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Visible Light Communication: A System Perspective—Overview and Challenges

Saeed ur rehman.

1 Department of Electrical and Electronic Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [email protected] (S.U.); [email protected] (P.H.J.C.)

Shakir Ullah

Peter han joo chong, sira yongchareon.

2 Department of Information Technology and Software Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [email protected]

Dan Komosny

3 Department of Telecommunications, Brno University of Technology, Technicka 12, 601 90 Brno, Czech Republic; zc.rbtuv.ceef@ynsomok

Visible light communication (VLC) is a new paradigm that could revolutionise the future of wireless communication. In VLC, information is transmitted through modulating the visible light spectrum (400–700 nm) that is used for illumination. Analytical and experimental work has shown the potential of VLC to provide high-speed data communication with the added advantage of improved energy efficiency and communication security/privacy. VLC is still in the early phase of research. There are fewer review articles published on this topic mostly addressing the physical layer research. Unlike other reviews, this article gives a system prespective of VLC along with the survey on existing literature and potential challenges toward the implementation and integration of VLC.

1. Introduction

In the 1980s, the development of high-efficiency red, orange and yellow light emitting diodes (LEDs) have fueled the idea of replacing the solid-state lighting for illumination purpose. It was until 1996 when the first white LED was commercially introduced in the market for sale [ 1 ]. LED lights are highly powered efficient, low carbon emissions, free from mercury, durable and produce good quality illumination. LED lights have 75% less power consumption and last 25% longer than traditional incandescent lamps [ 2 ]. With the decreasing prices and low power consumption of LED’s, it is estimated that the market share of LED lighting would increase to 69% in 2020 [ 1 ].

The exponential growth of data in the last two decades has raised concerns over the electricity consumption of information communication technology (ICT) infrastructure. It was estimated that ICT infrastructure accounted for 4.6% of worldwide electricity consumption in 2012 and projected to increase in the future despite emphasising on the introduction of power efficient technologies [ 3 , 4 ]. By 2030, the contribution of the ICT in greenhouse release would increase up to 23%, and at the worst, it can go up to 50% [ 5 ]. The future of the internet of everything (IoE) connecting people, processes, things, data and everything would require internet connectivity at all times. IoE would further increase the deployment of ICT infrastructure, thus increasing the power consumption. Apart from providing illumination at low-cost, LED lights have been used in several other applications, e.g., indoor farming and plantation [ 6 , 7 ], medical applications [ 8 , 9 ]. The easy availability of LED lights at home, offices and public spaces make it an affordable candidate to deal with the radio frequency (RF) spectrum scarcity as well as providing an energy efficient communication system. It is envisioned that existing lighting infrastructure should provide illumination as well as data connectivity.

Visible light communication (VLC) is a new paradigm that could revolutionise the future of wireless communication. In VLC, information is transmitted by modulating the visible light spectrum (400–700 nm) that is used for illumination. The information signal is superimposed on the LED light without introducing any flickering to the end user. Thus, it would be “green” as compared to providing two separate sources for illumination and communication network connectivity. On the other hand, the exhaustion of low-frequency bands to cope with the exponential growth for the highspeed wireless access is another reason for exploring new technologies. The visible light spectrum is unlicensed and hardware readily available, which can be used for data transmission. Furthermore, the exponential improvement in the high power light emitting diodes is an enabler for high data rate VLC Network. It has the potential to provide high-speed data communication with improved energy efficiency along with security/privacy. Standardisation efforts such as visible light communications association (VLCA) standards and IEEE 802.15.7 shows that VLC would augment existing wireless networks in coming years. VLC can have applications in indoor wireless communication [ 10 ], intelligent transport system [ 11 , 12 ], smart cities [ 13 ], localisation in warehouses/robotics [ 14 , 15 , 16 ], human sensing [ 17 ], safe and hazard-free data access in hospitals [ 18 ], toys and theme parks [ 19 ], indoor point to point (PPP) communication and vehicular communication [ 12 ].

VLC in its basic form like any other communication system in the downlink consists of a LED as a transmitter, a free space optical communication channel and a photodetector or an image sensor as a receiver. The uplink could be a WiFi transmitter or an IR transmitter or LED based VLC transmitter. Much of the work is focused on the downlink transmission to increase the data rate and improve VLC performance under different environmental conditions such as shadowing, non-LOS scenario and mobility. However, the uplink is equally essential for seamless integration of VLC in the existing ICT infrastructure. Unlike other review articles, this paper provides an overview of the VLC from the uplink and system perspective. This review article critically analyses the existing solutions of the VLC network from the system as well as uplink perspective. This paper has following contributions

• We have critically analysed the existing literature of the VLC regarding the uplink from the user device to VLC access point (VAP).
• We have discussed the open challenges associated with the use of existing RF spectrum for uplink connectivity.

The rest of the paper is organised as follow: Section 2 discusses the history and standardisation efforts. Section 3 discusses applications of VLC. Section 4 gives an overview of the different technologies used in the uplink and discusses its limitation. Section 5 summarises open research challenges toward the integration of VLC in the existing systems. Section 6 concludes the paper.

2. Visible Light Communication System

In the early 1990’s, mobile phones were mainly used for voice conversation or text messaging. However, the introduction of the iPhone in 2007 has started a new era in wireless communication [ 20 ]. Nowadays smartphones are equipped with all kinds of sensors and applications to provide health monitoring, video chatting, online streaming along with bank transactions and using cloud services. A larger bandwidth should be allocated for wireless communication in order to provide seamless connectivity and higher data rates. However, frequency spectrum below 5 GHz is well utilised, which leave no room to relocate spectrum for mobile communication. This led scientists to seek new wireless technologies that can fulfil the needs of the higher data rate at a low cost. One such candidate is the use of the visible light spectrum. It has the advantage of its availability (LED lights), link level security, higher bandwidth, and frequency reuse. Furthermore, the demand for data is higher in an indoor environments because 80% of the time people stay in an indoor environments [ 21 ]. Thus using the VLC in indoor applications for high data rate applications would free up the precious RF spectrum for other future applications such as autonomous cars and smart cities.

The history of VLC goes back to Romans when polish metallic plates were used to reflect sunlight and convey signals over a long distance. In 1794, Claude Chappe developed a semaphore system consisting of a series of towers equipped with mounted arms to transmit information. In the late 19th and early 20th Century, heliograph was used for long distance communication. In heliograph, the sunlight was reflected with a mirror to transmit Morse code. The British and Australian armies used it till 1960. Graham Bell is mostly known for his invention of the modern telephone, which uses electricity to transmit voice. However, Bell described the photo-phone as one of his important inventions [ 22 ]. Photophone uses voice to vibrate the mirror, which in turn is used to modulate the sunlight. It was the idea of Graham Bell, which led to fibre optic communication. The first commercial fibre optic communication system was deployed in 1975 and was capable of operating at a bit rate of 45 Mbit/s. VLC is a form of optical communication that instead of using a guided media (fibre optic) operates in the open air in close proximity of two to three meters. In 2003, the term VLC was coined first by Nakagawa Laboratory at Keio University, Japan [ 23 ]. The Nakagawa Laboratory demonstrates the first VLC system at Keio University in 2000. Light emitting diodes (LEDs) are used for transmitting the data.

Liang et al. have proposed a VLC system consisting of red-green-blue (RGB) LEDs as a transmitter and complimentary metal oxide semiconductor (CMOS) image sensor as a receiver [ 24 ]. The authors observe that RGB LEDs can increase the VLC transmission data rate with wavelength division multiplexing (WDM). For such systems, colour signals are isolated using colour filter array which relies on colour-sensitive sensing elements. However, the wide optical bandwidth of colour filters can increase spectral overlap between channels and can cause inter-channel interference (ICI). In order to solve this issue, the authors have applied CMOS image sensors with multiple input–multiple output (MIMO) to mitigate the ICI and demodulate the rolling shutter pattern.

In [ 25 , 26 ], Chow et al. have proposed a VLC based communication system that consists of an LED panel working as a transmitters and CMOS based camera as a receiver. In [ 27 ], Chow et al. have further improved their work using RGB LEDs to provide non-flickering for long-distance communication. The rolling shutter effect (RSE), and under-sampled modulation (USM) is analysed for VLC under different international standard organisation (ISO) values and distance. Results show that USM has better error performance over long distance compared to RSE.

Different research groups have demonstrated that a single gallium nitride (GaN)-based LED can achieve a data rate of up to 4 Gb/s [ 28 , 29 ]. A high data rate of up to 15 Gb/s and 13.5 G b/s using a GaN blue laser diode was achieved for a distance of 15 cm and 197 cm, respectively [ 30 ].

Zafar et al. have explored laser diodes (LDs) as an alternative to LEDs for visible light communication [ 31 ]. The use of micro LEDs can significantly improve modulation bandwidth and can provide data rates of up to 3 Gbps. However, at a high data rate, current LEDs might suffer from efficiency droop which can occur due to electron overflow. This might increase the cost of LEDs as LED will not be operating at optimum efficiency in high data rate scenarios. In contrast to LEDs, LD can offer higher direct modulation bandwidth and can provide good optical-to-electrical conversion efficiency without drooping. Therefore it is expected to provide high performance due to the characteristics of high optical power and light beam conversion. This could be used as an alternative to LEDs for illumination and data transmission. While LDs have several advantages over LEDs, LDs have many challenges of their own such as speckles, power limitations and cost that needs to be addressed. Nevertheless, VLC is still in its development phase, various features of LDs must be considered in future VLC scenarios.

Watson et al. have developed a VLC system based on GaN laser diodes aimed at unmanned underwater vehicles (UUV) [ 32 ]. The low loss of blue spectrum originating from laser makes LDs good option for underwater communications compared to traditionally used acoustic systems which are slow and proven to be susceptible to interception. Laser-based VLC systems which traditionally use non-return-to-zero on-off keying (NRZ-OOK) could achieve data rates of up to 4 Gibts/s. The authors of this study have used direct modulated GaN LD which emit light at 450 nm, and a data rate of 4.7 Gbits/s is achieved. Additionally, the underwater tracking system is developed, that, along with tracking, can provide data transmission as well.

2.1. Standarisation Efforts

In 2003, a VLC consortium (VLCC) was formed to speed up the research and commercialisation of VLC. The VLCC proposed two standards by 2007 [ 23 ]; JEITA CP-1221 (VLC system) and JEITA CP-1222 (VL ID system) that was later accepted by Japan electronics and information technology industries association (JEITA). CP-1223 was introduced as a VL beacon system in 2013. Both these standards have meagre data rates of up to 4.8 Kbps.

2.1.1. IEEE 802.15.7

Due to increasing interest of researchers in VLC, the VLCC has introduced the first IEEE 802.15.7 standard in 2009 [ 33 ]. The standard defines the physical and media access control (MAC) layer parameters for short-range optical wireless communication. It covers topics such as network topologies, modulation domain spectrum, MAC protocol specification, collision avoidance, addressing, performance, quality indicators, dimming support, coloured status indication, and stabilisation. The standard proposes one-off keying (OOK), color shift keying (CSK) and variable pulse position modulation (VPPM) techniques for indoor and outdoor communication [ 34 ]. The highest achievable data rate for indoor communication can go up 96 Mb/s. However employing the multiple input–multiple output (MIMO) system and other modulations scheme, the data rates can be improved significantly.

2.1.2. OpenVLC

Wang et al. have proposed and demonstrated an OpenVLC system, rapid prototyping, flexible and open source VLC system for the research community [ 35 ]. It provides an interface between VLC front-end with the embedded Linux platform. The hardware consists of beaglebone black board (BBB), and a transceiver front end consisting of a single LED which can serve both as the transmitter and receiver. A single LED in dual mode is used to reduce design complexity. A software-defined switch operates the transceiver. In transmitter mode (Tx) the LED is connected to the power amplifier, and in the receive mode (Rx) it is connected to a low noise amplifier. OpenVLC has implemented both the time-division duplex and IEEE 802.15.7 protocol that includes software programmability, carrier sensing, TCP/IP interoperability, encoding and decoding, preamble detection and signal sampling. The upgraded version of OpenVLC1.3 improves the data rate from UDP throughput of 100 kbps (in OpenVLC1.2) to 400 kbps without any modifications to the existing hardware [ 36 ]. It also reduces physical footprint (as it runs on off the shelf microcontroller) along with memory efficient frame detection technique. Techniques were introduced to reduce noise due to high-frequency components along with the synchronisation issues of the frame receptions. OpenVLC facilitates the research of VLC both for academia and industry.

In [ 37 ] authors have challenged the assumption that light sources are always static and users can expect LOS with many luminaries, and many scenarios together can provide deterministic localisation. The authors observe that these assumptions may not hold for many scenarios. For example, when the nodes consist of a single light and are mobile (e.g., bikes or swarms of robots) the localisation become non-deterministic. A framework is proposed to compute the relative position of objects when nodes are moving freely in all directions. The proposed framework has been implemented with OpenVLC platform. A good error rate of below 5cm is achieved through simulations.

