1、6G Visible Light CommunicationTechnology White PaperChina Mobile Research InstitutePrefaceWith the increasingly stringent communication requirements,more spectrum isurgently needed for mobile communication.Since the spectrum resources below6GHz has been depleted,and the millimeter-wave frequency ban

2、ds such as 26GHzand 39GHz have been allocated for 5G use,it is necessary to explore higherfrequency bands for communication,such as terahertz communication and visiblelight communication(VLC),to meet the requirements of higher capacity andultra-high experience rate.Visible light usually refers to el

3、ectromagnetic waves of380790THz(wavelength ranges is 380790nm).The candidate spectrum coversabout 400THz.The characteristicsof large bandwidth andeasy torealizeultra-high-speed communication make VLC become a potential supplement to futuremobile communication systems.This white paper aims to discuss

4、 the potential application scenarios of VLC in 6Gand the communication requirements expected to be met,and proposes the challengesand key technologies of VLC,so as to promote the discussion and thinking in theindustry.The white paper is supported by the partner teams of Fudan University,BeijingUnive

5、rsity of Posts and Telecommunications,Southeast University,etc.Thanks to thecontributions and assistance of researcher Chao Shen and Professor Nan Chi of FudanUniversity,Dr.Pan Tang and Professor Jianghua Zhang of Beijing University of Postsand Telecommunications,and Professor Jiaheng Wang of Southe

6、ast University,etc.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）1Contents1.Concept.31.1 Characteristics of VLC.31.2 Recent research.42.Application scenarios and requirements of VLC.62.1 Mobile communication scenarios.62.1.1 Hotspot high-capacity scenarios.62.1.2 Indoor hot-spot scenarios.72.2 Vertical industry c

7、ommunication scenarios.72.2.1 Traffic scenarios.82.2.2 Harsh electromagnetic scenarios.82.3 Requirements of VLC.93.Challenges of VLC.113.1 Channel modelling.113.2 Materials and devices.123.3 Transmission.143.4 Networking.154.Key technologies in VLC.164.1 Channel modeling.164.1.1 Recent research of V

8、LC channel modeling.164.1.2 Theory-based VLC channel modeling.164.1.3 Measurement-based VLC channel modeling.184.2 Key devices.194.2.1 Transmitter.194.2.2 Receiver.224.3 Transmission Technology.254.3.1 Transmission Theory.254.3.2 Modulation waveform.274.3.3 Equalization.28中国移动通信有限公司研究院6G 可见光通信技术白皮书（

9、2022）24.3.4 VLC-MIMO.294.4 Networking Technology.324.4.1 Integrated Wireless and Optical Networking.324.4.2 Visible light ultra-dense networking.355.Summary and Prospect.37References.39中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）31.ConceptVisible Light Communication(VLC)is a wireless communication technology th

10、atintegrates illumination and communication.In VLC systems,visible light source suchas light-emitting diode(LED)transmits information with high-speed flashing lightthat human eye cannot perceive,and photoelectric detector(PD)converts the receivedoptical signal into electrical signal to obtain inform

11、ation 1.As a new combinationmode of illumination and communication,VLC is an effective supplement to theexisting wireless radio frequency communication.1.1 Characteristics of VLCCompared with the traditional radio frequency(RF)communication,VLC mainlyhas the following advantages:1)Abundant spectrum:

12、the spectrum resources available for traditional wirelesscommunication are only about 300MHz,while the visible light candidatespectrum bandwidth is nearly 400 THz.Therefore,VLC can effectively solvethe problem of increasingly tight spectrum resources.Figure 1-1 Spectrum resource map2)Easy to deploy:

13、since the existing facilities such as lighting,display andimaging in the illumination industry are mature,transmitting and receivingdevices of VLC can be obtained by simple upgrade.Considering illuminationrequirements,low-cost ultra-dense deployment can be achieved to meet hightraffic density requir

14、ements.3)Green and energy-conservation:with the integration of illumination andcommunication,VLC has the advantages of low power consumption and highenergy efficiency.This is exactly in line with the national energy conservation中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）4and emission reduction strategy.4)Immun

15、ity to electromagnetic interference:there is no interference betweenVisible light and RF signal,so VLC is very suitable for electromagneticsensitive scenarios such as aircraft,hospital and smart factory to effectivelyavoid electromagnetic interference and ensure the normal operation ofequipment.Howe

16、ver,VLC also has some shortcomings.Due to the characteristics of visiblelight being easily blocked and having large propagation loss,and the low bandwidthof commercial VLC device,VLC is currently mainly used in short-distancepoint-to-point communication scenarios with medium or low data rate.On the

17、otherhand,the terminal lighting will bring great inconvenience in daily use,so thesupported scenarios are limited applying visible light uplink.1.2 Recent researchVLC sprouted in China and flourished in Japan 23.Grantham Pang of theUniversity of Hong Kong first proposed the concept of VLC in 1999,an

18、d then theresearch team of M.Nakagawa of Keio University in Japan proposed an accessscheme for LED based VLC.Since then,VLC has become a research hotspot inindustryandacademia.Japanestablishedthe VisibleLightCommunicationConsortium(VLCC)in 2003,and numerous VLC application forms such ashigh-speed in

19、terconnection of local area networks,intelligent transportation systems,and intelligent lighthouses have emerged.Europe launched OMEGA(the HomeGigabit Access)program in 2008,aiming to develop indoor interconnectiontechnologies with 1Gbit/s data rate,and VLC is one of the research priorities.In thesa

20、me year,the U.S.government launched the Smart Lighting Program to exploreadvanced and innovated technical schemes for VLC system.Domestic research in VLC started late.After more than ten years of hard work,wefinally caught up and reached the same level as the developed countries,and obtainedleadersh

21、ip in some directions.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）5In academia,there are mainly Fudan University,Tsinghua University,BeijingUniversity of Posts and Telecommunications,Southeast University,Institute ofSemiconductors,Chinese Academy of Sciences,Taiwan Jiaotong University,etc.participating in VLC r

22、esearch.The research directions include modulation technology,OFDM technology,LED equalization technology,VLC channel modelling,lightsource layout and so on 4.In the industry,the national and local governments have increased their investmentin the industrial development of VLC.The VLC application pr

23、oducts on the marketcover many fields such as indoor positioning,APP,secure communication,paymentand Internet of Vehicles.VLC standardization has followed the industrys footsteps.The Institute ofElectrical and Electronics Engineers(IEEE)is the first international organization toformulate VLC technic

24、al standards.The organization established the VLC workinggroup in 2009 and released the first version of VLC standard IEEE 802.15.7-20115.This standard has made detailed specifications in VLC modulation mode,networkingarchitecture,physical layer design and so on.In our country,the wireless personala

25、rea network standard working group of the National Information TechnologyStandardization Technical Committee published White Paper on Standardization ofVisible Light Communication in 2016,and began to formulate related standards.In2018,series of VLC standards called Long-distance communication and i

