Visible Light Communication System Technology Review: Devices, Architectures, and Applications

: Visible light communication (VLC) is an advanced, highly developed optical wireless communication (OWC) technology that can simultaneously provide lighting and high-speed wireless data transmission. A VLC system has several key advantages: ultra-high data rate, secure communication channels, and a lack of interference from electromagnetic (EM) waves, which enable a wide range of applications. Light-emitting diodes (LEDs) have been considered the optimal choice for VLC systems since they can provide excellent illumination performance. However, the quantum conﬁnement Stark effect (QCSE), crystal orientation, carrier lifetime, and recombination factor will inﬂuence the modulation bandwidth, and the transmission performance is severely limited. To solve the insufﬁcient modulation bandwidth, micro-LEDs ( µ -LEDs) and laser diodes (LDs) are considered as new ideal light sources. Additionally, the development of modulation technology has dramatically increased the transmission capacity of the system. The performance of the VLC system is brieﬂy discussed in this review article, as well as some of its prospective applications in the realms of the industrial Internet of Things (IoT), vehicle communications, and underwater wireless network applications.


Introduction
Electromagnetic waves have an extensive range of applications, including cell phone communications, wireless radio broadcast, Wi-Fi, etc. Depending on the wavelength, electromagnetic waves can be roughly divided into radio waves, infrared rays, visible light, ultraviolet rays, etc. Though radio frequency (RF) is not susceptible to low interference, it is capable of wide area coverage [1]. Nevertheless, RF has several disadvantages, such as interference, bandwidth limitations, safety issues, transmission power limitations, and a crowded radio spectrum [2][3][4][5]. Therefore, visible light communication (380 to 780 nm) has been developed to solve these challenges as an alternative solution to the problem mentioned above. Wireless optical communication technology is characterized by its emphasis on the realization of high-frequency bandwidth communication capacity, which is significantly greater than the current radio wireless communication frequency. In the past ten years, light fidelity (Li-Fi) technology, which combines illumination and communication, has risen to the forefront of study.
Optical Wireless communication (OWC) has two main advantages: one is that it won't be interfered with by electromagnetic waves. Despite the fact that current flight regulations allow passengers to use electronic devices on planes and vital medical equipment in hospitals, and copper nets are embedded in the walls to prevent mobile phone interference, there are still some chances for interference. On the other hand, wireless optical communication in these sensitive places can avoid interference. The other advantage is power saving; it is more power efficient to turn on and off a LED/LD than to use RF wireless radio frequency to transmit signals. Hand-held devices have limited battery power, so it is more advantageous to use optical wireless communication to transmit signals. Li-Fi technology design is directly combined with the LED bulb of the skylight board to enable hotspot for indoor wireless internet access, and most mobile phones have LED flashes and CMOS camera lenses, which means that they can potentially receive and send information. The beam steering and VLC position combination is a promising solution to directionality between the light source and the receiver [6]. Although the active components of the VLC system will inevitably increase the power consumption, alignment difficulties, cost, and volume of the mobile device, these can be mitigated by utilizing a modulated retroreflector (MRR) in the mobile device; this has been successfully demonstrated recently [7]. In the future, mobile phones may be able to easily exchange information; a schematic of a VLC System is shown in Figure 1. Visible light communication is mainly composed of two components: an optical transmitter (Tx) and optical receiver (Rx), as shown in Figure 1. After preprocessing and encoding, a binary bit stream drives the light source (LED/LD), and the electrical signals are converted into optical signals through modulation. Signal distortion generated by other channels is compensated by using pre-processing. The equalization technology can increase the response bandwidth of the LED and increase the transmission data rate. The receiver's post-equalization can compensate for other channel losses, such as phase noise. High-order coding modulation techniques were used to increase the transmission rate and spectrum efficiency.
The light sources that are predominantly used in VLC systems are LEDs and laser diodes. White light is preferred for indoor illumination; hence, red, green, and blue LEDs (RGB-LEDs) are mixed to generate white light sources, or yellow phosphors convert blue LED output into white light sources. When compared to RGB three-color hybrid LEDs, employing yellow phosphor Gallium Nitride (GaN) blue LEDs can minimize light source complexity and system cost. However, when the blue photon hits yellow phosphor [8][9][10][11], the transition causes a relatively reduced penetration power and frequency modulation bandwidth, thereby limiting the signal-to-noise ratio (SNR) and transmission capac- Visible light communication is mainly composed of two components: an optical transmitter (Tx) and optical receiver (Rx), as shown in Figure 1. After preprocessing and encoding, a binary bit stream drives the light source (LED/LD), and the electrical signals are converted into optical signals through modulation. Signal distortion generated by other channels is compensated by using pre-processing. The equalization technology can increase the response bandwidth of the LED and increase the transmission data rate. The receiver's post-equalization can compensate for other channel losses, such as phase noise. High-order coding modulation techniques were used to increase the transmission rate and spectrum efficiency.
The light sources that are predominantly used in VLC systems are LEDs and laser diodes. White light is preferred for indoor illumination; hence, red, green, and blue LEDs (RGB-LEDs) are mixed to generate white light sources, or yellow phosphors convert blue LED output into white light sources. When compared to RGB three-color hybrid LEDs, employing yellow phosphor Gallium Nitride (GaN) blue LEDs can minimize light source complexity and system cost. However, when the blue photon hits yellow phosphor [8][9][10][11], the transition causes a relatively reduced penetration power and frequency modulation bandwidth, thereby limiting the signal-to-noise ratio (SNR) and transmission capacity of Li-Fi systems. In 2020, J. Vučić  Multitone (DMT) format, achieving 513 Mbit/s of data rate [12]. In 2012, Khalid et al. reached a data rate of 1 Gbit/s with a phosphorescent white LED by the utilization of a DMT modulation scheme [13]. To improve spectrum utilization, Li-Fi systems mostly use carrierless amplitude and phase (CAP) and M-ary quadrature-amplitude-modulation (M-QAM) as the modulation format. In 2013, Wu et al. used CAP and QAM-orthogonal frequencydivision multiplexing (OFDM) to modulate an RGB-LED, reaching a data rate of 2.93 and 3.22 Gbit/s, respectively [14]. However, the coherence, noise performance, and direct modulation bandwidth of the traditional LEDs were all hampered by the spontaneously radiating photons of the active region stimulated by the photons. To overcome the problems mentioned above, resonant cavity LEDs or micro-LED arrays (µ-LEDs) have been proposed due to their high internal quantum efficiency (IQE), light extraction rate, and variable modulation bandwidth. Since the RC constant of the device cannot limit the bandwidth of the µ-LED [15] and the GaN µ-LED is a much smaller LED (<100 µm), it has been proven to provide better white light than traditional LEDs, with a higher output intensity and wider modulation bandwidth [16,17].
Laser diodes (LDs) are another light source often employed in VLC systems. They have stronger coherence, narrower spectral linewidth, and larger modulation bandwidth, making them more suited for data rates exceeding Gbit/s [18,19]. Unlike LEDs, which can only provide up to a few hundred MHz of usable bandwidth, a commercial transistoroutline can (TO-can)-packaged GaN blue LD was reported in 2013 carrying 2.5 Gbit/s of error-free on-off keying (OOK) transmission, which can provide a VLC system with 1.4 GHz of bandwidth [20]. In 2013, Chen et al. demonstrated a bidirectional VLC that can provide 2.5 Gbit/s 16-QAM OFDM, using a red laser pointer [21]. To increase the data rate, in 2013, Chi et al. achieved 9 Gbit/s VLC in a 5 m point-to-point (PtP) free space link by utilizing 64-QAM OFDM to modulate a GaN blue LD [22]. The lighting function of an LD is similar to an LED, which is realized by covering the output end of blue LD with yellow phosphor or using the collimated beam of mixed R/G/B LD. In 2015, Lee et al. demonstrated a non-return-to-zero OOK (NRZ-OOK) data stream that transmitted at a data rate of 2 Gbit/s using a blue LD covered with a yellow phosphor [23]. In the same year, Retamal et al. achieved a data rate of 4 Gbit/s by embedding the yellow fluorescence of the blue LD into remote color conversion [24]. In 2015, Chi et al. also illustrated a blue LD-based white light converted by a phosphorous diffuser, with a correlated color temperature (CCT) of 5217K, which can transmit 16-QAM OFDM data at a data rate of 5.2 Gbit/s in a 0.6 m free space channel [25]. Additionally, Tsonev et al. expected to achieve a data rate exceeding 100 Gbit/s by using an R/G/B hybrid LD light source for white light VLC carrying a QAM-OFDM signal [26]. In 2017, Wu et al. achieved an 8 Gbit/s data rate with 16-QAM OFDM over 0.5 m free space by using a commercial R/G/B TO-can-packaged LD [27]. These illustrations suggested that visible light LDs have greater data transmission potential than LEDs and that this potential will grow in the future. Mainstream white light VLC light sources will be achieved by using laser and color mixing technology, and a brief comparison of light source used in VLC systems is presented in Table 1. At present, most of the light sources used in VLC systems are white light LEDs, which are mainly divided into two categories, RGB-LED and phosphor-converted (pc)-LED. RGB-LED is realized by the utilization of three primary color phosphors which are excited Crystals 2021, 11, 1098 4 of 28 by violet or ultraviolet LEDs; another approach is to package red, green, and blue chips together to emit white light (RGB-LED), as shown in Figure 2a. On the other hand, a pc-LED converts the light source into white light by exciting the yellow phosphor with a blue LED chip, as shown in Figure 2b [28,29].

