Evolution of optical wireless communication for B5G/6G

Abstract

The research on optical wireless communication (OWC) has been going on for more than two decades. Particularly, visible light communication (VLC), as a means of OWC combining communication with illumination, has been regarded as a promising indoor high-speed wireless approach for short-distance access. Recently, lightwave, millimeter-wave (mmWave), terahertz (THz) and other spectrum mediums are considered as potential candidates for beyond fifth-generation/sixth-generation (B5G/6G) mobile communication networks. On the basis of previous studies, this review focuses on revealing how the research of next-generation OWC technology should be carried out to meet the requirements of B5G/6G for practical deployment. The research, development and engineering transformation of the OWC systems are a paragon of interdisciplinary. It involves a wide discussion on how to build a high-speed, multi-user, full-duplex, white-light OWC system based on existing technologies by showing the innovations and trade-offs at various levels with material, device, air-interface technology, system and network architecture. The compatibility of OWC is emphasized and some advanced heterogeneous OWC systems are presented, which involves the combination or integration of various functions such as sensing, near-infrared (NIR) beam-steering, positioning and coexistence with radio frequency (RF) communication. Finally, several potential directions are pointed out for the actual engineering deployment in the B5G/6G era.

Introduction

Driven by industries such as voice and data communications, the Internet of Things (IoT), smart manufacturing, home and cities, wireless communication technology has been iterated rapidly in the past few decades. The applications and services brought by wireless networks and devices have gradually changed the way people live and behave. Through the development of first to fourth-generation (4G), global mobile communication has continuously improved on the data transmission rate, coverage and stability which promoted the arrival of the consumer Internet era [1]. At present, global fifth-generation (5G) development has been commercially expanded rapidly for not only the increase of data rate, but also a wider-range application scenario, which is deeply integrated with many vertical industries rather than only focusing on mobile Internet [2]. However, as data traffic continues to grow and new vertical industries rapidly emerge, 5G will eventually encounter technical limitations in supporting high-speed, low-delay, safe and reliable distribution of massive data. Such a data-driven era not only demands extremely large capacity but also incurs huge energy consumption, which starves for next-generation communication - beyond fifth-generation/sixth-generation (B5G/6G) [3].

Focusing on meeting the application scenarios and business requirements of B5G/6G networks, the architecture will achieve greater performance than 5G not merely in wireless communication. With the massive interconnectivity of the network, some key indicators defined by 5G no longer apply under specific circumstances. For example, in holographic communication, the size of a single holographic photo is 7–8 Gigabyte, which is equivalent to 56–64 Gbit [4]. If the video is of the same resolution, considering 30 frames per second (Fps), the demanded converted data rate will reach the order of Terabits per second (Tbps). Meanwhile, emerging scenarios such as suburban communication service, as a counterpart to the current mobile network covering more than 2 billion residents, requires new performance indicators. Therefore, B5G/6G aims to build an integrated air-space-ground network, where network coverage will become a key performance indicator. In addition, applications such as high-precision industrial control and nano-medical robots also place high requirements on positioning accuracy. Such positioning accuracy will play a key role in the performance evaluation of B5G/6G compared to that in 5G. Therefore, B5G/6G will further improve the existing key performance indicators (KPIs): the peak rate will reach 100 Gbps ​∼ ​1 Tbps; the user experience rate will exceed 10 Gbps; the air-interface delay will be as low as 0.1 ​ms; the number of connections will support 10 million connection/square kilometers (km²). Based on the existing 5G indicators, B5G/6G will also evolute some new performance indicators, such as positioning accuracy to 1-cm indoors, 50-cm outdoors, delay jitter at±0.1 ​ns, and network coverage. In addition, the B5G/6G network will also have high-intelligence features. Through the combination between artificial intelligence (AI) and big data, it can meet the refined and personalized service needs of individuals or industries [5]. In addition, the B5G/6G network will effectively reduce costs and consumption, greatly improving energy efficiency to achieve sustainable development without impacting the KPIs of wireless communication.