In [ 38 ] an inexpensive receiver has been designed to cope with the issues arising due to optical noise and the mobility of users. This receiver is based on the OpenVLC platform; photodetectors are utilised to sense optical noise arising from the sun and other sources. The physical and data link layers are modified to adjust the receiver to the detected noise. Experiments were conducted for two nodes trying to maintain a link under different paths (e.g., straight and curved) and illuminations (e.g., night and day). Results validate that the noise sensing with photodetector outperforms LED only design in optical noise and mobility.

3. Applications

The concept of the IOE expands the network connectivity to the intelligent connection of people, data, processes, things, machine and everything. IoE would require internet connectivity for billions and trillions of sensors to provide ubiquitous, seamless services to people, machines, process, and things. All these applications would have a different set of requirements such as high data rate in Giga b/s, reliability, availability and security. The ease of availability, low cost and high data rates of VLC could make it a relevant wireless communication technology that would cater to all kind of future applications. Some of the potential applications of VLC are discussed as follow.

3.1. Intelligent Transport System (ITS)

Nearly 1.2 million people die in a traffic-related incident every year, and an estimated 50 million get injured [ 39 ]. Researchers have shown that most of the incidents are due to the slow response and inability of automobile drivers to take the right action at the right time [ 40 ]. In ITS, vehicle to vehicle (V2V) and infrastructure to vehicle (I2V) communication ensures the safety of people, traffic flow and comfort of drivers as shown in Figure 1 . ITS relies on reliable, robust and secure communication among vehicle and infrastructure (traffic lights, billboards). VLC is proposed for ITS communication to complement or replace the existing crowded RF-based communication [ 11 , 12 ]. All vehicles are equipped with head and tail lights that can be used for transmitting information. Traffic lights or billboards can also be used for sharing useful information about the road, traffic and weather conditions. These lighting sources can also be used for providing data connectivity to users and IoE entitites. Căilean et al. have discussed challenges facing VLC in the context of vehicular communication (VC) [ 41 ]. Increasing communication range, enhancing mobility and data rates are the main requirements for VC. The accomplishment of these objectives depends on the ability of communication channels to be resistant to parasitic light (PL). The outdoor channels are exposed to different kinds of PLs. It is observed that VLCs distance measuring and localisation capabilities could be beneficial in VC applications. Further, it is suggested that the development of heterogeneous systems consisting of VLC and dedicated short-range communication (DSRC) (or any other RF-based scheme) could lead to a reliable system for VCs as each of these technologies can make up for each other deficiencies. In this regard, a survey of VLC concerning 5 GHz DSRC in a hybrid arrangement is conducted. It is concluded that VLC systems aimed at VCs can be improved by exploring and integrating new technologies which include but are not limited to software-defined architecture, resource sharing, reconfigurable computing and integration of new materials. Ucar et al. have developed a hybrid 802.11p and VLC secure autonomous platoon system [ 42 ]. The autonomous platoon uses RF based 802.11p and consists of a platoon leader that controls other members to adjust the speeds stably. A 802.11p and VLC hybrid vehicular platoon communication protocol, named as SP-VLC is proposed. This protocol is aimed at addressing security vulnerabilities due to the exclusive use of RF communication. A simulation platform for the vehicle mobility and vehicle platoon managementis is developed. SP-VLC is evaluated under different security vulnerability scenarios. The simulation results validated the observations made in [ 41 ] that RF-VLC heterogeneous systems can provide many advantages over the RF only system.

An intelligent transport system using visible light communication (VLC).

Kunar et al. have proposed to integrate LED-based road side units (RSU) into the existing ITS infrastructure [ 43 ]. The RSUs are used to broadcast information in infrastructure to vehicle (I2V) mode using VLC concepts. A robust modulation technique based on a direct sequence spread spectrum (DSSS) and sequence inverse keying (SIK) is employed to minimise the effect of noise sources. The amount of data received by a car passing by RSU is considered as a performance metric. The experimental setup involves a movable receiver and a stationary emitter both separated by a distance of 1.5 m. Results show that packet error rate (PER) degrades linearly with distance during daylight while at night the packet error varies due to the local nature of artificial light.

In [ 44 ] performance evaluation of VLC based V2V systems has been conducted. For performance evaluation, a typical V2V VLC scenario is considered with left and right headlamps emitting light. The reflected rays are considered to have both line of sight (LOS) and non-line of sight (LOS) components with the Lambertian profile. Results show that depending on the headlamp location a data rate of 50 Mbps can be achieved at a separation distance of 70m between vehicles.

In [ 45 ] the authors have considered VLC-based ITS for accident avoidance, particularly, when lorry fleet are moving through intersections. VLC has been used to send signals related to acceleration, deacceleration and braking to ITS infrastructure (e.g., RSUs) which can trigger appropriate signal. For example, to reduce the number of emergency brakes and lane change in a complex environment, a lorry fleet can send VLC signal to RSU which can set a green signal or express path.

Yamazato et al. have used VLC with imaging sensor-based receiver for automotive applications [ 46 ]. Two scenarios I2V and V2V are considered. The first scenario consists of a transmitter designed from LED arrays (assumed to be RSUs) while the receiver is considered to be high frame rate CMOS imaging camera. In the second scenarios, a special CMOS sensor have been developed which can receive high-speed optical signals. In the field trials, a data rate of 32 kb/s and 10 Mb/s is achieved for I2V and V2V, respectively.

3.2. Smart Cities and Smart Homes

Smart cities are envisioned to provide seamless connectivity between people, government, infrastructure, economy and environment [ 13 , 47 ]. Most of the functional entities of a smart city are already available around us. However, the reliable, sustainable and high data rate wireless connectivity is the bottleneck to connect all the enablers. The already available infrastructure of lightning (street lights, parking lights, billboards) can be utilised to provide high-speed, low energy and sustainable network connectivity for some applications (e.g., utility services) in smart cities whereas freeing up the precious RF spectrum for other mobile applications. Street lights or other lighting sources could be used as a hotspot to provide extremely high data rates to the user.

A three-layer VLC based communication architecture is proposed to integrate different technologies in smart cities’ applications seamlessly [ 48 ]. Layer one uses VLC to allow user access and a sense of events. Layer two provides communication between different LEDs and sub gateways. The last layer provides communication between different sub gateways and the service gateway using optical communication. Based on this architecture several applications (such as intelligent communication, event surveillance, and object tracking) have been demonstrated. In [ 49 ], authors have investigated optimisation algorithms to provide consistent received optical power and signal to noise ratio (SNR) across a single receiver plane in smart homes.

In [ 50 ] a hardware design and location identification protocol are demonstrated for a VLC-based indoor positioning system for smart supermarkets. The hardware design considers the illumination flickering, brightness, signal synchronisation and noise in the indoor environment. Various practical user conditions have been used to optimise the hardware and software implementation in terms of location accuracy and bit error performance. It is concluded from the experimental work that location ID detectability is 95% for a distance of 0.7 s and under the illumination in diameter of 2 m.

3.3. Entertainment and Advertisement Industry

VLC requires line of sight (LoS) or semi line of sight. This can be used for applications requiring access to localise information. Theme parks, toy or advertisements are such applications, where the aim is to produce products with reducing costs. For example, most of the toys are battery powered and operates in LoS, and typically require a wireless connection to control the toy [ 51 ]. Some researchers have explored battery-free VLC communication system. Xu et al. have developed a new VLC based communication, called passive VLC that uses the backscatter communication for battery-free IoT applications [ 52 ]. This system is based on RetroVLC which tackles the challenges of directional communication and low optical coupling (mostly less than 20% via solar panel) for wideband VLC. RetroVLC uses the concept of retroflecting which involves sending light back, almost, along with the incoming path, achieving a pointed communication. Miller codes are used to double bandwidth utilisation compared to Manchester coding. Trend-based modulation and code-assisted demodulation are proposed for reducing the modulation time. Two proof of concept systems are developed to validate their proposed scheme. A data rate of 1Kbps was achieved with the RetroVLC system.

A single LED light can be used for transmission and reception as well as providing the visualisation to entertain kids. This would save energy and cost, also not polluting the precious RF spectrum. Another use of VLC could be in the advertising industry, where large billboards made of LED’s are used for advertisement. Such large billboards with high light intensity and equipped with hundreds of LED ’s can be used for providing free outdoor network connectivity that can work as hotspots for users, machines or smart cities applications.

3.4. Hospitals

VLC operates in 430–770 THz, and due to nanometer wavelength, it cannot penetrate in objects, therefore making it ideal for applications where confidentiality of data is of utmost importance. The inherent security and safety feature of VLC provides an alternate of wireless communication that could be used to minimise health risks associated with radio frequency radiations. One such area is hospitals, where VLC can be utilised for monitoring patients, machine to machine communication, record keeping of patients and all other networking applications [ 53 ]. Newborn babies are fragile and vulnerable to even small RF radiations of the ISM band and cellular frequencies. Implementation of VLC in newborn babies wards would be a natural choice for monitoring and indoor connectivity to the rest of the internal and external networks. To replace the existing wireless communication in hospitals, a broadband powerline and VLC with orthogonal frequency division multiplexing (OFDM) is demonstrated [ 53 ]. Abdaoui et al. have proposed another hybrid system consisting of VLC and 60 GHz RF for e-health applications [ 54 ]. The rationale of VLC for these applications as it is easily available in hospitals and can reduce communication system cost. As VLC offers only downlink communication, therefore, 60 GHz RF to provide uplink communication is used. Apart from uplink, 60 GHz can be used for localisation as well. It is concluded that this hybrid system could be used efficiently to collect a wide range of information from the patients.

Chow et al. [ 25 ] have demonstrated secure data transmission that can have applications in a hospital environment. In [ 25 ] authors have proposed a CMOS camera-based VLC system which encrypts emitted light. The transmitter consists of an encryption module which receives an optical signal that is encrypted using a private key or another advanced encryption scheme and is then emitted. The CMOS-based mobile camera is used as a receiver. The rolling shutter feature of the CMOS camera is used to enhance data rates. An additional Otsu thresholding module is used to get effective BER. The proposed scheme gives good results in terms of BER and encryption.

A communication system inspired by barcoding technology is proposed [ 55 ]. Unlike VLC, this system considers that without explicitly modulating a light source, the disturbances in the natural (e.g., sun) can be used to convey information and thus it is named as a passive communication system. It involves the design of a communication channel in which information is captured from the mobile elements with wear patterns consisting of distinctive surfaces. A user can be reduced to a “tiny-box” equipped with a commercial off-the-shelf (COTS) photodiode. Massive deployment of these tiny boxes can infer significant pieces of information from the surrounding. Such systems are deployed in hospitals where emergency, treatment and housing trolleys can report their physical locations. A key aspect of this system is sustainability in terms of infrastructure (as it needs of minimum infrastructure changes), energy efficiency (as the system uses only photodetectors which are energy efficient compared to the camera used in similar scenarios) and cost (as photodetectors are inexpensive). The passive communication system is demonstrated in the parking area and reported encouraging results.

4. Hybrid VLC

A VLC network arrangement is shown in Figure 2 a. The downlink consists of a LED driver which drives the LED illuminator to work as a VLC transmitter and a power line modem to connect the VLC transmitter to the network. For simplification, this downlink VLC transmitter is called a VLC access point (VAP). The power line communication (PLC) technology is typically utilised to connect all the VAPs to the rest of the network both internally and externally. In the user devices, the downlink VLC transmission can be received either through a photodiode or an image sensor. The uplink from the user device could be through either (a) VLC transmitter, (b) a wireless fidelity (WiFi) link, (c) an infrared (IR) link, (d) a combination of any two. Figure 2 b shows another arrangement of the VLC network. An IR transmitter is used on the uplink instead of a WiFi. In this scenario, VAP consists of PLC modem, VLC transmitter, and an infrared receiver (IR). The user device consists of a photodiode to receive the downlink VLC transmission and an IR transmitter to communicate on the uplink. Both IR and VLC require line of sight for communication. Instead of an IR transmitter, we could use a LED transmitter for the uplink to make a bidirectional VLC link. However, as light would be continuously emitting in an upward direction, which is aesthetically not pleasing from a user perspective.

VLC downlink and infrared is used for uplink in VLC Hetnet. ( a ) VLC downlink and WiFi for uplink in VLC Hetnet. ( b ) VLC–VLC communication.

In any case, the VLC would occur through a heterogeneous network that would be using more than one networking standard, i.e., a combination of PLC-VLC, PLC-VLC-WiFi, PLC-VLC-IR. There are limitations of each existing technology to support VAP. For example, PLC is used to connect the VAP with the rest of the network. PLC can achieve a data rate of 1 Gbit/s [ 56 , 57 ]. Thus creating a bottleneck to use VLC for high data rates although theoretically, VLC can provide high-speed data up to 15 Gbit/s [ 30 ]. Some researchers have considered fibre optic as an alternative to PLC [ 58 ]. However, it will increase infrastructure cost.

