrepresents the IWOC. for the IWOC.

(a) (b)

FigureFigure 15. The 15. The diﬀ differenterent schemes schemes of of IWOC-PLC IWOC-PLC systems:systems: (a (a) )DF DF relay relay hybrid hybrid IW IWOC-PLCOC-PLC scheme scheme [15], [15], (b) hybrid(b) hybrid IWOC-PLC IWOC-PLC scheme scheme [15 [15,16,19];,16,19]; ((cc)) multichannelmultichannel hybrid hybrid IWOC-PLC IWOC-PLC system system [15]; [(15d)]; full (d ) full duplexduplex DF IWOC-PLCDF IWOC-PLC scheme scheme [17 [17].].

Indoor multi-user downlink of hybrid circuit and IWOC must face the limited transmission bandwidth of power line and visible integrated channel. In order to improve the transmission spectrum eﬃciency of the above hybrid system, more and more researchers have proposed multi-carrier system Appl. Sci. 2020, 10, x FOR PEER REVIEW 17 of 45

Indoor multi-user downlink of hybrid circuit and IWOC must face the limited transmission Appl.bandwidth Sci. 2020 , 10of, 6463power line and visible integrated channel. In order to improve the transmission17 of 44 spectrum efficiency of the above hybrid system, more and more researchers have proposed multi-carrier system scheme to better cope with the frequency selectivity of the channel scheme[27,28,35,41,64,65], to better cope as shown with the in frequencyFigure 16. selectivity In this paper, of the the channel IWOC [ 27in, 28the,35 module,41,64,65 in], asFigure shown 16 inis Figurerepresented 16. In by this IWOC. paper, the IWOC in the module in Figure 16 is represented by IWOC.

Figure 16. Block diagram of the integrated PLC and IWOC multicarrier system [[15].15].

On the basisbasis ofof thethe aboveabove work,work, researchersresearchers developeddeveloped aa PLC-orientedPLC-oriented visiblevisible lightlight positioningpositioning system andand presentedpresented aa laboratorylaboratory prototypeprototype demonstrationdemonstration [[15],15], asas shownshown inin FigureFigure 1717.. TheThe aboveabove prototypeprototype leveragesleverages an an Android Android application application to achieveto achieve a few a few KBPS KBPS level level of low of speedlow speed links. links. This workThis willwork help will speed help upspeed the commercializationup the commercialization of such of systems such systems and prototypes. and prototypes. Such a power Such a wireless power opticalwireless positioning optical positioning system can system make can full make use of full the use orientation of the orientation of LED beam of LED and beam PLC and return PLC network return tonetwork provide to usersprovide with users customized with customized services based services on userbased location, on user including location, indoor including navigation, indoor devicenavigation, tracking, device and tracking, equipment and management equipment [management15,19]. In addition, [15,19]. existingIn addition, indoor existing monitoring indoor or sensingmonitoring device or sensing can also device realize can the also power realize supply the andpower communications supply and communications function of the function above system of the atabove the samesystem time, at the so same as to time, timely so as transmit to timely the tran monitoringsmit the monitoring or sensing or data sensing to the data indoor to the wireless indoor terminal.wireless terminal. In [1,12 ,15In ,[1,12,15,17,33,36,43,50,66],17,33,36,43,50,66], these studies these studies have shown have shown that the that cooperative the cooperative relay aided relay PLC-IWOCaided PLC-IWOC system system has not has been not thoroughly been thoroughly analyzed accordinganalyzed toaccording various statisticalto various parameters. statistical Inparameters. [12], the transmission In [12], the power transmission optimization power problem optimization of hybrid IWOCproblem/PLC /ofRF hybrid communication IWOC/PLC/RF system iscommunication discussed. system is discussed. ItIt mustmust be be pointed pointed out out that thethat research the research and implementation and implementation of power of wireless power optical wireless positioning optical technologypositioning istechnology still in its infancy.is still Somein its of infancy. the more Some complex of andthe potentiallymore complex high-precision and potentially visible locationhigh-precision technologies visible have location not beentechnologies fully explored have innot this been field. fully These explored algorithms in this usually field. need These to measurealgorithms the usually reception need intensity to measure information, the reception arrival intensity angle information, arrival arrival angle time information, orarrival arrival time time information, difference or information arrival time of difference light signals information from diff erentof light LED signals light from sources. different Based LED on suchlight informationsources. Based and on the such location information information and the of lo thecation light information source, the of spatial the light location source, of thethe mobilespatial terminallocation of can the be mobile reversely terminal estimated can be [ 6reversely]. The existing estimated research [6]. The work existing often research stops at work the positioning often stops algorithmat the positioning itself, without algorithm fully considering itself, without the actual fully factorsconsidering such as the interconnection actual factors relationship, such as physicalinterconnection characteristics, relationship, and synchronous physical characteri performancestics, ofand the synchronous power line networkperformance connected of the with power the LEDline network light source connected to the potential with the influenceLED light on source the performance to the potential of power influence wireless on the optical performance positioning of technology.power wireless Therefore, optical it positioning is necessary technology. to carry out Ther targetedefore, performance it is necessary improvement to carry out work targeted on the basisperformance of comprehensive improvement consideration work on the of basis the above of comprehensive factors, so as consideration to make the high-precision of the above factors, power wirelessso as to opticalmake the positioning high-precision technology power from wireless theoretical optical analysis positioning to practical technology application. from theoretical analysis to practical application. Appl. Sci. 2020, 10, 6463 18 of 44 Appl. Sci. 2020, 10, x FOR PEER REVIEW 18 of 45

(a) (b)

Figure 17. Demo and Android APP for indoor localization based on IWOC: (a) demo [12]; (b) receiver Figure 17. Demo and Android APP for indoor localization based on IWOC: (a) demo [12]; (b) (back and front) [12]; (c) signal detection in the Android APP [16]; (d) map localization in the Android receiver (back and front) [12]; (c) signal detection in the Android APP [16]; (d) map localization in the APP [19]. Android APP [19]. Paper [37] studies the link of relaying cascade PLC and IWOC with amplify and forward (AF) relay,Paper based [37] on that studies AF relaythe link has of not relaying been considered cascade PLC to connect and IWOC PLC with and IWOCamplify links and before.forward In (AF) the firstrelay, stage, based the on source that AF transmits relay has the not information been consid to theered relay to connect through PLC the and PLC IWOC link. In links the secondbefore. stage,In the first stage, the source transmits the information to the relay through the PLC link. In the second AF relay amplifies the received signal and forwards it to the target node via IWOC channel. Setting the stage, AF relay amplifies the received signal and forwards it to the target node via IWOC channel. total capacity of a hybrid IWOC/PLC system provides the opportunity to examine the performance of Setting the total capacity of a hybrid IWOC/PLC system provides the opportunity to examine the such a system and the effect of different system parameters such as relay position and relay distance to performance of such a system and the effect of different system parameters such as relay position the target. The accuracy of the results is verified by Monte Carlo simulation. and relay distance to the target. The accuracy of the results is verified by Monte Carlo simulation. For comparison, the performance of the hybrid system is compared with that of the hybrid/PLC, in which a linear combination is used to maximize the overall capacity of the system. In Appl. Sci. 2020, 10, 6463 19 of 44

For comparison, the performance of the hybrid system is compared with that of the hybrid/PLC, in which a linear combination is used to maximize the overall capacity of the system. In [67], the coordinationAppl. Sci. 2020, 10 between, x FOR PEER the REVIEW light sources and how to improve the performance of the IWOC19 of system45 are studied. The author veriﬁes that the implementation of decode and forward (DF) relay has better performance[67], the coordination than AF relay, between as shown the light in Figure sources 18 and. To how further to improve improve the theperformance performance of the of IWOC PLC and system are studied. The author verifies that the implementation of decode and forward (DF) relay IWOC cooperative system, researchers introduced relay technology into such IWOC indoor broadcast has better performance than AF relay, as shown in Figure 18. To further improve the performance of communicationPLC and IWOC system cooperative [37,38,68 system,–70]. In researchers [69], PLC introduced link is connected relay technology with IWOC into linksuch through IWOC DF relayindoor and becomes broadcast the communication backbone of system IWOC [37,38,68– link. This70]. paper In [69], investigates PLC link is both connected AF and with DF IWOC protocols for IWOC-basedlink through DF uplink relay and transmissions. becomes the backbone In traditional of IWOC cellular link. This mobile paper networks, investigates relay both AF technology and can signiﬁcantlyDF protocols improvefor IWOC-based network uplink throughput transmissions. and In extend traditional coverage. cellular Inspired mobile networks, by this, relay PLC and IWOCtechnology system can can also significantly be regarded improve as relay-assisted network thro dual-hopughput and communication extend coverage. systems. Inspired At by the this, signal transmissionPLC and IWOC level, system there arecan twoalso be main regarded relay as technologies relay-assisted to dual-hop choose co from,mmunication namely systems. decoding At and forwardingthe signal and transmission amplifying andlevel, forwarding. there are two main relay technologies to choose from, namely decoding and forwarding and amplifying and forwarding.

FigureFigure 18. System 18. System model model for for the the DF DF relaying relaying hybrid hybrid IWOCIWOC/PLC/PLC network network for for indoor indoor communication communication applications [67]. applications [67]. In the decoding, forwarding, and relay scheme, PLC signals need to be demodulated and In the decoding, forwarding, and relay scheme, PLC signals need to be demodulated and decoded decoded for PLC transmission before being loaded into LED light source, and then re-coded and for PLC transmission before being loaded into LED light source, and then re-coded and re-modulated re-modulated for IWOC transmission [38]. At the same time, in the amplification code forwarding for IWOCand relay transmission scheme, [the38]. analog At the PLC same signals time, insuperi the amplificationmposed with codenoise forwardingcomponents and are relaydirectly scheme, the analogamplified PLC and signals sent when superimposed loaded onto with the noise LED componentslight source, without are directly any coding amplified operation and sent [66]. when loadedSpecifically, onto the LEDThe PLC light subsystem source, without assumes any part coding of the operationreturn link [of66 IWOC]. Specifically, link, which The is PLCsusceptible subsystem assumesto log-normal part of the distribution return link fading of IWOC and additive link, which noise. is The susceptible former method to log-normal can completely distribution eliminate fading and additivethe cumulative noise. effect The formerof the above method noise can by DF, completely but the implementation eliminate the complexity cumulative is relatively effect of thehigh. above noiseAlthough by DF, but the the latter implementation scheme cannot complexity eliminate the is relativelyinfluence high.of noise, Although it does thenot latterinvolve scheme complex cannot eliminatedecoding the influence and encoding of noise, operation, it does notand involve the implementation complex decoding complexity and encoding is very low, operation, which andis the beneficial to significantly reduce the implementation cost of the system. At present, international implementation complexity is very low, which is beneficial to significantly reduce the implementation research teams have proposed to provide services for users through multiple IWOC access points in cost of the system. At present, international research teams have proposed to provide services for users IWOC link, as shown in Figure 18. The simulation indicates that the overall characteristics of system throughdepend multiple on the IWOC random access distribution points of in the IWOC multi-user link, as in shown the end in terminal. Figure 18By. selecting The simulation the diversity indicates that thecombination overall characteristics technology, the of appropriate system depend access on poin thet randomcan be selected distribution for the of user, the multi-userand then the in the end terminal.maximum By instantaneous selecting the SNR diversity can be provided combination for the technology, user. On the the basis appropriate of a single accessrelay node, point the can be performance gain can be further introduced into the system. Appl. Sci. 2020, 10, 6463 20 of 44 selected for the user, and then the maximum instantaneous SNR can be provided for the user. On the basis of a single relay node, the performance gain can be further introduced into the system.

ForAppl. theSci. 2020 sake, 10, ofx FOR enhance PEER REVIEW the throughput performance of integrated PLC and IWOC20 of systems, 45 more and more researchers are engaged in related research work. Speciﬁcally, the research team from the UniversityFor the sake of of Southampton enhance the throughput proposed aperformance NOMA joint of powerintegrated allocation PLC and strategy IWOC systems, for PLC and IWOCmore networks and more [13 ].researchers The results are show engaged that in NOMA related technology research work. has aSpecifically, higher throughput the research performance team thanfrom OMA the in University terms of of multiple Southampton minimum proposed rate a requirements NOMA joint power and userallocation density. strategy The for main PLC defectand of IWOC networks [13]. The results show that NOMA technology has a higher throughput IWOC technology is the performance degradation under LOS path missing. RF communication, performance than OMA in terms of multiple minimum rate requirements and user density. The withmain its higher defect penetration of IWOC technology capability, is can the overcome performance the abovedegradation deﬁciencies. under LOS In fact, path the missing. fusion RF of PLC technologycommunication, and wireless with its network higher pene hastration been proposed capability, incan many overcome application the above scenarios. deficiencies. This In fact, includes integratingthe fusion PLC of technology PLC technology into the and wireless wireless relay network system has in been smart proposed grid applications, in many application so as to realize long-distancescenarios. dataThis transmissionincludes integrating [66]. Based PLC ontechnology the above into considerations, the wireless relay researchers system in from smart Texas grid A&M Universityapplications, in Qatar so as proposed to realize a long-distance hybrid PLC, data IWOC, transmission and RF communications[66]. Based on the above system considerations, [66,71], as shown in Figureresearchers 19[ 12]. from To satisfy Texas the A&M required University quality in of Qatar service proposed (QoS), researchers a hybrid PLC, constructed IWOC, andand analyzed RF the overallcommunications transmission system power [66,71], minimization as shown in problem.Figure 19 [12]. To satisfy the required quality of service (QoS), researchers constructed and analyzed the overall transmission power minimization problem.

FigureFigure 19. 19.Hybrid Hybrid PLC PLC/IWOC/RF/IWOC/RF communication system system model model [12]. [12 ].