In order to meet the requirements of future B5G/6G networks, new key technologies should be introduced. Currently, the most frequently investigated technologies mainly include wireless access solutions (ultra-large-scale antennas, orbital angular momentum (OAM)), new spectrum resources (millimeter-wave (mmWave), terahertz (THz), visible light), and the new coverage methods of air-space-earth-sea integration [6]. The above-mentioned and other potential enabling technologies will greatly improve network performance and services, with complementing each other. In the development of wireless communication, among them, frequency spectrum congestion has become a major bottleneck, especially in the access of middle/short-distance mobile communication [7]. Meanwhile, traditional radio frequency (RF) has poor confidentiality and high-power consumption, and it always suffers from electromagnetic interference (EMI) especially in places that are full of electronic equipment such as hospitals, airports, data centers, etc. Because of increasing demands for high-speed and large-capacity data transmission as well as the development of semiconductor illumination, optical wireless communication (OWC), as a new method of wireless communication with faster data rate, larger capacity and higher reliability, is gradually being proposed and counted on breaking the bottleneck above [8]. Compared with traditional RF communications, OWC covers the entire waveband from ultraviolet to near-infrared (NIR) light. OWC is an ideal short-distance high-speed wireless access with an effective complementary advantage to RF [9]. Compared with mmWave communication with the useable bandwidth from 7 ​GHz to 9 ​GHz, OWC possess huge bandwidth which have a great transmission advantage after the short-distance line-of-sight (LOS) link is established. Limited by transmit power, detection sensitivity, and atmospheric attenuation, long-range terahertz communication (THzCom) is impractical, while OWC has the potential to build an earth-orbit-air communication network. In addition, for short-range high-speed underwater communication, underwater OWC (UOWC) has been proven to have unique advantages using the blue-green light band compared with others. At the same time, many governments clearly mention the construction of a new-generation information infrastructure to establish smart city clusters. Thus, OWC is also rapidly developing in the fields of intelligent transportation, street lamp communication and other new directions of intelligent transportation and smart cities [10,11].

Since studies have shown that more than 70% of wireless data exchanges occur in indoor environments, visible light communication (VLC), light fidelity (Li-Fi), is widely researched to construct an indoor communication system that can well meet the basic requirements for daily illumination without additional energy for information transmission [12]. Therefore, VLC is proposed to effectively meet the need for communication and illumination for indoor short-distance users [13]. With the introduction such as indoor IoT and smart homes, household appliances can also actively access the network and perform data exchange, and the number of network access sensor devices has increased significantly than before [14]. It can be seen that the indoor VLC is composed of dynamic input and output of multi-user devices. In addition, the structure and scale of the communication network have become more complicated over time. These electronic devices deployed in the indoor environment correspond to huge data traffic and bandwidth requirements [15]. How to make full use of relatively limited bandwidth and spectrum resources to expand the scale of indoor communication coverage networks and provide high-performance access services for multiple users is one of the current research hotspots, with great practical significance.

Secondly, the reported OWC systems mostly focus on realizing single-user point-to-point LOS framework, while the realization of multi-user systems is very limited [10,15]. Based on previous works, it is found that the main focus of them can be categorized into two types: one is to improve the reachable bandwidth of the system by improving optical devices and optimizing the design of the optical path system, and the other is to use high-order modulation such as m-order quadrature amplitude modulation (m-QAM), orthogonal frequency division multiplexing (OFDM) and other technologies to improve spectrum utilization. In current wireless optical communications, the bandwidth based on laser diodes (LD) and micro-size light-emitting diodes (micro-LED, μLED) is approximately GHz, which corresponds to a data rate of several Gbps using high-order modulation and coding [16]. Under the combined effect of these technologies, the highest data rate of point-to-point OWC architecture has continuously increased to tens of Gbps, which can meet the communication needs of the absolute majority of users, but how to use the existing settings to serve multiple users at the same time is still worth further investigations [17].

A single-function OWC implementation consisting of discrete components towards specific scenarios is the last theme. In addition to the above-mentioned indoor scenes, there are also OWC systems for underwater, optical camera communication (OCC) systems for outdoor vehicles [17], free-space optical (FSO) links for satellite communication or systems for indoor optical positioning functions. Or study specific problems on existing architectures, such as physical layer security, cross-media transmission, and general channel models [15]. These OWC systems involved are highly customized and have poor compatibility with existing systems and facilities with other functions. Therefore, at present, there are fewer existing multi-functional heterogeneous OWC systems, which limits the engineering transformation of the previous generation of OWC to a certain extent. In addition, the robustness, adaptability, and even intelligence of the OWC system need to be continuously improved [5,15].