Figure 3 shows a typical deployment of VLC network in a house. It consists of a heterogeneous network of PLC-VLC-WiFi. One WiFi AP is used to cover the entire house whereas VAP’s are distributed throughout the house and connected via PLC to the main gateway. This arrangement is cost effective and realistic as typically the links are asymmetric considering that uplink can have low data rates as compared to downlink transmission. VLC users at the boundary of WiFi would have high signal attenuation, which could deteriorate the overall throughput. A second arrangement is to deploy low power WiFi transmitters (WiFi Direct) in VAPs. However, it would increase the complexity of hardware and software in VAP. Furthermore, VAP’s are prone to interference as it cannot penetrate objects due to characteristics of its frequency. However, WiFi operates in 2.4 and 5 GHz range, and it can penetrate in walls. Therefore fine tuning WiFi transmitter to a very small area would be practically unachievable. This will increase the interference among wifi transmitters in close proximity. A third alternative is to deploy multiple WiFi to provide good coverage and throughput, but it would require excellent RF planning to mitigate interference issues. We will discuss this in detail later in the next subsection.

WiFi VLC communication network.

In most of the cases, photodetectors are employed as a receiver for the VLC. However CMOS sensor-based camera are also experimented with and demonstrated for reception of visible light and extracting information from it. Danakis et al. have developed a proof of concept system which shows that rolling shutter feature of CMOS sensors can be used to achieve higher data rates [ 59 ]. The overall system consists of an LED transmitter, a mobile camera-based receiver, and an Android application equipped with a decoder. The information is captured in a mobile phone as a light and dark band which can be decoded by mobile phones to retrieve information.

4.1. WiFi-VLC

WiFi (wireless fidelity) is a trademark of WiFi Alliance, based on the 802.11x family of Ethernet standard for wireless local area networks (WLANs). Devices can connect to the internet via a WLAN access points (APs). WiFi is primarily used for internet access in the indoor environment connecting via wire to the backbone network [ 60 ] as shown in Figure 3 . WiFi direct is another variant of WiFi, providing direct connectivity between user devices without any intermediate device [ 61 ].

The main application of VLC is to provide high-speed connectivity in an indoor environment. Light fidelity (LiFi) is one such indoor application of VLC, proposed by Herald Hass in 2011 [ 62 ]. LiFi promises a fully networked indoor system with bidirectional support for point-to-point (P2P) and multipoint-to-point communication (MP2P). It also provides the capability of multi-access points and the formation of wireless networks between them resulting in LiFi atto cells (comparable to femtocells). These atto cells provide seamless handover and promise to provide full user mobility. Different researchers have proposed different system models for LiFi [ 63 , 64 , 65 , 66 ]. One of the proposed models is to augment LiFi with WiFi, where the latter works as an overlay network over on the former. In another approach, a mixed WiFi and LiFi is used to provide an aggregated system providing aggregated channels for communication. Light radio (LiRa) is another application developed by augmenting VLC with legacy WiFi [ 64 ]. LiRa does not require VLC for the uplink transmission by the mobile client and instead employs a WiFi uplink. The system achieves VLC automatic repeat request (ARQ) feedback via WiFi without excessive delay with a newly introduced method, AP-spoofed multi-client ARQ (ASMA). This method provides the mechanism to minimise the impact of VLC control frames on legacy WiFi traffic while providing enough feedback for the visible light downlink.

Khreishah et al. [ 66 ] proposed a VLC-WiFi hybrid system for energy efficiency. An analytical framework for energy efficiency is proposed that is based on two requirements; (a) selection of the appropriate number of APs to provide the lighting, (b) to satisfy the user’s request for real-time communication. The results showed that the VLC-WiFi hybrid system is 75% more efficient than a standalone WiFi network. Kashef et al. carried out a similar investigation for energy efficiency and LOS availability of a VLC-RF hybrid system [ 67 ]. The system parameters that affect the energy footprint of the communication system are analysed. Results showed that energy efficiency improved at the cost of less LOS availability.

Shao et al. have proposed two variants of hybrid systems as proof of concept for the coexistence of WiFi and VLC [ 68 ]. In the first phase, a WiFi-VLC hybrid system is proposed where VLC is used for the downlink and WiFi for the uplink transmission. Results have shown that higher data rates can be achieved compared to the conventional WiFi systems. In order to take advantage of the higher bandwidth of VLC on the downlink and exploit the higher availability and range of WiFi, the hybrid systems are further enhanced by bundling together WiFi and VLC channels using bonding techniques based on the concept used in the Linux operating system. Practical experiments are performed in the crowded environments for comparing the hybrid system with WiFi only system. Robustness and throughput are used as performance metrics. Results have shown that hybrid systems are robust and provide higher throughput compared to WiFi systems.

In [ 69 ], a hybrid system has been developed which consists of optical femtocells overlaid over WiFi cells, named as WiFO. The WiFO exploits the high bandwidth of optical femtocells and high mobility of WiFi to provide high throughput along with seamless handover and good bit error rate (BER). This system involves, among other components, a custom network protocol with physical, link, network and transport layers, to support the developed system. While other layers are similar to traditional networking layers, the networking layer, however, is modified significantly to provide seamless mobility in addition to routing packets. The authors have performed experiments and have shown that the optical femtocells in the WiFO could achieve a data rate of up to 50 Mbps at a distance of 3 m.

In [ 70 ], a hybrid VLC and RF system is considered that exploits the longer range of RF to provide control functions and increase per user rate coverage of the VLC cells. It quantifies the minimum spectrum and power requirements which can achieve a certain per user outage performance. The hybrid system consists of central unit (CU) which assign users to either VLC or RF network based on the user’s channel conditions (users with the low data rate in the VLC network are migrated to RF network). The proposed system is evaluated for two scenarios, D1 and D2. In D1 power and spectrum are kept constant while in D2, CU dynamically adjusts power and spectrum according to the traffic demands. Results have shown that that D2 outperforms D1.

In [ 71 ] a hybrid VLC and RF system has been proposed which aims to support diverse quality of service (QoS) needs of users. The QoS needs are studied for limits on the buffer overflow and delay violation probabilities. Results show that RF performs better for arrival rate in some settings, while VLC has better delay performance.

In [ 72 ] a hybrid WiFi-VLC system has been implemented which consists of two duplex links. One is an aggregate of VLC and WiFi, and the other is WiFi duplex link. WiFi is used for the uplink transmission. The Linux bonding technique is used to achieve link layer aggregation using the Linux bonding driver to build a logical interface (LI) on the client. Throughput and round-trip time (RTT) of the hybrid systems are evaluated under different contention scenario and VLC operating distances. The proposed system was benchmarked against the conventional WiFi and a hybrid WiFi-VLC system (non-aggregate system with asymmetric VLC downlink and WiFi uplink). Experiments have shown that the throughput of the aggregate system outperforms the standalone WiFi and a conventional WiFi-VLC hybrid system. The RTT of the aggregate system shows considerable improvement over the benchmark system particularly when the WiFi is facing congestions.

Most of the existing work in VLC-WiFi hybrid system is based on a single VAP in a confined area. WiFi is used for uplink in close proximity similar to WiFi direct. As shown in Figure 3 , one WiFi AP can cover multiple VAP’s. However, WiFi signals attenuate over the distance, which would increase the errors in the uplink and thus feedbacking in the downlink to decrease the data rate. As the VAP is deployed at multiple locations to cover a large indoor area, then the collaboration between different VAP and one or more WiFi is not evaluated. This creates a bottleneck towards the scalability of VLC-WiFi network. Furthermore, there has been less reported work on the cost, fault tolerance, security and overall throughput of the systems.

4.2. IR-VLC

Most of the current research relies on IR or RF for uplink transmission [ 73 ]. Infrared (IR) indicates the light with a wavelength less than red light. It has a frequency range between 300 GHz and 430 THz. IR band is unlicensed and has the potential to provide high-speed connectivity. Since the frequency is above 300 GHz, it cannot penetrate objects, hence used mostly in the indoor LOS environment. According to Boucouvalas et al. [ 65 ], IR can achieve up to 1 Gbits/s of data rate over a distance of several meters. IR communication can take place in defused mode, also called scatter mode. In this type of communication transmitter and receiver do not need to be in the line of sight (LOS). However, for such communications devices need to be close to each other which can be an issue when used for developing indoor wireless networks. Researchers are now taking particular interests in using IR to complement VLC network in the uplink transmission.

Alresheedi et al. [ 74 ] have proposed fast adaptive beam steering IR (FABS-IR) system to enhance the received optical power signal, speed up the adaptation process and reduce the channel delay during high data rate operation. The proposed scheme employs IR for uplink transmission and VLC for the downlink transmission. FABS-IR was coupled with imaging receivers to improve the system performance. Results showed that employing the IR-VLC network increases the data rate up to 2.5 Gb/s for a mobile scenario in an indoor environment. Quintana et al. [ 75 ] have proposed a VLC-IR system for internet access to passengers on flights. The VLC-IR system can also be used to provide personalised entertainment using wireless media without interfering with airline radio systems. The proposed system claims to have minimal interference as every lamp serves as VAP dedicated to each seat. Experiments have shown significant minimisation in transmission error for a short distance (2 m), good downlink signal strength for receivers, and functional capability to work in the presence of other illumination sources. Further, the proposed system is tested for other onboard entertainment such as collaborative games and live video streaming.

Few researchers have investigated IR for uplink transmission in the VLC as compared to WiFi. WiFi is already available, and it has hardware and software support available, whereas integrating IR in experiments are difficult due to less support of hardware and software. IR has less data rate and a lower range of communication, which made it less attractive for the researcher to investigate. Furthermore, IR transceiver has to be integrated into the VAP and in the user device, which further increases the complexity of the VLC system. Nevertheless, researcher community has shown a high interest in improving the IR-VLC in an indoor communication system for specific applications such as in-flight communication system, point-to-point communication. IR communication could be one of the areas, which would complement VLC either in the uplink or during mobility.

4.3. PLC-VLC

For VLC to provide a fully functional indoor network, it requires connectivity among VAPs and external networks. The easy and straightforward solution is to connect VAP via separate cables. However, this will increase the cost of cabling, installation, labour and design. PLC utilises the already installed electrical power lines for broadband access [ 76 ]. PLC modems are typically plugged into power sockets to provide the interface between data and power network. For over a decade, PLC has been successfully used to transmit radio programs, home automation (remote control of lighting and appliances), home networking, internet access, utility company control switching and automated meter reading [ 77 , 78 ]. Thus like VLC, PLC has the advantage of having the readily available infrastructure.

Hu et al. [ 79 ] proposed PLiFi, a Hybrid WiFi-VLC that promises low-cost connectivity to the internet, interconnectivity between the VAPs and seamless integration of WiFi for the uplink transmission. The user mobility and change in device orientation is investigated. A smaller and distributed VAP along with a MAC protocol coordinated multi-point transmission (CoMP) is proposed to address the co-channel interference due to adjacent LEDs. In the experimental setup, PLC modems were used to synchronise VAPs.

Ndjiongue et al. [ 80 ] have developed a cascaded PLC and VLC channel model of hybrid PLC-VLC systems. The influence of PLC channel and VLC transmitters on the propagation of the signal is analysed. The available channel frequency response is used to determine the transfer function to characterise the channels over time. The cascaded channel path of PLC-VLC can have many sources of impairments including but not limited to attenuation and impulsive noise. The proposed model claims to analyse the cascaded channel attenuations accurately. However, noise is ignored in their analysis which can be a source of impairment in the PLC-VLC environment.

Yan et al. [ 81 ] have performed experiments with three-LED lamps to form a single frequency network (SFN) that showed the feasibility of VLC and PLC hybrid system for the indoor environment. Their system promises high-speed communication and greater signal coverage with less modification to the existing infrastructure. A similar experiment is performed by song et al. [ 82 ]. They have implemented a full duplex decode and forward (DF) hybrid PLC-VLC system with minimal protocols or medium modifications. A data rate of 5 Mbps was demonstrated with their experimental test bed, which can be extended theoretically up to 30 Mbps.

The research work as mentioned above provides some theoretical models as well as some practical implementations for setting up a PLC-VLC hybrid system. It provides an investigation into some essential components of hybrid systems such as channel modelling, noise removal, interference mitigation, and basic networking. However, hybrid systems are not evaluated for their benefits such as energy efficiency and scalability. The cost of electrical devices (e.g., PLC modems) is much higher compared to VAP, and the effect of adding these devices to the system have been ignored. According to Tonello et al. [ 57 ] data rates of up to 1 GB are possible by careful analysis of the PLC channel and appropriate selection of modulation schemes. For the current data requirements, 1 Gbps would suffice. However, VLC is capable of providing a data rate of up to 15 Gb/s [ 30 ], which is far higher than PLC. Thus, it would lead to a bottleneck. Future network application would require a backbone network that can support higher data rates. A viable solution might be to connect VAP through fibre optic, which will increase the system cost. Such VAP would potentially provide an ultra-high data rate for specific applications, where a user wants an ultra-high data rate.