Theoretical Background of LED for VLC Application
At present, most of the light sources used in VLC systems are white light LEDs, which are mainly divided into two categories, RGB-LED and phosphor-converted (pc)-LED. RGB-LED is realized by the utilization of three primary color phosphors which are excited by violet or ultraviolet LEDs; another approach is to package red, green, and blue chips together to emit white light (RGB-LED), as shown in Figure 2a. On the other hand, a pc-LED converts the light source into white light by exciting the yellow phosphor with a blue LED chip, as shown in Figure 2b [28,29].  [29]. Copyright 2011, American Chemical Society.
Studying the modulation characteristics of the LED device is critical to improving the performance of the new Li-Fi system. The modulation bandwidth of the LED determines the channel capacity and data rate of the communication system. The definition of LED modulation bandwidth is the frequency when the AC optical power output by the LED drops to 50% of a certain reference frequency value (−3 dB). As the photoelectric response of a pc-LED is relatively behindhand [30], the modulation bandwidth of the Li-Fi light source is limited to a few MHz, therefore limiting the data rate of the entire system; even the use of a blue filter failed to improve the defects of the light source.
Use of a higher modulation bandwidth by using wavelength division multiplexing (WDM) with three different wavelengths of light increases the channel capacity of RGB-LEDs. The terminal receives three wavelengths of light through filters of each wavelength, which significantly improves the transmission efficiency. However, different colors of RGB-LED have different operating temperatures, depending on the output light flux. To achieve independent color operating at different temperatures, the feedback loop and each single-color LED's driving current need to be controlled individually, which leads to higher cost and a more complex modulation circuit.
The response rate and modulation bandwidth of the LED are influenced by the carrier lifetime. In addition, increasing the radiation recombination rate of electrons and Studying the modulation characteristics of the LED device is critical to improving the performance of the new Li-Fi system. The modulation bandwidth of the LED determines the channel capacity and data rate of the communication system. The definition of LED modulation bandwidth is the frequency when the AC optical power output by the LED drops to 50% of a certain reference frequency value (−3 dB). As the photoelectric response of a pc-LED is relatively behindhand [30], the modulation bandwidth of the Li-Fi light source is limited to a few MHz, therefore limiting the data rate of the entire system; even the use of a blue filter failed to improve the defects of the light source.
Use of a higher modulation bandwidth by using wavelength division multiplexing (WDM) with three different wavelengths of light increases the channel capacity of RGB-LEDs. The terminal receives three wavelengths of light through filters of each wavelength, which significantly improves the transmission efficiency. However, different colors of RGB-LED have different operating temperatures, depending on the output light flux. To achieve independent color operating at different temperatures, the feedback loop and each single-color LED's driving current need to be controlled individually, which leads to higher cost and a more complex modulation circuit.
The response rate and modulation bandwidth of the LED are influenced by the carrier lifetime. In addition, increasing the radiation recombination rate of electrons and holes and reducing the spontaneous radiation lifetime of carriers is a conventional approach for designing a modulation circuit and minimizing the RC delay. The spontaneous emission lifetime of carriers has been decreased by increasing the injected carrier concentration, which can be achieved by increasing the injection current and delta doping. Under high injection current, as the injected carrier concentration increases, the exciton recombination probability increases, the radiation recombination carrier lifetime decreases, and the electro-optical conversion responds quickly. Delta doping technology also results in high carrier injection, thereby reducing carrier lifetime and improving modulation bandwidth at the same current density.
Due to the built-in piezoelectric polarization of the LED, the traditional polar LED grown on the c-plane has limited bandwidth, especially in the green spectral region [31]. The polarization in the c-plane-oriented InGaN quantum well produces a robust quantum confinement Stark effect (QCSE), which separates electrons and holes, thereby increasing the time for radiation recombination [31,32]. As a result, the device's performance will dramatically reduce, and the Auger effect QCSE will cause significant peak changes but also cause a significant drop in luminous efficiency under high injection currents [31,33]. To mitigate this polarization field, growing quantum wells in semipolar or non-polar directions allows better overlap of electrons and holes, which leads to faster radiation recombination, especially with thicker quantum wells [34][35][36][37][38][39][40]. It was expected that a higher electro-optical bandwidth and potentially reduced Auger effect could be achieved by crystal orientation.
With the advantages of semipolar LEDs in luminous efficiency and wavelength stability, researchers have expanded their applications; for instance, in high-resolution displays and high-speed visible light communications. Nevertheless, better implementation of VLC includes improving the modulation characteristics of LED light sources and the emission efficiency under high-speed modulation, which depend on their carrier recombination lifetimes.
Semipolar LEDs with less QCSE have a faster radiation recombination rate [30]. However, µ-LEDs have a better frequency response, low power consumption, and higher brightness [36]. The high brightness of a µ-LED leads to a faster transmission data rate and lower bit error rate (BER). By using a high-speed photodetector to detect the modulated LED photocurrent signal, the −3 dB frequency is related to specific diode parameters. The frequency response has been evaluated using a fixed current. The LED cut-off frequency was affected by the carrier lifetime and Resistor-Capacitor (RC) time constant composed of depletion layer capacitance and the junction differential resistance, which determines the transmission data rate of the LED.
The modulation bandwidth can determine the data rate of the VLC application, so the −3 dB frequency bandwidth of LED can be related to the resistance-capacitance time constant (τ RC ) and the minority carrier lifetime, as shown in the Equation (1) [37]: where τ r and τ nr represent the radiative and non-radiative carrier lifetime, respectively. The modulation bandwidth of GaN LEDs is mainly determined by the RC time constant and carrier lifetime [41][42][43][44]. The modulation bandwidth can be increased due to the decrement of the carrier lifetime by increasing the injected current density [38,45]. However, increasing the current density may cause the quantum efficiency and LED performance to decrease [46]. Several reports have indicated the carrier recombination process in GaN-based LEDs would be influenced by carrier localization [47][48][49] and the QCSE [50,51]. µ-LEDs can achieve a smaller carrier lifetime and higher modulation bandwidth because they can withstand higher injection current density. As the current density increases, QCSE will become less effective, increasing the optical power and modulation bandwidth [52]. Because the µ-LED has a smaller active area, the relative geometric capacitance and RC time constant, the main parameters limiting the modulation bandwidth, will be reduced. Growing LEDs on semipolar or non-polar substrates increased the modulation bandwidth, corresponding to a higher radiation recombination rate. GaN-based µ-LEDs have excellent performance in high brightness, high contrast, fast response time, extended service life, and low power consumption. Because of its ultra-small light-emitting area, a single µ-LED has a low power emission. By assembling multiple µ-LEDs into array, the output light power can be increased. A µ-LED array will be prepared for monochromatic and polychromatic emission [53,54]. In this section, we review the characteristics and applications of µ-LED arrays in the field of VLC, and a brief benchmark for µ-LEDs used in VLC systems is shown in Figure 3 [42,[55][56][57][58][59][60][61][62][63][64]. To utilize µ-LEDs in a VLC system, integrating multi-color emitters on a single chip is required. This can be achieved by combining blue µ-LEDs with color conversion materials (pc-LED) or RGB-LEDs.
tion bandwidth, will be reduced. Growing LEDs on semipolar or non-polar substrates increased the modulation bandwidth, corresponding to a higher radiation recombination rate. GaN-based μ-LEDs have excellent performance in high brightness, high contrast, fast response time, extended service life, and low power consumption. Because of its ultra-small light-emitting area, a single μ-LED has a low power emission. By assembling multiple μ-LEDs into array, the output light power can be increased. A μ-LED array will be prepared for monochromatic and polychromatic emission [53,54]. In this section, we review the characteristics and applications of μ-LED arrays in the field of VLC, and a brief benchmark for μ-LEDs used in VLC systems is shown in Figure 3 [42,[55][56][57][58][59][60][61][62][63][64]. To utilize μ-LEDs in a VLC system, integrating multi-color emitters on a single chip is required. This can be achieved by combining blue μ-LEDs with color conversion materials (pc-LED) or RGB-LEDs.

