visible light communication in 6g

6G networks are expected to provide extremely high capacity and satisfy emerging applications, but current frequency bands may not be sufficient. Moreover, 6G will provide superior coverage by

integrating space/air/underwater networks with terrestri-al networks, given that traditional wireless communica-tions are not able to provide high-speed data rates for nonterrestrial networks. Visible light communication (VLC) is a high-speed communication technique with an unlicensed frequency range of 400–800 THz and can be adopted as an alternative approach to solving these prob-lems. In this article, we present the prospects and chal-

lenges of VLC in 6G in conjunction with its advances in high-speed transmissions. Recent hot research interests, including new materials and devices, advanced modula-tion, underwater VLC (UVLC), and signal processing based on machine learning, are also discussed. It is envis-aged that VLC will become an indispensable part of 6G given its high-speed transmission advantages and will cooperate with other communication methods to benefit our daily lives.

Overview Following the commercial deployment of 5G at the end of 2019, research efforts on 6G are now being carried out in different countries and organizations. The Inter-national Telecommunication Union established a

VISIBLE LIGHT COMMUNICATION IN 6GAdvances, Challenges, and Prospects

Nan Chi, Yingjun Zhou, Yiran Wei, and

Fangchen Hu

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Network 2030 focus group with the aim to explore tech-nological development for 2030 and beyond. 6G net-works are supposed to provide better performance than 5G and satisfy emerging services for Industry 4.0, personalized health, virtual presence, and other chal-lenging applications. Thus, 6G is expected to support transmission rates 100–1,000 times higher than those for 5G [1]. A bandwidth up to 6 GHz has been adopted by 2G, 3G, and 4G, while 5G utilizes the range lower than 6 GHz as efficiently as possible by combining 24–100 GHz [1]. Nevertheless, researchers have realized that this range may not be adequate to meet growing demand based on the current frequency bands. Accord-ingly, it will be necessary to surpass 100 GHz for 6G and explore different frequency sources to solve the prob-lem of the scarce spectrum.

VLC employs unlicensed bands and high transmis-sion rates. Correspondingly, it is a promising technique intended to replace conventional wireless local area net-works for indoor communications because people spend more than 80% of their time indoors. Compared with oth-er optical wireless communication technologies, such as infrared communication, VLC is preferable because it

utilizes existing illumination systems, which can poten-tially reduce costs, and supports high-speed transmis-sion with good coverage due to illumination needs in daily life. Furthermore, VLC is safe for human eyes. VLC can also play an important role in outdoor terrestrial ap-plications; thus, car-to-car communication may become one of the first implementation paradigms. In addition, 6G will provide superior coverage by integrating space/air/underwater networks with terrestrial networks [2]. However, the environments of these three networks are different from terrestrial environments, indicating that traditional wireless communications are not able to provide high-speed data rates for them. VLC utiliz-es ultrahigh bandwidths within the frequency range of 400–800 THz, which is many orders of magnitude greater than the radio-frequency (RF) bandwidth, and so consti-tutes a suitable technique for these scenarios in 6G, as exhibited in Figure 1.

Advances in High-Speed VLC SystemsThe greatest advantage of VLC is that it can provide much higher transmission rates than traditional wireless communication systems. For this reason, researchers always focus on high-speed VLC studies. High-speed VLC systems can be classified as one of two types: offline and real-time processing. For offline processing, a real-time oscilloscope is required to record the received signals, and the signals are then processed by offline programs on personal terminals. Thus, it is necessary to investi-gate new techniques with the use of offline processing

Space Networks

Car Networks

Air Networks

Underwater Networks

Terrestrial Networks

Wi-Fi

VLC

VLC Links

mm-Wave

Figure 1 The applications of VLC in space, air, underwater, indoor, and car networks. mm-wave: millimeter-wave.

VLC utiLizes uLtrahigh bandwidths within the frequenCy range of 400–800 thz, whiCh is many orders of magnitude greater than the radio-frequenCy bandwidth.