In [ 83 ] optical fibre link has been used to connect central station (CS) to radio access points (RAP) for indoor applications. The system consists of an RF front-end which includes among other components, an antenna operating in 2.4 GHz, a power amplifier and a photodiode (as a receiver).

In [ 84 ] Gomez et al. have designed a fibre-wireless-fibre (FWF) link for indoor optical wireless communications. Through simulation data rates of 416 and 208 Gb/s can be achieved with a wide field of view of 40 and 60 degrees, respectively. Although these data rates are for the fibre optic communication to avoid the signal conversation from optic to electrical and vice versa. However, the same concept can be extended to the backbone access network of the VLC network to remove the bottle neck of PLC network.

4.4. LED-to-LED System

A typical VLC system uses LED as transmitter and photodetector as a receiver. Most of the research work has experimented with one-way VLC system in the downlink direction. To take full advantage of the large unregulated bandwidth of VLC, ideally, a bidirectional VLC system should be used for downlink and uplink communication. To build a two-way VLC-VLC system, an LED transmitter and photodetector would be required at both ends for downlink and uplink transmission. However, because of user device sensitivity to orientation changes and aesthetically not pleasing, it has not attracted much research for indoor wireless communication.

Schmid et al. [ 85 , 86 ] have developed a LED-to-LED VLC system based on the concept of Dietz et al. [ 87 ]. The VLC system is based on a single LED, which can be used both for transmission and reception. The discharging characteristic of LED was used for receiving the incoming light, which in turn translates to information. The LED capacitance changes with the intensity of receiving light. A threshold was set based on the experiments to determine the discharging for the different intensity of light to infer the incoming symbols of ON and OFF. In another reported work, smart phone was used to relay the VLC to a toy car [ 88 ]. The obvious drawback is that LED is less sensitive to light. A high-intensity light would be needed which will increase the cost and decrease the communication range. Also, a single LED for transmission and reception means that it would require multiplexing. Furthermore, fine tuning would be required to keep the LED illuminated without transmitting any useful information. This would decrease the maximum achievable data rate.

Wang et al. [ 58 ] have proposed a full fibre integrated LED-to-LED VLC system. A fiber optic link is used to connect all VAP’s to address the backbone bottleneck. A hybrid access protocol is designed using frequency division multiplexing for the uplink and downlink in fiber transmission, and time division multiplexing for bidirectional VLC transmission. Total system throughput of 8 Gb/s was achieved for eight VAP; each offered a 500 Mbps downlink and uplink transmission. The same network architecture can be extended to support upto 100 Gb/s wireless access system in the future.

Li et al. [ 89 ] have proposed a system which uses LED enabled devices to communicate on a two-way VLC channel without the use of an exclusive photodetector, while simultaneously providing illumination. Manchester coding with on/off keying (OOK) modulation is employed to achieve the best tradeoff between illumination and data rates. A prototype is developed to achieve data rates in the order of kbps at a distance of ten of centimetres.

Yeh et al. have used 682 nm visible-vertical-cavity surface-emitting laser (VCSEL) for the bidirectional VLC system [ 90 ]. The downstream of the proposed system is based on orthogonal frequency division multiplexing (OFDM) and quadrature amplitude modulation (QAM) modulation. To further enhance bandwidth efficiency bit-loading and power-loading algorithms are used. Results show that a single VCSEL bidirectional-VLC system can achieve 10.5 Gbits/s OFDM downstream traffic and 2 Mbits/s upstream.

Heydariaan et al. have investigated the VLC channel at the application level under different settings and scenario for low cost embedded VLC devices [ 91 ]. It investigates the effect of platform parameters such as transmitter and receiver position, the nature of receiver (whether the receiver is PD or LED-based), and the protocol level parameters such as symbol rate and frame payload size. The experimental setup involves adopting OpenVLC 1.0 with BeagleBone board and running a Debian Linux. The parameters investigated are throughput and packet reception ratio. Experiments show that channel conditions change for different scenarios and parameters need to be tuned for different scenarios such as day condition, mobility etc. The usage of a simple PD could reduce the impact of ambient light on the overall throughput.

Wang et al. [ 92 ] have developed a carrier sensing multiple access/collision detection and hidden avoidance (CSMA/CD-H) communication protocol, which provides reliable transmission for VLC networks consisting of nodes with different Field of View (FOV). Hidden node problem arises due to diverse FOVs of LEDs. CSMA/CA-H protocol enables intra frame bidirectional communication in the LED network. The proposed protocol is implemented using an OpenVLC platform consisting of beagleBone Black (BBB) board and a front-end transceiver. Experiments show that the CSMA/CD-H protocol can reduce hidden node collisions significantly while increasing throughput by 50% and 100% for two and four nodes networks respectively.

Giustiniano et al. [ 93 ] have addressed the issues of flickers elimination and efficient collision-free medium access control in bidirectional LED-to-LED networks. The system also consists of a method to measure the incoming light during data reception and transmission. Research showed that LED sensitivity, physical rate and flicker elimination are tightly correlated and lead to a tradeoff scenario. The proposed system is evaluated experimentally and shown that it can address flickers eliminations and collision-free communication with a throughput of 780 b/s.

The prevailing assumption in the VLC community is that LEDs emit constant light and can provide flicker-free constant throughput. Many of the current deployed LEDs are smart, dynamically adjusting emitted light to ambient conditions. Dynamic change can lead to fluctuating throughput and flickering, resulting in bad user experience and awkward lighting scenarios. Wu et al. have proposed a smartVLC system based upon the investigation of the trade-off between the fine-grained dimming level and high data rate throughput [ 94 ]. In order to achieve this trade-off, a new modulation scheme adaptive multiple pulse position modulation (AMPPM) is proposed which multiplexes individual symbols to achieve dimming granularity. Experiments show that an inexpensive smartVLC system provides flicker-free communication up to a distance of 3.5 m with improved throughput.

These studies have led to the development of very low-cost LED-to-LED VLC systems which can be readily applied is providing connectivity on the very low scale and cater to some specific applications. However, the reliability, scalability and robustness of such VLC based communication networks regarding handling interference, inter/intra LED-to-LED VLC communication and its energy efficiency are still less explored. Nevertheless, a LED-to-LED VLC system would be the end goal to tap into the large chunk of unregulated visible light spectrum. Some point to point applications would be potential applications for a full functional LED-to-LED VLC communications such as vehicle to vehicle and vehicle to infrastructure communication.

5. Discussion and Open Research Challenges

There has been increasing innovation in solid-state lighting which has resulted in the availability of efficient and low-cost devices available in the form of LED bulbs. This LED-based lighting infrastructure is now increasingly being used in indoor environments such as homes, offices and shopping malls [ 63 ], and in the outdoor environment such as for street and roads lighting [ 95 ]. Moreover, it has been shown that LED-based communication (which has been standardised by IEEE as 802.15.7) [ 33 , 96 ] offers higher data rates. According to some researchers, theoretical data rates for LED-based lighting communication is more than 15 Gbps [ 30 ]. While this inherent data communication capability is available on the downlink because of the readily available lighting infrastructure and user devices, equipped with photoreceivers and cameras, capable of detecting light. However, using VLC for the uplink has some issues. First, most modern devices do not have the transmitter for VLC and providing new ones will not only add to cost but also increase the device size. Second, using VLC for the uplink results in unusual lighting conditions from a user perspective which are aesthetically not pleasing. To provide seamless integration of VLC, researchers have focused on developing a hybrid system for seamless integration of VLC. In such systems, VLC is used for the downlink transmission and RF or IR for the uplink. Unlike VLC, IR and RF are already supported by most of the current networking devices. RF has a wider coverage range compared to VLC though susceptible to interference. Radio waves can penetrate objects and as a result can be a better substitute for the uplink transmission.

VLC is in the research and development phase. There are many research gaps need to be addressed before its commercial availability for home and enterprise users. Some of the research gaps is discussed as follow.

5.1. Inter-Cell Interference Mitigation

The small radius of a VAP cell provides higher capacity at the expense of dense deployments. Managing inter and intracell interference is a challenging task in a small and large indoor area, which is equivalent to densification of the cellular network to increase capacity. Visible light spectrum cannot penetrate objects and provides less interference out of its coverage area. However, the VAPs in the same area would cause interference leading to low signal to interference and noise ratio (SINR) and would degrade performance [ 97 ]. Beysens et al. have exploited dense deployment of LEDs and massive MIMO to propose a system, called DenseVLC [ 98 ]. It provides a battery-free VLC networking system by forming different beam spots to serve multiple users simultaneously. Optimisation techniques are used for allocating power among LEDs. DenseVLC prototype consists of 36 Tx and 4 Rx, developed from the commercial Off-the-shelf (COTS) devices and powered with the open source OpenVLC platform. The DenseVLC improves system throughput by 45% and the average power efficiency by 2.3 times in comparison with the existing techniques. It also significantly reduces inter-cell interference.

Network MIMO, joint transmission and VAP rearrangement are proposed as potential solutions for interference mitigation. In network MIMO and joint transmission, the interfering VAPs can coordinate their transmissions via interference nulling or synchronisation to ensure high SINR at the receivers. Also, interference could be decreased by rearranging the VAPs, which would be similar to cellular planning. However, the systematic design and analysis for optimising the communication performance under the illumination constraints is still an open research challenge which requires further investigation.

5.2. Backbone Network Design

To provide broadband access, all VAPs should be connected to the rest of the world. It would be a challenging problem to connect all the VAPs to internet considering the dense deployment for illumination purposes. Creating a new wired network of VAP with Ethernet or fibre is not a cost-effective solution. Wireless connectivity or power line communication could be a potential solution. However, it has limitations such as interference of wireless connections, the low data rate of PLC, the additional cost of integrating with VLC. A potential solution could be to free up the ISM band (2.4 and 5 GHz) providing approximately 200 MHz of a band that could be used to provide backbone connectivity to VAP.

5.3. Uplink Design

Most of the research in the VLC is focused on the downlink transmission to increase the data rate or mitigate interference issues. Less attention is given to uplink transmission from the user device. As mentioned earlier, a limiting factor in using the LED for the uplink from a mobile device is that it consumes not only significant energy but also causes visual disturbance to the user. Researchers propose RF and infrared. However, it would create heterogeneous networks due to different technologies at uplink and downlink. Coordination among multi-home clients, resource allocations and performance issues due to asymmetry heterogeneous networks are in the early phase of research.

6. Conclusions

In this article, we have discussed the visible light communication network, its history and applications. The existing literature is reviewed for the uplink transmission. Currently, most of the research work is focused on the downlink to present a strong argument to the research community about the potential of VLC regarding applications and high data rate. Most of the existing research have used RF and IR for the uplink. PLC is mostly used as a backbone in the access network. We highlighted the limitation of these uplink technologies. In order to efficiently utilise the vast bandwidth of visible light spectrum, a complete system-level evaluation is required. This evaluation should consider the limitation due to uplink and backbone. A scalable, reliable and high data rate VLC network would undoubtedly be a heterogeneous network. This article would facilitate the research community toward the system level analysis and implementation of the VLC system.

Acknowledgments

The authors are thankful to the editor and reviewers for their hard work which considerably improved the quality of this paper.

Author Contributions

S.U.R. conceptualised, reviewed and wrote the paper. S.U. carried out the literature review for hybrid VLC. P.H.J.C. supervised the whole review process and helped in the development of the figures. S.Y. and D.K. revised and assisted in the structure of the paper.

This research received no external funding

Conflicts of Interest

The authors declare no conflict of interest.

Theoretical and Experimental Analysis of LED Lamp for Visible Light Communications

• Open access
• Published: 20 May 2022
• Volume 125 , pages 3461–3477, ( 2022 )

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• Marcelo de Oliveira   ORCID: orcid.org/0000-0003-1932-1536 1 ,
• Fernando César Baraviera Tosta 1 ,
• David Esteban Farfán Guillen 1 ,
• Paulo P. Monteiro 2 &
• Alexandre de Almeida Prado Pohl 1  

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Visible light communications (VLC) are an emerging technology that uses light-emitting diodes (LEDs) and photodiodes to provide high-speed communication between devices employing the visible light spectrum. Taking advantage of a large unregulated spectrum with capacity for gigabit per second data rates, VLC has the potential to enable the more recent and future telecommunications technologies, such as IoT, 5G and beyond, to unlock their full capabilities, either acting as a sole solution or supporting traditional radiofrequency communications. One of the goals of VLC is to employ illumination LEDs as access points. In this paper we analyze the communication and illumination performance of an LED lamp. Both theoretical and experimental analysis are conducted and results compared. Theoretical analysis are conducted with VLC mathematical model and also with ray-tracing simulations employing the a commercial software. We show that the LED lamp is capable of providing both communication and illumination simultaneously under the proposed scenario. Our results present strong correlation between theoretical and experimental results, indicating that the theoretical model is robust.