Theoretical Background of Laser for VLC Application
Due to the low data transmission rate of the LED modulation, Professor Hardal Hass was the first to use laser diodes to replace LEDs for Li-Fi. By utilizing lasers' high energy and luminous efficiency, the data transmission rate can be ten times higher than LEDs. Laser lighting can produce white light through color mixing technology, similar to RGB LEDs. Although an LED-based VLC can achieve a transmission rate of 10 Gb/s and increase the upper limit of the data transmission rate of 7 Gb/s in Wi-Fi, the laser data transmission rate can easily exceed 100 Gb/s. The R&D team of Arizona State University developed a nano-scale white light laser which can be a potential Li-Fi light source. Laser diodes have several advantages in communication applications: faster response speed, direct modulation, and high coupling efficiency. For general semiconductor lasers, the LD can modulate light controlled by the input current when the injected current exceeds the threshold current ( ), as the modulation characteristics show in Figure 4. The linear region above the threshold current to the saturation area is the LD working area, the modulation range. Hence, reducing the device threshold current and obtaining a more extensive modulation working area is crucial.

Theoretical Background of Laser for VLC Application
Due to the low data transmission rate of the LED modulation, Professor Hardal Hass was the first to use laser diodes to replace LEDs for Li-Fi. By utilizing lasers' high energy and luminous efficiency, the data transmission rate can be ten times higher than LEDs. Laser lighting can produce white light through color mixing technology, similar to RGB LEDs. Although an LED-based VLC can achieve a transmission rate of 10 Gb/s and increase the upper limit of the data transmission rate of 7 Gb/s in Wi-Fi, the laser data transmission rate can easily exceed 100 Gb/s. The R&D team of Arizona State University developed a nano-scale white light laser which can be a potential Li-Fi light source. Laser diodes have several advantages in communication applications: faster response speed, direct modulation, and high coupling efficiency. For general semiconductor lasers, the LD can modulate light controlled by the input current when the injected current exceeds the threshold current (I th ), as the modulation characteristics show in Figure 4. The linear region above the threshold current to the saturation area is the LD working area, the modulation range. Hence, reducing the device threshold current and obtaining a more extensive modulation working area is crucial.
The threshold current density is shown in Equation (2): where J th is threshold current density; e is the electron element charge; d is the active layer thickness; I inj is the injection current; N is the transparent carrier concentration; α m and α i are the mirror loss and optical absorption loss, respectively; Γ g0 is Maximum mode gain; and B and C are the radiation recombination coefficient and Auger recombination coefficient, respectively.  The threshold current density is shown in Equation (2): where is threshold current density; is the electron element charge; is the active layer thickness; is the injection current; is the transparent carrier concentration; and are the mirror loss and optical absorption loss, respectively; is Maximum mode gain; and B and C are the radiation recombination coefficient and Auger recombination coefficient, respectively.
A laser-based VLC system reaches tens of gigabit-per-second of data rates, benefiting from its extensive modulation bandwidth. The modulation speed of the laser diode is controlled by the photon lifetime (~ps) rather than the carrier lifetime (~ns) in the LED. In addition, a laser-based VLC system enables narrow beam transmission for long-distance applications. A 100 m OOK-modulated VLC link was demonstrated with a data rate of 2.3 Gbps in 2017 [65]. While the OOK modulation technique is commonly used due to its simplicity, many advanced modulation schemes have been used in laser-based VLC to realize high data rates, with the feasibility to exceed 100 Gbps.
In a high-speed VLC system, the transmitter is critical to achieving a high data rate. A promising light source in a VLC system requires high optical output power, large modulation bandwidth, high energy conversion efficiency, low operation voltage, small form-factor, and long lifetime. Since the blue-green lasers were based on gallium nitride materials, the high-speed laser-based VLC system requires a fast GaN-based transmitter. Three types of lasers were commonly used as transmitters: edge emitting laser diodes (EELDs), super luminescent diodes (SLDs), and vertical-cavity surface-emitting lasers (VCSELs).