5G opens a new world of the interconnection of all things, realizes the comprehensive interconnection of people to people, people to things, and things to things, and gradually penetrates into various industries and fields of the economy and society, becoming a key infrastructure for the digital transformation of society. B5G/6G will fully support a leap towards the digital transformation of the whole society and realize the interconnection and intelligent connection of all things. On the basis of the previous technologies, we summarized the breakthroughs of emerging OWC technologies recently and proposes some issues that should be addressed in the coming future. The first part is about devices and materials, which mainly describes promising candidates for next-generation high-speed OWC, including micro-LED, vertical-cavity surface-emitting laser (VCSEL), advanced photodetector and color-converter materials. The second part focuses on the emerging air-interface technologies including multicarrier modulations, multiple access, machine learning (ML) and AI. These air-interface digital signal processing (DSP) technologies play an important role in OWC systems towards higher speed, more user access, and greater flexibility. The third part mainly presents the heterogeneous OWC systems, which combine OWC with sensing, fiber-wireless, cellular communication and visible light positioning (VLP) functions or systems. These heterogeneous systems greatly improve the compatibility of OWC as an important route for the practical deployment of B5G/6G. The last section summarizes, looks forward to, and proposes the research directions, evaluation indices, and design trade-offs of the next-generation OWC. In order to meet the requirements of B5G/6G mobile network applications, the traditional point-to-point OWC systems are bound to evolve to higher rates, wider mobility, multi-user access, indoor full-duplex, outdoor air-ground-sea integration, and to meet the needs of white illumination. OWC, as vertical interdisciplinary research, requires breakthroughs, coordination, and collaboration in many levels and fields including materials, devices, systems and networks, to meet the requirements of practical deployment. Finally, this review article further introduces some open issues to be solved in next-generation OWC research and draws a conclusion.

The developments of solid-state lighting (SSL) materials, illuminant and detecting devices drive a wide range of interests in FSO applications for wireless broadband communication. Fig. 1 Summarizes a general classification of the representative transmitters, receivers and materials from many kinds of literature. These materials and devices will be mentioned in the following description, which is applied in OWC for different indicators and scenarios, respectively. Historically, the development of VLC has benefited to a certain extent from the rapid development of the LED semiconductor industry due to the pursuit of progress for higher bandwidth. The light source of the transmitter is the top priority among them and deserves to be highlighted especially the micro-LED and VCSEL in the next section. Fig. 2 Roughly shows five kinds of commercial light sources with different electrical-optical (E-O) bandwidths and reachable data rates. At present, most of the light sources adopted in VLC are commercial white-light phosphor LED or red-green-blue (RGB) LED. Phosphor LED has a very low-frequency response and color shift due to the yellow phosphor which is unsuitable for high-speed transmission, although using a blue filter in front of the receiver can improve the frequency response. The channel capacity of VLC based on RGB-LED can be increased by wavelength division multiplexing (WDM), but limited by operating temperature dependencies of each monochromatic LED. Micro-LED should be a promising candidate which will be introduced detailed later [18]. Compared with LED, LD has a larger bandwidth (>GHz) and higher pumping efficiency. FSO communication systems based on visible or invisible light LDs are generally far-reaching due to their ability to emit highly coherent beams. Since the LED is without a resonator, the beam is more divergent and inferior to the LD/VCSEL in terms of coupling. VCSEL is relatively less reported in OWC, but the high-bandwidth feature, simple packaging and easy integration inside consumer electronics products make it attractive for next-generation OWC. Table 1 Summarizes and compares the characteristics, performance and mainstream applications of different devices.