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1 Introduction

The optical wireless communications (OWC) have been proposed as a high-capacity complementary technology to radiofrequency (RF) communications, widely employed today in wireless communications such as cellular networks, local area networks and radio and television broadcasting [ 1 ]. However, the RF spectrum, defined as wavelengths between a few kHz to 300 GHz, has its limitations. Although technology has demonstrated an increased spectral efficiency of RF systems in the last decades, the occupancy below 6 GHz is nearly saturated, and above 6 GHz almost all electromagnetic spectrum is regulated and it has been already allocated. This spectrum shortage is often referred to as spectrum crunch [ 2 ].

Among the OWC, the visible light communications (VLC) are a current research topic with the potential to unlock 5G and beyond technologies and alleviate spectrum crunch [ 2 , 3 ]. VLC uses the visible light spectrum (electromagnetic radiation frequencies between 400 and 790 THz) to transmit information by modulating the intensity of visible light [ 4 ].

In the last decades, VLC data rates have been increasing with more recent papers reporting transfer speeds in the order of gigabits per second, employing different techniques [ 5 , 6 ], but still at very short distances. Also, specific applications have also been studied, such as underwater communications [ 7 ], vehicular communication [ 8 ] and positioning [ 9 ]. Besides point-to-point communication between devices, a complete wireless communication protocol employing VLC has been proposed in the form of the Li-Fi standard [ 10 ].

Advantages of VLC include a wider and unregulated spectrum (near 10,000 times higher than the RF spectrum); inherent security, given that light waves do not pass through opaque structures such as walls; a low-cost solution to deploy, due to the fact that part of the infrastructure (the transmitter) is already installed; and is environmental-friendly, since LEDs are a green technology that has low power consumption and a longer lifespan than older illumination technologies [ 1 , 10 ].

Besides several benefits, VLC also has limitations that are challenging for researchers. The uplink channel, from the user side to the access point, is one of the main concerns, given that the user’s unit could consume too much power for a portable device, as well as may present a bright light near the user that could be unpleasant and not acceptable [ 1 ]. While VLC does not suffer from multipath fading [ 11 ], shadowing is a concern due to the fact that visible light does not pass through opaque structures [ 12 ]. Another challenge is the commercialization of VLC technologies, given that both transmitters and receivers must be deployed, requiring the illumination and telecom industries to work together [ 13 ], imposing a quest for a common standard.

VLC has the capability of offering both illumination and communication concurrently and, therefore, any LED lamp is a potential wireless access point [ 14 ]. However, when providing communication while illuminating, the system must be designed in a way that its modulation format would not cause flickering, i.e., the user must not perceive any variation in the illumination level [ 15 ]. This phenomenon may happen, for instance, when a modulation format such as OOK (on-off keying) presents several sequential equal bits, without a technique to prevent the flickering. While this dual capability is interesting, one of the concerns for the adoption of VLC is the need of always maintaining the lights on. In this sense, research has been conducted and it has been shown that (perceptively) lights-off VLC is possible while providing robust communication [ 16 ].

In VLC, the transmitters are light sources, mainly light-emitting diodes (LEDs). At the receiver side, photodiodes (PDs) are used to detect the emitted light and convert it to an electric signal to be demodulated and processed. In one standard way, the LEDs are modulated using intensity modulation (IM) and direct detection (DD) is employed at the receiver end. In this case, VLC systems are called IM/DD systems [ 14 ].

This work presents an analysis of a VLC link, both theoretically and experimentally. We use a LED module SP-02-T1 SinkPAD-II [ 17 ] equipped with seven LX18-P150-3 high power white LEDs. The light emitted by these LEDs is focused in a narrow-angle by using a polycarbonate concentrator, in order to improve performance in line-of-sight links. As for simulations, we employ the VLC channel model to evaluate mathematically our results. Finally, we model our light emission setup in the Apex add-in for SolidWorks software package [ 18 ] in order to run ray-tracing simulations and compare with theoretical and experimental results. We evaluated the received power, signal to noise ratio (SNR) and the bit error ratio (BER) for the communication link, comparing the different approaches.

Besides communication performance, given the illumination aspect of VLC, we also focus on evaluating the illumination features of the studied luminaire (LED lamp), both by ray-tracing simulations and also experimentally. We simulate the lamp in the Apex domain and also measure the illuminance distribution over a surface by using a lux meter. We present the measured illuminance distribution at a 1  \(\times\)  1 m surface for different consumed electrical power values. We also analyze the illumination impinging on a 5  \(\times\)  5 m room area by using various lamps.

Results for the communication and illumination aspects show that the lamp are suitable for both. Also, the theoretical, ray-tracing and experimental results are correlated, providing different approaches for the evaluation and design of VLC systems.

This work is organized as follows. In Sect.  2 our setup is presented, as well as the characterization of the luminaire, that is then reproduced in the Apex simulation software. In Sect.  3 , the optical wireless channel is described. Section  4 presents the analysis and results for illumination, while in Sect.  5 analysis and results of the communication performance are discussed. Finally, conclusions are given in Sect.  6 .

2 Characterization and Reproduction of the Luminaire in Apex

In order to compare theoretical and experimental results, we reproduced our components in SolidWorks. The employed luminaire is a SP-02-T1 SinkPAD-II [ 17 ] module containing seven LED LX18-P150-3 white LEDs, mounted in a mechanical holder to facilitate experimentation on a table. We attached an optic array, model PL121140 [ 19 ], to the LED module which works as a light concentrator, which allows focusing the output of the seven LEDs into a narrower beam of light. Both these pieces were reproduced in the SolidWorks domain in order to setup ray-tracing simulations using the Apex add-in [ 18 ]. The modeled components are shown in Fig.  1 .

Luminaire reproduced in SolidWorks and employed in APEX simulations. On the left the LEDs fitted to the concentrator element is observed. On the right, the whole set is seen in its support

The light sources were defined as Lumiled’s LXML-PW31 [ 20 ], which are available in the APEX software’s library and are similar to our physical components. Polycarbonate was employed as the material of the concentrator element. The LEDs were configured to generate 3 million rays at multiple wavelengths. To visualize the influence of the concentrator in the reception, we refer to Fig.  2 . In this simulation, we placed the luminaire at the center of a room and obtained the irradiation distribution over an area of 1 m \(^{2}\) , 2.5 m away from the light sources.

Irradiation distribution [W/mm \(^{2}\) ] on the floor in a 4.8  \(\times\)  4.8 \(\times\) 2.5 m room: a no concentrator and b with concentrator

3 Optical Wireless Channel

Figure  3 shows the geometry of the channel model. We consider the LED source as a Lambertian emitter [ 11 ]. As such, we define the Lambertian order of the source as

where \(\varphi _{1/2}\) is the half-power semiangle, i.e., the angle at which the optical power from the source is reduced by half of its maximum value. For the optical wireless channel model, we refer to Fig.  3 , where a line-of-sight (LOS) model consisting of a transmitter and a receiver is presented.

Geometry of line-of-sight VLC model

In the geometry depicted in Fig.  3 , \(\varphi\) is the emission angle, \(\theta\) is the incidence angle, d is the distance and FOV is the field of view of the photodetector, i.e., the maximum angle at which it can detect light. The received optical power \(P_{r}\) is defined as the transmit power \(P_{t}\) times the channel DC gain H (0).

The channel gain is given as

where A is the photodetector’s area, d is the distance between source and destination, and \(T_{s}(\theta )\) and \(g(\theta )\) represent, respectively, gains from the optical filter and concentrator, if they are implemented in the system.

3.1 Noise Model, SNR and BER

The signal-to-noise ratio (SNR) of a VLC system is calculated as [ 11 , 12 ]

where \(\gamma\) is the photodetector’s responsivity, given in A/W, and \(\sigma _{total}^{2}\) represents the total noise components, which is the combination of shot, thermal and intersymbol interference (ISI) noises [ 12 ]:

The intersymbol interference noise is mainly due to the reflection of light rays in the walls and surfaces of the surrounding environment. These light rays arrive with a delay at the photodetector, therefore acting as noise. The \(P_{rISI}\) means the total received power after the first ray of light and is modeled as

where T is the time for the first light ray to reach the detector, \(h_{i}(t)\) is the channel gain and X ( t ) is the transmitted signal.

The shot noise is inherent to photodiodes. It is caused by the signal and ambient light and describes the fluctuation in the number of detected photons and, therefore, the variation of the photocurrent generation [ 10 , 11 ]. This shot noise component is given by

in which q is the elementary charge, B is the equivalent noise bandwidth, \(I_{bg}\) is the current generated by the backlight and \(I_{2}\) is a noise factor.

The thermal noise is caused by the electronic pre-amplification in the receiver’s front-end, usually a transimpedance amplifier, to convert the photocurrent into a voltage [ 10 , 11 ], and is given by

in which the two terms represent, respectively, noise from the feedback resistor and from the FET channel. k is the Boltzmann’s constant, \(T_{k}\) is the absolute temperature, G is the open-loop gain, \(\eta\) is the fixed capacitance of the photodiode per area unit, \(\Gamma\) is the FET’s noise factor, \(g_{m}\) is the FET’s transconductance and \(I_{3}\) is also a noise factor [ 12 ].

In our experimental setups and simulations, we employ OOK (on-off keying) modulation, which is a usual VLC modulation format [ 10 , 21 ]. In this case, the bit error ratio (BER) for OOK is given by

4 Illumination Characterization and Analysis

4.1 ray-tracing analysis.

First, we characterize the luminaire’s illumination features in Apex. To determine the total luminous flux ( \(\psi _{v}\) ) for the luminaire, we refer to the LXML-PW31 LED’s datasheet, which states that its typical luminous flux is 105 lm. In order to define the radiant flux within the Apex domain, the source generated 10 wavelengths in the range of 350–800 nm and we defined the total radiant flux of the system containing only one LED to be 1 W, as a reference value. In this scenario we obtained a total luminous flux of 282.70 lm and, to obtain a luminous flux of 105 lm, in agreement with the LED’s datasheet, we calculate the total power of the system as

The increment of LED devices in the simulation must follow this premise, i.e., seven LXML-PW31 LEDs must be calibrated for the emission of a radiant flux of 2.6 W ( \(7\,\times \,371\)  mW). Therefore, when the seven LEDs are subjected to the adjustment of radiated power, the result is a total luminous flux, \(\psi _{v}\) , of 725.9 lm. In Fig.  4 simulations performed to obtain these results are presented.

Adjustment of the radiometric flux of the LXML-PW31 LED obtained with the APEX simulation. The circular shape in the images is due to the LED’s own radiating geometry

An important aspect of VLC is the capability of providing illumination and communication at the same time [ 10 ]. Thus, we have evaluated the luminaire’s illumination features by analyzing the illuminance distribution over a surface, similar to the previously presented irradiation distribution simulation, over a 1  \(\times\)  1 area, distant 1.7 m from the source. We do not employ the concentrator in this simulation and corresponding results are presented in Fig.  5 , considering the luminous flux of 725.9 lm, for two different power supply values.

Illuminance distribution over a 1 m \(^{2}\) surface, at a distance of 1.7 m, for electrical supply power values of a 1.9 W ( \(\psi _{v}\) = 207.2 lm) and b 5.5 W ( \(\psi _{v}\) = 599.1 lm)

In order to analyze the illumination in a whole environment, such as a room or office, we simulated the use of 9 luminaires equally distributed over the ceiling of a 5  \(\times\)  5 m room, as depicted in the Apex modelling of Fig.  6 . The floor is distant 2.5 from the ceiling. We considered scenarios with and without the use of the concentrator. Results are presented in Fig.  7 .

Room modeled in the Apex software for the simulation of illuminance coverage, considering the lamp a without concentrator and b with concentrator

Illuminance distribution over a 5  \(\times\)  5 m room, considering the lamp a without concentrator and b with concentrator

According to the EN 12464 standard, at least 20–50 lx are recommended for public areas with low occupation and at least 300–500 lx in spaces with high occupations [ 22 ]. In the scenario without the light concentrators, the illuminance values range from 170 to 300 lx. We see that, at this distance, the luminaires may be not considered suitable to illuminate a crowded room, but are suitable for a low occupation room. On the other hand, when employing the concentrator, there is a wider range of illuminance values, from 45 to 830 lx. In this case, the areas directly below the lamps are well within the crowded environment light requirement, while areas near the walls present much lower illuminance values.

4.2 Experimental Analysis

In order to observe the effect of the current supplied to the luminaire in the received optical power and the provided illuminance, we varied the applied current from 10 mA (light near off) to 280 mA, at incremental steps of 10 mA. In this experiment, we employed the concentrator element and the distance between the LEDs and the detector was 1 m. Results are presented in Fig.  8 .