26、nformationexchange between information technology systems Visible Light Communicationhave been released.China Mobile is setting up joint innovation projects with Fudan University,Tsinghua University,Beijing University of Posts and Telecommunications,SoutheastUniversity,focusing on core devices of VL

27、C systems for 6G,wireless and visiblelight fusion networking,air interface transmission schemes of VLC,VLC channelmodeling and prototype system for verification.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）62.Application scenarios and requirements of VLC2.1 Mobile communication scenariosWith the continuous devel

28、opment of mobile network services and the continuousenrichment of the types of network access devices,massive data needs to be accessedby air interface to the core network.Visible light provides the possibility of improvingthe 6G communication rate with its THz spectrum resources.The ultra-largeband

29、width can support more flexible time-frequency resource allocation schemes andmulti-user and multi-service transmission modes,which can not only become a newhotspot high-capacity scenario solution but also support high-speed user access insmall indoor scenarios.Ahigher user experience rate can be ac

30、hieved at the same time.Different from the existing visible light communication application scenarios,mobilecommunication scenarios need to focus on the users mobility and environmentalvariation.Visible light communication needs to provide users with a continuousservice experience.2.1.1Hotspot high-

31、capacity scenariosFigure 2-1 Typical hotspot high-capacity scenariosHotspot high-capacity scenarios include densely populated transportation hubs,中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）7shopping malls,stadiums,large conference rooms,etc.The target group is mainlyusers holding mobile devices.The main challe

32、nges faced by this scenario are lightbeam tracking and collaborative management,light noise and light interferenceelimination,and so on.2.1.2Indoor hot-spot scenariosFigure 2-2 Typical indoor hot-spot scenariosIndoor hot-spot scenarios include individual rooms,offices,conference rooms,etc.Such scena

33、rios have a small area,and the user movement area is limited to severalmeters.In these scenarios,the direction of the light beam does not need to switchfrequently,and the control information occupies relatively few resources.Morecommunication resources can be allocated to user data transmission.The

34、primarystudy items in these scenarios are spectral efficiency improvement and resourcescheduling optimization.2.2 Vertical industry communication scenariosWith the further optimization and upgrading of the future industry,the verticalindustry has put forward higher security,confidentiality,and relia

35、bility requirementsfor communication indicators.As a new technology integrated with illumination,visible light communication builds a network based on existing lighting nodes.It canavoid the impact of high-power RF signals on the vertical industrys services andimprove security.On the other hand,ther

36、e is a huge frequency gap between the visiblelight band and the present communication microwave band.The visible light canavoid interference with the existing system during the communication process and中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）8improve the robustness of communication.2.2.1Traffic scenariosFig

37、ure 2-3 Traffic scenariosTraffic scenarios mainly include ground,aerial and maritime transportationscenarios.In these scenarios,the existing light source in the traffic system could beused to transmit information and reduce the cost of mobile communication networkconstruction.Because visible light c

38、ommunication can also provide positioningfunction,which will help intelligent traffic management system obtain accurate roadinformation.The main challenges in traffic scenarios include visible light networkingand mobility management.2.2.2Harsh electromagnetic scenariosFigure 2-4 Harsh electromagneti

39、c scenariosHarsh electromagnetic scenarios mainly include hospitals,factories,and airplanes.Such scenarios have strict limits on electromagnetic power and frequency bands.Byintegrating visible light communication equipment and lighting equipment,and thelimitation of optical power,it is possible to p

40、rovide the necessary lighting whileeffectively avoiding electromagnetic interference and achieving safe and reliablewireless communication.The main challenges in such scenarios are optical power中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）9control,etc.2.3 Requirements of VLCIn 2030 and beyond,6G networks will sp

41、awn new application scenarios in threeaspects:intelligentlife,innovative production,and intelligentsociety.Theirapplications include human digital twins,holographic interaction,super transportation,synesthesia interconnection,and intellectual interaction.These scenarios will requireterabit-levelpeak

42、rates,sub-millisecondlatencyexperiences,centimeter-levelpositioning accuracy,and dozens of times improvements in network energy efficiency6,as shown in Figures 2-5.Figure 2-5 The overall requirements for 6G communicationsWith the help of its huge spectrum potential,visible light communication hasbec

43、ome one of the critical candidate technologies for 6G.We expect it to help 6Gnetworks improve user experience rate,peak rate,traffic density,network energyefficiency,and positioning accuracy.However,different scenarios have theirrespective communication requirements.For example,traffic density and m

44、obility areprioritized requirements in high-capacity scenarios.Small indoor rooms need toimprove user experience and peak rates;traffic light integration scenarios need higherpositioning accuracy;a harsh electromagnetic environment has strict transmissionpower and RF leakage power restrictions.In th

45、e future 6G application scenarios,visible light communication must first meet中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）10themobilityrequirements.Byintegratingwithtraditionalradiofrequencycommunication(such as low frequency,millimeter wave,terahertz,etc.),visible lightcommunication can provide mobile users wit

46、h continuous high-rate communicationservicesand abetter consumer experience.Itsbeneficialfor visible lightcommunications industry expanding in rapidly way.Besides,it is necessary to achievecommunication indicators in Table 2-1.Table 2-1 Visible light communication indicatorsIndicator namesValuesModu

47、lation bandwidth5 GHzPeak rate50 GbpsCoverage distance5 mCapacity density100 Mbps/m3中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）113.Challenges of VLCTo better meet the requirements of 6G communication,VLC technology is mainlyfaced with the following challenges at present.3.1 Channel modellingA wireless channel

48、is a mathematical representation of the wireless electromagneticenvironment.Its the foundation of designing a reliable and efficient communicationsystem.It builds an accurate simulation background for system evaluation andoptimization.Visible channel modeling needs to consider the frequency,channelf

49、ading,space,time,and other wireless channel characteristics.The visible light frequency band is much higher than the microwave band currentlyapplied.Since the channel characteristics are closely related to the carrier frequency,the visible light channel model doesnt follow the mobile channel model b

50、elow 100GHz.In terms of loss characteristics,the frequency increase of visible light willaggravate the propagation loss of the signal and produce atomic absorption effects inthe atmosphere.The additional rain and fog decay make the fading model morecomplicated.In addition,the frequency increase will

51、 also reduce the diffraction abilityoftheopticalsignal.Deepershadowfadingincreasestheerrorofthedistance-dependent path loss model,affecting the system evaluations accuracy.Interms of spatial and time characteristics,the optical signal propagation is limited bythe half-power angle of the light source

52、 and propagation loss.Thus,the primarypropagation mode is Line-of-Sight(LOS),and the secondary propagation mode isNonline-of-Sight(NLOS).The LOS paths occupy most of the received power.Theangular and delay spread decrease significantly.The low spread values indicate themultipath interference is weak