To coordinateTo coordinate the the collaboration collaboration betweenbetween RF RF ne networkstworks and and hybrid hybrid PLC and PLC IWOC and networks, IWOC networks, the the teamteam discusses discusses resource resource allocation allocation between between two twoparallel parallel links, links,namely namely RF links RF and links hybrid and PLC hybrid and PLC and IWOCIWOC links.links. The proposed proposed system system explores explores multiple multiple wireless wireless interfaces interfaces to accumulate to accumulate the parallel the parallel data streams received. The results show that the above optimization problems can be constructed as data streams received. The results show that the above optimization problems can be constructed convex optimization problems and can be solved by means of computational efficient behavior. The as convex optimization problems and can be solved by means of computational efficient behavior. quantization results show that the actual system parameters such as PLC channel model and RF The quantizationsignal availability results have show a significant that the influence actual system on network parameters performance. such as Therefore, PLC channel as a promising model and RF signalarchitecture, availability the have parallel a significant system of influenceRF, PLC, and on networkIWOC could performance. enhance the Therefore, rate and reliability as a promising of architecture,indoor wireless the parallel network. system of RF, PLC, and IWOC could enhance the rate and reliability of indoor wireless network.In [72], a new hybrid outdoor WOC/RF-IWOC system is proposed, which chooses to combine Inthe [ 72selection], a new between hybrid outdoor outdoor WOC WOC and/ RF-IWOCRF modules system to obtain is a proposed, single noise which with a chooses higher SNR. to combine At the selectionthe relay terminal, between the outdoor DF protocol WOC is andadopted RF for modules the signals to obtain with high a single SNR, and noise then with the signals a higher are SNR. At theforwarded relay terminal, to the multi-user the DF protocol through is the adopted IWOC link. forthe In [59], signals a hybrid with PLC/IWOC/RF high SNR, and future then for the the signals wired backhaul system based on optical fibers has been proposed to improve the realized SRC. Appl. Sci. 2020, 10, 6463 21 of 44 are forwarded to the multi-user through the IWOC link. In [59], a hybrid PLC/IWOC/RF future for the wired backhaul system based on optical fibers has been proposed to improve the realized SRC. Meanwhile, PLC-IWOC system also provides a lot of advantages, such as good security, economical and easy to deploy, good transmission efficiency, high capacity, and high data rate [20,73]. Thus, these systems could be a potential candidate to provide ubiquitous connectivity for Internet of Things (IoT) devices. In these three subsections, we consider the channel characteristics of PLC and IWOC system, the modulation of integrated PLC-IWOC system, and different transmission schemes. Table2 compares the different researches between PLC and IWOC system in these years.

Table 2. Comparison of diﬀerent researches between PLC and IWOC system.

Consideration Years System Types Key Techniques Distance Data Ref Factors Integrated 48 Mbps 2015 AF relay Multi-path eﬀects 3 m [12] PLC/IWOC (Data rate) Hybrid OFDM Satisfy the practical 48 Mbps 2015 8 m [16] PLC/IWOC modulation application (Data rate) 0.4 bit/symbol (BDM 1) DF Relay and Integrated Multi-path eﬀects and 0.13 bit/symbol 2015 BDM 2.8–3 m [18] PLC/IWOC attenuations (TDM 2) modulation (Data rate) Cascaded OFDM and FSK 105 2015 Impulsive noises 2.8–3 m [62] PLC/IWOC modulation (Data rate) PLC 10 Mbps (VLC) backhauling and Multi-user utility 2016 OFDM 2.8–3 m 5 Mbps (RF) [1] Hybrid function (Data rate) IWOC/RF Hybrid Duplex DF 5–30 Mbps 2016 Intercell interference Over3 m [14] PLC/IWOC underlying (Data rate) Hybrid OFDM 18.5 Mbps 2018 Power allocation 2.8–3 m [9] PLC/IWOC/RF modulation (Data rate) Relay location and Hybrid 0.04 bit/s/Hz 2018 AF Relay relay-to-destination 3.15 m [36] PLC/IWOC (Average capacity) distance Integrated User’s location and 0.04 bit/s/Hz 2018 DF Relay 2.75 m [60] PLC/IWOC transmit power (Average capacity) 1 BDM represents for bit division multiplexing. 2 TDM stands for time-division multiplexing.

2.4. Wireless Optical Sensor Monitoring of Power Lines High-speed communication on power industry allows for real-time monitoring of network. This allows to anticipate possible events, process those events more rapidly, and improve the quality of service to customers. Recently, communication infrastructures using broadband power lines, Wi-Fi, and optical fibers have been designed for power distribution network management services. The authors in [74] compared the speed performances of each communication. Experimental results indicated that optics fiber is the fastest communication network, and could provide a bidirectional delay of 3 ms. A significant factor to consider in the communications system is the costs of infrastructure investment. Sometimes slower solutions, for instance, PLC may be more appropriate for specific applications or when a large number of measuring devices must be installed in a distribution system. PLC is an example of a common and cost-effective means of communication for smart grid monitoring since it allows the power lines to be used for power and communication, thereby eliminating the need for special cables for transmission control and data signals. In reference [75,76], different overhead communication PLC lines were compared for several applications of power distribution networks. Appl. Sci. 2020, 10, 6463 22 of 44

Researchers in Brazil introduced wireless optical technology into the wireless transmission of Appl. Sci. 2020, 10, x FOR PEER REVIEW 22 of 45 sensor signals on high-voltage power lines and carried out a series of work [77–79]. Optical fiber sensors includea signal two fiber, types: or they distributed could sensorsbe used orin punctual lots of different sensors. Thefibers latter which could are be connected connected in in a signalsingle fiber,detection or they point could one be by used one, in for lots instance of different using fibers a distributor which are connected[80]. Specifically, in a single the detectioncurrent sensor point onetransmits by one, the for sensor instance signal using to athe distributor remote unit [80 ].fixe Specifically,d on the high-voltage the current sensorline. According transmits to the [77], sensor the signalexperiment to the applies remote a unit distributed fixed on thefeedback high-voltage laser wavelength line. According of 1530 to [nm.77], Aerial the experiment cables are applies used to a distributedconnect the feedback remote room laser wavelengthto the optical of power 1530 nm. phase Aerial conductor cables are (OPPC), used to which connect also the use remote single-mode room to the(SM) optical fibers. power Then phase multi-mode conductor (MM) (OPPC), fibers which are used also useto link single-mode to the collimator (SM) fibers. A Thenand collimator multi-mode B (MM)respectively. fibers areThe used FSO todevice link tois conne the collimatorcted to the A far and end collimator of the OPPC B respectively. cable. Then the The optical FSO device signal isis connectedtransmitted to from the far collimator end of the A OPPC to thecable. low-potential Then the collimator optical signal B via is FSO transmitted device. fromThe optical collimator signal A tois themeasured low-potential with a collimatorpower meter, B via and FSO data device. acquisit Theion optical (DAQ) signal board is measured collects data with recorded a power by meter, the andcomputer. data acquisition The block (DAQ)diagram board of the collects optical data wireless recorded outfield by the monitoring computer. device The block system diagram is shown of the in opticalFigure 20. wireless Next, outfieldthe optical monitoring signal carries device the systemsensing isinformation shown in Figureto the control20. Next, unit the through optical the signal MM carriesfiber. the sensing information to the control unit through the MM fiber.

FigureFigure 20.20. Block diagram of ﬁeldfield testtest ofof thethe opticaloptical wirelesswireless monitoringmonitoring devicedevice [[77].77].

ConsideringConsidering thatthat there there is is a hugea huge potential potential diff erencedifference between between the aligners,the aligners, andthe and typical the typical value canvalue be can as high be as as high 138 kV,as 138 the researcherskV, the researchers introduced introduced the wire postthe wire insulator post insulator and suspended and suspended it vertically it fromvertically the high from voltage the high line voltage in the air.line Meanwhile,in the air. Meanwhile, the two collimators the two collimators were fixed onwere the fixed upper on and the lowerupper endsand oflower the insulatorends of the respectively insulator respectively to achieve insulation to achieve protection. insulation In protection. addition, theIn addition, scheme also the introducedscheme also that introduced high power that laserhigh lightpower power laser technologylight power senttechnology 830 nm sent light 830 signals nm light by multimode signals by fibermultimode collimator, fiber throughcollimator, the through wireless the optical wireless link sentoptical to thelink collimator sent to the and collimator then sent and to then the remotesent to unit,the remote and through unit, and the internalthrough rechargeable the internal powerrechar supplygeable power unit for supply the remote unit unitfor power.the remote Refer unit to Figurepower. 20 Refer for the to specificFigure 20 device for the implementation specific device block implementation diagram, and block the monitoring diagram, and device the and monitoring its main componentsdevice and its are main given components in Figure 21 are. The given scheme in Figure uses wireless 21. The opticalscheme signals uses wireless to transmit optical high-voltage signals to sensingtransmit signals, high-voltage avoiding sensing the huge signals, safety avoiding risks caused the by huge wired safety schemes risks directly caused guiding by wired signals schemes from highdirectly potential guiding to low signals potential, from as high well aspotential the complexity to low of potential, introducing as customizedwell as the hollow complexity insulators of tointroducing reduce risks. customized hollow insulators to reduce risks. In the electrical system, it is necessary to measure the current, voltage, and temperature of the high voltage overhead transmission line, which requires the special monitoring device to be completely isolated from the ground potential. Power over fiber (PoF) includes the transmission of light energy, electronics, or electrical equipment for remote power supply. PoF has the following advantages when used in high pressure environment: no spark, lightning, electromagnetic pulse durability, can be used in extreme temperature, humidity, and other harsh environments. The PoF/OWC system and its components are shown in Figure 22. In addition, PoF can be used to replace metal cables and battery power in remote areas to improve system reliability and safety. One of the major advantages of using PoF technology in substations is that conductive copper wire does not need to be inserted in high ground areas. Appl. Sci. 2020, 10, x FOR PEER REVIEW 23 of 45

Appl. Sci. 2020, 10, 6463 23 of 44 Appl. Sci. 2020, 10, x FOR PEER REVIEW 23 of 45

Figure 21. Optical wireless monitoring device and its main elements [77].

In the electrical system, it is necessary to measure the current, voltage, and temperature of the high voltage overhead transmission line, which requires the special monitoring device to be completely isolated from the ground potential. Power over fiber (PoF) includes the transmission of light energy, electronics, or electrical equipment for remote power supply. PoF has the following advantages when used in high pressure environment: no spark, lightning, electromagnetic pulse durability, can be used in extreme temperature, humidity, and other harsh environments. The PoF/OWC system and its components are shown in Figure 22. In addition, PoF can be used to replace metal cables and battery power in remote areas to improve system reliability and safety. One of the major advantages of using PoF technology in substations is that conductive copper wire does not need to be inserted in high ground areas. Figure 21. Optical wireless monitoring devi devicece and its main elements [[77].77].

FigureFigure 22. 22. PoF/OWCPoF/OWC system system and and its its components components [78]. [78].

Aiming at the above power line wireless optical monitoring system solution, the researchers conducted a prototype implementation and carried out field tests for 8 months, as shown in Figure 23. The researchers found that rainwater residing in collimator packages can significantly affect the wireless optical loss, especially the coupling loss. In addition, the experimental results show that when optical sensing technology does not use amplitude variations, but frequency or planned variations, power oscillations introduced by thermal variations will not be a critical factor. In the laboratory testing phase, researchers also focused on the sensitivity of the above wireless optical sensing transmission system to lateral migration [77–79]. Two commercial collimator pairs, F810APC-1550 and F260APC-1550, were used in the experiment. In the experimental test of the two

Figure 22. PoF/OWC system and its components [78]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 24 of 45

Aiming at the above power line wireless optical monitoring system solution, the researchers conducted a prototype implementation and carried out field tests for 8 months, as shown in Figure 23. The researchers found that rainwater residing in collimator packages can significantly affect the wireless optical loss, especially the coupling loss. In addition, the experimental results show that when optical sensing technology does not use amplitude variations, but frequency or planned variations, power oscillations introduced by thermal variations will not be a critical factor. In the laboratory testing phase, researchers also focused on the sensitivity of the above wireless Appl.optical Sci. 2020 sensing, 10, 6463 transmission system to lateral migration [77–79]. Two commercial collimator24 of pairs, 44 F810APC-1550 and F260APC-1550, were used in the experiment. In the experimental test of the two kinds of collimators, the classification test was carried out respectively under the coupling kinds of collimators, the classification test was carried out respectively under the coupling connection connection condition of fiber core diameter 62.5 micron multi-mode fiber and fiber core diameter 9 condition of fiber core diameter 62.5 micron multi-mode fiber and fiber core diameter 9 micron micron single-mode fiber. The experimental results show that compared to the small diameter single-mode fiber. The experimental results show that compared to the small diameter collimator collimator F260APC-1550, the large diameter collimator F810APC-1550 can support a greater degree F260APC-1550, the large diameter collimator F810APC-1550 can support a greater degree of lateral of lateral deviation. Specifically, the collimator F810APC-1550 can support a horizontal offset higher deviation. Specifically, the collimator F810APC-1550 can support a horizontal offset higher than 3 mm than 3 mm under the constraint of not less than −10 dBm received optical power. Correspondingly, under the constraint of not less than 10 dBm received optical power. Correspondingly, the collimator the collimator F260APC-1550 can− only support an offset of 1 mm. At the same time, when the large F260APC-1550 can only support an offset of 1 mm. At the same time, when the large diameter diameter collimator is combined with the multi-mode fiber, the support degree of the system to the collimator is combined with the multi-mode fiber, the support degree of the system to the offset can offset can be further improved. The above results are of great practical significance for improving be further improved. The above results are of great practical significance for improving the wireless the wireless optical sensor transmission system in the harsh sensing environment such as outdoor optical sensor transmission system in the harsh sensing environment such as outdoor high temperature high temperature and high wind for reliability and reducing the link interruption frequency and and high wind for reliability and reducing the link interruption frequency and interruption duration. interruption duration.