The frequency response and luminescence characteristics of a light source are key indicators for their applications as transmitters in OWC. In the past few years, micro-LEDs have begun to attract the attention of many researchers in the field of OWC due to their high E-O bandwidth and the potential for UOWC, parallel communication, VLC-display convergence and other novel applications [19]. The comparison of various LED devices including PC-WLED, RGB-LED, micro-LED and OLED [20] is shown in Table 1. Regardless of bandwidth and power consumption, micro-LEDs are much better than other kinds of LEDs even though they have more complex fabrication processes. As it shows in Fig. 3, the typical structure of multiple quantum well (MQW) micro-LED is mainly divided into two types: the top-emitting chip and the flip-chip. The top-emitting structure in Fig. 3(a) usually has a thin, low-reflectivity p-shaped metal contact layer, and thus the light can easily be emitted from the top. For the flip-chip structure in Fig. 3(b), a mirror is placed on top of the p-shaped metal contact layer to increase the light output power of the device, and the higher light output power indicates a better resolution [21]. In general, the fabrication process of a standard micro-LED device or micro-LED array device mainly includes photo etching, pattern transfer, and metal deposition. In this process, micro-LED devices are usually prepared in the form of array structure including matrix addressing and independent addressing [[18], [22], [23]].

As we mentioned before, an important advantage of the micro-LED-based VLC system is the ability to support Gbps VLC. Using non-return-to-zero on-off keying (NRZ-OOK), the data rate of the system can reach 1 Gbps. Also, multi-Gbps data rates can be achieved by quadrature-phase shift keying (QPSK), QAM, or OFDM. Among them, J.W. Shi et al. used a micro-LED emitting light at 520 ​nm to show us a communication system up to 330 ​MHz [24]. D. Tsonev et al. have demonstrated a 3-Gbps single micro-LED based VLC link using OFDM, and then the data rate of OFDM-based VLC is extended to 3.32 Gbps using violet and UV GaN-based micro-LEDs [25]. A micro-LED was used to transmit data over free-space at the data rates of 1.7 Gbps, 3.4 Gbps, and 5 Gbps with different modulation schemes, respectively [26]. At present, single-pixel GaN-based micro-LEDs with 655 ​MHz modulation bandwidth have been able to achieve the highest data rates of 10 Gbps by using modified OFDM [27]. Long-distance VLC and UOWC using micro-LED have also been experimentally demonstrated [28,29]. P. Tian et al. used an 80-μm micro-LED with 160-MHz bandwidth to make a high-rate UOWC system with a transmission rate higher than 800 Mbps over a 0.6-m transmission link [30]. In addition, green and deep-ultraviolet GaN-based micro-LEDs have also been used in FSO communication applications [31]. An 80-MHz modulation bandwidth white-light illumination and communication system with a data rate of 0.3 Gbps using single blue micro-LED and liquid perovskite quantum dot materials was realized [32]. E. Xie et al. adopted a series-biased 3 ​× ​3 micro-LED array with 18-mW emitting optical power to realize a VLC system and then achieved data rates of 2.1 Gbps and 5.18 Gbps using NRZ-OOK and QAM-OFDM, respectively [33]. Although the modulation bandwidth was only 285 ​MHz, the large modulation depth of the input signal enhanced the SNR performance of the VLC system benefited by the series structure. In addition, micro-LED also can be used in the VLC tracking systems or adopted as detectors in a LD-LED VLC system [[34], [35], [36]]. J. F. C. Carreira et al. fabricated several on-chip GaN-based dual-color micro-LED arrays and then demonstrated the application in VLC [37]. Recently, InGaN quantum dots grown by self-assembly were used as the active layer of high-speed LED by Z. Wei et al., and the modulation bandwidth of −3 dB could be achieved up to 1.1 ​GHz. The results were much better than any other records based on c-plane epitaxy and were comparable to the best results based on reported semi-polar/non-polar GaN substrates. The measured results were based on a single-pixel micro-LED VLC system, which worked at the highest system modulation bandwidth without any equalization [38,39]. Then, using the QD micro-LED device, Z. Wei et al. built a work of the high-speed UOWC system which showed a record results of modulation bandwidth, data rate and BER performance of the UOWC system. Fig. 4(a) and Fig. 4(b) are the schematic and image of the proposed micro-LED, and the modulation bandwidth and BER performance are also presented in Fig. 4(c) and (d), respectively [40]. The product of modulation bandwidth with communication distance was the highest among all existing UOWC systems based on a single LED. This work demonstrated a 2-Gbps NRZ-OOK UOWC link with a 2.03 ​× ​10⁻³ BER at a distance of 3 ​m combining 1 ​m of free space and 2 ​m of underwater. In addition, by deploying micro-LED on the ceiling and integrating VCSEL into consumer devices, a full-duplex OWC system was built, which was faster and supported more mobilities compared with other full-duplex construction methods [41]. Besides LEDs grown on c-plane, LEDs based on semi-polar or non-polar substrates can also exhibit good E-O bandwidth characteristics. Devices with E-O bandwidths exceeding 1 ​GHz and their constructed VLC systems have been reported successively [[42], [43], [44], [45]]. By using semi-polar green micro-LED and micro-LED array, Y.-H. Chang et al. respectively demonstrated a 4.343-Gbps green-light and 2.473-Gbps white-light VLC systems, which has a measured system modulation bandwidth around 1 ​GHz [43,44]. However, wafers based on semi-polar or non-polar substrates are limited in size and fabrication cost, which further limits large-scale growth to the VLC market. L. Wang et al. designed and fabricated blue wetting-layer micro-LED and green QD micro-LED on c-plane GaN substrate, both of which have broken the bandwidth bottleneck [46,47]. This part of the work continues to be rapidly refreshed, and Table 2 shows a comparison of the performance of selected typical micro-LED-based high-speed OWC systems recently.