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VLC systems in laboratory experiments. In 2000, a scheme was proposed by Tanaka et al. at Keio University to use VLC based on white LEDs for indoor applications. In addition, in 2003, the Visible Light Communication Consortium, a Japanese organization, was established with the aims of publicizing and standardizing VLC tech-nology. In 2011, the first standard (802.15.7-2011 for short-range VLC) was published by IEEE, and the term light fidelity (LiFi) was proposed to represent wireless net-works using VLC. In 2016, a topic interest group was cre-ated for IEEE 802.11, and, in 2017, the European Union H2020 Wireless Optical/Radio Terabit Communications project was approved with the goal of bringing industry and academia together to develop new techniques for VLC. Since then, VLC has become increasingly popular around the world.

Currently, many companies are focusing on bringing high-speed VLC to commercial markets. PureLiFi devel-oped an optical front end operating with 802.11 base-band solutions to achieve LiFi integration into smart devices. More than 1-Gb/s downlink rates and lower than 600-Mb/s uplink rates were demonstrated with the optical front end. The Fraunhofer Heinrich Hertz Insti-tute created a conference room using VLC-based LiFi networks on Mainau Island with more than 1-Gb/s data rates. ByteLight developed an indoor positioning system based on VLC that used smartphone cameras to control consumers’ locations. Oledcomm demonstrated LiFi with top speeds of 100 Mb/s per seat on a commercial Air France flight and said that 1-Gb/s speeds could be achieved with next-generation products.

There are two types of commonly used light sources for VLC systems, i.e., LEDs and laser diodes (LDs). Ap-plication of LD-based VLC systems can easily achieve high data rates and long-distance transmissions owing to the intrinsic large bandwidth of the LDs. However, these systems require precise alignments between LDs and the corresponding receivers. LEDs have much wider divergence than LDs. Therefore, they can be applied in shorter-distance links for both point-to-point and point-to-multipoint applications. Furthermore, the price of LEDs is much lower than that of LDs. Accordingly, LEDs can be integrated into large-scale arrays to achieve hun-dreds of watts of illumination power [3]. There are two basic ways to generate white light illumination based on LEDs. The first is based on the use of blue emitters with a phosphor layer, while the second is based on the use of multicolor chip-integrated emitters, such as red-green-blue (RGB) and RGB-yellow (RGBY) emitters. The multicolor chip-integrated method is preferable to phosphor-based LEDs, owing to the higher bandwidth and the ability to offer wavelength division multiplexing (WDM) transmission, and thus can be used to improve data rates. The bandwidth of phosphorescent white LEDs is only a few tens of megahertz even with a blue

filter because of the slow response phosphors. However, considering their lower cost, lower complexity, and mar-ket dominance, phosphor-based LEDs are sometimes more attractive for VLC commercialization. Gallium ni-tride (GaN) microLEDs (μLEDs) are another type of com-monly used LEDs that have a photoactive device area at the micrometer scale; therefore, the plate capacitance is relieved, and the bandwidth is increased. However, the reduction of the photoactive area results in a dramatic reduction of optical power and transmission distance. The optical power of μLEDs is significantly less than that of standard phosphor-based LEDs.

The main drawback of LED-based VLC systems is the limited modulation bandwidth of LEDs. Thus, many spectrally efficient techniques [such as multilevel car-rier-less amplitude and phase modulation (CAP); pulse amplitude modulation (PAM); orthogonal frequency di-vision multiplexing (OFDM); discrete multitone modu-lation (DMT); multiple-input, multiple-output (MIMO); and WDM] are used to increase system data rates by improving bandwidth efficiency. Meanwhile, analog pre-equalization circuits can also be applied to extend the modulation bandwidth of VLC systems and enhance overall transmission data rates. Recent representative research advances for high-speed VLC systems are summarized in Figure 2. Note that free-space data rate results using LEDs are separated into two parts to pro-vide a clear data rate comparison between the multicol-or integrated white LED and the phosphorescent LED. In addition, the high-speed results of UVLC systems using two kinds of LEDs are combined into a group for a bet-ter view. It is clear that the supported data rates of VLC systems have developed rapidly over the years. The highest data rates for free-space VLC systems based on multicolor integrated LEDs, phosphor-based LEDs, and GaN μLEDs are 15.73 Gb/s [4], 3.0 Gb/s [5], and 7.91 Gb/s [6], respectively.