53、ened,but it also shows that we need to pay more for thescene coverage.The noise characteristic is another critical feature in visible channelmodeling.Due to the small front-end incident power,the background noise,scatter中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）12noise,and receiver circuit noise which are com

54、mon in the environment will impactsignal reception.Because noise is random and can be affected by opticalcommunication devices,it isnt easy to quantify and model through experiments,which brings new challenges to channel modeling.3.2 Materials and devicesThe current mainstream optical transmitters a

55、re LED and laser diode(LD).Although LED is much safer and lower-cost,its frequency response is not ideal.The3dB bandwidth of the existing commercial LED is generally less than 100MHz,which is far from meeting the required high-speed transmission of 6G.Moreover,LED is nonlinear device,and the nonline

56、ar performance becomes more serious withthe increase of transmitted power.As for LD,the emitted light is laser of goodcoherence,and LD bandwidth can reach over 1GHz,meeting the data raterequirement well.However,the transmitter and the receiver need to be aligned in thecoupling process,which puts for

57、ward high requirements for the stability of the spaceenvironment.Also,the laser emitted at high power can pose potential risks to thehuman eye.At the receiving end,the visible light signal is generally received by aphotodetector(PD).Currently,Positive-Intrinsic-Negative(PIN)photodiodes andAvalanche

58、Photodiodes(APD)are more commonly used.PIN is of relatively low cost,but its sensitivity and frequency response bandwidth are limited,which cannotsupport high-speed and long-distance VLC systems.APD utilizes the avalanche effectof the diode to multiply the excited photocurrent,and the introduced noi

59、se isamplified at the same time,so it is difficult to achieve high signal-to-noise ratio.To sum up,VLC for 6G place high demands on devices.For the new generation ofoptical transmitters and receivers,large bandwidth,wide coverage,low cost,convenient coupling,good security,and high sensitivity are re

60、quired.Therefore,further breakthroughs in the materials and manufacturing process of devices are inurgent need.In order to accelerate the development and selection of devices,it has中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）13significant instruction to establish an indicator system for VLC devices.For light so

61、urce devices,it is necessary to focus on communication indicators andtake lighting indicators into account.Core indicators in communication includemodulation bandwidth,power,and linearity.Among them,modulation bandwidthtends to be the bottleneck of the system bandwidth,which is related to many facto

62、rssuch as light source structure and drive circuit.It is the most important indicator.High-speed VLC systems often require the modulation bandwidth of the light sourceto reach hundreds of MHz or even GHz.In addition,the optical power of the lightsource has a great influence on the working distance o

63、f the system,and is also animportant basis for selection.Indicators related to illumination indicators includevolt-ampere characteristics dominant electrical indicators,thermal indicators,and aseries of optical indicators,such as driving voltage and current,luminous efficiency,brightness,color tempe

64、rature,stroboscopic and so on.Since driving voltage andcurrent determine the driving circuit design,and affect other indicators such asmodulation bandwidth and linearity,they are worthy of attention.Luminousefficiency directly reflects the energy efficiency of the light source,and affectscharacteris

65、tics such as junction temperature.It is also an important parameter.For detection devices,the main indicators are bandwidth,sensitivity,and spectralresponse distribution.In addition,factors such as driving voltage and dark current alsoneed to be considered under certain circumstances.Bandwidth of de

66、tector directlyaffects the communication performance of the system,which can reach hundreds ofMHz in the industry up to now.In the future,with the development of high-speedVLC systems,the demand for high-bandwidth detectors will be further strengthened.The sensitivity of the detector is also an impo

67、rtant indicator,which has a greatinfluence on the working distance of the system.The long-distance VLC systemespecially requires high-sensitivity detectors such as APD.Besides,the distribution ofresponse to different frequency bands also has a certain influence on the systemperformance.High sensitiv

68、ity and large bandwidth in the visible light frequency bandare required at the same time.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）143.3 TransmissionSince the bandwidth of VLC transceiver faces huge challenges,it is necessary toselect a suitable modulation scheme to further improve the bandwidth utilization 7

69、.Single-carrier modulation schemes such as On-Off Keying(OOK),pulse positionmodulation(PPM),andpulseamplitudemodulation(PAM)areoflowimplementation complexity,but the spectrum efficiency is not high enough.OrthogonalFrequencyDivisionMultiplexing(OFDM)modulationishighspectral-efficient and resistant t

70、o multipath effects.The existing OFDM modulationschemes for VLC are of various types,each with its own advantages anddisadvantages.Howtoselectamodulationschemefordifferentkindsofcommunication scenarios and physical channels needs research.In addition,adoptionof high-order modulation in VLC will caus

71、e the following problems:signal-to-noiseratio(SNR)needs to be further improved due to the sensitivity to noise of the system;change of the spatial position could result in uneven received power of the quadraturesignal;constellation diagram distortion will be introduced due to the nonlinear effectof

72、the visible light emitting device.All the problems need advancements ofwaveforms and modulation schemes.Furthermore,the bandwidth of VLC transceivers is affected by the frequencyresponse characteristics.To solve the problem of significant difference in frequencyresponse characteristics between high

73、and low frequency bands,the commonsolutions are equalization and bit energy loading.Equalization scheme can make thefrequency response of the entire bandwidth almost the same.Bit energy loadingscheme divides the entire bandwidth into several subbands,the power of which isconsistent.An optimal modula

74、tion order is selected for each subband to maximize thecapacity.Multiple Input Multiple Output(MIMO)technology is an important technology forVLC in the future.Under the condition of limited bandwidth,MIMO improves theoverall communication rate by spatial multiplexing.MIMO system has multipletransmit

75、ters and multiple receivers.On the one hand,it can achieve higher capacity中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）15and improve the data rate by spatial multiplexing.On the other hand,it can solve theocclusion problem in VLC caused by user movement and complex environment.However,in VLC MIMO systems,the MIM

76、O channel has strong spatial correlation,because the size of the receiver is much larger than the signal wavelength.And thesignals from multiple light sources can easily interfere with each other,which affectsthe overall transmission rate.Therefore,how to design a reasonable spatialdecorrelation alg

77、orithm at the receiving end is a key problem that needs to beovercome.3.4 NetworkingVLC can take advantage of its characteristics of low power consumption,low cost,and easy deployment,combined with lighting functions,to achieve wider coveragethrough the ultra-dense deployment of visible light nodes.

78、However,visible lightaccess points have a higher density and are closer to the user than traditional RFcommunication.If the visible light network adopts the common cellular architecturein mobile communication,there will be problems such as frequent handover,lowspectral efficiency and complex interfe

79、rence management.On the other hand,theultra-dense deployment of visible light nodes also increases the requirements anddeployment difficulty of fronthaul and backhaul.In addition,VLC has the problems of difficult uplink implementation,easydownlink interruption and poor mobility,which greatly limit t

80、he application scenariosof independent VLC.It is necessary to consider VLC as a part of the mobile cellularcommunication network to realize the integration of VLC and traditional RFcommunication in the cellular network,so as to have the ability to support mobileservices and form an industrial scale.