VCSEL has advantages of low manufacturing cost, low-power consumption, high-pumping efficiency, high E-O bandwidth and easy integration which results in a wide application in optical communication and data center optical interconnection, especially in the parallel transmission of local area networks (LANs) and very short reach (VSR) system as a mainstream transmitter [[48], [49], [50]]. M. Yoshikawa et al. and T. Tsujimura et al. have adopted single-pixel VCSEL or VCSEL array as the transmitter in OWC, but the performance of the system was limited by the weak optical power of the sources [51,52]. I.-C. Lu et al. used a red-light VCSEL to set up a free-space VLC system with 1-GHz modulation bandwidth at 11.1 Gbps using QAM-OFDM over 1.2-m transmission distance [53]. Z. Wang et al. presented an 8.23-Gbps data rate OWC system based on a single-pixel 850-nm VCSEL using QAM-OFDM [54]. Additionally, a 940-nm VCSEL array has recently been applied in the three-dimension (3D) sensing such as structured light or optical time of flight (ToF) for consumer electronic devices which has greatly expanded the market size [55]. The VCSEL array, especially at 940 ​nm, integrated in consumer electronic devices as biometric identification or distance detector loses the communication function, while current consumer electronic equipment is not closely integrated with the OWC system. Therefore, a flexible integrated OWC transceiver module is needed which can be well compatible with existing electronic equipment. If the integrated VCSEL array can be used for communication while maintaining its sensing function, it may be a solution to the relatively discrete problem of current OWC systems. Moreover, sensing and communication integration is also conducive to the realization of the optical Internet of Things (OIoT) in the future which should be a mainstream of the next-generation OWC systems in the B5G/6G mobile network. Z. Wei et al. firstly proposed this concept and demonstrated an 850-nm VCSEL-based OWC system over a 3.1-m link [56]. For the safety of human eyes, the high-power wide-diffuse-angle 940-nm VCSEL array is a better choice [57]. Z. Wei et al. improved the traditional Monte-Carlo algorithm to make it suitable for indoor OWC systems based on the NIR VCSEL array. In a typical indoor scenario, diffuse OWC is more common than point-to-point OWC. It was the first work to combine Monte-Carlo numerical simulation with experiments to prove the feasibility of a high-speed, wide-coverage diffuse reflection OWC system based on NIR VCSEL array in Gigabit Ethernet access. The correctness of the Monte-Carlo model was fed back by the intensity and distribution of received light power. All three systems using various materials with ∼1 ​GHz bandwidth could achieve rates of around 2 Gbps and ensure wide coverage over 1-m communication distance [58]. Y. Chen et al. adopted QPSK OFDM and convolutional code to achieve a two-user OWC system [59], and then Z. Cao et al. increased the data rate by using bit-loading and power-loading QAM-OFDM over a >10-m free-space link [60]. L. Zhang et al. proposed a high-speed multi-users 940-nm VCSEL array-based OWC system using NOMA to support two users, simultaneously [61]. Then, L. Zhang et al. experimentally proved a two-user high-speed QAM orthogonal frequency division multiple access (OFDMA) OWC system based on 940-nm VCSEL. Over a 2-m link, the measured −3 dB bandwidth was ∼1.13 ​GHz for both independent links of two users. For two users with different bit error rates (BER) and different channel conditions respectively, the maximum total rate recorded reached 2.7 Gbps. The demonstration of the modified system described potential solutions for high-speed OWC or direct device-to-device (D2D) connection between devices [62].