Optical power and illuminance measured as function of electrical power variation consumed by the luminaire at 1 m distance

Similar to the simulation presented in Fig.  5 , we experimentally measured the illuminance distribution over a 1  \(\times\)  1 m surface. Results are presented in Fig.  9 . As in the simulation of Fig.  5 , the distance between the luminaire and the illuminated area is 1.7 m. The maximum illuminance at the center was 248 lx and 689 lx, respectively, when 1.9 W and 5.5 W, are consumed by the luminaire. The minimum values obtained are 33 lx (1.9 W) and 106 lx (5.5 W), located at the borders. According to the EN 12464 standard illuminance requirements, as mentioned in the previous subsection, even with the use of the concentrator, the luminaire could satisfy lighting requirements in less crowded scenarios. As for the measurement equipment, we employed the Criffer X-08 Flex Sensor lux meter.

Measured illuminance distribution over a surface of 1 m \(^{2}\) for power consumption values of a 1.9 W and b 5.5 W. The distance between luminaire and illuminated area is 1.7 m

5 Communication Simulations and Experimental Analysis

We will now evaluate the communication aspect of our setup. Typically, LED datasheets only provide luminous flux or luminous intensity information, which is useful for illumination design, but not for communication, where optical power is the main parameter of interest [ 23 ]. The scenario is depicted in Fig.  10 .

Environment for communication simulation in Apex. The 7.1 mm \(^2\) photodetection area is placed at the center

5.1 Received Power Analysis

First, we simulate the received power as a function of the transmission distance. Results are presented in Fig.  11 . In these simulations, the seven LEDs of the luminaire were adjusted for the emission of four different consumed electrical power values by the luminaire and radiometric fluxes ( \(\psi _{e}\) ): 1.9 W (209 lm), 3.9 W (425 lm), 5.5 W (605 lm) and 7 W (770 lm), considering a luminous efficiency of 110 lm/W. To evaluate this analysis, in the same figure the theoretical curves (employing Eq.  3 ) are drawn as reference for comparison purposes, considering \(\phi _{1/2}\) = 20 \(^{\circ }\) .

Detected optical power in a round surface of 7.1  mm \(^{2}\) area as a function of the distance related to the luminaire for different transmitted power values. Comparison between theoretical curves and Apex ray-tracing simulations

5.2 Communication Performance Analysis

Lastly, we analyze the parameters for the communication performance of the setup. In Table  1 we list the noise parameters employed in the theoretical simulations. We used parameters that correspond to our experimental setup wherever possible; otherwise, we employed reference parameters as listed in [ 12 ].

Our communication experimental setup is represented in Fig.  12 . The LED module is driven by a bias-t that combines the modulated signal from an arbitrary waveform generator with the power supply. The OOK-NRZ signal is generated in Matlab and embedded into the AWG. As the signal from the AWG is generated with a small amplitude, the signal is amplified before being combined at the bias-T. Our receiver is a Hamamatsu C12702-12 module [ 24 ], which has an APD with a 7.1 mm \(^2\) photodetection area. After the transmission in the optical wireless channel, the received signal at the photodetector module is captured with an oscilloscope and then processed off-line in Matlab.

Block diagram for BER versus distance measurement setup

In Fig.  13 , theoretical and experimental BER versus distance curves are plotted for a power input of 7 W, evaluated for distances from 1.4 to 2.2 m with OOK modulation at 2 MHz data rate.

BER versus distance for OOK modulation between theoretical and experimental values

We note that the two curves have a similar behaviour. The experimental measured results have a slightly worse performance than the theoretical model, which may be the result of not ideal component behaviour and other unforeseen effects in the communication channel.

6 Conclusions

In this paper, we analyzed the illumination and communication features of a LED luminaire. While VLC is emerging as a communication solution for future high-performance links, it is important to develop evaluation tools for the performance of the components. As one of the main features of VLC is its double ability to illuminate and to communicate, we focused on both these aspects for a more embracing evaluation. By applying theoretical analysis employing the VLC channel model and ray-tracing simulations, we obtained a comprehensive understanding of the setup, which was then validated through experimental measurements.

The LiFi standard is a proposal of a complete wireless networking solution employing VLC while also providing illumination to the environment [ 25 ]. In this sense, the analysis of the capabilities of communication and illumination of off-the-shelf LED consumer luminaires, as proposed in this work, is relevant.

The evaluated luminaire showed satisfactory communication and illumination performance for small to medium distances, which can be expanded by using more lamps or high power LED devices. As the studied luminaire is not designed towards environmental lighting, illumination results were mixed depending on the scenario, being suitable for illuminating small areas at relatively small distances, but not so appropriate for a whole room illumination. Experimental results for the communication link presented a BER near 10 \(^{-6}\) , which may be considered a satisfactory target ratio for some applications [ 12 , 16 ], at distances up to 1.4 m. It should be noted that these communication performance values may be improved by amplifying the photocurrent at the receiver, by installing lenses before the photodetector or by using more robust modulation formats.

The simulated scenarios showed correlation between each other, indicating that these approaches may be employed to support one another in the design of VLC systems. Further studies related to the adopted approach may include fine-tuning in the theoretical to experimental transposition and also analysis considering other luminaire setups available in the market, as well as scenarios where multiple light sources are employed.

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Acknowledgements

Authors thank the Multi-User Photonics Facility-UTFPR-CT used during the experimental work.

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 and by the European Regional Development Fund (FEDER), through the Competitiveness and Internationalization Operational Programme (COMPETE 2020) of the Portugal 2020 framework, Project LANDmaRk (POCI-01-0145-FEDER-031527) and Financial Support National Public (FCT)(OE).

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de Oliveira, M., Tosta, F.C.B., Guillen, D.E.F. et al. Theoretical and Experimental Analysis of LED Lamp for Visible Light Communications. Wireless Pers Commun 125 , 3461–3477 (2022). https://doi.org/10.1007/s11277-022-09720-z

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Title: linking vision and multi-agent communication through visible light communication using event cameras.

Abstract: Various robots, rovers, drones, and other agents of mass-produced products are expected to encounter scenes where they intersect and collaborate in the near future. In such multi-agent systems, individual identification and communication play crucial roles. In this paper, we explore camera-based visible light communication using event cameras to tackle this problem. An event camera captures the events occurring in regions with changes in brightness and can be utilized as a receiver for visible light communication, leveraging its high temporal resolution. Generally, agents with identical appearances in mass-produced products are visually indistinguishable when using conventional CMOS cameras. Therefore, linking visual information with information acquired through conventional radio communication is challenging. We empirically demonstrate the advantages of a visible light communication system employing event cameras and LEDs for visual individual identification over conventional CMOS cameras with ArUco marker recognition. In the simulation, we also verified scenarios where our event camera-based visible light communication outperforms conventional radio communication in situations with visually indistinguishable multi-agents. Finally, our newly implemented multi-agent system verifies its functionality through physical robot experiments.

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Experimental demonstration of a visible light communications system based on binary frequency-shift keying modulation: a new step toward improved noise resilience.

1. Introduction

2. state of the art in noise-resilient visible light communications systems, 3. binary frequency-shift keying visible light communications concept presentation, 4. design and implementation of the visible light communications system with bfsk, 4.1. vlc bfsk emitter, 4.2. vlc bfsk receiver, 5. experimental results, 5.1. experimental procedure and methods, 5.1.1. the initial calibration for the experiment, 5.1.2. the resilience to optical noise test, 5.2. experimental results in ambient optical noise, 5.3. debate on the experimental results, 6. conclusions and discussions, author contributions, institutional review board statement, informed consent statement, data availability statement, conflicts of interest.

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Beguni, C.; Done, A.; Căilean, A.-M.; Avătămăniței, S.-A.; Zadobrischi, E. Experimental Demonstration of a Visible Light Communications System Based on Binary Frequency-Shift Keying Modulation: A New Step toward Improved Noise Resilience. Sensors 2023 , 23 , 5001. https://doi.org/10.3390/s23115001

Beguni C, Done A, Căilean A-M, Avătămăniței S-A, Zadobrischi E. Experimental Demonstration of a Visible Light Communications System Based on Binary Frequency-Shift Keying Modulation: A New Step toward Improved Noise Resilience. Sensors . 2023; 23(11):5001. https://doi.org/10.3390/s23115001

Beguni, Cătălin, Adrian Done, Alin-Mihai Căilean, Sebastian-Andrei Avătămăniței, and Eduard Zadobrischi. 2023. "Experimental Demonstration of a Visible Light Communications System Based on Binary Frequency-Shift Keying Modulation: A New Step toward Improved Noise Resilience" Sensors 23, no. 11: 5001. https://doi.org/10.3390/s23115001

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Asia Communications and Photonics Conference 2021

• Technical Digest Series (Optica Publishing Group, 2021),
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• • https://doi.org/10.1364/ACPC.2021.W2B.3

Visible Light Communication Channel Modeling by Experiment-Data-Driven Deep Learning

Yanwen Zhu, Dahai Han, Chuan Yang, Shengnan Li, Xiaoyun Li, and Min Zhang

Author Affiliations

Yanwen Zhu, 1, * Dahai Han, 1 Chuan Yang, 1 Shengnan Li, 1 Xiaoyun Li, 1 and Min Zhang 1

1 State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications Beijing, 100876, China

* Corresponding author: [email protected]

• Shanghai China
• 24–27 October 2021
• ISBN: 978-1-957171-00-5

From the session Visible Light Communication and Positioning Systems (W2B)

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• Asia Communications and Photonics Conference 2021 , C. Chang-Hasnain, A. Willner, W. Shieh, P. Shum, Y. Su, G. Li, B. Eggleton, R. Essiambre, D. Dai, and D. Ma, eds., Technical Digest Series (Optica Publishing Group, 2021), paper W2B.3.%0Ahttps://opg.optica.org/abstract.cfm?URI=ACPC-2021-W2B.3%0A---------------------------------------------------------------------------%0AThis is sent to you as an email notification feature from Optica Publishing Group: https://opg.optica.org"> Email
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An experiment-data-driven channel-modeling method based on bidirectional long short-term memory is proposed for visible light communication channels. The resulting mean absolute error is stable in a small range, reaching 0.009 without occupying spectrum resources.

© 2021 The Author(s)

Danshi Wang, Yuchen Song, and Min Zhang M3B.3 Asia Communications and Photonics Conference (ACP) 2020

Wenbin Chen, Danshi Wang, Dongdong Wang, Yuchen Song, Jin Li, and Min Zhang S4B.2 Optoelectronics and Communications Conference (OECC) 2021

Zhenquan Zhao, Zixian Wei, Zhaoming Wang, Yuan Zhang, Mutong Li, Faisal Nadeem Khan, and H. Y. Fu JS2B.12 Optoelectronics and Communications Conference (OECC) 2021

Biao Zhou, Jing He, Yiting Yang, Lin Xu, and Jing He T4A.36 Asia Communications and Photonics Conference (ACP) 2021

Shencheng Ni, Feng Wang, Shuying Han, Xiang Li, Wu Liu, Cai Li, and Shanhong You SW4L.1 CLEO: Science and Innovations (CLEO:S&I) 2020

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• Visible Light Communications

A 10.7 Km visible light communications experiment

• Conference: 2016 Eighth International Conference on Ubiquitous and Future Networks (ICUFN)

• Beijing University of Posts and Telecommunications

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Visible Light as a Communication Medium

In this experiment, you will learn how to use the VLC - visible light communication - devices on the WITest testbed, and how to capture and visualize the light levels measured at the receiver.

It should take about 2 hours to run this experiment, but you will need to have reserved that time in advance. This experiment uses wireless resources, and you can only use wireless resources on GENI during a reservation.

To reproduce this experiment on GENI, you will need an account on the GENI Portal , and you will need to have joined a project . You should have already uploaded your SSH keys to the portal . The project lead of the project you belong to must have enabled wireless for the project . Finally, you must have reserved time on WITest and you must run this experiment during your reserved time.

With growing demand for wireless access and limited availability of traditional radio frequency spectrum, has come increasing interest in visible light as a communication medium. This has resulted in a great deal of recent research activity on visible light communication (VLC).

However, despite this increased research activity, it is not easy to start experimenting with VLC. A variety of platforms have been developed for experimentation with VLC, such as Shine [1], EnLighting [2], DarkLight [3], and OpenVLC [4]. However, there is no commercially available VLC network interface card. To begin working with VLC, a research team has to set up a local VLC testbed, and possibly also build devices themselves, even if they are only interested in VLC protocols or applications and do not intend to innovate in hardware. This creates a barrier to entry for VLC research (compared to other wireless technologies, for which devices are generally available, and for which open-access testbeds already exist).