Free-space VLC systems based on real-time process-ing can achieve simultaneous information transmission, reception, and processing, and, thus, can facilitate real-time processing. It is also crucial to carry out research studies on real-time VLC high-speed systems because these are essential for future commercial applications of VLC. A 1-Gb/s real-time VLC system based on nonreturn to zero on/off keying (OOK) with a free-space distance of 1.5 m was presented in 2018, and a 2.5-Gb/s real-time VLC system with DMT modulation over a free-space

free-spaCe VLC systems based on reaL-time proCessing Can aChieVe simuLtaneous information transmission, reCeption, and proCessing, and, thus Can faCiLitate reaL reaL-time proCessing.



distance of 1.7 m was reported in 2019. Further work should be carried out to improve the data rates of real-time VLC systems.

Research Interests in High-Speed VLC Systems

New Materials and DevicesIn recent years, many researchers have carried out investigations on new materials and devices for VLC sys-tems to achieve breakthrough results. Superluminescent diodes (SLDs) combine the beam directionality of LDs with the wide divergence characteristics of LEDs. SLDs have other advantages, including large bandwidth, increased brightness, and speckle-free characteristics. A high-power indium GaN-based SLD emitting blue light with a bandwidth of ~800 MHz was presented in 2016. As the manufacturing processes mature further, SLDs are expected to become promising light-emitting devices for future VLC systems.

Apart from the μLED mentioned previously, three other types of LEDs [silicon (Si)-based LEDs (Si-LEDs), surface plasmon-coupled LEDs (SP-LEDs), and off-the-shelf LEDs] have gained attention in VLC systems over the last few years. Si-LEDs have the advantages of strong antistatic ability, long lifetime, and high production ef-ficiency. A common-anode, GaN-based, five-primary-color Si-LED reported in [3] employed 1) a hemispherical

and pyramidal pattern surface-textured GaN to enhance the light extraction efficiency, 2) a complementary elec-trode to reduce light absorption, and 3) a silver reflec-tor to improve single-side luminescence. Moreover, a 15.17-Gb/s bit-loading DMT VLC transmission through a 1.2-m underwater channel was successfully achieved based on the use of this LED, where the room tempera-ture was 20°C [3]. Utilizing SP-LEDs is another effective way to increase the modulation bandwidth by increas-ing spontaneous emission rate. SP-LEDs are attractive solutions for high-speed VLC systems, given that they afford high-modulation bandwidths and optical powers without the need of high current densities. Cheap off-the-shelf LEDs are available for <50 U.S. cents and can be applied in free-space VLC systems with a record data rate of 15.73 Gb/s [4].

Among VLC-receiving devices, positive intrinsic neg-ative (PIN) diodes and avalanche photodiodes (APDs) are currently the mainstream optical receivers. PIN diodes have a lower cost than APDs, but APDs have a higher sensitivity than PIN diodes and need a high volt-age for the bias circuit. To improve the sensitivity of PIN diode, a 3 × 3 integrated PIN array was proposed in 2015. Its performance was validated by a 1.2-Gb/s VLC demonstration and shown to be superior to a single PIN diode. Focusing optical devices, like lenses, are com-monly used in VLC systems to match the small active

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figure 2 Recent representative research advances for high-speed VLC systems.



areas of photodetectors. However, the signal-to-noise ratio gain comes at the cost of a reduced field of view. Correspondingly, a novel nonimaging optical concentra-tor, referred to as a compound parabolic concentrator-shape luminescent solar concentrator (CPC-shape LSC), was proposed in 2017 to solve this problem [7]. Based on these experiments, CPC-shape LSCs doubled the op-tical gain of their rectangular counterparts. This clearly shows their potential for high-speed VLC systems with smart terminals.