(a) (b)

Figure 23. (a) Optical wireless (OW) device anchored in a tower of the transmission line, (b) a technician aligningFigure the 23. collimators (a) Optical on thewireless top of (OW) the high device voltage anchored tower [in77 ]a. tower of the transmission line, (b) a technician aligning the collimators on the top of the high voltage tower [77]. In [80], laboratory tests were carried out according to the scheme in Figure 24 to observe the sensitivityIn [80], oflaboratory the outdoor tests WOC were systemcarried toout lateral accord misalignment.ing to the scheme For in each Figure type 24 of to collimator, observe the twosensitivity conditions of were the tested,outdoor that WOC is, coupling system withto lateral a diameter misalignment. of 62.5 µm For through each atype MM of fiber, collimator, and with two SMconditions fiber whose were core tested, diameter that is 9is,µ m.coupling They capturedwith a diameter the optical of power62.5 µm and through spectral a dataMM (collimatingfiber, and with gapSM Z passed fiber twice),whose changingcore diameter the transverse is 9 µm. displacement They captured (transverse) the optical X for dipowerfferent and distances spectral (Z). data (collimatingIn the beginning, gap Z thepassed lateral twice), displacement changing Xthe is tran keptsverse under displacement optimal conditions, (transverse) and the X for possible different couplingdistances for each(Z). interval Z. Then the collimator moves in the X direction (through symmetry, only one direction is needed). The experiment is to send a light signal through the fiber grating interrogator of the existing broa band light source, and measure: the optical power at point A, the optical power at point B before the fiber grating sensor, the optical power at point B, the peak value of the spectrum after the fiber grating sensor, and the values. The sensor captured the interrogator. We noticed that when only SM fiber is used, there is no need to measure the optical power of point A in front of the FBG sensor, and the optical power readings of point A and point B coincide. Appl. Sci. 2020, 10, x FOR PEER REVIEW 25 of 45

In the beginning, the lateral displacement X is kept under optimal conditions, and the possible coupling for each interval Z. Then the collimator moves in the X direction (through symmetry, only one direction is needed). The experiment is to send a light signal through the fiber grating interrogator of the existing broadband light source, and measure: the optical power at point A, the optical power at point B before the fiber grating sensor, the optical power at point B, the peak value of the spectrum after the fiber grating sensor, and the values. The sensor captured the interrogator. Appl.We noticed Sci. 2020, that10, 6463 when only SM fiber is used, there is no need to measure the optical power of point25 of 44A in front of the FBG sensor, and the optical power readings of point A and point B coincide.

Figure 24. Experimental setup used for coup couplingling efficiency eﬃciency measurement [[79].79].

Generally speaking,speaking, it it is observedis observed that that the lateralthe lateral displacement displacement supported supported by the largestby the diameter largest collimatordiameter collimator (F810APC-1550) (F810APC-1550) is greater than is greater that of thethan smallest that of diameter the smallest collimator diameter (F260APC-1550). collimator In(F260APC-1550). fact, the lateral In displacement fact, the lateral of the displacement10 dB peak of amplitude the −10 dB of peak the F260APC-1550 amplitude of the collimator F260APC-1550 is about − 2–3collimator mm, while is about the lateral2–3 mm, displacement while the lateral of the displacement F810APC-1550 ofcollimator the F810APC-1550 is about 4–5collimator mm. Moreover, is about contrary4–5 mm. Moreover, to the imagination, contrary to the the use imagination, of multimode the use fiber of does multimode not seem fiber to does bring not significant seem to bring gain. Insignificant reality, for gain. the collimatorIn reality, F260APC-1550,for the collimator single-mode F260APC-1550, fiber supports single-mode greater fiber lateral supports displacement greater thanlateral multi-mode displacement fiber. than Actually, multi-mode with fiber. the peak Actual valuely, with of the the frequency peak value spectrum of the frequency falling at spectrum a level of falling10 dB, at about a level 3 mm of requires−10 dB, lateral about displacement 3 mm requires and lateral 2 mm ofdisplacement fiber mm. However, and 2 mm for the of collimationfiber mm. − F810APC-1550However, for the there collimation is an increase F810APC-1550 in lateral displacement there is an increase from 4 in to lateral 4.5 mm displacement to 5 mm [79 ].from 4 to 4.5 mm to 5 mm [79]. 2.5. Optical Wireless Power Inspection 2.5. Optical Wireless Power Inspection 2.5.1. Two Ways of Power Inspection 2.5.1.Compared Two Ways with of Power the traditional Inspection power inspection methods, unmanned aerial vehicle (UAV) power detection line is more efficient and safer to operate in the forests, deep mountains and forests, rivers and Compared with the traditional power inspection methods, unmanned aerial vehicle (UAV) lakes. In terms of operation precision, the UAV power line also gradually achieves the effect of accuracy power detection line is more efficient and safer to operate in the forests, deep mountains and forests, that is comparable to the professional camera. UAV aerial survey could clearly study and judge the line rivers and lakes. In terms of operation precision, the UAV power line also gradually achieves the loss parts that are hard to find by human eye. There are two main methods for power inspection: one effect of accuracy that is comparable to the professional camera. UAV aerial survey could clearly is the visible/photoelectric pod detection carried by a multi-rotor UAV, mainly solves these problems study and judge the line loss parts that are hard to find by human eye. There are two main methods such as insulator damage, needle, wire damage, temperature anomaly detection problems. The other for power inspection: one is the visible/photoelectric pod detection carried by a multi-rotor UAV, is the secondary power channel data with multi-rotor/v TOL equipped LiDAR, which mainly solves mainly solves these problems such as insulator damage, needle, wire damage, temperature anomaly the problems of tree barrier detection, cross crossing detection, tree growth analysis, power detection, detection problems. The other is the secondary power channel data with multi-rotor/v TOL and so on. Combined with the two inspection methods, how to solve the need of regular inspection equipped LiDAR, which mainly solves the problems of tree barrier detection, cross crossing and fine inspection at the same time to improve the efficiency of operation. detection, tree growth analysis, power detection, and so on. Combined with the two inspection Compared with fixed-wing UAVs, multi-rotor UAVs fly at a low speed and have the function of hover on the fixed point. They could observe specific inspection targets in detail, which is conducive to defect discovery, such as insulator burst, wire broken strand, wire icing, bolt detection, foreign body suspension, etc. Since the defect inspection of the tower needs to observe the insulators, bolts, wires, and other special details, the traditional manual inspection has to climb up the tower to inspect with electricity, which is extremely dangerous. The multi-rotor UAV may hover around the tower and take pictures of the tower from all directions, which is undoubtedly a revolution of power inspection. Appl. Sci. 2020, 10, 6463 26 of 44

The multi-rotor UAV inspection system consists of two parts: UAV flight platform and ground station. The flight platform includes the fuselage, power supply, propeller, flight control system, cradle head, camera, and image transmission system. The ground station includes surveillance and observation systems that monitor the UAV’s flight status (such as batteries, number of satellites, flight path, altitude, etc.,) and receive high-definition video signals sent by the UAV. The operator could use this information to control the UAV to capture target images and video information in real-time; the evening after sunset and when on a cloudy day it does not rain. Generally, it is better to fly along the wire above the wire side when the plane is patrolling, and the plane and the sun should be on the same side of the wire to avoid backlight. The axis of the lens should be at an angle with the horizontal plane and less than 100 m away from the wire. The orientation of the lens (usually autofocus) is usually fixed on the ground in advance and does not change during flight. If change is needed, it is achieved by changing the flight path of the aircraft. If there is a very complex gyro stabilization system platform on a moving vehicle, it can realize automatic positioning and use images to accurately analyze the transmission route. Finally, the use of autonomous satellite navigation control, geographic matching line automatic control, automatic tracking of the line tower, and flight control functions, so that the drone can patrol to follow the flight control according to the transmission line, altitude, and direction angle of the fully automatic line, making the drone close to the body patrol work on transmission lines.

2.5.2. Optical Wireless Electric Tunnel Inspection In the course of tunnel inspection, power tunnel detection is a dangerous and complicated task. Since the length of the tunnel is long and relatively closed, general network signals cannot be transmitted, patrol officers may be out of touch, not to mention the real-time detection of data sent back. The inspectors hope to keep communication with the ground centralized control center. In this way, in the event of a line failure or accident, it will keep in touch with the ground centralized control center, and work together to troubleshoot and carry out on-site rescue. At the same time, since the tunnel patrol is a dangerous work, it has a strong demand for personnel positioning and navigation. However, because the cable tunnel is usually located at a depth of more than 10 m underground, and the influence of the structural characteristics and electromagnetic characteristics of the tunnel, all kinds of RF signals in the tunnel are seriously attenuated, which makes it difficult to satisfy the above requirements. IWOC-PLC system could solve the above problems of substation and cable tunnel patrol. This is because when LED lights are used to communicate in substations or cable tunnels, they neither generate radio waves nor interfere with equipment in substations or cable tunnels. Using the LED as the communication node in the workstation, inspectors can always maintain a network connection with the background operating system, obtain information services and conduct confidential communication during the inspection. At the same time, it can precisely locate inspectors and the equipment they operate. In [81], Song et al. proposed an intelligent detection network structure for substations or cable tunnels. Where the working data of the business system is transmitted through the network to the lighting LEDs in different positions in the substation or cable tunnel; the developed light source transceiver modulates the data into light signals and loads them on the LEDs for transmission. On-site inspection personnel receive VLC signals through user transceivers to realize data communication services and high-precision positioning services. At the same time, the user transceiver uses visible light signals to uplink location information and communication data to realize real-time monitoring and interaction between on-site operators and background managers. Another possible solution is a LiFi+PLC-IoT solution. Since power tunnels lack wires the most, allowing the wires to transmit data and tunnel ceiling lights to act as transmitters form a complete solution. Wherever they go, the inspectors can send back data in real-time and talk to the people behind them in real time, at speeds currently as high as 20 Mbps. Appl. Sci. 2020, 10, 6463 27 of 44

3. Wireless Power Transfer of Power Industry Optical Wireless Communications Wired power transmission has some unsafe factors, such as line aging and tip discharge, which cannot be used on some special occasions, such as seabed and mines. In addition, cable transmission requires too many lines, which brings inconvenience to people’s lives. Therefore, wireless power transfer (WPT) technology arises at the historic moment. Recent development of WPT provides a new node model for wireless sensor networks technique to extend their lifetime by the redistribution of energy [82]. It does not require a power transfer from the power supply conductors to the energy receiver technology. WPT technology development focused on applications requiring unidirectional power transmission [83]. WPT technology consists of two types: near field and far field [84]. The former is suitable for applications where the distance between transmitter and receiver is within a few millimeters or centimeters, such as handheld device charging, radio frequency identification technology, induction cooking technology, and wireless charging or continuous WPT of implanted medical devices. In [85], authors validated the use of WPT technology in medical settings and for the elderly and disabled. On the country, the latter approach allows for longer distances. It is applicable to applications where the distance between transmitter and receiver is within kilometers [86]. The development of WPT technology is mainly concentrated in application areas that require unidirectional power transmission. In recent years, bidirectional WPT (BD-WPT) systems in electric vehicles (EVs) grid integration of vehicle to grid (V2G) to realize the concept has been widely applied [83]. In order to reduce the impact caused by the intermittency of renewable energy systems and promote dynamic demand management, the V2G concept can be used as a cost-effective alternative energy storage unit. There have been many reports on the converter design, compensation topology and control technology of BD-WPT systems. One of the key issues of the Beidou-WPT system is to use external communication links to realize information sharing between main and auxiliary components. In [87], the secondary side controller is used to achieve synchronization between the two parties and control the power transmission. In [88], the author accurately modeled the BD-WPT system and proposed several methods to analyze the power flow. In order to realize the concepts of V2G and grid to vehicle, a two-way converter between the power grid and electric vehicles is needed to facilitate the transmission of electric energy between the power grid and electric vehicles. In an electric vehicle charging system, it is necessary to ensure that the working characteristics of the converter do not affect the grid-side current. In [89], a single-phase DQ controller for grid-side bidirectional converters is proposed. In [90], an optical WPT system was proposed, which could finish fast charging over a long distance while satisfying human body safety. A single junction gallium arsenide solar cell is designed to maximize the output power of the solar cell. In the rechargeable battery of portable equipment, a step-up direct current-direct current (DC-DC) converter is used to offer a stable current and convert it to the desired DC voltage.

3.1. Optical Wireless Power Transfer Recently, WPT technology has received extensive attention and has rapidly developed from a theory to commercial products [91]. The existing WPT technology can be roughly divided into transmission based on non-radiative coupling and transmission based on radiated radio frequency [92]. Transmission technologies based on non-radiative coupling include inductive coupling [93], magnetic resonance coupling [91], and capacitive coupling [94]. The technology based on non-radiative couplers has high transmission efficiency and has been diffusely applied in people’s household appliances, for instance wireless chargers for smart phones. However, the effective transmission distance of non-radiative coupling technology is less than a few meters. Although RF based transmission technology could transmit power over longer distances, the transmission efficiency is inversely proportional to the square of the distance [95]. Although the transmission technology based on RF-beam forming has been proposed to increase the transmission distance [96], there is still a limit to the effective transmission distance. Appl. Sci. 2020, 10, x FOR PEER REVIEW 28 of 45

magnetic resonance coupling [91], and capacitive coupling [94]. The technology based on non-radiative couplers has high transmission efficiency and has been diffusely applied in people’s household appliances, for instance wireless chargers for smart phones. However, the effective transmission distance of non-radiative coupling technology is less than a few meters. Although RF based transmission technology could transmit power over longer distances, the transmission efficiency is inversely proportional to the square of the distance [95]. Although the transmission

Appl.technology Sci. 2020, 10based, 6463 on RF-beam forming has been proposed to increase the transmission distance28 of[96], 44 there is still a limit to the effective transmission distance. Furthermore, it can cause electromagnetic interference (EMI) to other electronic devices. To makeFurthermore, the transmission it can cause distance electromagnetic longer, optical interference wireless (EMI) power to other transfer electronic (OWPT) devices. is a Tofeasible make thesolution. transmission As a wireless distance energy longer, transmission optical wireless technolo powergy that transfer uses light (OWPT) as an isenergy a feasible transmitter, solution. in AsOWPT, a wireless the transmitting energy transmission terminal technologyconverts the that el usesectrical light energy as an energyinto optical transmitter, power, in and OWPT, the thereceiving transmitting terminal terminal uses an converts optical thepower electrical receiver energy to receive, into optical for instance power, anda solar the cell, receiving to convert terminal the usesreceived an optical optical power power receiver into electrical to receive, energy. for instance a solar cell, to convert the received optical power into electricalDifferent energy. from that of the RF beams, the size and the power of a laser beam could maintain overDi longerfferent distance. from that Thus, of the RFthe beams, transmission the size andefficiency the power of aof single-wavelength a laser beam could laser maintain does over not longerdecrease distance. with the Thus, increase the transmission of distance. e ffiMoreover,ciency of ait single-wavelength does not create laserEMI doeson existing not decrease radio withcommunications. the increase ofThe distance. paper [97] Moreover, explains it doesthat the not expected create EMI DC on to existing DC transmission radio communications. efficiency of TheWPT paper technology [97] explains is a function that the of expecteddistance. DCAlthough to DC transmissionOWPT is less eefficientfficiency over of WPT short technology distances than is a functionother technologies, of distance. it Although is expected OWPT to be is more less eefficifficientent overover shortlong distancesdistances. than So far, other there technologies, have been itmany is expected studies toon be OWPT more [90,98–102]. efficient over Most long of distances. them are Sobased far, on there a laser have emitter been many and a studies solar cell on OWPTreceiver, [90 while,98–102 some]. Most OWPT of them uses area LED based transmitter on a laser that emitter has been and aproven solar cell[98,99]. receiver, Although while OWPT some OWPTis a solution uses a suitable LED transmitter for long-distance that has beenwireless proven power [98 ,transmission,99]. Although research OWPT is on a solutionOWPT is suitable still rare. for long-distanceIn the experiment wireless power of [103], transmission, similar to researchother traditional on OWPT OWPT is still work, rare. authors make a LD as an opticalIn thepower experiment transmitter. of [ 103However,], similar they to otherproved traditional that both OWPT a PD and work, a solar authors cell makecould abe LD used as an to opticalreceive powerlight power. transmitter. After However,that, the PD they and proved solar that cells both as athe PD performance and a solar cellof the could optical be used power to receivereceiver light also power. were compared. After that, Figure the PD 25 and illustrates solar cells the as theconstruction performance of OWPT. of the optical At the powertransmitter, receiver as alsothe energy were compared. is usually Figure electrical 25 illustrates energy, a thelight construction source (such of as OWPT. laser, AtLED, the etc.,) transmitter, is needed as the to convert energy isthe usually electrical electrical energy energy, into optical a light power. source When (such tran as laser,sferred LED, to the etc.,) light is needed source toof convertelectricity the for electrical PEt, and energylight source into optical the light power. output When power transferred for POt1, the to the electric-to-optic light source of (E/O) electricity power for conversion PEt, and light ratio source of the theoptical light source output is power CE/O. for POt1, the electric-to-optic (E/O) power conversion ratio of the optical source is CE/O.