This creates a barrier to entry for VLC research, and also serves as a barrier to reproducibility. Because there is no common hardware platform that everyone has access to, it is difficult to reproduce published results, to directly compare results from different experiments, or to build on other researchers' work. Furthermore, because VLC links are so sensitive to blocking and the position of the transmitter and receiver relative to one another, it can be difficult to reproduce a result even with the same hardware platform. Factors in the experiment environment, such as nearby reflective surfaces or the intensity of surrounding ambient light, can have a major impact on the resulting measurements. This, too, is a barrier to reproducibility.

To address this problem, we have integrated a VLC testbed (using the OpenVLC platform) into WITest. By enabling access to a common platform and common environment for experiments, we hope to lower the barrier to entry, encouraging further research on VLC protocols and applications and use of VLC experiments in education.

Three VLC transceivers are available through the WITest facility, each with a BeagleBone Black (BBB) board with the OpenVLC driver installed, an OpenVLC 1.1 cape, and a 5mm LED. Experimenters have full access to the BBB via SSH, and can modify the VLC driver or run VLC applications from there.

With VLC links, the position of the transceivers relative to one another (distance and orientation) is an important factor in link quality. We have put each LED on a pan/tilt platform with two servo motors, which experimenters can control remotely to change the direction of the light beam.

A VLC transceiver on the WITest testbed, with the LED transmitter mounted on a pan/tilt platform.

Experimenters can also visually monitor an experiment by streaming video from a webcam connected to another testbed node.

Overhead view of the VLC devices from the webcam. You can see VLC3 (on the left), VLC2 (on the right), and VLC1 (middle, on the bottom). Experimenters can watch a live stream of their experiment, like the one shown above, via the webcam.

As a shared, remote-access platform for VLC experiments, this testbed has several limitations with respect to the kinds of experiments that are supported. Experimenters cannot change the VLC hardware or the external environment (e.g. ambient light, physical layout of the room), and they have limited ability to change the position of the VLC transceivers. However, we anticipate that this facility may support experiments involving applications that use VLC links, or experiments on protocols for VLC communication (which can be implemented by modifying the OpenVLC driver ). This facility can also support experiments involving hybrid networks with VLC and also conventional wireless technologies, such as WiFi or cellular networks, or links between software defined radio devices.

In this experiment, we will send data between two VLC devices. We'll also record the light levels received at the receiver during "sensing" periods. In the following image, we plot the recorded light levels over time as black points. We also show when the device is in transmitting mode (in green) and when it is receiving a frame (in purple):

In the recorded light levels, we can pick out the pattern of the preamble. At the beginning of each frame, the transmitter sends the pattern 10101010 three times. We can see this pattern as high/low light levels recorded at the receiver:

Run my experiment

In order to run our experiment, you first have to reserve time on the WITest testbed .

Set up the testbed

At the reserved time, open a terminal and log into the appropriate testbed console using SSH:

using your own GENI wireless username and password (typically, "geni-X" where X is your regular GENI username).

Then continue by loading a disk image onto node24, which has a webcam attached to it that will allow us to visually monitor the experiment. On the WITest console, run:

After this is complete, turn it on:

and wait a few minutes for the node to boot.

Next, turn on the VLC devices. All three devices are powered on and off at the same time. To power on the VLC devices, run:

Set up the webcam to watch your experiment

To monitor the experiment using the webcam, we will set up an SSH tunnel to node24, which has a webcam attached to it.

In a new terminal window, run

(using your own GENI wireless username). This will set up a tunnel between TCP port 8081 on your local host, and TCP port 8081 on node24 on the testbed, where the webcam stream is. You'll have to keep this terminal window open in order to keep using the tunnel.

From the WITest console, log in to node24:

and then try to log in again.)

From the console on node24, you need to install some software and set up a config file to use the webcam.

First, to install the motion software, run

Then download the config file from this gist :

Finally, to start the software, run

Now, you can view a live stream of your experiment in your local browser at http://localhost:8081 .

You may notice that the camera focus is lost when the LEDs are flashing. To fix the focus of the camera, you have to turn off the auto focus. On the node24 terminal, run

Setup VLC devices

Now, we will set up the VLC devices.

Open two more terminals, and log into the WITest console.

Then, from the WITest console, log on to the "vlc2" device in one terminal:

Use "temppwd" as the password when prompted.

Next, we want to set up the VLC link. On both boards, we will navigate to the directory where the openvlc driver is saved, load the driver with the appropriate arguments, and set up an IP address for the VLC interface.

On vlc2 run:

and on vlc3 run:

When we load the VLC driver with the option show_msg=1 , the light levels received during the sensing periods will be printed to the system log. We can use this to produce a figure like the one in the Results section.

Then we want to setup the low power LEDs as the transmitter and the photodiode as the receiver. On both boards run:

Test the visible light link

To test the visible light link, on vlc2 run:

This will send 10 "ping" messages to vlc3. When in range, vlc2 sends an echo request to vlc3, and vlc3 sends an echo reply back.

If there is no line of sight between the boards, no responses will be received, and you will have to attempt to use the pan/tilt platform to change the position of the transceivers in relation to one another. To move the position of the transmitter, run

where X and Y are numeric values. Use the webcam view to adjust iteratively adjust the positions:

• If you see both LEDs light up when you send the ping, then the requests are received at vlc3, and you don't have to move vlc2's transmitter. You should adjust the position of vlc3 until the replies are received at vlc2. (You can start with the "known good" positions listed below, then make slight adjustments as needed.)
• If you only see the LED of vlc2 light up, then you'll have to adjust its position until vlc3's LED also lights up, indicating that the requests are received at vlc3. Then, you may still need to adjust vlc3's position so that the replies will be received at vlc2. (You can start with the "known good" positions listed below, then make slight adjustments as needed.)

The following video shows the general procedure:

We have observed successful two-way communication with these settings, so you should find these useful as a starting point (you may have to make slight adjustments):

• On vlc2: sudo pan-tilt --pan 9 --tilt 6.5
• On vlc3: sudo pan-tilt --pan 8 --tilt 7.5

Similarly, we have observed two-way communication between vlc1 and vlc2 with these settings:

• On vlc1: sudo pan-tilt --pan 4.5 --tilt 4
• On vlc2: sudo pan-tilt --pan 11 --tilt 3

And we have observed two-way communication between vlc1 and vlc3 with these settings:

• On vlc1: sudo pan-tilt --pan 11.5 --tilt 4
• On vlc3: sudo pan-tilt --pan 6.75 --tilt 6.5

Inspect the received light levels

During operation, the VLC device is either in the sensing state (looking for a preamble), receiving a frame, or transmitting. While in the sensing state, it will print received light levels to the system log.

To record these values, on the BBB console run

to power cycle the VLC devices.

[1] L. Klaver and M. Zuniga. Shine: A step towards distributed multi-hop visible light communication. In 2015 IEEE 12th International Conference on Mobile Ad Hoc and Sensor Systems, pages 235–243, Oct 2015.

[2] S. Schmid, T. Richner, S. Mangold, and T. R. Gross. EnLighting: An indoor visible light communication system based on networked light bulbs. In 2016 13th Annual IEEE International Conference on Sensing, Communication, and Networking, SECON, pages 1–9, June 2016.

[3] Z. Tian, K. Wright, and X. Zhou. The DarkLight rises: Visible light communication in the dark. In Proceedings of the 22Nd Annual International Conference on Mobile Computing and Networking, MobiCom ’16, pages 2–15, 2016.

[4] Q. Wang, D. Giustiniano, and D. Puccinelli. OpenVLC: Software-defined visible light embedded networks. In Proceedings of the 1st ACM MobiCom workshop on visible light communication systems, VLCS ’14.

Acknowledgments

This research was supported by the NYU Tandon School of Engineering Center for K12 STEM Education via the ARISE program, and by the Pinkerton Foundation.

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• Open access
• Published: 07 September 2024

A metal-free cascaded process for efficient H 2 O 2 photoproduction using conjugated carbonyl sites

• Tiwei He 1 ,
• Hongchao Tang 1 ,
• Jiaxuan Wang 1 ,
• Mengling Zhang 1 ,
• Cheng Lu 1 ,
• Hui Huang   ORCID: orcid.org/0000-0002-9053-9426 1 ,
• Jun Zhong 1 ,
• Tao Cheng   ORCID: orcid.org/0000-0003-4830-177X 1 ,
• Yang Liu   ORCID: orcid.org/0000-0002-6872-6815 1 &
• Zhenhui Kang   ORCID: orcid.org/0000-0001-6989-5840 1 , 2  

Nature Communications volume  15 , Article number:  7833 ( 2024 ) Cite this article

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• Chemical bonding
• Nanoscale materials
• Photocatalysis

Carbon-based metal-free catalysts are promising green catalysts for photocatalysis and electrocatalysis due to their low cost and environmental friendliness. A key challenge in utilizing these catalysts is identifying their active sites, given their poor crystallinity and complex structures. Here we demonstrate the key structure of the double-bonded conjugated carbon group as a metal-free active site, enabling efficient O 2 photoreduction to H 2 O 2 through a cascaded water oxidation − O 2 reduction process. Using ethylenediaminetetraacetic acid as a precursor, we synthesized various carbon-based photocatalysts and analyzed their structural evolution. Under the polymerization conditions of 260 °C to 400 °C, an N -ethyl- 2 -piperazinone-like structure was formed on the surface of the catalyst, resulting in high photocatalytic H 2 O 2 photoproduction (2884.7 μmol g −1 h −1 ) under visible light. A series of control experiments and theoretical calculations further confirm that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts.

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Introduction.

The metal-free photoelectrocatalysts are the most popular, green and renewable catalysts for the present critical energy and environmental challenges 1 . Many recent works have suggested that the carbon-based metal-free catalysts have significant potential for cost reduction and high efficiency and stability 1 . Additionally, these catalysts have exhibited various advantages, including enhanced electronic conductivity, adjustable structure, abundant availability, and robust tolerance to acidic/alkaline environments 2 , 3 . For these metal-free carbon-based catalysts, the porosity, functional groups, specific surface area, π-conjugated and donor-acceptor (D-A) structure, and incorporated heteroatoms (N, B, O, P, S, Cl, Se, Br, and I) significantly impact their catalytic performance 4 , 5 , 6 . On the other hand, hydrogen peroxide (H 2 O 2 ) is recognized as a sustainable oxidizing agent and is a crucial chemical compound in industry 7 , 8 , 9 , 10 . The photocatalytic process utilizing solar energy is considered as an alternative method for H 2 O 2 production due to its cost-effectiveness and environmental friendliness 2 , 11 , 12 . Carbon-based photocatalysts such as covalent organic frameworks (COFs) 13 , graphitic carbon nitride (G-C 3 N 4 ) 14 , 15 , polymer resins 5 , 16 , carbon dots (C-Dots) 17 , 18 and graphene oxide (GO) 19 , 20 have been widely studied for photocatalytic H 2 O 2 production due to their appropriate energy band structure, optical excitation characteristics and low price. Through photocatalysis, H 2 O 2 is mainly generated via O 2 reduction by the photogenerated electrons on the catalyst surface 21 , 22 , 23 , which is also obtained by the reaction between H 2 O and the photogenerated holes 24 , 25 , 26 . Many efforts such as the elucidation of reaction mechanisms 11 , 27 , including carrier migration patterns 28 , 29 , band structure regulation 5 , and surface modification during catalytic reactions 30 , 31 , have been reported. However, there is still no clear understanding of the real structure of their active sites. As a result, the exact mechanisms are contentious, and the advancement of high-performance metal-free catalysts still relies on trial-and-error methods, rendering the investigation of metal-free catalysts for H 2 O 2 photoproduction highly challenging.

Here, we report the synthesis of various carbon-based materials derived from ethylenediaminetetraacetic acid (EDTA) using a one-step thermal polymerization method involving decarboxylation, cracking, and polymerization processes. Specifically, within the synthesized temperature range of 260 °C to 350 °C, a conjugated carbonyl structure, namely, N -ethyl- 2 -piperazinone, was locally formed and exhibited highly efficient photocatalytic activity for H 2 O 2 production, with the highest rate of 2884.7 μmol g −1  h −1 at 260 °C. Conversely, at synthesized temperatures below 240 °C or above 400 °C, structural analysis indicated the preservation of the original EDTA structure and the formation of an aromatic ring structure, respectively, both of which lack photocatalytic activity for H 2 O 2 production. Notably, in situ characterization and density functional theory (DFT) calculations revealed that the double-bond conjugated carbonyl groups serve as adsorption sites for water molecules, providing active sites for water oxidation reaction (WOR) and subsequent oxygen reduction reaction (ORR). The universal applicability of this conclusion was confirmed through the use of several small molecules as model photocatalysts. Our findings underscore the significance of conjugated carbonyl groups as active sites for visible light-driven H 2 O 2 production by a cascaded WOR−ORR process, namely, a key and universal feature of the active site of metal-free photocatalysts.