81、中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）164.Key technologies in VLC4.1 Channel modeling4.1.1Recent research of VLC channel modelingThe equivalent baseband model of VLC can be represented by the followingformula:where the channel impulse response(CIR)can be used to characterize thechannel characteristics.It

82、can also be used to analyze the impact of channel distortion.is the noise.Depending on the different modeling methods,the VLC channelmodeling can be divided into two types:theory-based and measurement-based.4.1.2Theory-based VLC channel modelingThe theory model of CIR includes two types:deterministi

83、c channel model andstochastic channel model 8.The deterministic channel model is a detail descriptionof specific propagation environment,channel scenario and location of transceiver,which has a high accuracy.Compared with the deterministic channel model,the lattermodel reduces the computation comple

84、xity and maintain a high flexibility.However,the accuracy is degraded.The recursive model was first used to describe the CIR of high-order reflectionpath of wireless infrared communication in deterministic models.It is used tocharacterize the line of sight(LOS)path of VLC.Besides,the Lambertian radi

85、ationmodel is used to describe LED lighting.The total CIR is the sum of LOS CIR and NLOS CIR:中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）17The LOS CIR is represented by a series of delayed impulse functions:In the calculation of NLOS path,the reflection surface is divided into smallLambertian reflection units.E

86、ach surface unit can be viewed as the receiving unitand transmitting unit.The impulse response of NLOS can be obtained by traversinghigh-order reflection path.Therecursivemodelprovidesagoodideaandhighaccuracyforthecharacterization of VLC channel.However,it is less used in theoretical modeling ofVLC

87、in practice due to its high time complexity.On the basis of recursive model,the iterative model changes the order ofconvolution in high-order reflection path,which can reduce computation:Due to its high accuracy and relatively low time complexity,the iterative model is acommonly used method in the t

88、heoretical modeling of VLC channel 9.Besides,based on the geometric optic and coherent diffraction theory,the raytracing-based geometric deterministic modeling utilizes the software(such as Zemax10)to analyze the channel under the specific situation.The ray tracing technologyensures the accuracy of

89、specifications of transceiver antenna,the surface reflectioncharacteristic of materials and the geometry of the created environment.Comparedwith iterative model,the ray tracing software requires a detailed description of thepropagation environment,so it cannot be easily applied to other scenarios.Bu

90、t it canrestore the actual scenarios more accurately and model complex scenarios more中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）18flexibly.Thestochasticmodelincludesgeometric-basedstochasticmodelandnon-geometric-based stochastic model,where the former includes spherical model(Acommon approach in VLC noise mode

91、ling.),regular-shaped geometric randomnessmodel and Carruthers model,and the latter includes the Monte Carlo Algorithm(MCA),Modified Monte Carlo algorithm(MMCA)and Modified Ceiling BounceModel(MCBM).There are three kinds of noise in VLC,which are shot noise,thermal noise andamplifier noise,where the

92、 shot noise can also be divided into the signal-dependentpart and signal-independent part.For the VLC noise modeling,there are two typicalmethods:one is the equivalent noise source method,where the thermal noise andamplifier noise are unified as the circuit thermal noise 11.The other is to modelthes

93、e three noises separately 12.Both methods are commonly used in VLC systemanalysis.However,there is still necessary to verify the accuracy and applicability ofthese two models.4.1.3Measurement-based VLC channel modelingFigure 4-1 VLC Measurement PlatformMeasurement-based VLC modeling uses the VLC mea

94、surement platform to collectmeasurement data in practice.Based on the pseudo-random sequence correlationprinciple,the transmitter sends a pseudo-random sequence.After OOK or quadratureamplitude modulation(QAM),LED will convert the information to an intensityvariation.The receiver detects the signal

95、strength through PD,and decorrelates the中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）19information to obtain the CIR.Since the channel impulse response can be obtained directly from the time domainresponse or obtained by inverse Fourier transform from the frequency domainresponse,the VLC measurement platform can

96、 also be divided into two categories:time domain(TD)detection and frequency domain(FD)detection.In TD detection,the transmitter sends the PN code with arbitrary waveform generator and the receiverdecorrelates the TD signal sampled by oscilloscope to obtain the CIR.In FD detection,the transmitter use

97、s a port of vector analyzer to send a frequency sweep signal.Thesecond port of vector analyzer will receive the signal after channel,so that the channelfrequency response(CFR)can be obtained.CIR is the inverse Fourier transform ofCFR.Some researches have been carried out for the measurement of VLC c

98、hannel.9compared the characteristics of millimeter-wave and visible light channels in aparking lot.They found that the frequency response of the millimeter-wave channel iseasily affected by the parking lot structural and surrounding vehicles,while thefrequency response of the VLC channel is not grea

99、tly affected by these two reasons.The main limitation of VLC measurement platform is the device performance.Reference 13 studied the impacts of human body occlusion on received power andRMS delay in indoor scenario.Reference 14 measured the CIR and path loss inindoor scenario.And references 15,16 st

100、udied the impact of different weather onthe VLC system in the vehicle-to-vehicle scenario.At present,there is no measurement work for VLC noise,and there is a lack ofactual measurement and verification of theoretical VLC channel noise model.Theactual noise distribution in VLC is still a problem that

101、 needs to be solved.4.2 Key devices4.2.1TransmitterThe transmitter in VLC systems generally uses white light as the light source,and中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）20the light source mainly includes LED,PD and Superluminescent Diode(SLD).LEDThe modulation bandwidth of LED is an important determinant

102、 of the channelcapacity and transmission rate of VLC system.In order to meet the requirements of6G-oriented visible light communication,the next generation led needs to improve themodulation bandwidth.Phosphor white-light LEDs is a commonly used led in visible light system.The blueLED chip emits blu

103、e light，and part of the blue light excites the phosphor to produceyellow light,which is mixed to obtain white light.This type of white-light LED islimited by the fluorescence lifetime of its phosphor(such as CE:YAG),so itsbandwidth is generally limited to about 2MHz.In order to alleviate the influen

104、ce ofthe long fluorescence lifetime,a blue light filter can be added in front of the detector,so that only the blue light radiated by the LED chip itself reaches the detector,thereby increasing the bandwidth of the component.However,since only part of thelight reaches the detector,the optical power

105、received by the detector is reduced andthe communication distance is limited.The multi-color chip white-light LEDs can also improve the modulation bandwidth,which encapsulates multiple color chips together to form a white light source,andthe more common is the RGB three-color chip white-light LED.Th