To solve the current LED bandwidth limitation, low sensitivity, and nonlinearity issues associated with the detectors, future high-speed VLC systems will need new light sources, detectors, and optoelectronic devices based on new materials. New VLC light sources should have wider modulation bandwidths and higher light ef-ficiencies. Furthermore, new VLC detectors need to improve the selective absorption of visible light and in-ternal and external quantum efficiencies. In the future, as demonstrated in Figure 3, VLC systems will require more advanced optoelectronic devices, which will include ex-ternal modulators, amplifiers, multiplexers/demultiplex-ers (Mux/Demux), optical switches, and transceivers. Note that this article attempts to provide only a brief overview of new materials and devices for VLC systems;

thus, some materials and devices displayed in Figure 3, such as the single-photon avalanche diode (SPAD) and multipixel photon counter (MPPC), are not introduced.

Advanced ModulationApart from the materials and devices, advanced modula-tion is also critical for achieving a high-speed VLC sys-tem with high spectral efficiency. Four dimensions can be utilized for modulation in VLC systems, including amplitude, frequency, phase, and polarization. Further-more, typical 1D modulations, such as traditional OOK and frequency shift keying, can be combined with other dimensions to implement multidimensional modulation schemes. The application of multilevel modulation is another way to achieve high spectral efficiency, i.e., the use of 64 quadrature amplitude modulation (QAM) or 128 QAM to replace OOK. Detailed information on imple-menting higher-order QAM in high-speed VLC systems can be found in [3]. Reduction of the signal bandwidth is also an important method used to improve spectral effi-ciency. For a regular WDM system, the channel spacing is usually larger than the baud rate, but it is equal to or even less than the baud rate in a Nyquist or super-Nyquist system. Therefore, more data can be transmitted in cases where the system bandwidth is limited.

State of Art Future Development

APDReceiver

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LED

Off-the-ShelfLED

figure 3 The materials and devices for VLC systems.



In recent years, new technologies, such as probabilis-tic or geometric shaping and polar codes, have emerged and been successfully applied to the implementation of high-speed VLC systems. However, imperfect modula-tion methods cause the current systems to have capaci-ties that are far from the Shannon limit. For instance, commonly used wavelength WDM VLC systems do not make full use of spectral resources: the latter are wast-ed owing to the gaps between different channels. Thus, additional research studies on advanced modulation should be carried out in the next phase.

UVLC SystemsUnderwater communication technologies are important to achieve 6G network integration. As presented in Fig-ure 4, the interconnection of ocean observation sensors, ultrahigh-speed noncontact data communications among different marine equipment, and functional implementation, such as wireless fusion networking of submarine optical cable networks and underwater wire-less optical communications, all require support from underwater communication technologies.

Compared with underwater wired communication, underwater wireless communication technology is preferable because it does not require the transmis-sion media and is currently mainly implemented by acoustic waves and RF. Acoustic waves can be used to implement low-rate and long-distance underwa-ter transmissions owing to their small attenuation in seawater. However, they are associated with several

disadvantages, i.e., narrow bandwidth, low carrier frequency, large propagation delay, and poor security. RF transmission is suitable for underwater short-dis-tance and high-rate communications. However, this method also has drawbacks, such as the skin effect in seawater, limited penetration depth, and high trans-mission power requirements. Therefore, the develop-ment of new underwater communication technologies has become an urgent need. In 1963, it was found that the attenuation of blue-green light in the range of 450–550 nm was much smaller than that in other light bands. The discovery of this physical phenomenon has laid a theoretical foundation for the development of UVLC. Compared with underwater acoustic and RF communications, UVLC has the advantages of low cost, high transmission rates, strong anti-interference ability, and high security and, thus, has become the focus of international competition.