The uplink scheme of indoor VLC has been a difficult problem. Until now, there is still no flexible, mobile and rate-matching solution for VLC downlink based on LED [9,41]. Generally, these systems use an LED fixed to the indoor ceiling as an antenna for signal transmission, and broadcast data to the receiving terminal equipped with a photoelectric converter. However, in the previously proposed solutions, most only solved the problem of the downlink, and there is no feasible solution for the uplink. And a complete communication system should not be a one-way transmission link, but must have a cooperative working uplink. As one of the important challenges of VLC, technologies such as RF, visible light, or NIR light have been pointed out as data carriers for the uplink, but these methods have their inherent shortcomings. The indoor RF technology represented by wireless fidelity (Wi-Fi) is one of the most mature technical solutions for indoor wireless communication at present which can be used as a solution for the uplink. RF-OWC convergence systems have been widely discussed and analyzed but most works stay in theory. RF can provide hundreds of megabits per second of uplink data, cooperating with VLC downlinks, or further deeply integrating with VLC to form a heterogeneous network [63,64]. However, the adoption of the RF uplink scheme introduces EMI, which directly leads to the inability to form an all-optical full-duplex communication system in the room. It is difficult to reflect the advantages that the VLC system does not need to occupy a new spectrum and has good confidentiality, but the solution also has some inevitable shortcomings. If the LED installed in the mobile terminal is in a constantly lit state, visible light becomes a visual interference, which greatly affects the user experience. Therefore, this scheme can only be used in some specific scenarios. In the scheme of optical wave uplink, visible light uplink is a widely mentioned scheme, which can achieve the effect consistent with the indoor downlink [[65], [66], [67]]. Compared to the LED-based visible light uplink scheme, another widely mentioned light-wave uplink solution is the 780–950 ​nm NIR light uplink scheme. For example [68], shows a full-duplex discrete multi-tone (DMT) modulation all-optical communication system in which visible light LEDs and 850-nm diodes constitute a bidirectional 400-Mbps link [69]. Although the NIR light in this solution has no visual interference, it is still close to the visible light band, which is sensitive to human eyes. Therefore, if a NIR LED is used as the emission source, the transmission power must be strictly limited, otherwise, it will be harmful to the user. This directly leads to the limited coverage and transmission rate of the NIR uplink scheme. In addition, the uplink scheme based on 1550-nm light source integrated optical fiber and other accessories has also been proposed [70]. To build an indoor full-duplex communication system that matches the VLC downlink data rate, the uplink needs to be wireless, mobile, high-speed, and integrated. Therefore, the NIR VCSEL array will be a strong candidate for building indoor communication networks in terms of device development and integration. Fig. 5(a) shows a full-duplex OWC system based on the VCSEL array integrated into the mobile terminal with communication performances of downlink and uplink, respectively [41]. The downlink using bit-loading OFDM is presented in Fig. 5(b). Fig. 5(c) and (d) show the BER performances of downlink and uplink, respectively. Table 3 also gives a comparison of current uplink solutions including RF, visible light and NIR light, in which the uplink solution based on the VCSEL array has achieved a clear advantage.