106、e multi-color chipdoes not have the problem of long fluorescence lifetime,and the wavelength divisionmultiplexing technology is used to modulate each monochromatic light chip in thewhite light components separately,and transmit multiple signals in parallel,whichcan greatly improve the channel capaci

107、ty.Unfortunately,it will increase thecomplexity of driving design,and also affect the color temperature of white lightafter combining.In addition to using different monochromatic light to improve the transmissionrate of the component,Micro-LED as the emission source of the visible light systemcan al

108、so greatly improve the modulation bandwidth of the component.The mainreason is that the reduction of the active area greatly increases the current density ofthe carriers in the active area and shortens the carrier life in the active area.At the中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）21same time,the smaller

109、active area reduces the junction capacitance of thecomponent,thereby reducing the RC time constant.We can increase the modulationbandwidth of the component while ensuring the total output power by fabricating aMicro-LED array.LDLD has a higher modulation bandwidth and thus has a faster informationtr

110、ansmission rate.At present,the modulation bandwidth of commercial LDs canreach 2.6 GHz,which is two orders of magnitude higher than that of commercialLEDs.Compared with LED,LD has the advantages of fast response,direct modulationand high coupling efficiency.SLDBased on spontaneous radiation and ampl

111、ified spontaneous radiation,SLD is acomponent between LED and LD,which has the characteristics of broad spectrum,weak temporal coherence,low noise intensity,and high efficiency.Compared withLED,SLD has higher modulation bandwidth and luminous efficiency.Compared withLD,it does not form a laser with

112、great power density,which is harmful to human eyes,and has a wider spectrum,which is more suitable for lighting.Table 4-1 Comparison of Different Types of Light SourcesTypeoflightsourceLinewidthMethodofphotongenerationCoherencelengthDivergenceangleModulationbandwidthLEDwidespontaneousradiationshortl

113、argelowLDnarrowamplifiedspontaneousradiationlongsmallhighSLDmiddlestimulatedmiddlemiddlemiddle中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）22radiationThe development trend of different light source types:The basic bandwidth of phosphor-based WLED is difficult to increase,and itneeds to rely on equalization techn

114、ology and filters.The only advantage is low cost,and it is not recommended as a future research focus.Multicolor LED have good technical maturity,flexibility and industrialfoundation,and can be used as a focus component in the initial stage.Micro-LED can achieve high basic bandwidth,but the total po

115、wer is low,andthe high-power technology is not mature.Its application in the field of lighting hascertain challenges,which can be focused on in the future.SLD can basically be attributed to the category of special high-bandwidthLEDs,in which stimulated radiation dominates,and the bandwidth character

116、istics areideal.4.2.2ReceiverThe common visible light detectors include PIN photodetectors,avalanchephotodiode(APD)photodetectors,andmetal-semiconductor-metal(MSM)photodetectors.Their mainly used materials are silicon(Si),germanium(Ge),siliconcarbide(SiC),gallium nitride(GaN),aluminum nitride(AlN),p

117、erovskite,and so on.Si-based photodetectorsSi-based photodetectors are widely used in communication in the visible lightband because the detection band of Si materials is between 400-1100 nm.The mostpopular silicon-based detectors are PIN photodetectors and APD photodetectors.PINphotodiode detectors

118、 have a lower cost.But their sensitivity and the responsebandwidth are limited,which cannot meet the requirement for long-distancecommunication.Compared with the Si-based detector,APD photodetectors havehigher sensitivity(internal gain up to 102104),higher response speed,and widerfrequency band(freq

119、uency band bandwidth up to 100 GHz).But they may introduceconsiderable noise and have poor stability in high/low temperatures.So they are not中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）23suitable for scenarios with high signal-to-noise ratios or significant temperaturedifferences.III-V photodetectorsCompared wi

120、th Si-based semiconductors,III-V compound semiconductors havethe advantages of high carrier mobility,high direct energy gap,adjustable band gap,and good stability.InGaN detectors using third-generation semiconductor InGaNmaterials can realize optical signal detection throughout the whole visible lig

121、ht bandbecause of their adjustable band gap.It can be integrated with the same opticalcommunication system to achieve efficient reception of optical signals by matchingLED light sources.With the development of zero bias self-drive detector research,theoperating voltage of InGaN-based detectors can r

122、educe to zero,making the devicework with low power consumption.MSM photodetectors resemble back-to-back Schottky photodiodes.When theapplied bias voltage increases and an avalanche breaks down of the Schottky diode onthe reverse deflection side,the current increases dramatically.MSM photodetectorsar

123、e responsive,simple,compatible with CMOS processes,and easy to integrate.However,the metal on their surface has strong absorption of light,which impactstheir internal gain and limits their application scenarios.Perovskite photodetectorThe chemical formulas of Perovskite can be represented as the for

124、m of ABX3.The A is a cation divided into organic and inorganic.Organic cation includesCH3NH3+(referred to as MA+),HC(NH2)2+(referred to as FA+),while inorganiccation is often Cs+.The B denotes metal cation such as Pb2+,Sn2+,etc.The Xrepresents the halogen anion I-,Cl-,Br-.Perovskites have short resp

125、onse times andhigher bandwidths.It has the advantages of being lightweight,having good flexibility,high carrier mobility,large light absorption coefficient,long exciton diffusion length,and adjustable band gap width.They can realize light detection in different bands.However,because perovskites are

126、organic materials,their life and stability are far lessthan Si and III-V compound semiconductor detectors.The application scenarios aremore limited.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）24PIN and APD are relatively mature.The corresponding commercial products caneven reach 1 GHz bandwidth.Their output pow

127、er can meet various gain requirements.But most Si-based detectors respond at peaks in the infrared band.Although the peakresponse of the III-V group detector is closer to the visible light band,the sensitivityof the III-V group commercial products is insufficient.Table 4-2 Comparison of the performa

128、nce of several photodetectorsMaterialSize(nm)Bandwidth(nm)Responsivity(A-W1)Detectivity(Jones)Si3.0-7.Ge2-13980-12001.51.21011PbS2.5-7.2800-1700-.1.81013PbSe3-171200-25000.67-PbTe2.6-8.31100-2150-CdTe/CdS6-11480-8200.31.2108InAs2-7970-15003.812.21O10InSb33-6.51300-1850-3.71012Ag2S5.4-

129、101200-MoS23.1-5.91080-13300.858.01011CuS3-6.13HgTe3--38.71011The responsivity,response bandwidth,and dark current of a visible light detectorare mainly affected by its material and structure.By matching light source anddetector/detector array,semi-conductive visible li

130、ght detectors can adapt to morescenarios.With the development of VLC technology,the effective transmission ofinformation has become particularly important.The new VLC systems will achievehigher transmission efficiency and a wider transmission spectrum,which puts forwardhigher requirements for detect