Edge Emitting Laser Diodes (EELDs)
A typical white laser transmitter is constructed using a blue laser exciting yttrium aluminum garnet (YAG) phosphor. Nakamura et al. performed the first demonstration of an InGaN/GaN-based room-temperature EELD [66], utilizing the injection locking technique and demonstrating that the bandwidth of the laser diode can be enhanced, making it promising for high data rate VLC systems [67].  A laser-based VLC system reaches tens of gigabit-per-second of data rates, benefiting from its extensive modulation bandwidth. The modulation speed of the laser diode is controlled by the photon lifetime (~ps) rather than the carrier lifetime (~ns) in the LED. In addition, a laser-based VLC system enables narrow beam transmission for long-distance applications. A 100 m OOK-modulated VLC link was demonstrated with a data rate of 2.3 Gbps in 2017 [65]. While the OOK modulation technique is commonly used due to its simplicity, many advanced modulation schemes have been used in laser-based VLC to realize high data rates, with the feasibility to exceed 100 Gbps.
In a high-speed VLC system, the transmitter is critical to achieving a high data rate. A promising light source in a VLC system requires high optical output power, large modulation bandwidth, high energy conversion efficiency, low operation voltage, small form-factor, and long lifetime. Since the blue-green lasers were based on gallium nitride materials, the high-speed laser-based VLC system requires a fast GaN-based transmitter. Three types of lasers were commonly used as transmitters: edge emitting laser diodes (EELDs), super luminescent diodes (SLDs), and vertical-cavity surface-emitting lasers (VCSELs).

Edge Emitting Laser Diodes (EELDs)
A typical white laser transmitter is constructed using a blue laser exciting yttrium aluminum garnet (YAG) phosphor. Nakamura et al. performed the first demonstration of an InGaN/GaN-based room-temperature EELD [66], utilizing the injection locking technique and demonstrating that the bandwidth of the laser diode can be enhanced, making it promising for high data rate VLC systems [67]. In 2018, Mukhtar et al. demonstrated a prism-based self-injection locked (SIL) external cavity diode laser based on a violet LD with a continuous wavelength tunability of 5.15 nm [68]. In 2020, Holguin-Lerma et al. demonstrated that a novel InGaN-based distributed feedback (DFB) laser diode with narrow-linewidth emission at approximately 480 nm enabled a 10.5 Gbps data rate using a 16-QAM OFDM modulation scheme [69].
The phosphor material is laser-pumped to produce eye-safe white light emission, which has significant advantages over the non-human-eye-safe beam from a single-wavelength coherent emission high-power LD chip [70,71]. Figure 5 shows an integrated surfacemount device (SMD) package for laser lighting applications. The laser beam hits the phosphor, and the combination of photons from the LD and the phosphor converts the photons to produce a white light spectrum. These laser SMDs can produce a directional beam of 400-500 lumens from a 300-400-micron spot, producing a brightness level beyond 1000 cd/mm 2 [72]. gle-wavelength coherent emission high-power LD chip [70,71]. Figure 5 shows an integrated surface-mount device (SMD) package for laser lighting applications. The laser beam hits the phosphor, and the combination of photons from the LD and the phosphor converts the photons to produce a white light spectrum. These laser SMDs can produce a directional beam of 400-500 lumens from a 300-400-micron spot, producing a brightness level beyond 1000 cd/mm 2 [72].

Super Luminescent Diodes (SLDs)
SLDs shown in Figure 6 are light emitters generating amplified spontaneous emission (ASE), combining optical characteristics of LEDs and LDs. An SLD combines the advantage of both an LD and LED. That is, the diffraction-limited, spatially coherent, excellent coupling to external component, and small form factor of the LD, and the wideband spectrum and time incoherence of the LED [74].
Since developing high-power and high-speed violet-blue SLDs, VLC systems based on SLD transmitters have been demonstrated with data rates of 3.8 Gbps with a DMT modulation scheme by Hu, F. et al. in 2020 [75]. Studies have shown that GaN-based SLDs can achieve high power emission power, such as over 100 mW blue SLD in continuous wave operation [76].

Vertical Cavity Emitter Lasers (VSCELs)
VCSELs, illustrated in Figure 7, are ideal for high-speed communication systems owing to their high optical beam quality and short cavity. However, the GaN-based violet-blue-green emitting VCSELs suffer from the large lattice-mismatch AlGaN/GaN distributed Bragg reflectors (DBRs). Research efforts are required to develop room-temperature continuous-wave operation high power GaN-based VCSELs. Exper-

Super Luminescent Diodes (SLDs)
SLDs shown in Figure 6 are light emitters generating amplified spontaneous emission (ASE), combining optical characteristics of LEDs and LDs. An SLD combines the advantage of both an LD and LED. That is, the diffraction-limited, spatially coherent, excellent coupling to external component, and small form factor of the LD, and the wideband spectrum and time incoherence of the LED [74].
Since developing high-power and high-speed violet-blue SLDs, VLC systems based on SLD transmitters have been demonstrated with data rates of 3.8 Gbps with a DMT modulation scheme by Hu, F. et al. in 2020 [75]. Studies have shown that GaN-based SLDs can achieve high power emission power, such as over 100 mW blue SLD in continuous wave operation [76].  [73] with permission from SPIE.

Super Luminescent Diodes (SLDs)
SLDs shown in Figure 6 are light emitters generating amplified spontaneo sion (ASE), combining optical characteristics of LEDs and LDs. An SLD com advantage of both an LD and LED. That is, the diffraction-limited, spatially excellent coupling to external component, and small form factor of the LD, wideband spectrum and time incoherence of the LED [74].
Since developing high-power and high-speed violet-blue SLDs, VLC system on SLD transmitters have been demonstrated with data rates of 3.8 Gbps wit modulation scheme by Hu, F. et al. in 2020 [75]. Studies have shown that Ga SLDs can achieve high power emission power, such as over 100 mW blue SLD tinuous wave operation [76].

Vertical Cavity Emitter Lasers (VSCELs)
VCSELs, illustrated in Figure 7, are ideal for high-speed communication owing to their high optical beam quality and short cavity. However, the GaN-b let-blue-green emitting VCSELs suffer from the large lattice-mismatch AlGaN/ tributed Bragg reflectors (DBRs). Research efforts are required to room-temperature continuous-wave operation high power GaN-based VCSEL Figure 6. Schematic of a SLD [77]. © 2016 Optical Society of America.

Vertical Cavity Emitter Lasers (VSCELs)
VCSELs, illustrated in Figure 7, are ideal for high-speed communication systems owing to their high optical beam quality and short cavity. However, the GaN-based violet-bluegreen emitting VCSELs suffer from the large lattice-mismatch AlGaN/GaN distributed Bragg reflectors (DBRs). Research efforts are required to develop room-temperature continuous-wave operation high power GaN-based VCSELs. Experimental results have revealed the modulation bandwidth of violet-blue InGaN/GaN quantum-well VCSEL beyond GHz, confirming that GaN-based VCSELs have great potential for high-speed VLC transmitters [78].
However, a laser diode could be a possibility for a Li-Fi light source; Nobel Laureate in Physics Nakamura Shuji anticipated that laser illumination would eventually replace LED lighting, but the current lighting technology is led by the rather established and low-cost LED. The development of high-brightness, high-efficiency, high-speed modulation micro-LED modules and laser diodes, among other features, may accelerate Li-Fi technology's commercialization. We summarize the recent research achievements in LED and laserbased VLC in Table 2.
imental results have revealed the modulation bandwidth of violet-blue InG quantum-well VCSEL beyond GHz, confirming that GaN-based VCSELs have g tential for high-speed VLC transmitters [78]. However, a laser diode could be a possibility for a Li-Fi light source; Nobel L in Physics Nakamura Shuji anticipated that laser illumination would eventually LED lighting, but the current lighting technology is led by the rather establis low-cost LED. The development of high-brightness, high-efficiency, high-speed tion micro-LED modules and laser diodes, among other features, may acceler technology's commercialization. We summarize the recent research achievement and laser-based VLC in Table 2.