At present, UVLC is mostly achieved with LDs and LEDs. Recent research results of high-speed UVLC sys-tems are presented in Figure 2. Longer UVLC transmis-sion distances and higher data rates can be achieved with LDs than with LEDs. Since the first experimental demonstration of a high-speed LD-based UVLC system (on the order of gigabits per second) in 2008, many re-search works have concentrated on exploring higher data rates and longer transmission ranges for UVLC sys-tems with LDs. A UVLC transmission of 14.8 Gb/s over 1.7 m with OFDM and a 3.7-GHz modulation bandwidth was reported in 2018. Fei et al. [8] presented a UVLC

Optical FiberNetwork

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UnderwaterVehicle

UnderwaterSensor Network

UnderwaterHuman Activities

SensorNodes

BaseStation

Smart CityVLC Links

ONU

ONU

Figure 4 A schematic of future UVLC networks.



system with transmission rates of 16.6 Gb/s over a 5-m, 13.2 Gb/s over a 35-m, and 6.6Gb/s over a 55-m water channel with adaptive bit-power-loading DMT modula-tion and a Volterra series-based nonlinear equalizer. In addition, a 25-Gb/s turbid water transmission with a two-stage injection-locked, vertical-cavity-surface-emitting laser with three LDs was realized by Li et al. [9]. At the same time, LED-based UVLC systems with higher data rates have also been successfully implemented. A 20.09-Gb/s WDM transmission was reported in [10] and constitutes the highest data rate for UVLC systems based on LEDs reported to this day. Moreover, a 2.34-Gb/s real-time transmission in a UVLC system was reported in [11] and represents the highest data rate for real-time UVLC applications ever reported.

Based on ongoing researcher efforts, UVLC has achieved higher transmission rates over larger transmis-sion distances. However, the adverse conditions of the underwater environment, such as attenuation and the scattering of visible light, water temperature changes, underwater bubbles, and turbulence, can interfere in a profound way with the state of the UVLC channel and cause performance uncertainties. Therefore, modeling UVLC channels is critical. The spatial effects of multiple scattering on an underwater laser beam with experi-mental validations were investigated in 2018. A closed-form expression of the double-gamma-function model was presented in 2014 to model the impulse response of UVLC channels. Zedini et al. [12] reported a unified statistical model in 2019 to characterize the fading char-acteristics of underwater optical channels in the pres-ence of bubbles, turbulence, and temperature gradients in fresh and salty water. This is the first channel model capable of solving underwater beam irradiance fluc-tuations caused by bubbles and temperature gradients. However, additional studies on the complete modeling of UVLC channels need to be conducted, and further devel-opment is also required.

Machine Learning-Based Signal ProcessingIn recent years, machine learning has been widely applied in VLC systems. As seen in Figure 5, machine learning can be used for tasks such as system nonlineari-ty mitigation, modulation format identification, indoor positioning, channel estimation, phase estimation, and MIMO. At present, machine learning algorithms are used mainly as effective research tools to provide new ideas and pursue the optimum system performance in VLC systems. However, with the rapid development of GPUs, they may play an active role in future commercialization of VLC.

Nonlinearity in high-speed VLC systems can be miti-gated by employing neural network (NN) and cluster-ing schemes. Artificial NN (ANN)-based equalizers are becoming increasingly popular because of advances in digital signal processing technologies. Several types of ANNs can be applied as equalizers, including multilay-er perceptron (MLP), radial basis function (RBF), and functional link ANN (FLANN) [13]. MLP is an ANN with minimum complexity. MLP and FLANN equalizer ap-plications in VLC systems were presented in 2015 and 2019, respectively. Nowadays, deep NNs (DNNs) are also considered an effective way to achieve equalization for VLC systems and mitigate nonlinearity, but real-time ap-plications of these types of equalizers are restricted by the required training. A DNN based on a Gaussian ker-nel (GKDNN) was proposed in [14] for postequalization in UVLC. The addition of the Gaussian kernel increased the network convergence speed and reduced the num-ber of training times by 47.06%. The first application of long short-term memory (LSTM) in VLC systems was reported in 2019. Application of the LSTM-based equal-izer improved the system’s quality (Q) factor by 1.2 dB and extended the transmission distance, while reduc-ing system complexity. Some clustering-based machine learning techniques have also been applied to mitigate nonlinearity in VLC systems, i.e., K-means and clustering

Machine Learning for VLC

� Channel Estimation

� PBL/TTHnet

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OtherApplications

Figure 5 Machine learning applications in VLC. KNN: k-nearest neighbors.



algorithm perception decision (CAPD). In 2017, CAPD was applied in a multiband CAP VLC system with a Q-factor improvement of 1.6–2.5 dB. A K-means-based predistort-er was proposed in 2018, and at least 50% performance improvement was achieved using this scheme.