131、or bandwidth,high sensitivity/response,integration,flexibility,and self-power supplication.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）254.3 Transmission Technology4.3.1Transmission Theory1)Signal Distribution and Capacity BoundA typical VLC system is shown in Figure 4-2.It can be divided into two partsaccordin

132、g to the physical transmitted signals,which are electrical domain and opticaldomain.Figure 4-2 VLC Measurement PlatformThe data stream generated by the source drives the LED after passing throughmodules such as channel coding,rate matching,interleaving and modulation.LEDconverts the input positive s

133、ignal from the electrical domain to the optical domain,and its amplitude is carried by the optical power.The PD at the receiver converts thesignal from the optical domain to the electrical domain to complete the demodulationof the signal.In the linear scale of the LED,the optical power of the LED is

134、approximately proportional to the electrical power,which means that the opticalpower emitted by LED is proportional to the driving current 17.Since the physical forms of signals are different,the constraints of differentphysical signals are also different,so that the analysis of VLC signal distribut

135、ion canbe divided into electrical domain and optical domain.When the signal source is acontinuous source,the analysis based on the electrical domain is consistent with RFsignal.The optimal distribution of the received electrical signal obeys the Gaussiandistribution when the total electrical power i

136、s constrained.The analysis of opticaldomain needs to consider the impact of the LED linear scale and human eye safety.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）26Therefore,the distribution interval of the optical power signal is a fixed value,and thepower of the optical signal is constrained by its first-orde

137、r moment.The optimaldistribution of optical signal obeys an exponential distribution.The distribution of signal directly affects the system capacity,which could berepresented by the performance of capacity bound mathematically.The currentresearches on VLC signal distribution and capacity bounds are

138、mainly based on theoptical domain.The Swiss Federal Institute of Technology Zurich proposed a series ofwireless optical communication channel capacity bound from SISO to MIMO 18.Southeast University and Nanjing University of Posts and Telecommunications haveproposed a variety of VLC signal distribut

139、ion and capacity bounds based on dimmingcontrol 19.Although the analysis of signal distribution on the electrical domain isconsistent with RF,it still needs to consider the characteristics of the LED,such asnegative values removing and DC bias adding.Based on the analysis of signal distribution in e

140、lectrical and optical domain,thegeneration of VLC signal can be further designed.For example,different distributionscan be selected for different channels to obtain better performance.In addition,thecapacity bound analysis based on signal distribution can be used for subsequentsystem design and perf

141、ormance optimization.2)Signal-Dependent NoiseThe received noise of VLC is usually divided into shot noise and circuit thermalnoise,where the shot noise can be further divided into relative intensity noise andbackground light noise 20.Due to the physical characteristics of the PD and therandomness of

142、 the photons emitted by the LED at the transmitter,the actual receivednoise of the VLC system consists of two parts:signal-independent noise(relativeintensity noise)and signal-dependent noise(background light noise and circuitthermal noise)21.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）274.3.2Modulation wavefor

143、mThe multi-carrier modulation technology can effectively overcome the frequencyselection characteristics of the channel,so it is widely used in VLC.There are manymulti-carrier modulation schemes based on OFDM.The basic schemes mainlyinclude Direct Current Biased Optical-OFDM(DCO-OFDM),Asymmetrically

144、Clipped Optical-OFDM(ACO-OFDM),PulseAmplitude Modulation-Discrete MultiTone(PAM-DMT),etc.Combining the advantages of different modulation schemes,scholars have studied the combination of the above modulation schemes,mainlyincluding LACO-OFDM,ADO-OFDM,HACO-OFDM and so on.The threewaveforms ACO-OFDM,D

145、CO-OFDM,and LACO-OFDM are the mostrepresentative.Multi-carrier modulation of VLC requires that the transmitted signal is anon-negative real number 22.Hermitian symmetry makes the transmitted signal areal number.There are two main ways to generate a non-negative signal 23:1)AddDC bias,such as DCO-OFD

146、M;2)Clips negative signals in the time domain,such asACO-OFDM,LACO-OFDM.Adding bias does not affect subcarrier utilization,butincreases power overhead;clipping does not affect power overhead but wastesbandwidth.The nonlinear effect of VLC transmitting component will distort the constellationof the s

147、ignal.Therefore,in order to ensure that the transmitting component works inthe linear region,it is necessary to limit the maximum power of the signal.Limitingthe maximum power will cause clipping of the signal,resulting in clipping noise.Decreasing the signal amplitude can reduce the clipping noise,

148、but it will also reducethe received signal-to-noise ratio.Therefore,an optimal trade-off between signalamplification and clipping reduction is required to obtain the maximum receivedsignal-to-noise ratio.The coverage performance ofACO-OFDM under low-ordermodulation is better than DCO-OFDM,while it i

149、s the opposite under high-ordermodulation.Therefore,different waveforms can be selected according to the中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）28performance requirements of simultaneous interpreting VLC applications anddifferent transmission channels.In ACO-OFDM and LACO-OFDM systems,negative signal clippi

150、ng results in alarge number of zeros in the transmitted signal.Due to the influence of noise andinterference,the receiver may receive some negative signals.Therefore,filtering thenegative signals at the receiver can significantly reduce the noise interference.The above analysis is instructive for wa

151、veform selection in different scenarios andsystem performance improvement.Further,more waveforms can be analyzed in thefuture,and the recommended waveforms and clipping ratios in different scenarios canbe given,so as to realize high-speed visible light communication transmission.4.3.3EqualizationThe

152、 high-frequency response attenuation of VLC transmitting componentsseriously limits the signal transmission rate.Therefore,it is necessary to useequalization technology to expand the available bandwidth.According to the stage ofimplementing equalization,equalization can be divided into pre-equalizat

153、ion at thetransmitter and post-equalization at the receiver.Pre-equalization refers to raising the high-frequency part of the signal at thetransmitter to widen the available modulation bandwidth.Generally,it is achieved inthe frequency domain by applying the inverse function of the system frequencyr

154、esponse:Aweight is added to each sub-carrier according to the measured responsefunction,so that the amplitude of the low-frequency part is reduced,and theamplitude of the high-frequency part is increased,thereby making the frequencyresponse more flat and achieving the purpose of equalization.The sch

155、eme is simpleand easy to implement.Post-equalization refers to compensating for the transmission loss of the signal bytransmitting a known signal as a pilot and processing the received signal with anadaptive equalizer.Post-equalization can be divided into time-domain equalizationand frequency-domain

156、 equalization.Time domain equalization is to consider the中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）29impulse response of the time domain,and eliminate the crosstalk between symbols inthe time domain through a time domain filter with a finite length tap.The frequencydomain equalization is considered from the f

157、requency response,so that the totaltransfer function after the channel and the equalizer can satisfy the distortion-freetransmission.The application of post-equalization technology requires accuratechannel estimation,but the uneven frequency response of VLC components willaffect the accuracy of chan