VLC Receiver Technology
The following are the basic requirements of photodetectors in a visible light communication system:

1.
The wavelength has a sufficiently high responsivity. For a certain incident optical power, the output photocurrent can be as large as possible.

2.
Fast enough response speeds to be implemented to a high-speed broadband system.

3.
A sufficiently fast response speed can be applied to high-speed broadband systems.
The noise level must be as low as possible to reduce the impact on the device's signal.

4.
According to a good linear relationship, the signal conversion process has been guaranteed to spread without distortion.

5.
Small size and has a long service lifetime.
There are three critical parameters for photodetector: responsivity, bandwidth, and dark current [101]. Responsivity is a wavelength-dependent metric that measures the gain of an output electrical signal per unit of incident optical power incident on the detector. The responsivity, R, shown in Equation (3) where incident optical power is P, output electrical signal current is I, is usually given in amperes per watt (A/W): The quantum efficiency η, which is relative to the maximum possible current, is also comparable to responsivity. When the photodetector picks up every electron-hole pair generated by the incident photons, the quantum efficiency will be denoted as 100%. Hence, Equation (3) can be rewritten into Equation (4), where q is the electron charge, λ is the wavelength of the incident light, h is the Plank constant, and c is the speed of light.
The bandwidth (BW) corresponds to the rise time (t r ) of the photodetector, which is shown in Equation (5): Two crucial parameters influence the bandwidth of a detector: the transit time and the RC parasitic response. Transit time is the time it takes for carriers generated by an optical signal to be swept out of the detector's active region. The velocity of the carriers is given by the following Equation (6): where µ is the carrier mobility and E is the electric field. When biased with a high enough electric field, the carriers will reach a saturation velocity, v sat . The mobility is thus modified to be as Equation (7): The bandwidth is then given by Equation (8) [102]: where h PD is the full height of the photodetector. Another parameter in determining the bandwidth of a detector is the electrical impedance characteristic of the detector circuit, also known as the RC parasitic response. The bandwidth due to RC limitations is shown in Equation (9): The total bandwidth, which includes both RC and transit time effects, is given by Equation (10): DC current may occur in the detector, even with the absence of the photons, which is known as dark current. Sufficient levels of dark current can result in a decrease in the signal-to-noise ratio. Bulk generation and surface generation are considered when calculating the total dark current. Bulk generation is a volume-dependent mechanism in which a significant lattice mismatch between germanium and silicon causes threading dislocations, allowing for mid-bandgap states, and results mainly from the Shockley-Read-Hall process [103]. Although the bulk current density is relatively constant at a low electric field, as the electric field increases, band bending will result in a higher bulk current density that increases exponentially with the applied electric field. Surface generation is another factor that leads to dark current, and results from surface defects such as dangling bonds. Surface passivation is more difficult with germanium than silicon as germanium is not fully passivated by silicon dioxide. Other materials such as germanium oxide have been used for passivation with some success [104].
The total dark current is given by Equation (11): where A is the junction area. Suppose we assume that bulk current dominates; then, the dark current scales with detector area. There is an approximately linear increase of dark current with the area of the detector. The shot noise is usually the dominant noise factor in germanium on silicon photodetectors. The shot noise due to the dark current is given by Equation (12): The avalanche photodiode (APD), PIN photodetector, metal-semiconductor-metal photodetector (MSM-PD), superlattice avalanche photodetector (SL-APD), waveguide photodetector (WGPD), and cavity-enhanced photodetector (CE-PD), are the most used photodetectors in optical communications. The free-space VLC receiver with large-area APD has a −3 dB modulation bandwidth of 420 MHz [105]. With a data rate of 56 Mbps, triple cationic perovskite solar cells can also be used as VLC receivers [106].
The bandwidth is limited by the RC time constant in the perovskite solar cell receiver. A high-performance surface-absorbing semipolar InGaN/GaN PIN-based micro-PD demo a −3 dB modulation bandwidth of 347 MHz, suggesting its promising performance for VLC applications [107]. A black silicon photodetector is developed that shows high gain in the visible-NIR regime (400-1200 nm). A high responsivity of 1097.60 A/W at 1080 nm is reported, indicating its advantages for weak light detection. Table 3 summarizes the recent research achievements in receiver technology for VLC.

Modulation Technology in VLC System
Due to the low bandwidth of the light-emitting diodes (LEDs) used in visible light communication (VLC), complex modulation schemes are a potentially attractive approach to offer high-speed data transmission. Different from other types of communication, the modulation scheme of VLC must achieve a higher rate and meet the requirements of human perception of light. The following two attributes can characterize these requirements for perceiving light.

•
Dimming: Performing different types of activities that require different levels of illuminance. For example, the illuminance in the range of 30-100 lux is sufficient to perform simple visual tasks in most public places. If the LED can be adjusted to any level, it is necessary to understand the impact of human perception of light, meaning that the human eye expands to adapt to a lower illuminance at the pupil to allow more light to enter the eye. The characteristics of the perceived light of the human eye are shown in Figure 8.  [114].

•
Flicker reduction: The VLC modulation scheme requires that humans should not perceive th ness of the flicker. The flicker can cause harmful and severe physiological chang human body, as shown in [115]. Therefore, it is necessary to change the light i faster than the human eye can perceive the flicker or change the light intensity fas 200 Hz to avoid any harmful effects, according to the IEEE 802.15.7 standard [1 means that the VLC Modulation scheme should reduce flicker while providing data rate.
After preprocessing, encoding and modulation, a binary bitstream drives th LED), and the electrical signal implemented the intensity modulation to conver optical signal. Each modulation technique has a finite number of symbols in wh can be encoded. Having more symbols allows the representation of more bits by symbol. For example, if an eight-symbol modulation technique is used, each sym represent a set of three bits because each set can have one of eight possibilities. In each symbol of an M-symbol scheme can represent k ¼ log2 M bits, and these k mapped such that adjacent symbols differ by only one bit (gray encoding). There incorrect selection of adjacent symbols results in a single bit error. However, th such techniques comes with an increase in the power required or a decrease in

•
Flicker reduction: The VLC modulation scheme requires that humans should not perceive the brightness of the flicker. The flicker can cause harmful and severe physiological changes in the human body, as shown in [115]. Therefore, it is necessary to change the light intensity faster than the human eye can perceive the flicker or change the light intensity faster than 200 Hz to avoid any harmful effects, according to the IEEE 802.15.7 standard [116]. This means that the VLC Modulation scheme should reduce flicker while providing a higher data rate.
After preprocessing, encoding and modulation, a binary bitstream drives the LD (or LED), and the electrical signal implemented the intensity modulation to convert into an optical signal. Each modulation technique has a finite number of symbols in which data can be encoded. Having more symbols allows the representation of more bits by a single symbol. For example, if an eight-symbol modulation technique is used, each symbol can represent a set of three bits because each set can have one of eight possibilities. In general, each symbol of an M-symbol scheme can represent k 1 ⁄4 log2 M bits, and these k bits are mapped such that adjacent symbols differ by only one bit (gray encoding). Therefore, the incorrect selection of adjacent symbols results in a single bit error. However, the use of such techniques comes with an increase in the power required or a decrease in the immunity of error. The five common modulation schemes of VLC are:

Multilevel Pulse Amplitude Modulation (M-PAM) Technique
Multilevel pulse amplitude modulation (M-PAM) is a conventional multilevel modulation scheme and is the generalization of non-return-to-zero (NRZ) on-off keying (OOK) from a set of two symbols to a set of M symbols, as illustrated in Figure 9. For example, when the LED/Laser is turned off, the light intensity will not be completely switched off, but the optical intensity will be diminished. Where Q(.) is the Q function, the BER and SNR relation of M-PAM can be determined by Equation (13) [117]:

Phase Shift Keying (PSK) Modulation Technique
Phase shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Although Binary-PSK (BPSK) is the simplest form of PSK, which uses two phases separated by 180°, it can only modulate at 1 bit/symbol. The motivation behind M-ary PSK (MPSK) is to increase the bandwidth efficiency of the PSK modulation scheme. Figure 10 illustrated the constellation diagrams for BPSK, QPSK and 8PSK. Gray encoding was also used to map the sets of bits to the appropriate symbol. Where . is the error function, the BER and SNR relation of BPSK and QPSK can be determined by Equations (14) and (15) [118], respectively:

Phase Shift Keying (PSK) Modulation Technique
Phase shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Although Binary-PSK (BPSK) is the simplest form of PSK, which uses two phases separated by 180 • , it can only modulate at 1 bit/symbol. The motivation behind M-ary PSK (MPSK) is to increase the bandwidth efficiency of the PSK modulation scheme. Figure 10 illustrated the constellation diagrams for BPSK, QPSK and 8PSK. Gray encoding was also used to map the sets of bits to the appropriate symbol. Where er f c(.) is the error function, the BER and SNR relation of BPSK and QPSK can be determined by Equations (14) and (15) [118], respectively: 15) and SNR relation of BPSK and QPSK can be determined by Equations (14) and (15) [118], respectively:

M-ary Quadrature Amplitude Modulation (QAM) Technique
QAM combines both PSK and amplitude-shift keying by changing the two parameters of the carrier. For that reason, this method is also known as amplitude phase keying

M-Ary Quadrature Amplitude Modulation (QAM) Technique
QAM combines both PSK and amplitude-shift keying by changing the two parameters of the carrier. For that reason, this method is also known as amplitude phase keying [120]. With the same average signal strength, the number of symbols can be raised to represent more bits by a single symbol, resulting in improved bandwidth efficiency compared to MPSK. The constellation diagrams of 8-QAM and 16-QAM are shown in Figure 11. The BER and SNR relation of M-QAM can be determined by Equation (16) [121]:  [120]. With the same average signal strength, the number of symbols can be raised to represent more bits by a single symbol, resulting in improved bandwidth efficiency compared to MPSK. The constellation diagrams of 8-QAM and 16-QAM are shown in Figure 11. The BER and SNR relation of M-QAM can be determined by Equation (16

Orthogonal Frequency Division Modulation (OFDM) Technique
OFDM uses multiple subcarriers with orthogonal frequencies to utilize the available bandwidth efficiently. The OFDM diagram transmission and reception is shown in Figure 12 [119]. The serial data stream is converted to a parallel stream and mapped using a modulation scheme, such as PSK or QAM. The data flow is then imposed with Hermite symmetry to ensure that the inverse fast Fourier transform (IFFT) output in the following block is true valued. The use of PSK or QAM is therefore possible when OFDM is employed. If the size of the IFFT is N, N/2 symbols going into the IFFT will represent the data to be transmitted, and the other N/2 symbols will be their conjugates. This ensures that the IFFT generates only real values. The analytical expression between the BER and SNR of M-QAM OFDM can be determined by Equation (17) [122]:

Orthogonal Frequency Division Modulation (OFDM) Technique
OFDM uses multiple subcarriers with orthogonal frequencies to utilize the available bandwidth efficiently. The OFDM diagram transmission and reception is shown in Figure 12 [119]. The serial data stream is converted to a parallel stream and mapped using a modulation scheme, such as PSK or QAM. The data flow is then imposed with Hermite symmetry to ensure that the inverse fast Fourier transform (IFFT) output in the following block is true valued. The use of PSK or QAM is therefore possible when OFDM is employed. If the size of the IFFT is N, N/2 symbols going into the IFFT will represent the data to be transmitted, and the other N/2 symbols will be their conjugates. This ensures that the IFFT generates only real values. The analytical expression between the BER and SNR of M-QAM OFDM can be determined by Equation (17) [122]: ure 12 [119]. The serial data stream is converted to a parallel stream and mapped using a modulation scheme, such as PSK or QAM. The data flow is then imposed with Hermite symmetry to ensure that the inverse fast Fourier transform (IFFT) output in the following block is true valued. The use of PSK or QAM is therefore possible when OFDM is employed. If the size of the IFFT is N, N/2 symbols going into the IFFT will represent the data to be transmitted, and the other N/2 symbols will be their conjugates. This ensures that the IFFT generates only real values. The analytical expression between the BER and SNR of M-QAM OFDM can be determined by Equation (17)

Color Shift Keying (CSK) Modulation Technique
The blue LED chip excites the yellow phosphor to produce white light, but its switching speed capability slows down. Therefore, green, blue, and red are used to achieve white light through light mixing technology. CSK is mentioned in IEEE 802.15.7 and lower data rate is used in other modulation schemes [123]. The modulation of CSK is performed using the intensities of the three colors of RGB LED. CSK is sampled in the color space chromaticity diagram [114]. It maps the eye-perceivable color to two chromaticity factors, x and y. Table 4 illustrates the seven bands of human-visible wavelengths, the centers of which are marked in Figure 13. Where k is the number of bits (k = log 2 M), M is the modulation order and N 0 is one-sided noise power spectral density, the bit error probability (BEP) of a 4-CSK three color LEDs (Trichromatic LEDs, TLED) can be determined by Equation (18) [124]: lengths, the centers of which are marked in Figure 13. Where is the number of bits ( = ), is the modulation order and is one-sided noise power spectral density, the bit error probability (BEP) of a 4-CSK three color LEDs (Trichromatic LEDs, TLED) can be determined by Equation (18)

Power Requirements and Spectrum Efficiency
The power requirement for an OWC system is proportional to the square root of the SNR, as shown in Equation (19)

Power Requirements and Spectrum Efficiency
The power requirement for an OWC system is proportional to the square root of the SNR, as shown in Equation (19) [117]: where R is the responsivity of the photodetector and σ 2 N is the total noise power in the detector current.
Hence the normalized average power requirement of M-PAM, BPSK, QPSK is shown in Equations (20)- (22), respectively: Nowadays, the most commonly used modulation schemes are the M-QAM OFDM, NRZ-OOK, and M-PAM. Among them, M-PAM has better spectral efficiency but requires higher SNR to maintain targeted BER, as illustrated in Figure 14.
Nowadays, the most commonly used modulation schemes are the M-QAM OFDM, NRZ-OOK, and M-PAM. Among them, M-PAM has better spectral efficiency but requires higher SNR to maintain targeted BER, as illustrated in Figure 14.

Optical Wireless Communication Standards
VLC is the most promising technology for communications. Due to LED lighting's rapid development, the cost is rapidly falling; however, some of the challenges that must

Optical Wireless Communication Standards
VLC is the most promising technology for communications. Due to LED lighting's rapid development, the cost is rapidly falling; however, some of the challenges that must be addressed are listed as follows: 1.
Integration of VLC system with existing communication standards.
VLC should properly consider mobility issues such as handover.