FigureFigure 25.25. SystemSystem blockblock diagramdiagram ofof OWPTOWPT [[100].100].

InIn [[104],104], KimKim etet al.al. demonstrateddemonstrated underwaterunderwater OWPTOWPT usingusing LDLD asas powerpower transmittertransmitter throughthrough experiments.experiments. TheyThey studied studied the the properties properties of solar of sola cellsr andcells PD and aspower PD as receivers. power receivers. They also optimizedThey also LD,optimized PD, and LD, solar cellsPD, forand maximum solar cells transmission for maxi effimumciency. transmission The maximum efficiency. transmission The e ffimaximumciency of back-to-backtransmissionOWPT efficiency measured of back-to-back with the PDOWPT receiver meas wasured 4.3%. with They the PD then receiver demonstrated was 4.3%. OWPT They in then tap waterdemonstrated and seawater. OWPT The in resulttap water shows and that seawater. the attenuation The result in seashows water that is the 3 dB attenuation/m. in sea water is 3 dB/m.The application of OWPT in indoor environment was described in [104,105]. In these studies, ambientThe light application was determined of OWPT to in be indoor negligible, environment so the concept was ofdescribed OWPT toin SCs [104,105]. was investigated In these studies, under darkambient conditions. light was In determined [104], by using to be high negligible, brightness sowhite the concept LED, a of collimating OWPT to large SCs parabolicwas investigated mirror and a-Si solar panel placed at 5 m, an OW link was created. Measure the maximum link efficiency and harvest power were only 0.1% and 18.3 mW, respectively [100]. Raavi and his researchers in [90] proposed a solar cell wireless light energy transmission system based on optical antenna, as detected in Figure 26. As can be shown, the light source is located on the ceiling of the room and emits light signals that comply with the US Federal Communications Commission (FCC) human safety regulations which is less than 1 mW/cm2 [89]. The central station connected with the light source receives the guide signal from the equipment to be charged and controls the activation and deactivation of the light source. The mobile device to be charged contains an optical Appl. Sci. 2020, 10, x FOR PEER REVIEW 29 of 45 under dark conditions. In [104], by using high brightness white LED, a collimating large parabolic mirror and a-Si solar panel placed at 5 m, an OW link was created. Measure the maximum link efficiency and harvest power were only 0.1% and 18.3 mW, respectively [100]. Raavi and his researchers in [90] proposed a solar cell wireless light energy transmission system based on optical antenna, as detected in Figure 26. As can be shown, the light source is located on the ceiling of the room and emits light signals that comply with the US Federal 2 CommunicationsAppl. Sci. 2020, 10, 6463 Commission (FCC) human safety regulations which is less than 1 mW/cm 29 [89]. of 44 The central station connected with the light source receives the guide signal from the equipment to be charged and controls the activation and deactivation of the light source. The mobile device to be chargedantenna contains array and an a optical DC conversion antenna circuit array integratedand a DC conversion on top of the circuit solar integrated cell, which on can top be anywhereof the solar in cell,the housewhich to can receive be anywhere energy from in the the house light source. to receive The energy system willfrom realize the light a safe source. and e ffiThecient system intelligent will realizecharging a safe system. and efficient intelligent charging system.

FigureFigure 26. 26. DiagramDiagram of of the the proposed proposed wirele wirelessss energy energy transfer transfer system system [90]. [90].

InIn order order to to realize realize this system, aa nano-structurednano-structured opticaloptical antenna antenna using using the the enhancement enhancement e ffeffectect of ofplasmonic plasmonic electric electric field field was was designed designed and and analyzed analyzed [90]. [90]. In addition, In addition, an e ffiancient efficient single-junction single-junction GaAs GaAssolar cellsolar was cell designed was designed to maximize to maximize the output the outp powerut ofpower the solar of the cell. solar In thecell. rechargeable In the rechargeable batteries batteriesof portable of portable devices, adevices, step-up a DC-DC step-up converter DC-DC converter is used to provideis used to a stableprovide current a stable and current convert and it to convertthe required it to the DC required voltage. DC Because voltage. the Because optical antennathe optical can antenna enhance can the enhance plasma the electric plasma field electric effect, fieldthereby effect, promoting thereby thepromoting efficiency the of efficiency the energy of transfer the energy system transfer without system conflict without with humanconflict safetywith humanrules. Theysafety simulated rules. They the realizationsimulated ofthe optical realization antennas, of optical solar cells, antennas, and DC-DC solar conversioncells, and DC-DC circuits. conversionOn the basis circuits. of this simulation,On the basis the of solarthis simulation cell output, ofthe the solar proposed cell output technology of the isproposed 40 times technology higher than isthat 40 times of the higher traditional than solarthat of cell the system. traditional solar cell system.

3.2.3.2. Optical Optical Wireless Wireless Power Power Transmission Transmission with with Mirror Mirror TheThe OWPT OWPT system system consists consists of of a a power power transmitting transmitting unit unit (light (light source), source), a a power power receiving receiving unit unit (solar(solar battery), andand aa DC DC operating operating circuit. circuit. Generally, Generally, a light a light source source such assuch a laser as convertsa laser converts electrical electricalenergy into energy a highly into a collimated highly collimated beam, which beam, can which travel can within travel a within few meters a few with meters very with little very loss. Besides, solar cells convert light energy into electrical energy. The light source is driven by a DC working circuit to output a specific and stable solar battery voltage [106]. On this basis, a compact system can be used to achieve long-distance transmission, and no electromagnetic waves will leak into the environment outside the beam. By using this method, when the irradiated area is larger than the solar cell, light will be wasted and the power transmission efficiency will be reduced. In addition, when the solar cell area is larger than the illuminated area, the conversion efficiency will be reduced Appl. Sci. 2020, 10, 6463 30 of 44 because the solar cell is not fully irradiated at this time. Therefore, to maintain high efficiency, an appropriate irradiation pattern is required [107]. To improve the efficiency of the transfer system OWPT in remote and steep, authors propose a new system image OWPT numerical and experimental investigation and analysis that put forward the divergence angle of the power generation efficiency and system variables in the model space of the beam [106]. Furthermore, the OWPT system with multiple mirrors was considered, and the number of mirrors required for rooms of different sizes was studied. In this paper, the beam can be transmitted through the mirror to a device far away from the light source and with a small inclination angle, which avoids the energy waste caused by the beam shining on the outside of the solar cell and improves the energy transmission efficiency [106]. Moreover, the experimental results show that the WPT system using mirrors can effectively extend the application range of OWPT system and improve the flexibility of room effective charging [106].

3.3. Other Technologies of Wireless Power Transfer

3.3.1. Optical Power Transmission Optical power transmission (OPT) is one of the least studied technologies for wireless transmission of power to biomedical implants [107–111]. In the literature, there are few examples of OPT, even in other applications [88,90,101,112]. An external TX unit in a complete OPT system uses an external laser to power the implant. The light emitted by the external laser is received by the CMOS PD array implanted under the skin and converted into electrical energy, using the power management unit. The advantage of this WPT method is to provide a compact solution, because the CMOS-based power receiving system can be smaller than other systems [100]. The authors suggest in [111] that implantable sensors could be used for short-term in vivo monitoring after surgery, and that the use of biodegradable materials could avoid secondary surgery to remove the devices. Therefore, OWPT is considered to be applied in human skin to complete communication with a broad prospect. Compared with other methods, the OPT system for biomedical implant devices is still in the early stages of development, and the potential of this technology depends on the results of further research. In addition to the design and manufacturing challenges, more system-based implementation challenges need to be solved before considering using the system for complex implants. OPT requires a complete analysis of organizational security. Furthermore, misalignment and limited output power are important issues established by OPT in biomedical implant applications. OPT is the least researched WPT technique presently. To support most IMDs, PTE is much lower. Besides, any misalignment of Rx photodiodes with TX can significantly reduce PTE. Moreover, WPT technology still requires a rigorous organizational security analysis before its use in IMDs can be considered. The generation ratio of OWPT system with reflector is obtained when the reflector is 95% [106]. Figure 27b shows the power generation ratio of the system under a light source of 5 square mm. It can be seen that within the range of 150–650 cm, the power generation ratio can reach more than 0.7. Combined with Figure 27a, the result shows that the energy transfer efficiency above 0.7 can be achieved within a distance of 650 cm after the installation of the reflector, which causes the extension of the effective distance. Furthermore, when the distance is between 150 cm and 300 cm, the beam is transmitted more flexibly through direct irradiation of light source and indirect irradiation of mirror, even if the direct beam is blocked by obstacles. In this system, the reflection loss is a disadvantage because of the need of multi-plane mirror module. Appl. Sci. 2020, 10, x FOR PEER REVIEW 31 of 45

Appl.irradiation Sci. 2020, 10of, 6463mirror, even if the direct beam is blocked by obstacles. In this system, the reflection31 of 44 loss is a disadvantage because of the need of multi-plane mirror module.

(a) (b)

FigureFigure 27. 27.Power Power generationgeneration ratio obtained: (a ()a )irradiated irradiated directly directly by by light light source; source; (b) (irradiatedb) irradiated by by mirrormirror [ 106[106].].

3.3.2.3.3.2. Laser Laser Emission Emission WirelessWireless Power Transfer LaserLaser WPT WPT technology technology research research is moving is moving toward to theward high the transmission high transmission power, highpower, transmission high effitransmissionciency, and longefficiency, transmission and long distance transmission development distance [ 113development]. The transmitting [113]. The part transmitting of laser transmitting part of WPTlaser system transmitting mainly WPT includes system laser mainly driver, includes laser and laser optical driver, transmitting laser and optical antenna. transmitting The laser isantenna. a key part ofThe laser laser emission. is a key Lasers part of with laser a wavelengthemission. Lasers of around with a 800 wavelength nm are known of around to be better800 nm suited are known for wireless to energybe better transmission. suited for wireless Currently energy available transmission high-power. Currently lasers available with a wavelength high-power of lasers approximately with a 800wavelength nm include of solid-stateapproximately lasers 800 and nm semiconductor include solid-state lasers. lasers As forand conversion semiconductor efficiency, lasers. solid-stateAs for conversion efficiency, solid-state lasers only reach 40%, while semiconductor lasers can reach lasers only reach 40%, while semiconductor lasers can reach 55–60%. Semiconductor lasers have the 55–60%. Semiconductor lasers have the advantages of high electro-optical conversion efficiency, advantages of high electro-optical conversion efficiency, small size, light weight, and low cost. In recent small size, light weight, and low cost. In recent years, the electro-optical conversion efficiency of years, the electro-optical conversion efficiency of AlGaAs semiconductor lasers has been improved AlGaAs semiconductor lasers has been improved steadily. Experiments show that the efficiency can steadily. Experiments show that the efficiency can reach 70% and will be higher in the future [114]. reach 70% and will be higher in the future [114]. The transmitting antenna of laser-transmitting TheWPT transmitting can be designed antenna to ofapproach laser-transmitting the diffraction WPT limit can and be designedobtain the to minimum approach receiving the diffraction point as limit andfar obtainas possible. the minimum In order receivingto reduce pointthe influence as far as of possible. receiving In aperture order to and reduce increase the influence the distance, of receiving it is aperturenecessary and to increasereduce the the emission distance, angle it is of necessary laser beam. to reduce the emission angle of laser beam. Thus,Thus, in in the the transmittingtransmitting system, it it is is usually usually necessary necessary to to converge converge the the launching launching lasers lasers and and achieveachieve the the requirementsrequirements of the collimation system system through through beam beam forming forming launching launching technology. technology. KimKim et et al. al. studied studied the the wireless wireless optical optical energy energy transmission transmission based based on on optical optical beam beam forming, forming, and and the the beam formingbeam forming technology technology realized byrealized the spatial by the light spatial modulator light realized modulator wireless realized energy wireless transmission energy [ 98]. Intransmission order to support [98]. theIn powerorder to and support distance the requirements power and ofdistance the Internet requirements of Things of and the mobileInternet devices, of literatureThings and [115 mobile] proposed devices, a distributed literature [115] laser propos charginged a (DLC) distributed system. laser The charging self-aligning (DLC) featuresystem. ofThe DLC providesself-aligning a more feature convenient of DLC way provides to charge a more the conven Internetient of way Things to charge and mobile the Internet devices of without Things specificand positioningmobile devices or tracking, without asspecific long aspositioning the transmitter or trac andking, receiver as long areas the within transmitter LOS of eachand receiver other. In are 2017, Japanwithin in LOS [116 ]of put each forward other. In a scheme2017, Japan in which in [116 a] photovoltaicput forward a cell scheme is fixed in which on the a topphotovoltaic of the car cell and a solar-pumpedis fixed on the laser top of is the used car on and both a solar-pumped sides of the road laser to is receiveused on the both solar sides lamp of the and road directly to receive convert itthe into solar a laser, lamp and and then directly the convert laser is it irradiated into a laser, to and the photovoltaicthen the laser cellis irradiated to provide to the energy photovoltaic for the car, cell to provide energy for the car, as shown in Figure 28 [113]. as shown in Figure 28[113]. Appl. Sci. 2020, 10, 6463 32 of 44

Appl. Sci. 2020, 10, x FOR PEER REVIEW 32 of 45

(a)

(b)

FigureFigure 28. The 28. The solar solar panels panels on on the the roof roof of of electric electric vehiclesvehicles (EVs) (EVs) receive receive laser laser light light from from infrastructure infrastructure alongalong arterial arterial roads; roads; (a) sunlight(a) sunlight illuminates illuminates directly directly atat photovoltaic cells cells on on car car roof; roof; (b) (photovoltaicb) photovoltaic cells oncells the on car the roof car roof illuminated illuminated by by laser laser generated generated fromfrom solar-pumpedsolar-pumped [113]. [113 ].