158、nel estimation.In order to estimate the channel moreaccurately,adaptive pilot design can be adopted to allocate more pilot signals forhigh-frequency carriers,so as to improve the accuracy of channel estimation andenhance the post equalization performance.Similar to the equalization technology,the ad

159、aptive bit energy loading scheme canalso be used to expand the available bandwidth range and improve the bandwidthutilization efficiency.There are many technical solutions for bandwidth expansion such as equalizationand adaptive bit energy loading.In practical application,the appropriate scheme canb

160、e selected according to different scenarios,comprehensively considering the factorssuch as algorithm complexity,engineering implementation difficulty and transmissionrate,so as to effectively improve the performance of visible light communicationsystem and improve the data transmission rate.4.3.4VLC

161、-MIMOThe illumination and modulation bandwidth of a single LED is limited andinsufficient,which makes it difficult to meet indoor illumination requirement andcommunication requirement.Therefore,LED integration is usually adopted inpractical deployment,where multiple LEDs are formed into LED arrays,a

162、nd multipleLED arrays are deployed in practice.As one of the important technologies of VLC,the MIMO transmission using multiple LED arrays can significantly increase thecapacity,overcome the occlusion problem and indoor blind spot problem caused bymobility,improve communication quality,and reduce th

163、e difficulty of physical中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）30alignment.1)Classification of VLC-MIMOAccording to whether the receiver equips imaging lens,VLC-MIMO can bedivided into non-imaging MIMO and imaging MIMO,as shown in Figure 4-3.Figure 4-3 non-imaging MIMO(left)and imaging MIMO(right)The diffe

164、rence between non-imaging MIMO and imaging MIMO is mainly at thereceiver.Non-imaging MIMO is similar to traditional MIMO technology.However,since VLC signal does not contain phase information,the VLC-MIMO channelcorrelation is very large.For example,when the PD is located at the center or axis ofthe

165、 room,the channel matrix may not be full of rank,and the gain and delay ofmultiple subchannels are the same.Moreover,due to the limitation of space andequipment installation complexity,it is difficult to realize multi-stream transmissionin the same LED array.Therefore,each LED array usually transmit

166、s different datastream in VLC-MIMO transmission,while different LEDs in the same LED arraytransmit the same data stream.Imaging MIMO is a good solution to the solve the location limitations ofnon-imaging MIMO.In the imaging MIMO system,the imaging lens projects theimage of the LED array on the recei

167、ving imaging sensor detector array.The use ofimaging lenses reduces the receiver size compared to the concentrator used innon-imaging MIMO.The imaging sensor detector array consists of multiple detectorpixels,and the projected image will hit multiple pixels on the array to excite current.Each pixel

168、can be regarded as a receiver,so the channel from each pixel to the LEDarray constitutes a channel matrix.The sensor detector array is composed of a largenumber of PDs,and the pixels refer to the effective area that the receiver could中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）31identify,so a pixel point may co

169、ntain one or more PDs.Even if the received image isoverlapping,the receiver can still restore the signal by some specific algorithms.However,limited by the refresh speed of the detection array,it is difficult to furtherimprove the transmission rate.According to the transmitter,VLC-MIMO also includes

170、 the transmitted lens-basedMIMO,as shown in Figure 4-4.Figure 4-4 VLC-MIMO system with transmitted lensThe transmitted lens-based VLC-MIMO uses the transmitted lens to refract thelight emitted by different LEDs to different directions and angles,so as to realize theenergy concentration and improve t

171、he spatial resolution.The light emitted by the LEDis firstly refracted on the lens,and then refracted on the spherical surface of lens,sothe transmitted lens can be used to distinguish different light signals.The light will berefracted by lens and concentrate into a small angle range,which can form

172、a narrowbeam.This means that the VLC-MIMO system using the transmitted lens can servemultiple users at the same time,which is called the beam division multi-access(BDMA).However,adding a transmitted lens does not significantly change thedistribution of LED light intensity.This MIMO scheme is meaning

173、ful for the asymptotic characteristics of large-scaleLEDs,andcanovercomeinter-userinterferenceandrealizemulti-usercommunication.On the other hand,due to the use of transmitted lens,the cost is highand the configuration flexibility will be limited.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）322)VLC-MIMO precodin

174、gVLC-MIMO precoding could be considered in the view of electrical domain andoptical domain.The precoding of electrical domain is consistent with RF-MIMO,thusthe commonly used linear precoding can be considered,such as maximum ratiotransmission(MRT)precoding,zero-forcing(ZF)precoding,block diagonaliz

175、ation(BD)precoding 24 and singular value decomposition(SVD)based precoding 25.Due to the impact of the precoding matrix specific value,the precoding signals aredifficult to meet the positive value requirement,so it is still necessary to add a DCbias to ensure that the signal is positive on LED.The p

176、recoding analysis of opticaldomain is still researching.Since the signal distribution is no longer Gaussiandistributed,it may be necessary to consider the design of nonlinear precoding toapproach the channel capacity bound.Limited by the space diversity gain,the precoding design of VLC-MIMO needsfur

177、ther exploration.Besides,when MIMO is combined with OFDM,the analysisbased on the electrical domain is more general,and it is a common idea for precodingdesign.4.4 Networking Technology4.4.1Integrated Wireless and Optical NetworkingIn the cellular network-based RF and visible light heterogeneous fus

178、ion networking,it is necessary to research in-depth how to integrate the VLC network of differenttypes,different levels and overlapping coverage and the RF communication networkbased on the cellular network,and realize the deep coupling of communication,lighting and positioning to form an end-to-end

179、 technical system.1)RF and visible light fusion accessIn terms of converged access,it is necessary to consider the process that the RFbase station assists the terminal to access the visible light node.One way is that theterminal accesses the RF base station first,the RF base station configures candi

180、date中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）33visible light nodes for the terminal,the terminal selects the appropriate visible lightnode based on the downlink reference signal sent by the visible light node,and reportsthe RF base station through the RF uplink,and finally the RF base station configuresthe v

181、isible light transmission node for the terminal and perform the visible lightdownlink transmission.Another way is that the terminal selects one or more suitablevisible light nodes by measuring the downlink reference signals sent by the visiblelight nodes,and initiates a random access request to the

182、RF base station through theRF uplink.At the same time,the selection result is reported to the network,and theRF base station responds and associates one or more visible light nodes for theterminal.In this process,the configuration information of the network to the terminalmay be transmitted through

183、the RF downlink or the visible light downlink.2)RF and visible light fusion transmissionIn terms of converged transmission,it is necessary to consider key processes suchas RF network-assisted visible light network for HARQ feedback,channelmeasurement feedback,optical beam switching management,and do

184、wnlink controlsignal transmission.For example,the visible light node sends data to the terminal,andthe terminal sends ACK feedback to the RF base station through the RF uplink afterreceiving the data successfully.If the terminal does not receive data or demodulationfails,it sends NACK feedback to th