K-means and CAPD are also suitable for modulation format identification in VLC systems. Apart from these, density-based spatial clustering of applications with noise (DBSCAN), 2D DBSCAN, and 3D DBSCAN can also be applied to carry out identification. In 2018, a DBSCAN-based method was proposed to distinguish different signal levels in a PAM VLC system. This method was fur-ther extended into in-phase/quadrature 2D space and in-phase/quadrature/time 3D space in 2019; these were referred to as 2D DBSCAN and 3D DBSCAN, respective-ly. Machine learning techniques can be used for other applications in VLC systems as well. Probabilistic Ba -yesian learning (PBL) and two tributary heterogeneous neural networks (TTHnets) have been proposed and utilized for channel estimation. The PBL tech-nique used as a VLC channel estimator in 2017, reduced the required training overhead significantly. In 2019, the TTHnet-based channel estimator was proposed for both single-carrier and multicarrier UVLC systems. Sup-port vector machines (SVMs) and K-means were verified as phase estimators. In 2018, an SVM-based method was applied in a two-band CAP VLC system to achieve phase estimation with an aggregated data rate of 400 Mb/s. K-means clustering was used to correct the phase of special-shaped 8-QAM signals in 2019. Independent

component analysis (ICA) was proposed as a MIMO demultiplexer in 2020, and Q-factor improvements of 2.5 dB and 4.9 dB are achieved with data rates of 750 Mb/s and 900 Mb/s, respectively. It is also impor-tant to note that machine learning is a powerful tool in indoor VLC positioning systems, and both single classifiers and multiple classifiers can be used to achieve positioning [15].

Challenges and Prospects of VLC in 6GDuring the last 20 years, VLC has experienced rapid development with the enabling techniques summarized in Figure 6. As a new type of communication technology, it has attracted the interest of many researchers around the world, which has led to gratifying progress. At pres-ent, effective analysis of user application needs can pro-mote a country’s long-term economic growth and optimize the industrial layout. VLC can be combined with user needs in indoor positioning, heterogeneous networking, high-definition video transmission, and the integration of visible light and Wi-Fi. Figure 7 shows the demand distributions of VLC transmission distance and data transmission rates in different application fields.

VLC is an important component in the future 6G blue-print. Thus, research on VLC still should be carried out to further broaden its application scenarios and improve its communication performance. Future development of VLC requires more research on the evolution of its fun-damental transmission devices, as summarized in Fig-ure 8. High-speed VLC systems are mainly restricted by the limited bandwidths of the light sources. Accordingly, this indicates that super-high-bandwidth light sources with new materials and mechanisms should be investi-gated. The commonly applied Si-based detectors for VLC systems are mainly sensitive to infrared waves and result in low sensitivities to visible light. Thus, the use of detectors based on aluminium gallium arsenide (AlGaAs) and single-

photon detectors with high respon-sivity would be a solution. Another problem is the lack of application-specific integrated circuits (ASICs) for VLC baseband processing. There-fore, analog front-end circuitry, such as drivers, transimpedance amplifi-ers, and digital chips, are necessary for baseband processing. Currently, field programmable gate arrays with power consumption greater than 10 W are widely applied to imple-ment signal processing for real-time VLC systems due to the lack of ASICs. In the future, power consumption can be reduced using ASICs with smaller size and lower power require-ments. Nowadays, VLC systems are

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figure 6 The enabling techniques in VLC. GS: geometric shaping; PS: probabilistic shaping; CNN: convolutional neural network; LMS: least mean-square; RLS: recursive least square; PPM: pulse position modulation.

maChine Learning is a powerfuL tooL in indoor VLC positioning systems, and both singLe CLassifiers and muLtipLe CLassifiers Can be used to aChieVe positioning.