3.4. Dynamic3.4. Dynamic Optical Optical Wireless Wireless Power Power Transfer Transfer OWPTOWPT is one is ofone the of mostthe most appropriate appropriate ways ways to to extend extend the the transmission transmission distancedistance while keeping keeping the the relevant system components relatively simple [117]. Some companies, for example, [118], are relevant system components relatively simple [117]. Some companies, for example, [118], are already already investigating the technology for use in consumer electronics and other areas. At the same investigatingtime, some the research technology institutions for use and in consumer institutions electronics around the and world other are areas. conducting At the far-field same time, WPT some researchresearch institutions on OWPT, and such institutions as [119,120]. around the world are conducting far-ﬁeld WPT research on OWPT, suchAn asOWPT [119 ,120system]. consists of three parts: the transmitter of light, transmitting environment, Anand OWPT receiver system of light. consistsTherefore, of the three OWPT parts: system the efficiency transmitter is deduced of light, from transmitting [98], that is: environment, and receiver of light. Therefore, the OWPTλλλλ system=×× eﬃciency is deduced from [98], that is: OWPT T C R (3)

where λ stands for the light transmitter energy efficiency, which is for the ratio of output energy T λOWPT = λT λC λR (3) λ × × to consumed energy. C is the optical transfer through the environment, and shows how much whereenergyλT stands is lost for when the light light is transmitter traveling in energythe considering efficiency, environment which is forsuch the as ratioair, pure of output water or energy sea to λ consumedwater, energy. etc. R λ ofC isthe the optical optical receiver; transfer respectively. through the The environment, correctness of and the shows formula how is muchverified energy by is lost whenexperimental light is tests traveling and could in the be consideringused to explain environment the measurement such efficiency as air, pure of OWPT water system. or sea water, etc. λR of the opticalTo verify receiver; the proposed respectively. dynamic The optical correctness WPT co ofncept, the formula Nguyen is et verified al. first bycreated experimental a simple tests and couldexperiment, be used as toshown explain in Figure the measurement 29. Here, solar e ffienerciencygy is ofcollected OWPT and system. stored in a device including ToDC-DC verify and the DC-alternating proposed dynamic current converters, optical WPT and concept, Li-ion batteries. Nguyen Then et al.utilizing first createdthe device a simpleto experiment,power the as LED shown light in bulbs Figure of a29 small. Here, EV. solarThis LED energy bulb is consumes collected 8 andW electricity stored in as a a device light emitter, including while a small solar panel is connected to another toy electric car as an energy receiver. The DC-DC and DC-alternating current converters, and Li-ion batteries. Then utilizing the device to power the LED light bulbs of a small EV. This LED bulb consumes 8 W electricity as a light emitter, while a small solar panel is connected to another toy electric car as an energy receiver. The LED-solar panel Appl. Sci. 2020, 10, x FOR PEER REVIEW 33 of 45 Appl. Sci. 2020, 10, 6463 33 of 44

LED-solarAppl. Sci. panel 2020, 10 WPT, x FOR systemPEER REVIEW introduced could work as far as 5 cm. The maximum WPT33 ofefficiency 45 is 2% when the distance is almost zero, and the maximum WPT efficiency is 1.57% when the WPT systemLED-solar introduced panel WPT could system work introduced as far could as 5 cm.work Theas far maximum as 5 cm. The WPTmaximum eﬃ ciencyWPT efficiency is 2% when the distance is 3 cm [117]. distance isis almost2% when zero, the anddistance the is maximum almost zero, WPT and etheﬃ ciencymaximum is 1.57%WPT efficiency when the is 1.57% distance when is the 3 cm [117]. distance is 3 cm [117].

Figure 29. Experiment setup for dynamic optical wireless power transfer (WPT) between ground Figure 29. Experiment setup for dynamic optical wireless power transfer (WPT) between ground EVs FigureEVs 29. using Experiment visible light setup [117]. for dynamic optical wireless power transfer (WPT) between ground using visible light [117]. EVs using visible light [117]. Figure 30a shows the situation where the blade is at a standstill without WPT, while Figure 30b Figureshows 30a that shows after thethe situationLED bulb moves where to thea certain blade distance, is at a WPT standstill does occur without due to WPT, the rotation while of Figure 30b Figure 30a shows the situation where the blade is at a standstill without WPT, while Figure 30b shows thatthe after blade. the Similar LED to bulb the movestesting of to ground a certain vehi distance,cles, the dynamic WPT doesOWPT occur system due of tothis the UAV rotation is of the showsinefficient. that after Since the its LED instrument bulb moves is simple, to ait iscertain predictable. distance, WPT does occur due to the rotation of blade.the Similar blade. toSimilar the testing to the of testing ground of vehicles,ground vehi thecles, dynamic the dynamic OWPT OWPT system system of this of UAV this isUAV ineﬃ iscient. Sinceinefficient. its instrument Since isits simple, instrument it is is predictable. simple, it is predictable.

(a) (b)

Figure 30. Experiment setup for demonstration of dynamic OWPT between a moving light source and a moving object: (a) no WPT; (b) with WPT [117]. (a) (b)

FigureFigure 30. Experiment 30. Experiment setup setup for demonstrationfor demonstration of of dynamic dynamic OWPT OWPT between between aa moving moving lightlight source and a movingand a object: moving (a object:) no WPT; (a) no ( bWPT;) with (b) WPT with [WPT117]. [117].

Described above experimental OWPT efficiency is very low, it can also be seen in other systems, for instance used in underwater systems [121]. However, using high quality solar cells and light sources, higher OWPT efficiency can be expected. Theoretically, the upper limit of the energy conversion efficiency of traditional single-junction solar cells is 30%. This is because most of the solar energy that impacts the battery is not absorbed or converted into heat. Scientists are studying various solar cell Appl. Sci. 2020, 10, 6463 34 of 44 designs to eliminate these limitations in conversion efficiency and reduce energy loss. The current world record for four-junction solar cells is 46% [122]. In [123,124], the GaSb-based solar cell designed by researchers at George Washington University in 2017 has a photoelectric conversion efficiency of 44.5%. If the energy conversion efficiency of solar cells exceeds 50%, it will have a great impact on the cost of power generation [122]. This is still far behind the efficiency of resonant inductively coupled systems, but is acceptable in certain applications, for example, wireless charging for consumer electronics devices. In recent years, optical wireless technology has become more and more important. Check the freepatantsononline website about OWC, IWOC, PLC patents, there are as many as several million in each field, enough to see that optical wireless technology is widely used in today’s society, which is a research hotspot. There are also hundreds of thousands of patents related to power industry and optical wireless communication, indicating that the combination of power industry and optical wireless technology is a feasible solution. Similarly, according to CNKI statistics, there are 200 Chinese patents related to optical wireless communications from 1986 to 2020. Furthermore, Chinese patents related to visible light communication amounted to nearly 14,000, with an average of more than 1000 patents per year in the past eight years. It covers the power industry, telecommunications technology, computer software and computer applications, automation technology, radio electronics, and other multidisciplinary fields. There are also nearly 10,000 patents related to PLC, with an average of more than 800 articles per year in the past eight years. In addition, Chinese patents for the combination of visible light communications and PLC can be traced back to 2011. There are still relatively few patents on the combination of smart power and wireless optical technologies. Optical wireless and wireless optical have always been used by researches in the communication field, meanwhile power grid and smart grid belong to the categories of the power industry. Therefore, on the World Intellectual Property Organization (WIPO) official website we searched “smart grid,” “optical wireless,” “power line,” “visible light communication,” “free space optical,” “power grid,” and so on, and realized the search of related patents in the field of power optical wireless from 2012 to 2020, and obtained 45 patents with these themes, as shown in Figure 31[ 125]. It can be seen from the figure that in the past five years, power industry optical wireless technologies have been more and more widely used. Therefore, exploring the power technology under optical wireless technologies has great development prospects. Appl. Sci. 2020, 10, x FOR PEER REVIEW 35 of 45

Figure 31. 2012–2020 statistical analysis chart based on power industry optical wireless patents from Figure 31. 2012–2020 statistical analysis chart based on power industry optical wireless patents from WIPO [125]. WIPO [125]. 4. Challenges and Potential of Power Industry Optical Wireless Communications At present, optical wireless technology for smart power is still in the broadcast technology paradigm. In this case, the power line transmits data from the data source to the data driving module, and the data driving module loads the same data stream on all light sources, and broadcasts the signal to the area covered by the light signal through the light source. This means that even if there are multiple distributed light sources, the data transmission potential of each light source cannot be fully released. In view of these problems, it is feasible to introduce MIMO technology into optical wireless technology. The basic idea is to realize multiple data streams separately to multiple distributed sources through the indoor power line network. The light source completes the loading of each data stream through its own efforts and transfers the optical signal from different data information. At the receiving end, the optical signals from different light sources are converted into electrical signals by multiple detectors, and then the original multiple streams are restored through various demultiplexing techniques. Theoretically, MIMO technology paradigm can double the transmission capacity of the system. It must be pointed out that, unlike the radio frequency MIMO technology in the current commercial cellular network, the wireless optical MIMO technology based on the indoor distribution network must comprehensively consider the MIMO characteristics of the distribution network itself [126–129]. Generally speaking, mainstream alternating current power supply networks use a three-phase power supply. How to eliminate the transmission interference between the neutral line, the live line, and the ground line may become the key to the practical application of high-power optical wireless technology in MIMO technology. In addition, since OMA only allocates a single wireless resource to one user, such as by frequency or time, NOMA scheme allows multiple users to use the same resource. In some of the scenes, such as the near-far effect scenario and the scenario of multi-node access with wide coverage, NOMA approach using power multiplexing has obvious performance advantages over traditional OMA and is more suitable for future system deployment. In the field of power industry optical wireless channel modeling and characterization, the existing research works cannot fully cover the diverse power grid configurations at home and abroad, nor can it be applied to the extensive non-Lambert wireless optical link configurations. Existing PLC studies show that due to the different standards of power industry, the transmission characteristics of smart grid in different countries and regions are obviously different, and it is difficult to use a single model to represent [128–130]. At the same time, as the original function of the electric power network is not used for data transmission, power grid load and electricity demand in Appl. Sci. 2020, 10, 6463 35 of 44

4. Challenges and Potential of Power Industry Optical Wireless Communications At present, optical wireless technology for smart power is still in the broadcast technology paradigm. In this case, the power line transmits data from the data source to the data driving module, and the data driving module loads the same data stream on all light sources, and broadcasts the signal to the area covered by the light signal through the light source. This means that even if there are multiple distributed light sources, the data transmission potential of each light source cannot be fully released. In view of these problems, it is feasible to introduce MIMO technology into optical wireless technology. The basic idea is to realize multiple data streams separately to multiple distributed sources through the indoor power line network. The light source completes the loading of each data stream through its own efforts and transfers the optical signal from different data information. At the receiving end, the optical signals from different light sources are converted into electrical signals by multiple detectors, and then the original multiple streams are restored through various demultiplexing techniques. Theoretically, MIMO technology paradigm can double the transmission capacity of the system. It must be pointed out that, unlike the radio frequency MIMO technology in the current commercial cellular network, the wireless optical MIMO technology based on the indoor distribution network must comprehensively consider the MIMO characteristics of the distribution network itself [126–129]. Generally speaking, mainstream alternating current power supply networks use a three-phase power supply. How to eliminate the transmission interference between the neutral line, the live line, and the ground line may become the key to the practical application of high-power optical wireless technology in MIMO technology. In addition, since OMA only allocates a single wireless resource to one user, such as by frequency or time, NOMA scheme allows multiple users to use the same resource. In some of the scenes, such as the near-far effect scenario and the scenario of multi-node access with wide coverage, NOMA approach using power multiplexing has obvious performance advantages over traditional OMA and is more suitable for future system deployment. In the field of power industry optical wireless channel modeling and characterization, the existing research works cannot fully cover the diverse power grid configurations at home and abroad, nor can it be applied to the extensive non-Lambert wireless optical link configurations. Existing PLC studies show that due to the different standards of power industry, the transmission characteristics of smart grid in different countries and regions are obviously different, and it is difficult to use a single model to represent [128–130]. At the same time, as the original function of the electric power network is not used for data transmission, power grid load and electricity demand in the different periods have an obvious dynamic difference; these factors make the electric model to be further points area, the scene, the difference of the period of time to discuss, and integrate wireless optical link corresponding channel measurement and statistical analysis. Meanwhile, to meet the diverse lighting needs of roads, assembly halls and exhibition halls, the actual commercial LED light sources usually need to use the encapsulation lens technology to distribute the light beam, improve the light efficiency and improve the lighting uniformity [131–133]. This kind of technology processing results in the beam of LED light that no longer match the Lambert model in the existing theoretical analysis, and ultimately leads to the reduction of the accuracy of the power wireless optical modeling analysis results, and even the analysis of the practical significance of modeling representation. Therefore, in the next power wireless channel characteristic study, light must consider electric model, and the integrated lambert the diversity of the respective characteristics of the beam, carried out a series of specific power optical wireless channel measurements, and then through data fitting, statistical analysis methods, for instance, constructing can face a series of typical scenario high reliable power wireless optical channel model, which provide a basis for comparison in a variety of technical scheme evaluation. In the reliability of power optical wireless link, the existing technical scheme and prototype system still have great room for improvement. Especially in the part of optical wireless link, as the light source often needs to meet the requirements of lighting function, the semi-power of the beam is relatively large, which makes the optical signal emitted by the light source unable to focus on the local spatial position where the data receiver is located [134–136]. At the same time, a considerable part of the Appl. Sci. 2020, 10, 6463 36 of 44 signal power is projected to the space where there are no users, resulting in the actual waste of data capacity and reducing the overall energy efficiency of the system. In future research, beam forming technology can be tried to be introduced into the field of power wireless optical technology [72,137]. With the user location information fed back by the uplink, the remote data source loads the user data stream onto multiple light sources adjacent to the user space location by selecting the power line path. At the same time, the algorithm is designed to scale the above light source to different degrees, and on the premise of maintaining the same energy consumption level, light signal energy is concentrated in the area where the user is, so as to effectively enhance the user’s business experience and improve the overall energy efficiency of the system. Another outstanding disadvantages of OWPT is that it cannot be transmitted over a long distance. Because of its mobility and flexibility, UAV is used in the new technologies such as monitoring and emergency rescue and military applications. The endurance of UAV is always an urgent problem to be solved. The possibility of using OWPT technology to power UAV systems has recently been recognized as a very attractive approach that could lead to alternative and effective charging solutions [84,137–141]. Since the OWPT technology does not require electricity, and then it makes future communications technology possible to a large extent. In addition, there are few researches on OWPT, so it is a feasible scheme and has a broad prospect to consider the combination of OWPT technology and power industry.