185、e RF base station.After receiving theACK/NACKinformation from the terminal,the RF station feeds it back to the visible light node.Ifthe RF base station does not receive the feedback from the terminal within a presettime,it feeds back the NACK information to the visible light node.In addition,conside

186、ring that VLC technology not only meets the communication needs,but alsomeets the lighting needs,it is necessary to consider the fusion transmission problembased on dimming.3)RF and visible light fusion connection managementIn terms of mobility management,it is necessary to consider the related tech

187、nicalsolutions of the RF base station assisting the terminal to perform visible lightdownlink multi-connection management.In the RF and visible light fusionnetworking,the multi-connection technology includes not only the multi-connection中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）34between multiple visible ligh

188、t base stations,but also the multi-connection between thevisible light base station and the RF base station.In the RF and visible light fusion connection management based on the cellularnetwork,it is necessary to consider the change of the multi-connection configurationcaused by the change of the vi

189、sible light service node.The movement of the terminaland the change of the environment will lead to the signal strength and service qualityof the visible light node to change,thus triggering the switching of the visible lightservice node.The switch of the downlink transmission function of the visibl

190、e lightnode or the RF base station means that the downlink transmission function of thevisible light node or the RF base station can be dynamically switched according to theload of the downlink service or the power consumption of the network to optimize theoverall transmission performance or power c

191、onsumption of the network.For example,when the visible light service quality is good,the visible light node can be used fordownlink services preferentially for transmission.In this case,more RF resources canbe released to support uplink service transmission or provide higher-quality servicesfor user

192、s in areas with poor visible light coverage.Therefore,the results of networkperformance optimization may lead to changes in visible light service nodes.Foranother example,considering the low power consumption of visible light nodes andthe requirement of illumination,the network can comprehensively o

193、ptimize thenetwork power consumption by considering the overall requirements of lightingpower consumption,visible light communication power consumption and RF basestation power consumption,and then dynamically switch the downlink transmissionof the visible light node or the RF base station to achiev

194、e network energy saving.Atthe same time,the visible light service node may change.Regardless of the reasons for the multi-connection change,when the terminalselects the visible base station to access and switches the visible base station,if theswitch is based on the visible light link,the communicat

195、ion will be interrupted and theterminal cannot access due to the visible occlusion and instability.It is necessary toconsider the cellular network-based RF base station assisting the visible lightdownlink multi-connection management.The terminal reports the measurement report中国移动通信有限公司研究院6G 可见光通信技术白

196、皮书（2022）35to the RF base station through the RF link.RF base station decides whether theterminal to perform handover and the target base station according to certain decisionconditions.The stability of the RF link can ensure the service continuity of theterminal when moving in the network,and improv

197、e the efficiency of multi-connectionmanagement.In addition,since the visible light downlink transmission is easy to be blocked andthe visible light communication is easily interrupted,the uplink and downlinkbidirectional links of RF can assist the rapid recovery of the visible lightcommunication,and

198、 provide continuous services for users during the interruption ofthe visible light communication.4.4.2Visible light ultra-dense networkingVisible light nodes can be deployed ultra-densely in combination with lightingrequirements,but will also bring problems such as frequent switching,low spectraleff

199、iciency,and complex interference management.According to the characteristics ofvisible light channel and network,the user-centered visible light cellular networkarchitecture is considered.By establishing the formation mechanism of amorphousvisible light cell and efficient configuration of visible li

200、ght network resources,thespectral efficiency is improved and the visible light communication from point topoint transmission to multi-cell seamless coverage is realized.Amorphous cellular network architectureIn the future visible light network,the user-centric network architecture allowsmultiple acc

201、ess points(APs)to simultaneously serve a user and constructs anamorphous cell for each user based on the uses channel quality and interferencecharacteristics,which can realize the most reasonable utilization of space,time,frequency,and user domain resources.The network structure is shown in Figure 4

202、-5.Compared with the 5G network based on RF communication,the VLC network ismore conducive to the realization of user-centric network architecture.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）36Figure 4-5User-centric cellular network architecture in multi-light source VLC systemsAmorphous cell formation mechanis

203、mIn user-centric visible light cellular networks,scheduling,that is,the pairingbetween APs and users,is the key to exerting network performance.The matchingbetween users and APs must consider the interference between multiple light sources,the acquisition and interaction of channel state information

204、,and find the bestcompromise between overall network performance and user fairness.Through anefficient resource scheduling algorithm,a resource allocation scheme with theadvantages of low interference,large diversity gain and high user service rate can beselected.In addition,in the ultra-dense visib

205、le light deployment architecture,due tothe limited modulation bandwidth and illuminance of a single LED lamp,the MIMOtransmission method is usually used to meet the communication and lightingrequirements at the same time.Optimization issues such as energy efficiencymaximization under the constraint

206、of minimum illuminance need to be considered.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）375.Summary and ProspectVLC is expected to meet 6G requirements such as area traffic density,userexperience data and other important requirements,and improve network performancein peak data rate,network energy efficiency,po

207、sitioning accuracy and other aspects.However,it also faces considerable challenges in channel modeling,devices,transmission and networking.For channel modeling in different scenarios,there isstill a lack of model optimization and application testing for actual working scenarios.In terms of VLC devic

208、es,most of the high-performance key devices are still in thestage of laboratory trial production,and there is still a lack of mature industrializeddevices,which need industrial pilot test.In terms of transmission and networkingtechnology,it is necessary to combine the characteristics of high-perform

209、ance visiblelight devices and visible light channel for targeted design,and form a wireless visiblelightfusionheterogeneousnetworktogetherwithtraditionalRFfrequencycommunication(such as medium and low frequency,millimeter wave,terahertz,etc.)to support continuous high rate services in mobile scenari

210、os.In terms ofstandardization,it is necessary to consider the unification of VLC standards and 6Goverall communication standards to maximize the industrial scale of 6G,but alsoconsider the formation of complete VLC device standard system to form an industrialsynergy.How to miniaturize and integrate

211、the system is also one of the challenges tobe overcome in the industrialization of VLC.Facing the time of 6G commercialization in 2030,we propose that academia andindustry start the overall research on VLC for 6G as soon as possible,plan thedirection of industry promotion and standardization researc

212、h for VLC,and jointlyestablish and perfect the visible light channel model and complete key devicetechnologies route selection and research,and form key technical frameworks such astransmission and networking.It lays the foundation for the standardization andindustrial application of 6G-oriented VLC

213、 in the future.中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）38Figure 5-1 6G VLC Technology Research Timeline中国移动通信有限公司研究院6G 可见光通信技术白皮书（2022）39References1 Nan Chi.Key Devices and Application of LED Visible Light CommunicationM.Posts&Telecom Press,2015.2 Tanaka Y,Haruyama S,Nakagawa M.Wireless optical transmission

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