extensively applied based on point-to-point communication using single transmitters and detectors, while MIMO communications based on transmitter and detector arrays constitute the future trend. Current transmission and reception antennas for VLC systems require a large lens group, which presents difficulties for future integration. This problem can be solved based on the use of Fres-nel lenses using beamforming and steering based on nano-optical an-tennas. Additionally, the modeling of VLC systems requires further inves-tigation. Current VLC channel mod-eling is based on only the light-field distribution and spatial characteris-tics of LED or LD devices. However, the actual VLC system channels also include receiver frequency response characteristics, optical antennas, spa-tial light-field distribution, atmospheric turbulence, back-ground light noise, scattering, diffraction, and reflection. VLC channel modeling considering these factors will pro-vide theoretical guidance for future high-speed space and UVLC systems.

VLC is a reliable communication method in 6G and will be adopted to generate a novel heterogeneous network with other communication techniques to provide high-capacity, high-rate, stable, and reliable transmissions. In this network, a single access point can support access

from multiple terminals simultaneously with a switching time of more than 10 ms, an uplink rate of more than 10 Gb/s, and a single point-to-point link transmission rate of at least 100–200 Gb/s. To adapt to the complex data process-ing needs of future systems, intelligent machine learning should become the focus of the next phase of research and should be used to implement advanced signal-pro-cessing algorithms in VLC systems. It is undeniable that 1) VLC has very important theoretical and practical signifi-cance and 2) the application prospects for high-speed VLC

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figure 8 The contemporary challenges and problems of VLC devices and their prospective solutions.

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figure 7 The distribution demands of VLC transmission distances and data transmission rates in different application fields. V2X: vehicle to everything.



technology in 6G are very broad. As long as a reasonable and stable development plan is formulated, VLC technol-ogy will surely be of great use in the future.

ConclusionsIn this study, the prospects of VLC in 6G were introduced in conjunction with the latest high-speed VLC research advances. The development of new materials and devic-es, advanced modulation, UVLC, and machine learning-based signal processing were also summarized. Utilization of VLC has achieved a series of remarkable results based on the efforts of researchers worldwide. However, significant challenges still exist in various aspects. For example, existing optical devices still limit the performance of VLC systems. In the future, new devic-es should be investigated to break the performance bot-tlenecks. Current VLC channel models for both free-space and underwater transmission do not cover all of the influ-ential factors in actual channels, and a comprehensive VLC theoretical channel model will need to be studied further. A heterogeneous network architecture based on VLC should also be proposed. The applications of machine learning in VLC are still not sufficient, i.e., net-work monitoring has not been fully studied until now. More research on new machine learning algorithms is required to realize the intelligence of VLC systems. Given these research directions, it is anticipated that more rapid development will be achieved for VLC in 6G.

AcknowledgmentsThis work was partially supported by the National Key Research and Development Program of China grant 2017YFB0403603 and National Natural Science Founda-tion of China project 61925104.

Author InformationNan Chi ([email protected]) is a profes-sor in the School of Information Science and Technology, Fudan University, Shanghai, China. She is also with the Shanghai Insti-tute for Advanced Communication and Data Science, Key Laboratory for Information Sci-

ence of Electromagnetic Waves, and the Academy for Engi-neering and Technology. Her current research interests include advanced modulation formats, optical packet/label switching, optical fiber communication, and visible light communication. She is a fellow of the Optical Society of America.

Yingjun Zhou (15110720015@fudan .edu.cn) received her B.S. degree in 2015 and is now working toward her Ph.D. degree at Fudan University, Shanghai, China. Her current research interests include high-speed visible light systems

and advanced digital signal processing.

Yiran Wei ([email protected] .cn) received his M.E. degree in 2018 and is now working toward his Ph.D. degree at Fudan University, Shanghai, China. His research interests include optical fiber and free-space optical

communication. Fangchen Hu (18110720018@fudan .edu.cn) received his B.S. degree in 2018 and is now working toward his Ph.D. degree at Fudan University, Shanghai, China. His research interests include visible light communication and nonlin-

ear effects.

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visible light communication in 6g

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