5. Conclusions With the further deepening of the demand for information and communication technology in the electric power industry, the emerging wireless optical technology will show its natural advantages in the traditional electric power industry in terms of large transmission capacity, strong confidentiality, and free from electromagnetic interference. Most of the existing researches focus on the theoretical level, so it is necessary to accelerate the practice and large-scale application of wireless optical technology in the power industry. Further, future work should focus more on the integration of power lines and indoor wireless optical transmission of high-speed, highly reliable two-way communication. With the key enabling technologies such as multi-input and multi-output, Non-Lambert beam characterization, beam forming, and wireless power transfer technologies, the transmission and coverage performance of power industry optical wireless technologies will be significantly improved and play a significant role in the construction of smart power and power industry.

Author Contributions: Conceptualization, J.D. and W.L.; methodology, J.D. and W.L.; validation, J.D. W.L., and C.-L.I.; formal analysis, J.D. and W.L.; investigation, J.D., W.L. C.-L.I., H.Z., and H.M.; writing—original draft preparation, J.D. and W.L.; writing—review and editing, J.D., W.L., and H.M.; supervision, J.D., W.L., C.-L.I., and H.Z. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by the National Natural Science Foundation of China (Grants No. 62061043), Natural Science Foundation of the Xinjiang Uygur Autonomous Region (Grants No. 2019D01C020), High-level Talents Introduction Project in Xinjiang Uygur Autonomous Region (Grants No. 042419004), Tianchi Doctor Program of the Xinjiang Uygur Autonomous Region (Grants No. 04231200728), Natural Science Foundation of Xinjiang University (Grants No. 62031224624), National Natural Science Foundation of China (Grants No. 61401420), and Postgraduate Research and Innovation Project of Xinjiang Uygur Autonomous Region (Grants No. XJ2020G063). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Abbreviations The following abbreviations are used in this manuscript:

AF Amplify and Forward APD Avalanche Photodiode BDM Bit Division Multiplexing BD-WPT Bidirectional Wireless Power Transfer BER Bit Error Rate Appl. Sci. 2020, 10, 6463 37 of 44

BPSK Binary Frequency Shift Keying BS Base Station CFR Channel Frequency Response CSK Color Shift Keying DAQ Data Acquisition DC Direct Current DF Decode and Forward DLC Distributed Laser Charging EMI Electromagnetic Interference EV Electric Vehicle FCC Federal Communications Commission FSK Frequency Shift Keying FSO Free Space Optical IoT Internet of Things IWOC Indoor Wireless Optical Communications HDTV High-Deﬁnition Television HV High Voltage LED Light-Emitting Diode LD Laser Diode LOS Line of Sight LV Low Voltage MIMO Multi-Input and Multi-Output MM Multi-Mode MV Medium Voltage NOMA Non-Orthogonal Multiple Access OFDM Orthogonal Frequency Division Multiple OFDMA Orthogonal Frequency Division Multiple Access OMA Orthogonal Multiple Access OOK On Oﬀ Keying OPPC Optical Power Phase Conductor OPT Optical power transmission OW Optical Wireless OWC Optical Wireless Communications OWPT Optical Wireless Power Transfer PAPR Peak to Average Power Ratio PD Photodiode PLC Power Line Communications PoF Power over Fiber PSK Phase Shift Keying QoS Quality of Service QPSK Quadrature Phase Shift Keying RF Radio Frequency S-FSK Spread Frequency Shift Keying SC-BPSK Sub-Carrier Binary Shift Keying SIC Successive Interference Cancellation SM Single Mode SO-OFDM Spatial Optical Orthogonal Frequency Division Multiplexing SNR Sight-to-Noise Ratio TDM Time Division Multiplexing UAV Unmanned Aerial Vehicle V2G Vehicle to Grid VLC Visible Light Communications WIPO World Intellectual Property Organization WPT Wireless Power Transfer Appl. Sci. 2020, 10, 6463 38 of 44

References

1. Komine, T.; Nakagawa, M. Integrated system of white LED visible-light communication and power-line communication. IEEE Trans. Consum. Electron. 2003, 49, 71–79. [CrossRef] 2. Song, X.; Wang, M.; Xing, S.; Zhao, Z. Progress of orthogonal frequency division multiplexing based on visible light communication. Laser Optoelectron. Prog. 2018, 55, 100–109. 3. Jia, K.; Zhang, S. Inﬂuence of multiuser interference on performances of direct-current-biased optical orthogonal frequency division multiplexing code division multiple access system for visible light communication. Laser Optoelectron. Prog. 2019, 56, 110604. 4. Yang, Y.; Jiang, M.; Zhang, Y.; Zhang, H. Design of full duplex visible light communication system based on single light source. Laser Optoelectron. Prog. 2019, 56, 010603. [CrossRef] 5. Ding, J.; Chih-Lin, I.; Zhang, C.; Yu, B.; Lai, H. Evaluation of outdoor visible light communications links using actual LED street luminaries. In Chinese Conference on Biometric Recognition; Springer: Cham, Switzerland, 2018. 6. Luo, J.; Fan, L.; Li, H. Indoor positioning systems based on visible light communication: State of the art. Commun. Surv. Tutor. IEEE 2017, 19, 2871–2893. [CrossRef] 7. Chowdhury, M.Z.; Hossan, M.T.; Islam, A.; Jang, Y.M. A comparative survey of optical wireless technologies: Architectures and applications. IEEE Access 2017, 6, 9819–9840. [CrossRef] 8. Tsonev, D.; Videv, S.; Haas, H. Towards a 100 Gb/s visible light wireless access network. Opt. Exp. 2015, 23, 1627–1637. [CrossRef] 9. Khalighi, M.A.; Uysal, M. Survey on free space optical communication: A communication theory perspective. IEEE Commun. Surveys Tuts. 2014, 16, 2231–2258. [CrossRef] 10. Ndjiongue, A.R.; Ferreira, H.C.; Song, J.; Yang, F.; Cheng, L. Hybrid PLC-VLC channel model and spectral estimation using a nonparametric approach. Trans. Emerg. Telecommun. Technol. 2017, 28, 1–13. [CrossRef] 11. Nlom, S.M.; Ndjiongue, A.R.; Ouahada, K. Cascaded PLC-VLC channel: An indoor measurements campaign. IEEE Access 2017, 6, 25230–25239. [CrossRef] 12. Kashef, M.; Abdallah, M.; Al-Dhahir, N. Transmit power optimization for a hybrid PLC/VLC/RF communication system. IEEE Trans. Green Commun. Netw. 2018, 2, 234–245. [CrossRef] 13. Feng, S.; Bai, T.; Hanzo, L. Joint power allocation for the multi-user NOMA-downlink in a power-line-fed VLC network. IEEE Trans. Veh. Technol. 2019, 68, 5185–5190. [CrossRef] 14. Ma, H.; Lampe, L.; Hranilovic, S. Hybrid visible light and power line communication for indoor multiuser downlink. J. Opt. Commun. Netw. 2017, 9, 635–647. [CrossRef] 15. Song, J.; Ding, W.; Yang, F.; Yang, H.; Yu, B.; Zhang, H. An indoor broadband broadcasting system based on PLC and VLC. IEEE Trans. Broadcasting 2015, 61, 299–308. [CrossRef] 16. Gao, S.; Zhang, J.; Yang, H. A reference channel model of indoor integrated power line communication and VLC system. Electr. Power Inf. Commun. Technol. 2018, 40, 1062–1066. 17. Song, J.; Liu, S.; Zhou, G.; Yu, B.; Ding, W.; Yang, F.; Zhang, H.; Zhang, X.; Amara, A. A cost-eﬀective approach for ubiquitous broadband access based on hybrid PLC-VLC system. In IEEE International Symposium on Circuits and Systems (ISCAS); IEEE: Montreal, QC, Canada, 2016. 18. Chen, M.; Feng, Y.; You, S.; Wang, Y.; Wang, Z.; Chen, J. Performance analysis of channel estimation for hybrid PLC&VLC system. Study Opt. Commun. 2017, 43, 3–6. 19. Ding, W.; Yang, F.; Yang, H.; Wang, J.; Wang, X.; Zhang, X. A hybrid power line and visible light communication system for indoor hospital applications. Comput. Ind. 2015, 68, 170–178. [CrossRef] 20. Ma, X.; Gao, J.; Yang, F.; Ding, W.; Yang, H.; Song, J. Integrated power line and visible light communication system compatible with multi-service transmission. IET Commun. 2017, 11, 104–111. [CrossRef] 21. Gao, J.; Yang, F.; Ding, W. Novel Integrated power line and visible light communication system with bit division multiplexing. In Proceedings of the 2015 International Wireless Communications and Mobile Computing Conference (IWCMC), Dubrovnik, Croatia, 24–28 August 2015; IEEE: Piscataway, NJ, USA, 2015. 22. Yang, L.; Li, J.; Zhang, J. Hybrid visible light communications (VLC) and PLC system. In Proceedings of the Opto-Electronics and Communications Conference (OECC) and Photonics Global Conference (PGC), Singapore, 31 July–4 August 2017. 23. Liu, H.; Zhu, P.; Chen, Y.; Huang, M. Power allocation for downlink hybrid power line and visible light communication system. IEEE Access 2020, 8, 24145–24152. [CrossRef] Appl. Sci. 2020, 10, 6463 39 of 44

24. Pittolo, A.; Tonello, A.M.; Versolatto, F. Performance of MIMO PLC in measured channels affected by correlated noise. In Proceedings of the 18th IEEE International Symposium on Power Line Communications and Its Applications, Glasgow, UK, 12 May 2014; pp. 261–265. 25. Gao, S.; Zhang, J.; Song, J.; Yang, H.; Chu, X. Random channel generator of the integrated power line communication and visible light communication. In Proceedings of the 2017 IEEE International Symposium on Power Line Communications and its Applications (ISPLC), Madrid, Spain, 3–5 April 2017; pp. 1–7. 26. Zimmermann, M.; Dostert, K. A multipath model for the powerline channel. IEEE Trans. Commun. 2002, 50, 553–559. [CrossRef] 27. Ma, H.; Lampe, L.; Hranilovic, S. Integration of indoor visible light and power line communication systems. In Proceedings of the IEEE International Symposium on Power Line Communications & Its Applications, Vancouver, BC, Canada, 21 24 October 2013; IEEE: Piscataway, NJ, USA, 2013. − 28. Ma, H.; Lampe, L.; Hranilovic, S. Subcarrier allocation in hybrid visible light and power line communication system. In Proceedings of the IEEE International Symposium on Circuits & Systems, Montreal, QC, Canada, 22–25 May 2016; IEEE: Piscataway, NJ, USA, 2016. 29. Kuhn, M.; Berger, S.; Hammerstrom, I.; Wittneben, A. Power line enhanced cooperative wireless communications. IEEE J. Sel. Areas Commun. 2006, 24, 1401–1410. [CrossRef] 30. Ndjiongue, A.R.; Ferreira, H.C.; Shongwe, T. Inter-building PLC-VLC integration based on PSK and CSK techniques. In Proceedings of the International Symposium on Power Line Communications & Its Applications, Bottrop, Germany, 20–23 March 2016; IEEE: Piscataway, NJ, USA, 2016. 31. Nlom, S.M.; Ouahada, K.; Ndjiongue, A.R.; Ferreira, H.C. Evaluation of the SFSK-OOK integrated PLC-VLC system under the influence of sunlight. In Proceedings of the International Symposium on Networks, Computers and Communications (ISNCC), Marrakech, Morocco, 16–18 May 2017; IEEE: Piscataway, NJ, USA, 2017. 32. Ndjiongue, A.R.; Ferreira, H.C.; Ouahada, K.; Vinck, A.J.H. Low-complexity SOCPBFSK-OOK interface between PLC and VLC channels for low data rate transmission applications. In Proceedings of the IEEE International Symposium on Power Line Communications & Its Applications, Glasgow, Scotland, 20 March–2 April 2014; IEEE: Piscataway, NJ, USA, 2014; pp. 226–231. 33. Komine, T.; Haruyama, S.; Nakagawa, M. Performance evaluation of narrowband OFDM on integrated system of power line communication and visible light wireless communication. In Proceedings of the 2006 1st International Symposium on Wireless Pervasive Computing, Phuket, Thailand, 16–18 January 2006; pp. 6–11. 34. Song, J.; Ding, W.; Yang, F.; Yang, H.; Wang, J.; Wang, X.; Zhang, X. Indoor hospital communication systems: An integrated solution based on power line and visible light communication. In Proceedings of the IEEE Faible Tension Faible Consommation, Monaco, Monaco, 4–6 May 2014; pp. 1–6. 35. Alavi, S.E.; Rezaie, H.; Supat, A.S.M. Application of OFDM on integrated system of visible free space optic with PLC. In Proceedings of the IEEE Asia–Pacific Conference on Applied Electromagnetics, Port Dickson, Malaysia, 9–11 November 2010; pp. 1–5. 36. Ma, H.; Lampe, L.; Hranilovic, S. Integration of indoor visible light and power line communication systems. In Proceedings of the 2013 IEEE 17th International Symposium on Power Line Communications and Its Applications, Johannesburg, South Africa, 24–27 March 2013; pp. 291–296. 37. Gheth, W.; Rabie, K.M.; Adebisi, B.; Ijaz, M.; Harris, G. Performance analysis of integrated power–line/visible–light communication systems with AF relaying. In Proceedings of the 2018 IEEE Global Communications Conference (GLOBECOM), Abu Dhabi, United Arab Emirates, 9–13 December 2018; pp. 1–6. 38. Gheth, W.; Rabie, K.M.; Adebisi, B.; Ijaz, M.; Harris, G. On the performance of DF–based power–line/visible–light communication systems. In Proceedings of the 2018 International Conference on Signal Processing and Information Security (ICSPIS), Dubai, United Arab Emirates, 7–8 November 2018; pp. 1–4. 39. Basnayaka, D.A.; Haas, H. Hybrid RF and VLC systems: Improving user data rate performance of VLC systems. In Proceedings of the 2015 IEEE 81st Vehicular Technology Conference (VTC Spring), Glasgow, UK, 1–14 May 2015; pp. 1–5. 40. Kashef, M.; Ismail, M.; Abdallah, M.; Qarage, K.; Serpedin, E. Energy efficient resource allocation for mixed RF/VLC heterogeneous wireless networks. IEEE J. Sel. Areas Commun. 2016, 34, 883–893. [CrossRef] Appl. Sci. 2020, 10, 6463 40 of 44