4.
Specification of a forward error correction plan to improve communication system performance.
As the number of VLC devices increases, there will be interference between different VLC devices. The Electronic Information Technology Industry Association developed the standard 802.15.7, which is a standard established by the IEEE for the physical layer and the MAC layer [123]. Objectives of this standard are: 1.
Establish access to hundreds of terahertz frequency bands.

3.
Communication of additional services that supplement the current visible light equipment.

4.
VLC communication that specifies a forward error correction (FEC) scheme, modulation form, and transmission rate.

5.
Channel access mechanism, as visibility support also describes channel access, and contention access period (CAP) and contention-free period (CFP). 6.
Physical layer specifications, such as optical mapping, TX-RX, RX-TX turnaround time, flicker, and dimming relief. IEEE 802.15.7 is new product development standard.
Three different types of devices used by VLC are vehicles, mobile equipment, and infrastructure, as shown in Table 5 [125].

VLC Application
VLC provides many applications from high-speed internet chains with LED bulbs to interplanetary communication and quantum communication. VLC also brings a new perspective to computing that is considered ubiquitous. In this section, we discuss the potential of visible light communications, focusing on these applications: indoor VLC [2,10,12,88], ID and position systems [6,126], in-vehicle communications [127], and underwater communication [6,128].

Li-Fi
Li-Fi is similar to Wi-Fi, but it is a bidirectional, visible light wireless communication system. In 2011, Harald Haas first created Light Fidelity (Li-Fi). Wi-Fi signals have the problem of interference with other radio signals, such as interfering with the airplane navigation equipment. Hence, Li-Fi can be a better solution in areas sensitive to electromagnetic radiation, such as airplanes and hospitals, which is illustrated in Figure 15.

Li-Fi
Li-Fi is similar to Wi-Fi, but it is a bidirectional, visible light wireless communication system. In 2011, Harald Haas first created Light Fidelity (Li-Fi). Wi-Fi signals have th problem of interference with other radio signals, such as interfering with the airplan navigation equipment. Hence, Li-Fi can be a better solution in areas sensitive to electro magnetic radiation, such as airplanes and hospitals, which is illustrated in Figure 15.

Vehicle to Vehicle Communication
Front-end collision warning, pre-collision sensing, emergency stop lights, lan change warning, parking movement assistance, left turn assist, signal violation warning Figure 15. LiFi application in an airplane cabin. Reprinted from [28], Copyright 2019, with permission from Elsevier.

Vehicle to Vehicle Communication
Front-end collision warning, pre-collision sensing, emergency stop lights, lane change warning, parking movement assistance, left turn assist, signal violation warning, and corner speed limit warning are among the most important vehicle safety communication initiatives [129]. A Li-Fi high-speed visible light communication system can be employed for these, since all high-priority applications require extremely low latency reachability and the permissible delay of vehicle safety communication is ultra-low.

Underwater Communication
Due to the extremely high attenuation of radio frequency waves in water, RF or nearinfrared OWC cannot propagate well in sea water [130]. Therefore, an underwater optical communication (UWOC) network should adopt VLC communication as shown in Figure 16. and corner speed limit warning are among the most important vehicle safety communication initiatives [129]. A Li-Fi high-speed visible light communication system can be employed for these, since all high-priority applications require extremely low latency reachability and the permissible delay of vehicle safety communication is ultra-low.

Underwater Communication
Due to the extremely high attenuation of radio frequency waves in water, RF or near-infrared OWC cannot propagate well in sea water [130]. Therefore, an underwater optical communication (UWOC) network should adopt VLC communication as shown in Figure 16.
where is the output power of the light source, is the received power over certain distance , and is coefficient of water associated with attenuation. By evaluating Equation (23) and by assuming the output power to be 1 mW we can obtain the propagation distance of visible light in ocean water, as illustrated in Figure 17. In clear ocean water, blue and green light are usually preferable to red light, as illustrated in Figure  17a; however, the attenuation and scattering of red light are lower for certain environ- For underwater application, the attenuation of the light can be evaluated through Equation (23) [132]: where P 0 is the output power of the light source, P d is the received power over certain distance d, and c(λ) is coefficient of water associated with attenuation. By evaluating Equation (23) and by assuming the output power to be 1 mW we can obtain the propagation distance of visible light in ocean water, as illustrated in Figure 17. In clear ocean water, blue and green light are usually preferable to red light, as illustrated in Figure 17a; however, the attenuation and scattering of red light are lower for certain environments such as a highly turbid harbor [133], as illustrated in Figure 17b.
where is the output power of the light source, is the received power over certain distance , and is coefficient of water associated with attenuation. By evaluating Equation (23) and by assuming the output power to be 1 mW we can obtain the propagation distance of visible light in ocean water, as illustrated in Figure 17. In clear ocean water, blue and green light are usually preferable to red light, as illustrated in Figure  17a; however, the attenuation and scattering of red light are lower for certain environments such as a highly turbid harbor [133], as illustrated in Figure 17b.

Information Displaying Signboards
LEDs are utilized as light sources on signboards set in a specific order at airfields, bus stations, and other locations where information must be broadcasted. In venues such as airports, museums, and hospitals, signs are utilized to transfer data and can also be used for instructions [134], as shown in Figure 18.

Information Displaying Signboards
LEDs are utilized as light sources on signboards set in a specific order at airfields, bus stations, and other locations where information must be broadcasted. In venues such as airports, museums, and hospitals, signs are utilized to transfer data and can also be used for instructions [134], as shown in Figure 18.

Visible Light ID and Position System
Visible light can be used as an identification system and to determine indoor position, similar to a global position system (GPS). For instance, if we were standing in room 15 of a certain building, the system could be used to identify the room number and its building [6,126].

Wireless Local Area Networks (WLANs)
In Figure 19, an ultra-high-speed full-duplex local area network with star topology is shown, with LED visible light communication set up for the LAN, using VLC to provide speeds exceeding 10 Gb/s, and is tested by a large number of users.

Visible Light ID and Position System
Visible light can be used as an identification system and to determine indoor position, similar to a global position system (GPS). For instance, if we were standing in room 15 of a certain building, the system could be used to identify the room number and its building [6,126].

Wireless Local Area Networks (WLANs)
In Figure 19, an ultra-high-speed full-duplex local area network with star topology is shown, with LED visible light communication set up for the LAN, using VLC to provide speeds exceeding 10 Gb/s, and is tested by a large number of users.
Copyright 2019, American Chemical Society.

Visible Light ID and Position System
Visible light can be used as an identification system and to determine indoor position, similar to a global position system (GPS). For instance, if we were standing in room 15 of a certain building, the system could be used to identify the room number and its building [6,126].

Wireless Local Area Networks (WLANs)
In Figure 19, an ultra-high-speed full-duplex local area network with star topology is shown, with LED visible light communication set up for the LAN, using VLC to provide speeds exceeding 10 Gb/s, and is tested by a large number of users.