As mentioned above, novel light sources such as micro-LED, VCSEL, and superluminescent diode (SLD) are experimentally verified in VLC [71], but there is still a dearth of matching receiver counterparts considering the precious cost efficiency. The free-space photodetector, as another key component in OWC, has encountered bottlenecks in photoelectric linearity, photosensitive area, optical-electrical (O-E) bandwidth, frequency and time response, detection efficiency and sensitivity. Most of the conventional photodetector-based receivers used in previous OWC are made of silicon (Si) or indium gallium arsenic (InGaAs) and other III-V compound semiconductors, corresponding to the absorption of visible and NIR spectra, respectively [72,73]. For the PD structure, positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APD) have been adopted since the superiority for dynamic and linearity characteristics, with >GHz O-E bandwidth and large-range output amplitude. The bottleneck problems are also different because of different detection materials and structures in UV, visible, and NIR bands. For example, InGaAsP and InGaAs MQW-based PDs have shown more than 20-Gbps data rate in the NIR region but limited by the light collection and coupling on the active region [74,75], but such PDs lapse when it comes to UV and visible color region because of the bandgap limitation. The <2 ​GHz O-E bandwidth of Si-based PD is limited by the front-end junction capacitance and the size of the active region. Si-based PIN has intricate difficulty in alignment, interference immunity and sensitivity due to its limited active size and low output of photocurrent, while APD has a larger noise index especially in a harsh environment in space or salty seawater but with lower O-E bandwidth, higher cost on device, and driver [76]. Photomultipliers such as single-photon avalanche diode (SPAD) and multi-pixel photon counter (MPPC) modules are then applied in the OWC system to enhance the sensitivity and complex channel tolerance [77,78]. Those devices with expensive costs not only impose complexity to the setup, but face limited O-E bandwidth (<100 ​MHz), which is mostly applied in long-range satellite and underwater OWC. On the other hand, III-nitride-based PD can be tailored for specific UV or visible regions to enhance received optical power and signal-to-noise ratio (SNR), which is a fundamental condition for high order MPSK or m-QAM modulation. Previous III-nitride-based PDs researches including metal-semiconductor-metal (MSM) [[79], [80], [81], [82], [83], [84]], PIN [[85], [86], [87], [88]], and multiple quantum-wells (MQW) [[89], [90], [91], [92], [93]] structures have mainly focused on the typical characteristics like photoelectric linearity and sensitivity performance, etc. In 2018, K. Ho et al. presented the first demonstration of InGaN MQW based micro-photodetectors (micro-PD, μPD) used as the optical receiver of near-UV to the violet region to achieve a 3.2 Gbps VLC link via 16-QAM OFDM [94]. Later on, C. H. Kang et al. used semipolar InGaN/GaN μPD in VLC link to further enhance the bandwidth up to 347 ​MHz with a data rate of 1.55 Gbps [95]. There are also some other components and structure-based PD used in VLC. The booming organic photodetectors (OPD) research brings new opportunities to OWC. Because of great compatibility with flexible substrates like PDMS and PET, OPD can theoretically be integrated with any shape, which satisfies the requirement of IoT. C. F. Hernandez et al. proposed optimized OPD surpassing Si-PD in most metrics within the visible spectral range. The work showed the great potential of OPD as a substrate of Si-PD while the −3 dB frequency response less than MHz was limited for daily VLC application [96]. L. Salamandra et al. made an OPD with a bandwidth ∼1 ​MHz, which was possible for high-speed VLC [97]. Meanwhile, R. Deng et al. used a solution-processed method to fabricate high-performance photodetectors without thermal and UVO treatments, and the O-E bandwidth of the OPD reached 2.8 ​MHz [98]. Thus, OPD can bring potential choices for low cost, flexible, and large-area electronics in VLC-based IoT.