41. Kubjana, M.D.; Shongwe, T.; Ndjiongue, A.R. Hybrid PLC-VLC based on ACO-OFDM. In Proceedings of the 2018 International Conference on Intelligent and Innovative Computing Applications (ICONIC), Plaine Magnien, Mauritius, 6–7 December 2018; IEEE: Piscataway, NJ, USA, 2018. 42. Kubjana, M.D.; Ndjiongue, A.R.; Shongwe, T. Impulsive noise evaluation on PLC-VLC based on DCO-OFDM. In Proceedings of the 11th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP), Budapest, Hungary, 18–20 July 2018. 43. Mapfumo, I.; Shongwe, T. A comparison of FFT and DHT hybrid PLC-VLC systems based on ACO-OFDM in impulsive noise. In Proceedings of the 30th International Conference Radioelektronika (RADIOELEKTRONIKA), Bratislava, Slovakia, 15–16 April 2020; pp. 1–7. 44. Kizilirmak, R.C.; Rowell, C.R.; Uysal, M. Non-orthogonal multiple access (NOMA) for indoor visible light communications. In Proceedings of the 4th International Workshop on Optical Wireless Communications (IWOW), Istanbul, Turkey, 7–8 September 2015; pp. 98–101. 45. Yin, L.; Popoola, W.O.; Wu, X.; Haas, H. Performance evaluation of non-orthogonal multiple access in visible light communication. IEEE Trans. Commun. 2016, 64, 5162–5175. [CrossRef] 46. Lin, B.; Tang, X.; Ghassemlooy, Z. Optical power domain NOMA for visible light communications. IEEE Wirel. Commun. Lett. 2019, 8, 1260–1263. [CrossRef] 47. Liu, X.; Wang, Y.; Zhou, F.; Hu, R.Q. BER Analysis for NOMA-Enabled Visible Light Communication Systems with M–PSK. In Proceedings of the 2018 10th International Conference on Wireless Communications and Signal Processing (WCSP), Hangzhou, China, 18–20 October 2018; IEEE: Piscataway, NJ, USA, 2018. 48. Ye, Q.; Rong, B.; Chen, Y.; AL-Shalash, M.; Caramanis, C.; Andrews, J.G. User association for load balancing in heterogeneous cellular networks. IEEE Trans. Wirel. Commun. 2013, 12, 2706–2716. [CrossRef] 49. Lee, J.W.; Mazumdar, R.R.; Shroﬀ, N.B. Joint resource allocation and base–station assignment for the downlink in CDMA networks. IEEE/ACM Trans. Netw. 2006, 14, 1–14. [CrossRef] 50. Amirshahi, P.; Kavehrad, M. Broadband access over medium and low voltage power-lines and use of white light emitting diodes for indoor communications. In Proceedings of the Consumer Communications and Networking Conference, Barcelona, Spain, 23–29 April 2006; IEEE: Piscataway, NJ, USA, 2006; pp. 897–901. 51. Yoon, S.G.; Jang, S.; Kim, Y.H.; Bahk, S. Opportunistic routing for smart grid with power line communication access networks. IEEE Trans. Smart Grid 2014, 5, 303–311. [CrossRef] 52. Ferreira, H.C.; Lampe, L.; Newbury, J.; Swart, T.G. Power Line Communications: Theory and Applications for Narrowband and Broadband Communications over Power Lines; Wiley: Hoboken, NJ, USA, 2010; p. 11. 53. Anatory, J.; Theethayi, N. Broadband Power-line Communication Systems: Theory & Applications; WIT Press: Cambridge, MA, USA, 2010; p. 1. 54. Luján, R.D.; Edenor, S.A.A. Harmonics disturbances survey in distribution networks. In Proceedings of the 21st International Conference on Electricity Distribution, Frankfurt, Germany, 6–9 June 2011; pp. 1–4. 55. Liu, Q.; Sun, D.; Zang, Z.; Wang, X.; Zhang, S.; Liu, J.; CG. Co., Ltd. Design of visible light and power line communication protocol for electric power tunnel inspection. Autom. Instrum. 2018, 11, 133–137. 56. Xu, Z. An autonomous intelligent UAV line inspection system with LIDAR navigation. Zhejiang Electr. Power 2017, 36, 44–47. 57. Zhang, J.; Yang, Y.; Liu, B.; Chen, X.; Chen, H. Design of home network based on visible light communication and power line carrier communication. Laser Optoelectron. Prog. 2011, 48, 100604. [CrossRef] 58. Reddy, Y.S.; Panda, M.; Dubey, A.; Kumar, A.; Panigrahi, T.; Rabie, K.M. Optimisation of indoor hybrid PLC/VLC/RF communication systems. IET Commun. 2020, 14, 117–126. [CrossRef] 59. Saraﬁ, A.M.; Stavropoulos, G.I.; Cottis, P.G. Hybrid wireless-broadband over power lines: A promising broadband solution in rural areas. IEEE Commun. Mag. 2009, 47, 140–147. [CrossRef] 60. Pan, Q.W.; Kathnaur, A. Power–line networks to extend ranges of 2.4 GHz wireless communications inside multi–storey buildings. In Proceedings of the IEEE Radio and Wireless Symposium (RWS), New Orleans, LA, USA, 10–14 January 2010; pp. 621–624. 61. Güzelgöz, S.; Çelebi, H.B.; Arsian, H. Analysis of a multi–channel receiver: Wireless and PLC reception. In Proceedings of the European Signal Processing Conference, Aalborg, Denmark, 23–27 August 2010; pp. 1106–1110. Appl. Sci. 2020, 10, 6463 41 of 44

62. Gheth, W.; Rabie, K.M.; Adebisi, B.; Ijaz, M.; Harris, G.; Alfitouri, A. Hybrid power–line/wireless communication systems for indoor applications. In Proceedings of the 11th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP), Budapest, Hungary, 18–20 July 2018; pp. 1–6. 63. Gupta, A.; Sharma, N.; Garg, P.; Alouini, M.S. Cascaded FSO-VLC communication system. IEEE Wirel. Commun. Lett. 2017, 6, 810–813. [CrossRef] 64. Chen, M.; You, S.; Wang, Y.; Wang, Z.; Bian, S.; Chen, H.; Ding, J.; Wu, H.; Shi, R.; Zhou, S.; et al. Performance analysis of pilot-based OFDM channel estimation for hybrid PLC&VLC system. In Proceedings of the 15th International Conference on Optical Communications and Networks (ICOCN), Hangzhou, China, 24–27 September 2016; IEEE: Piscataway, NJ, USA, 2016. 65. Ndjiongue, A.R.; Shongwe, T.; Ferreira, H.C.; Ngatched, T.M.N.; Vinck, A.J.H. Cascaded PLC-VLC channel using OFDM and CSK techniques. In Proceedings of the IEEE Global Communications Conference (GLOBECOM), San Diego, CA, USA, 6–10 December 2015; IEEE: Piscataway, NJ, USA, 2015. 66. Kashef, M.; Abdallah, M.; Al-Dhahir, N.; Qaraqe, K. On the impact of PLC backhauling in multi-user hybrid VLC/RF communication systems. In Proceedings of the IEEE Global Communications Conference (GLOBECOM), Washington, DC, USA, 4–8 December 2016; pp. 1–6. 67. Narmanlioglu, O.; Kizilirmak, R.C.; Uysal, M. Relay-assisted OFDM-based visible light communications over multipath channels. In Proceedings of the 2015 17th International Conference on Transparent Optical Networks (ICTON), Budapest, Hungary, 5–9 July 2015; pp. 1–4. 68. Jani, M.; Garg, P.; Gupta, A. Modeling and outage analysis of DF relay assisted mixed PLC-VLC system. In Proceedings of the Twenty Fourth National Conference on Communications (NCC), Hyderabad, India, 25–28 February 2018; IEEE: Piscataway, NJ, USA, 2018. 69. Jani, M.; Garg, P.; Gupta, A. Performance analysis of a co-operative PLC/VLC system with multiple access points for indoor broadcasting. Aeu-Int. J. Electron. Commun. 2019, 103, 64–73. [CrossRef] 70. Jani, M.; Garg, P.; Gupta, A. Outage analysis of an asymmetric dual hop PLC-VLC system for indoor broadcastin. In Proceedings of the National Conference on Communications (NCC), Bangalore, India, 20–23 February 2019; IEEE: Piscataway, NJ, USA, 2019. 71. Kashef, M.; Torky, A.; Abdallah, M.; Aldhahir, N.; Qaraqe, K. On the achievable rate of a hybrid PLC/VLC/RF communication system. In Proceedings of the IEEE Global Communications Conference, San Diego, CA, USA, 6–10 December 2015; IEEE: Piscataway, NJ, USA, 2015. 72. Deka, R.; Verma,A.; Anees, S. Performance analysis of decode-and-forward based hybrid RF/FSO-VLC system. In Proceedings of the IEEE International Conference on Advanced Networks and Telecommunications Systems (ANTS), GOA, India, 16–19 December 2019. 73. Jani, M.; Garg, P.; Gupta, A. On the performance of a cooperative PLC-VLC indoor broadcasting system consisting of mobile user nodes for IoT networks. IEEE Trans. Broadcasting 2020, 1–10. [CrossRef] 74. Saldaña-González, A.E.; Sumper, A.; Aragüés-Peñalba, M.; Smolnikar, M. Advanced distribution measurement technologies and data applications for smart grids: A review. Energies 2020, 13, 3730. [CrossRef] 75. Della Giustina, D.; Rinaldi, S. Hybrid communication network for the smart grid: Validation of a field test experience. IEEE Trans. Power Deliv. 2015, 30, 2492–2500. [CrossRef] 76. Sharma, K.; Saini, L.M. Power-line communications for smart grid: Progress, challenges, opportunities and status. Renew. Sustain. Energy Rev. 2017, 67, 704–751. [CrossRef] 77. Rosolem, J.B.; Penze, R.S.; Floridia, C.; Dini, D.C.; Valadares, J.E.F. FSO applied to optical fiber sensing: A field test demonstration. In Proceedings of the SBMO/IEEE MTT-S International Microwave and Optoelectronics Conference (IMOC), Águas de Lindoia, SP, Brazil, 27–30 August 2017; IEEE: Piscataway, NJ, USA, 2017. 78. Rosolem, J.B.; Bassan, F.R.; Penze, R.S.; Leonardi, A.A.; Fracarolli, J.P.V.; Floridia, C. Optical sensing in high voltage transmission lines using power over fiber and free space optics. Opt. Fiber Technol. 2015, 26, 180–183. [CrossRef] 79. Floridia, C.; Rosolem, J.B.; Leonardi, A.A.; Hortencio, C.A.; Fonseca, R.F.; Moreira, R.O.C.; Souza, G.C.L.; Melo, A.L.; Nascimento, C.A.M. Temperature sensing in high voltage transmission lines using fiber Bragg grating and free-space-optics. In SPIE Defense, Security, and Sensing; International Society for Optics and Photonics: Bellingham, DC, USA, 2013. Appl. Sci. 2020, 10, 6463 42 of 44