Challenges
While VLC has many advantages over RF communication, it also faces unprecedented challenges that need to be mitigated. One of these challenges is commercialization, in which lighting companies and phone manufacturers have to develop their future devices to accommodate the current VLC technologies for use in future applications. Another challenge is creating new VLC standards and developing the current ones by considering the latest technological evolutions in the field [126]. In addition, the performance of VLC can be degraded or distorted due to background noise. Manchester coding mitigates this background noise [6]. Moreover, to cater for dual-functionality LD-or SLD-based SSL-VLC applications, the quality of the produced white light still needs to be improved using stable light converters that can be used for prolonged periods of time. The use of LDs as data transmitters also faces issues with misalignment and outages because there are many ways in which the line-of-sight link between the transmitter and the receiver might be lost or blocked. A critical concern is the hazards related to eye safety, as long-term exposure to high-intensity light is potentially harmful to human vision and circadian rhythm. In the case of using conventional LEDs as transmitters, the limitation in allowable bandwidths would potentially limit the transmission rate in Li-Fi systems; thus, micro-LEDs and LDs are more promising alternatives, offering higher bandwidths and transmission rates. Another updated modulation technology, quantum communication, which is also being applied to the VLC system, will introduce the principle of quantum communication.
The following section discussed the state of the art of OWC systems and potential techniques for improving the performance or the data rate. The possible solutions for solving problems of the current VLC system mention in [136] are discussed, such as: µ-LED operated under low current density, decreasing the power consumption of the transmission system [137].

Si-Based Detectors Are Mainly Sensitive to Infrared Waves
In 2020, Huang et al. demonstrated a black silicon-based photodetector with high responsivity up to 100 A/W at 400 nm and broad response spectrum from 400 to 1600 nm. In their work, the broad-bandgap responsivity and SNR increase, and the dark current, are mitigated through the utilization of rapid thermal annealing and hydrogenated surface passivation [112], which may be a promising candidate for Si-based detectors for VLC systems.

Point-to-Point Communication Based on a Single Transmitter and Detector
Multiple light sources are needed to accomplish uniform lighting and elevate the data rate in a mesh multiple-point network. Nevertheless, spatial multiplexing (SMP) is the most commonly used MIMO scheme that is restricted by channel correlation, causing difficulties for receivers to distinguish a single data stream. In 2020, a novel odd-order 32-QAM constellation scheme for 2 × 2 MIMO VLC systems by superposing two independent signals modulated with 4-QAM and 8-QAM was illustrated by Guo et al. as a promising candidate for mitigating the channel correlation of SMP MIMO VLC system [138].

Transmission and Reception Antennas Require a Large Lens Group
This problem could be solved via the combination of a Fresnel lens and an optical antenna that is capable of radiating directionally and switching to wide-angle rapidly. In 2018, Sabouri et al. proposed a SiN-based optical phase antenna (OPA); the performance of such a design is estimated to have a ±12.7 • beam steering in Azimuth angle, 15.6 • in polar angle using wavelength tuning in a range of 60 nm [139]. OPA is a promising candidate due to the advantages of its small size, light-weight, rapid switching speed, and multi-beam steering [140].

Quantum Communication
Quantum communication is a way to transfer quantum states from the transmitter (Alice) to the receiver (Bob). Quantum Key Distribution (QKD) is a well-known case of quantum cryptography, first proposed by Bennett and Brassard in 1984 [141,142], also known as the BB84 protocol, and was first experimentally demonstrated in 1991 [143]. Different photon states, such as polarization, period, frequency, phase, and space, can realize various QKD protocols. The essence of BB84 is to pressure eavesdropper (Eve) to perform a quantum measurement that will expose her existence, by sending a series of individual photons to Bob; many of the photons never arrive or are not detected. Of the resulting bits, some are kept and used as key material, the remaining are used to hunt for Eve.
A schematic of a polarization-based BB84 QKD protocol and the four polarization states that were applied are shown in Figure 20. A polarization-based BB84 QKD protocol is often used in free-space optical (FSO), since when used over optical fiber the polarizationdependent loss, polarization mode dispersion, and the fiber loss restricted the transmission distance [144]. The beam steering system allows Alice to direct the beam to Bob without physically moving the system, and can be realized by a liquid crystal spatial light modulator (LC-SLM), LCoS-SLM, and micro-electromechanical systems (MEMs) [7,145]. The single photon source required by the BB84 protocol is realized by a high attenuation laser, which can generate weak coherent state (WCS) pulses [146]. tion-dependent loss, polarization mode dispersion, and the fiber loss restricted the transmission distance [144]. The beam steering system allows Alice to direct the beam to Bob without physically moving the system, and can be realized by a liquid crystal spatial light modulator (LC-SLM), LCoS-SLM, and micro-electromechanical systems (MEMs) [7,145]. The single photon source required by the BB84 protocol is realized by a high attenuation laser, which can generate weak coherent state (WCS) pulses [146]. The weak coherent state output by the highly attenuated laser source can guarantee that the coherent state has a high probability that there is no more than one photon. The number state | ⟩ can be expressed in terms of the ground state (|0⟩) with the creation operator by Equation (24): The weak coherent state output by the highly attenuated laser source can guarantee that the coherent state has a high probability that there is no more than one photon. The number state |n can be expressed in terms of the ground state (|0 ) with the creation operator by Equation (24): where a is the annihilation operator. The density operator of a coherent state is given by Equation (25): where a is a complex eigenvalue of the annihilation operator, and |α is the coherent state vector. The probability that n-photons can be found in coherent state |α follows the Poisson distribution shown in Equation (26): where µ = |α| 2 is the photon number. Considering that the chance of more than one photon is sent is not zero, by applying a photon number splitting (PNS) attack, it is possible for Eve to obtain information. To solve the problem above, Hoi-Kwong Lo et al. proposed a decoy-state-based quantum key distribution in 2005 [147]. In decoy-state QKD, Alice and Bob can detect whether Eve is capturing photons when more than one photon is sent since the average number of photons sent is increased during random timeslots. The quantum mechanics laws guarantee QKD's security. According to the method utilized on Bob's side, there are two common QKD schemes: discrete variable (DV)-QKD and continuous variable (CV)-QKD, which utilize a single-photon detector (SPD) and homodyne/heterodyne detection to measure field quadrature on Bob's side, respectively. Different QKD schemes can also be classified as entanglement assist or preparation and measurement types. For DV-QKD, the unclonability theorem and in distinguishability theorem of arbitrary quantum states make absolute security possible. The unclonability theorem declares that any quantum state is unique and cannot be copied, which means that even with the help of the most powerful computer in the world, Eve cannot clone a non-orthogonal quantum state [148]. Furthermore, the distinguishability theorem states that non-orthogonal states cannot be distinguished clearly. That is to say, it is inevitable that constancy of the quantum states will be disturbed when Eve is attempting to gain information on transmitted bits and this will be detected by Bob. The CV-QKD applies the uncertainty principle and states that the in-phase and quadrature components of the coherent state cannot be measured completely and accurately at the same time. Recently, the development of QKD has greatly improved not only theoretically but also experimentally; an LED-based decoy-state QKD was proposed recently by Jian-Wei Pan et al. in 2019 [146]. By replacing the WCS laser source with an LED, they lower the cost of the QKD system, providing a tolerable price and sufficient security.

Conclusions
Here, we present the recent advances in high-speed visible light communication: transmitter, modulation technology, receiver standard, and application. Micro-LED/laserdiode-based VLC are promising approaches towards next-generation wireless data communication networks, which have caught the attention of both academia and industry. VLC technology provides many advantages over RF and has become an alternative to RF communication, despite the disadvantage of blind spots due to light spacing and line-ofsight blocking. The successful development of the VLC system addresses the immediate multi-billion-dollar markets in beyond-5G optical wireless communication. Empowering the Internet of Underwater and Underground things (IoU2T), VLC will unfold many new ideas and enable future applications related to energy, connectivity, and the environment.

Conflicts of Interest:
The authors declare no conflict of interest.