One of the key features distinguishing VLC from other free-space technologies is that illumination can be satisfied during communication [99]. However, at present, the vast majority of OWC systems maintaining high communication performances are based on monochromatic light sources. Generally, conventional illumination with LDs or LEDs usually involves at least three primary colors mixing of RGB. In recent years, with the development of photo-luminescent materials, blue or violet LEDs combined with yellow or RGB phosphors respectively have become mainstream solutions for indoor illumination [100]. Many methods are studied to improve the bandwidths and data rates to remove the influence of phosphors inside white LEDs, such as blue-filtering, pre-equalization, and post-equalization [[101], [102], [103]]. C. Lee et al. achieved a 2-Gbps white-lighting transmission using blue GaN LD combining with Y³⁻_xAl₅O₁₂:xCe³⁺ (YAG: Ce) phosphors without filtering, expanding the white-lighting potential from LEDs to LDs [104]. However, conventional phosphors, typically yttrium aluminum garnet phosphor, usually have long relaxation time, long spontaneous carrier lifetimes of microseconds, and large RC parasitic, which hinders the system bandwidth (3–12 ​MHz) for high-speed VLC transmission [105]. Soon after, many fluorescent organic semiconductors were proposed and experimentally demonstrated in VLC systems, such as BODIPY, MEH-PPV, BBEHP-PPV, and so on [[106], [107], [108]]. Due to high photoluminescence quantum yield (PLQY) and direct radiative recombination compared to phosphors, the generation of that organic fluorescence always happens in a time range of 10⁻⁷-10⁻⁹ ​s, which correlates to a theoretical −3 dB frequency response up to tens or hundreds of MHz scale (usually ​∼ ​40–200 ​MHz). Y. Zhang et al. applied aggregation-induced emission illuminates as color converters for VLC to realize a high data rate of 493 Mbps and prove the inversely proportional relationship between bandwidth and effective lifetime [109]. Meanwhile, the quantum dots (QDs) and nanocrystal have also been tried for enhancing the capacity of white-lighting VLC based on high-bandwidth LEDs and LDs. For LEDs, many colors-converted white LEDs have been proposed, including CdSe/ZnS QDs, perovskite QDs, and AgInS₂/ZnS QDs, etc. [32,110,111], with higher-bandwidth of hundreds of MHz compared to conventional phosphor-converted white LEDs without filters and equalization. H. Cao et al. used a GHz bandwidth blue LED chip combined with CdSe/ZnS semiconductor QDs to reach 637.6 ​MHz white LED with maximum optical power up to 7.2 ​mW [112]. Moreover, K. Wang et al. and R. Wang et al. cooperated to improve the modulation bandwidth of a QD-LED at 74.19% by using CdSe/ZnS core/shell QDs, and a relationship between the illumination performance and overall bandwidth was derived and experimentally verified [113,114]. Although previous color-converted white LEDs have been reported to support Gbps transmission, the optical power at GHz bandwidth still remains several milliwatts, which is hard to satisfy the illumination requirement of VLC. On the other hand, blue or violet LDs are alternative pump sources to provide large optical power white light. I. Dursun and C. Shen et al. collaborated on a 2 Gbps high-speed VLC system using solution-processed CsPbBr₃ perovskite nanocrystals (NCs) and 450-nm blue LDs, with a modulation bandwidth of 491 ​MHz [115]. In addition, G. Lin et al. combined violet LDs with CdSe/ZnS core-shell QDs and BEHP-PPV ​+ ​MEH-PPV polymer blend respectively for high-speed white-light VLC, achieving qualified warm white lights with high color rendering index CRI (˃ 60) and suitable correlated color temperature (CCT) ranging from 3000 ​K to 9000 ​K [116,117]. Besides, the modulation algorithm can be introduced into white-light system to compensate the limited bandwidth. A single channel of 6.915-Gbit/s white-light VLC system has been reported by using optimized bit-loading algorithm onto the direct-current optical OFDM signal in a free-space propagation distance of 1.5 ​m [118]. Despite high-power white light and high-speed data link reached, LDs-based white lights are still confronted with aggregation-caused quenching and precipitation caused by concentrated thermal damage of unstable QDs in long term. Recently, as shown in the following Fig. 6(a), Z. Wang et al. realized a white-light VLC system with a >GHz modulation bandwidth and a data rate of 1.7 Gbps based on the combination of single-pixel micro-LED and perovskite nanocrystal-polymethyl methacrylate (PNC-PMMA) films [119]. Fig. 6(b) and (c) show the modulation bandwidths of different combinations and the overall NRZ-OOK communication performance with and without PNC-PMMA films. Table 4 compares different color-converter white-light OWC systems based on various LEDs or LDs.