80. Rosolem, J.B.; Dini, D.C.; Penze, R.S.; Floridia, C.; Leonardi, A.A.; Loichate, M.D.; Durelli, A.S. Fiber optic bending sensor for water level monitoring: Development and field test: A review. IEEE Sens. J. 2013, 13, 4113–4120. [CrossRef] 81. Sun, D.; Xu, D.; Gong, T.; Song, J.; Liu, R. The design of VLC system for intelligent patrol inspection. Distrib. Util. 2016, 11, 27–30. 82. Gao, Z.; Chen, D.; Wu, H.-C. Energy loss minimization for wireless power transfer based energy redistribution in WSNs. IEEE Trans. Veh. Technol. 2019, 68, 12271–12285. [CrossRef] 83. Pandey, G.; Narsa, R.T. Power flow study of grid connected bidirectional WPT systems for EV application. In Proceedings of the IEEE International Conference on Power Electronics, Smart Grid and Renewable Energy (PESGRE), Cochin, India, 2–4 January 2020. 84. Zhou, Y.; Liu, C.; Huang, Y. Wireless power transfer for implanted medical application: A review. Energies 2020, 13, 2837. [CrossRef] 85. Abdullahi, Q.S.; Joshi, R.; Podilchak, S.K.; Khan, S.R.; Chen, M.; Rooney, J.; Rooney, J.; Sun, D.; Desmulliez, M.P.Y.; Georgiadis, A.; et al. Design of a wireless power transfer system for assisted living applications. Wirel. Power Transf. 2019, 6, 41–56. [CrossRef] 86. Nguyen, M.T.; Nguyen, C.V.; Truong, L.H.; Le, A.M.; Quyen, T.V.; Masaracchia, A.; Teague, K.A. Electromagnetic field based WPT technologies for UAVs: A comprehensive survey. Electronics 2020, 9, 461. [CrossRef] 87. Tang, Y.; Chen, Y.; Madawala, U.K.; Thrimawithana, D.J.; Ma, H. A new controller for bidirectional wireless power transfer systems. IEEE Trans. Power Electron. 2018, 38, 9076–9087. [CrossRef] 88. Mohamed, A.A.S.; Berzoy, A.; Mohammed, O.A. Experimental validation of comprehensive steady-state analytical model of bidirectional WPT system in EVs applications. IEEE Trans. Veh. Technol. 2017, 66, 5584–5594. [CrossRef] 89. Thrimawithana, D.J.; Madawala, U.K.; Shi, Y. Design of a bi-directional inverter for a wireless V2G system. In Proceedings of the IEEE International Conference on Sustainable Energy Technologies (IC-SET), Kandy, Sri Lanka, 6–9 December 2010; pp. 1–5. 90. Raavi, S.; Arigong, B.; Zhou, R.; Jung, S.; Jin, M.; Zhang, H.; Kim, H. An optical wireless power transfer system for rapid charging. In Proceedings of the Texas Symposium on Wireless and Microwave Circuits and Systems (WMCS), Waco, TX, USA, 4–5 April 2013; pp. 1–4. 91. Kurs, A.; Karalis, A.; Moffatt, R.; Joannopoulos, J.D.; Fisher, P.; Soljacic, M. Wireless power transfer via strongly coupled magnetic resonances. Science 2007, 317, 83–86. [CrossRef] 92. Han, Z.; Kim, I.D.; Lu, X.; Niyato, D.; Wang, P. Wireless charging technologies: Fundamentals standards and network applications. IEEE Commun. Surv. Tutor. 2016, 18, 1413–1452. 93. Ho, S.L.; Wang, J.; Fu, W.N.; Sun, M. A comparative study between novel Witricity and traditional inductive magnetic coupling in wireless charging. IEEE Trans. Magn. 2011, 45, 1522–1525. [CrossRef] 94. Kline, M.; Izyumin, I.; Boser, B.; Sanders, S. Capacitive power transfer for contactless charging. In Proceedings of the Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC), Fort Worth, TX, USA, 6–11 March 2011; p. 1398. 95. Shinohara, N. Beam efficiency of wireless power transmission via radio waves from short range to long range. J. Korean Inst. Electromagn. Eng. Sci. 2010, 10, 224. [CrossRef] 96. Yedavalli, P.S.; Riihonen, T.; Wang, X.; Rabaey, J.M. Far-field RF wireless power transfer with blind adaptive beamforming for Internet of Things devices. IEEE Access 2017, 5, 1743–1752. [CrossRef] 97. Rhee, D.-H.; Kim, S.-M. Unknown. J. Korea Inst. Electro. Commun. Sci. 2016, 11, 1219. [CrossRef] 98. Kim, S.-M. Wireless optical energy transmission using optical beamforming. Opt. Eng. 2013, 52, 1–5. [CrossRef] 99. Haydaroglu, I.; Mutlu, S. Optical power delivery and data transmission in a wireless and batteryless microsystem using a single light emitting diode. J. Microelectromechanical Syst. 2015, 24, 155–165. [CrossRef] 100. Fakidis, J.; Videv, S.; Kucera, S.; Claussen, H.; Haas, H. Indoor optical wireless power transfer to small cells at nighttime. J. Lightwave Technol. 2016, 34, 3236–3258. [CrossRef] 101. Pani, C.; Ray, O.; Ghosh, A.; Qadir, Z.; Sarkar, M.M.; Reja, W. Wireless power transfer using free space photonics. In Proceedings of the 2nd International Conference on Advances in Computing, Communication, & Automation (Fall), Bareilly, India, 30 September–1 October 2016; pp. 1–5. Appl. Sci. 2020, 10, 6463 43 of 44

102. Saha, A.; Iqbal, S.; Karmaker, M.; Zinnat, S.F.; Ali, M.T. A wireless optical power system for medical implants using low power near-IR laser. In Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Seogwipo, Korea, 11–15 July 2017; pp. 1978–1981. 103. Kim, S.-M.; Choi, J.; Jung, H. Experimental demonstration of underwater optical wireless power transfer using a laser diode. Chin. Opt. Lett. 2018, 10, 080101. 104. Fakidis, J.; Ijaz, M.; Kucera, S.; Claussen, H.; Haas, H. On the design of an optical wireless link for small cell backhaul communication and energy harvesting. In Proceedings of the IEEE 25th Annual International Symposium Personal Indoor Mobile Radio Communication, Washington, DC, USA, 4–8 December 2014; pp. 58–62. 105. Fakidis, J.; Kucera, S.; Claussen, H.; Haas, H. On the design of a free space optical link for small cell backhaul communication and power supply. In Proceedings of the IEEE International Conference Communication Workshop, London, UK, 15–17 April 2015; pp. 1428–1433. 106. Tang, J.; Miyamoto, T. Numerical and experimental analysis of power generation characteristics in beam direction control of optical wireless power transmission with mirror. In Proceedings of the 24th Microoptics Conference (MOC2019), Toyama, Japan, 17–20 November 2019; pp. 164–165. 107. Tang, J.; Matsunaga, K.; Miyamoto, T. Numerical analysis of power generation characteristics in beam irradiation control of indoor OWPT system. Opt. Rev. 2020, 27, 170–176. [CrossRef] 108. Khan, S.R.; Pavuluri, S.K.; Cummins, G.; Desmulliez, M.P.Y. Wireless power transfer techniques for implantable medical devices: A review. Sensors 2020, 20, 3487. [CrossRef] 109. Mujeeb, U.; Rahman, M.; Adalian, D.; Chang, C.-F.; Scherer, A. Optical power transfer and communication methods for wireless implantable sensing platforms. J. Biomed. Opt. 2015, 20, 95012. [CrossRef][PubMed] 110. Goto, K.; Nakagawa, T.; Nakamura, O.; Kawata, S. An implantable power supply with an optically rechargeable lithium battery. IEEE Trans. Biomed. Eng. 2001, 48, 830–833. [CrossRef][PubMed] 111. Santis, M.D.; Cacciotti, I. Wireless implantable and biodegradable sensors for postsurgery monitoring: Current status and future perspectives. Nanotechnology 2020, 35, 252001. [CrossRef][PubMed] 112. Dregely, D.; Lindfors, K.; Lippitz, M.; Engheta, N.; Totzeck, M.; Giessen, H. Imaging and steering an optical wireless nanoantenna link. Nat. Commun. 2014, 5, 4354. [CrossRef][PubMed] 113. Fan, X.; Gao, L.; Mo, X.; Zhao, Q.; Jia, E. Overview of research status and application of wireless power transmission technology. Trans. China Electrotech. Soc. 2019, 34, 1353–1380. 114. Luo, W.; Dong, W.; Yang, H.; Xu, P.; Tang, Z. Development trend of high power lasers. Laser Infrared 2013, 43, 845–852. 115. Liu, Q.; Wu, J.; Xia, P.; Zhao, S.; Yang, Y.; Chen, W.; Hanzo, L. Charging unplugged: Will distributed laser charging for mobile wireless power transfer work? IEEE Veh. Technol. Mag. 2016, 11, 36–45. [CrossRef] 116. Motohiro, T.; Takeda, Y.; Ito, H.; Hasegawa, K.; Ikesue, A.; Ichikawa, T.; Higuchi, K.; Ichiki, A.; Mizuno, S.; Yamada, N.; et al. Concept of the solar-pumped laser-photovoltaics combined system and its application to laser beam power feeding to electric vehicles. Jpn. J. Appl. Phys. 2017, 56, 08MA07. [CrossRef] 117. Nguyen, D.H. Optical wireless power transfer for moving objects as a life-support technology. In Proceedings of the 2nd Global Conference on Life Sciences and Technologies, Kyoto, Japan, 10–12 March 2020; pp. 405–408. 118. Wi-Charge. Available online: https://wi-charge.com (accessed on 13 September 2020). 119. Japan Aerospace Exploration Agency. Research on Laser Wireless Power Transmission Technology. Available online: http://www.kenkai.jaxa.jp/eng/research/ssps/ssps-lssps.html (accessed on 13 September 2020). 120. Fraunhofer Institute for Solar Energy Systems ISE, Germany. Science Blog. Available online: https: //blog.innovation4e.de/en/2016/09/12/power-by-light (accessed on 13 September 2020). 121. Jin, M.H.C.; Pierce, J.M.; Lambiotte, J.C.; Fite, J.D.; Marshall, J.S.; Huntley, M.A. Underwater free-space optical power transfer: An enabling technology for remote underwater intervention. In Proceedings of the Offshore Technology Conference, Houston, TX, USA, 30 April–3 May 2018; pp. 1–9. 122. Kobe University. Solar cell design with over 50% energy-conversion efficiency. In PHYS& ORG, Technology, Energy & Green Tech; Kobe University: Kobe, Japan, 2017. 123. Lumb, M.P.; Mack, S.; Schmieder, K.J.; González, M.; Bennett, M.F.; Scheiman, D.; Meitl, M.; Fisher, B.; Burroughs, S.; Lee, K.T.; et al. GaSb-Based Solar Cells for Full Solar Spectrum Energy Harvesting. Adv. Energy Mater. 2017, 7, 1700345. [CrossRef] 124. Zhu, P. Solar Cell with a Photoelectric Conversion Efficiency of 44.5%. Available online: http://blog.sciencenet. cn/blog-212210-1066142.html (accessed on 13 September 2020). Appl. Sci. 2020, 10, 6463 44 of 44

125. WIPO IP PORTAL. Available online: http://patentscope2.wipo.int/search/en/result.jsf?_vid=JP1-KDI0PI- 61891 (accessed on 13 September 2020). 126. Hashmat, R.; Pagani, P.; Zeddam, A.; Chonavel, T. MIMO communications for inhome PLC networks: Measurements and results up to 100 MHz. In Proceedings of the IEEE International Symposium on Power Line Communications & Its Applications, Rio de Janeiro, Brazil, 28–31 March 2010; IEEE: Piscataway, NJ, USA, 2010. 127. Schneider, D.; Schwager, A.; Speidel, J.; Dilly, A. Implementation and results of a MIMO PLC feasibility study. In Proceedings of the Power Line Communications and Its Applications (ISPLC), Udine, Italy, 3–6 April 2011. 128. Cano, C.; Pittolo, A.; Malone, D.; Lampe, L.; Tonello, A.M.; Dabak, A.G. State-of-the-art in power line communications: From the applications to the medium. IEEE J. Sel. Areas Commun. 2016, 34, 1935–1952. [CrossRef] 129. Laguna, G.; Barron, R. Survey on indoor power line communication channel modeling. In Proceedings of the Power Line Communications and Its Applications (ISPLC), Electronics Robotics and Automotive Mechanics Conference, Morelos, Mexico, 30 September–3 October 2008; IEEE: Piscataway, NJ, USA, 2008. 130. Zimmermann, M.; Dostert, K. Analysis and modeling of impulsive noise in broad-band powerline communications. IEEE Trans. Electromagn. Compat. 2002, 44, 249–258. [CrossRef] 131. Ding, J.; Chih-Lin, I.; Chen, X.; Lai, H. Asymmetrical emission beams based visible light communication access points design. In Proceedings of the 28th Wireless and Optical Communications Conference (WOCC), Beijing, China, 9–10 May 2019; IEEE: Piscataway, NJ, USA, 2019. 132. Ding, J.; Chih-Lin, I.; Zhang, H.; Chen, X.; Lai, H. Cells planning of VLC networks using non-circular symmetric optical beam. In Proceedings of the 2019 IEEE International Conference on Communications (ICC), Shanghai, China, 20–24 May 2019; IEEE: Piscataway, NJ, USA, 2019. 133. Ding, J.; Chih-Lin, I.; Xu, Z. Indoor optical wireless channel characteristics with distinct source radiation patterns. IEEE Photonics J. 2016, 8, 1–15. [CrossRef] 134. You, C.; Hu, Q.; Li, S.; Ma, Y.; Chen, H. Light source distribution for visible light communication in coal mine working face based on positioning and power co-optimization. Chin. J. Lasers 2019, 46, 0406001. 135. Chen, Y.; Shen, Q.; Liu, H. Optimization algorithm of receiver optical power uniformity in indoor visible light communication. Chin. J. Lasers 2018, 45, 0506003. [CrossRef] 136. Kim, S.M.; Baek, M.W.; Nahm, S.H. Visible light communication using TDMA optical beamforming. Eurasip J. Wirel. Commun. Netw. 2017, 2017, 1–6. [CrossRef] 137. Campi, T.; Cruciani, S.; Maradei, F.; Feliziani, M. Near-field reduction in a wireless power transfer system using LCC compensation. IEEE Trans. Electromagn. Comput. 2017, 59, 686–694. [CrossRef] 138. Campi, T.; Cruciani, S.; Santis, V.D.; Feliziani, M. EMF safety and thermal aspects in a pacemaker equipped with a wireless power transfer system working at low frequency. IEEE Trans. Microw. Theory Tech. 2016, 64, 375–382. [CrossRef] 139. Jawad, A.M.; Nordin, R.; Gharghan, S.K.; Jawad, H.M.; Ismail, M. Opportunities and challenges for near-field wireless power transfer: A review. Energies 2017, 10, 1022. [CrossRef] 140. Nair, V.V.; Choi, J. An efficiency enhancement technique for a wireless power transmission system based on a multiple coil switching technique. Energies 2016, 9, 156. [CrossRef] 141. Feliziani, M.; Campi, T.; Cruciani, S.; Maradei, F.; Grasselli, U.; Macellari, M.; Schirone, L. Robust LCC compensation in wireless power transfer with variable coupling factor due to coil misalignment. In Proceedings of the IEEE 15th International Conference on Environment and Electrical Engineering (EEEIC), Rome, Italy, 10–13 June 2015; pp. 1181–1186.

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