Structural characterization of the catalysts

In this work, a carbon-based catalyst, named EA- x , was prepared via EDTA thermal polymerization, where x is the synthesis temperature. To determine the structural information of the carbon skeleton structure and functional group, a series of structural characterization experiments on these samples (EA- x ) were first conducted, including the Fourier transform infrared spectroscopy (FTIR), carbon-13 cross polarization/magic angle spinning nuclear magnetic resonance ( 13 C CP/MAS NMR), thermogravimetric analysis (TGA), and time-of-flight secondary ion mass spectrometry (TOF−SIMS).

The FTIR spectra of the pristine EDTA and as-prepared catalysts (EA- x , x  = 200, 240, 260, 300, 350 and 400) are shown in Fig.  1a and Supplementary Figs.  1 and 2 . In Supplementary Fig.  1 , the FTIR spectrum of EA-200 (blue line) is consistent with that of pristine EDTA. Specifically, the peak centered at 3000 cm −1 is ascribed to the stretching vibration of C−H, while the peak located at 1700 cm −1 is assigned to the stretching of C=O 14 . The peaks located at 1413 cm −1 and 1320 cm −1 are ascribed to COO − , and the peaks located at 1213 cm −1 and 1140 cm −1 are attributed to the absorption vibration mode of C−N 32 . The absorption bands at 1087 cm −1 , 1050 cm −1 and 1010 cm −1 are ascribed to C−C stretching vibrations 33 . For EA-260 (black line in Fig.  1a ), an increase in the peak at 3350 cm −1 compared to EA-200 (blue line in Fig.  1a ) indicates the formation of −NH, suggesting that the secondary amine was produced by the cleavage reaction 3 . Simultaneously, the intensity of the C−H peak decreases, accompanied by a redshift to 2941 cm −1 . This observation suggested the transformation of methylene group into a double-bond conjugated structure 34 . Compared with that of EA-200, the C=O characteristic peak of EA-260 shifts from 1700 cm −1 to 1736 cm −1 , demonstrating the formation of cycloketone components 3 . The peak at 1650 cm −1 is attributed to C=N and C=C 3 . Compared with those of EA-200, the intensities of the −COO − , C−C, and C−N peaks of EA-260 decreased, which indicates the destruction of the carboxyl group and the breakage of the alkane chain. Since the vibrational mode of the acid anhydride was not observed in the FTIR spectrum of EA-260, carboxylic acid polymerization was excluded. When the synthesis temperature reached 400 °C (EA-400, red line in Fig.  1a ), almost all the infrared absorption vibrational peaks disappeared, and the peak at 1638 cm −1 was attributed to C=C or C=N on the aromatic ring. The infrared peak is derived in the second order, and the derivative result is integrated to obtain the relative content of the corresponding functional groups. The relative content of the obtained functional groups is shown in Fig.  1b , and it can be intuitively seen that with increasing reaction temperature, the number of unsaturated bonds (C=C, C=N) first increases and then decreases, and the sample synthesized at 260 °C (EA-260) has the maximum content of unsaturated bonds (source data).

a FTIR spectra of EA-200, EA-260, and EA-400. b The change trend of the functional group content with synthesis temperature. The peaks of the FTIR spectra of different samples were fitted to obtain the corresponding second derivative spectra. Then, the derived second derivative spectra were used to determine the exact location of the absorption and shoulder peaks in the original spectrum, and the peak area of each subpeak was calculated using the fitted smoothed spectrum of the original spectrum and compared for analysis. 13 C CP/MAS NMR spectra of EA-200 ( c ), EA-260 ( d ), and ( e ) EA-400 Inset: Three chemical structures inferred separately from the analysis. All the illuminations were based on the research. f The thermogravimetric curve of EDTA and possible reactions in the corresponding temperature range; ① to ⑤ show the synthesis temperatures of the five catalyst samples. Source data are provided as a Source Data file.

Figure  1 c– e and Supplementary Fig.  3 show the 13 C CP/MAS NMR spectra of these carbon-based catalysts. The 13 C spectra of EA-200 and EA-240 show five signals like those of pristine EDTA, indicating that both the EA-200 and EA-240 still maintain the EDTA structure (Fig.  1c and Supplementary Fig.  3a ). Specifically, the peak at 53.2 ppm corresponds to saturated carbon atoms connected to carbon and nitrogen atoms, while the peaks at 58.46 and 61.46 ppm corresponds to saturated carbon atoms connected to carboxyl groups 4 . The chemical shifts at δ  = 168.81 ppm and 174.58 ppm correspond to the carbon on the carboxyl group 4 . The signals in the 13 C NMR spectra of EA-260 and EA-300 can be deconvoluted into nine carbon components, a to i, which can be classified as edge-saturated carbon linkers (methyl or methylene) produced by breaking C−C or C−N bonds (a–d), unsaturated pyrazine conjugated systems (e, f), and C=O or C=N in carbocyclic structures (g, h), respectively (Fig.  1d and Supplementary Fig.  3b ) 35 , 36 . All these structural features suggested that EDTA undergoes cleavage, decarboxylation and dehydration reactions, producing secondary amines as intermediate substrates for further condensation reactions during thermal polymerization at 260 °C (Supplementary Fig.  4 ). Ultimately, a pyrazine ring bearing a double-bond conjugated carbonyl structure was synthesized from compound EA-260. The 13 C NMR spectrum of EA-400 shows two signals ascribed to the aromatic π-conjugated structure (Fig.  1e ). This suggested that at 400 °C, due to further carbonization, the saturated carbon linker and amide structure were destroyed and polymerized to form an aromatic π-conjugated system.

Thermogravimetric analysis (TGA) of EDTA was performed to elucidate the pyrolysis characteristics of EDTA across a temperature range of 50 to 400 °C. As depicted in Fig.  1f and Supplementary Fig.  5 , the pyrolysis process can be divided into three stages. The first stage (stage I), between 50 °C and 243 °C, exhibited negligible weight reduction. Subsequently, the following stage (stage II) occurs from 243 °C to 261 °C, during which EDTA undergoes rapid pyrolysis, resulting in a significant mass reduction of 70%. The weight loss process at this stage and the substantial decrease in the number of −COOH groups and C−N bonds illustrated in the results of IR and NMR may be attributed to decarboxylation and cracking reactions during this phase, and the presence of amide structures indicates the potential for dehydration condensation reactions. Beyond 261 °C (stage III), the pyrolysis action becomes weaker, with the total weight loss reaching 83% at 400 °C.

Furthermore, based on the TGA curve, we investigated the various structures of the carbon-based catalysts at five specific temperatures ( ① to ⑤ for 200 °C, 240 °C, 260 °C, 350 °C, and 400 °C, respectively, in Fig. 1f ), and the molecular weight distributions of these samples were determined by gel permeation chromatography (GPC). As shown in Supplementary Fig.  6 , with increasing temperature, the molecular weight increases, and the molecular weight distribution becomes wider. Specifically, the molecular weight distribution of EA-260 ranged from 8.2 × 10 4  g/mol to 5.1 × 10 6  g/mol, with a peak molecular weight of 8.5 × 10 4  g/mol, indicating substantial polymerization of EDTA.

The hydrogen component transformation of EDTA during thermal polymerization was elucidated by nuclear magnetic resonance (1H NMR) analysis (Supplementary Fig.  7 ). The peak at 2.77 ppm observed in the spectrum of EA-200 (black line) corresponds to the methylene proton of ethylenediamine in EDTA. Furthermore, the signal at 3.48 ppm is attributed to the protons of the ethylenediamine methylene grafted to the β ammonium groups 37 . However, the absence of methylene protons in EA-260, EA-350 and EA-400 indicates complete dehydrogenation or cleavage of the C−N bond at synthesis temperatures exceeding 260 °C. Simultaneously, as the synthesis temperature increases, the number of branched methylene hydrogen molecules in β ammonium shifts toward the low magnetic field region due to the α-carbon electron drawing effect 37 .

To characterize the structure of EA- x , TOF−SIMS of EA- x ( x  = 200, 260, 350 and 400) was performed. The TOF−SIMS of EA-200 showed that the largest monomer fragment (positive ion) was identified as EDTA (positive ion) with m / z  = 293 (Supplementary Fig.  8 ). The TOF−SIMS spectra of EA-260 and EA-350 exhibit similar secondary ion fragment compositions; moreover, based on the above analysis, the fragment structure, i.e., m / z  = 127, is derived as N -ethyl- 2 -piperazinone (Supplementary Fig.  9 ). The characteristic fragment structure at m / z  = 159 in the TOF−SIMS spectrum of EA-400 was simulated and defined as 2 -methyl- 8 -hydroxyl-quinoxaline (Supplementary Fig.  10 ).

The local electronic structure and chemical structure of EA- x were investigated by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). As depicted in Supplementary Fig.  11 , no discernible peaks corresponding to elements other than C, N and O were detected in survey XPS spectra of EA- x ( x  = 200, 240, 260, 350 and 400). Concurrently, the inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of the EA- x sample revealed an absence of detectable signals associated with metallic elements (The specific elements measured are detailed in Supplementary Table  2 ). Supplementary Figs.  12a and 13a show the XPS spectra of C 1  s of EA- x ( x  = 200, 240, 260, 350 and 400), and the peaks of EA-200 centered at 284.8 eV, 286.1 eV, and 288.6 eV are attributed to graphitic carbon (C−C/C=C), C−N/C−O, and C=O, respectively 22 . As the temperature increased, the intensity of the C−C/C=C peak increased, while the intensities of the C−N/C−O peak and the C=O peak decreased. This suggests that the pyrolysis process destroys the carboxyl group and C−N, providing unsaturated bonds for polymerization. The peak at 284.8 eV observed in the C K-edge XAS spectrum of EA-200 is attributed to the π*, which is a typical out-of-plane C=C bond related to interlayer bonding, and the peak located at 288.1~288.4 eV is attributed to 1  s  → σ C=O/C−O * (Supplementary Fig.  14 ) 38 . The σ C=O/C−O * peaks of EA-260 and EA-400 are negatively shifted, which is due to the participation of endogenous oxygen in autooxidation, the formation of carbon oxides by the original carbon species, and the increase in delocalized π bonds in the aromatic carbon layer 38 . As demonstrated in the N K-edge region of EA-200, the peak located at 407.2 eV is ascribed to 1  s  → σ N−C * (Supplementary Fig.  15 ). In the N K-edge XAS spectrum of EA-260, two new signals at 399.4 and 402.5 eV are attributed to the 1  s  → π* transitions of heterocyclic aromatic nitrogen atoms (π C=N−C *) and sp 3 N−C bridges between triazine groups (π N−C *), respectively 38 . These results indicate the formation of nitrogen species in prominent triazine-like structures from EA-200 to EA-260, which is beneficial for charge conduction 39 . From the XPS N 1  s spectrum in Supplementary Figs.  12b and 13b , the transition from C 3 −N to C=N−C and C−N−H can be observed, which proves the cleavage of C−N bonds on EDTA and the generation of pyrazine structures from EA-200 to EA-400. A negative shift of the peak attributed to C 3 −N indicates a decrease in the electron density of nitrogen, which may be attributed to the C 3 −N group being adjacent to the conjugated carbonyl structure. The O K-edge XAS and O 1  s XPS spectra further reveal the changes in surface oxygen species during continuous heat treatment (Supplementary Figs.  12c , 13c and 16 ). Compared to those of EA-200, no new features are observed in the O K-edge XAS spectra of EA-260 and EA-400 (Supplementary Fig.  16 ). The peak at 532.7 eV is attributed to the 1  s  → π* transition in the carbonyl structure 38 . In addition, the peak at 539 eV is attributed to the O 1  s  → σ C−O * transition, which gradually decreases from EA-200 to EA-400. These conclusions are also supported by the XPS O 1  s peak (Supplementary Figs.  12c and 13c ).

Next, electron microscopy was used to characterize the morphology and microstructure of the as-synthesized catalysts (EA-200, EA-260 and EA-400). As revealed in Fig.  2a , the transmission electron microscopy (TEM) image of EA-200 shows its block-shaped amorphous morphology. The spherical aberration corrected transmission electron microscopy (AC-TEM) image of EA-200 illustrates that EA-200 has a similar structure to EDTA and a clear edge with 0.193 nm and 0.328 nm crystal lattices, corresponding to the (604) and (−213) planes of the monoclinic structure of EDTA (Fig.  2b ) 40 . From the TEM image of EA-260, a flower-like morphology can be observed (Fig.  2c ). The AC-TEM image of EA-260 shows that its surface contains of triazine-like carbocyclic rings, and the lattice spacing measured from the AC-TEM image is 0.26 nm which can be indexed to the (110) of the in-plane pyrazine ring unit (Fig.  2d and Supplementary Fig.  17 ). The X-ray diffraction (XRD) pattern of EA-260 shows a broad peak at 2θ = 17.5° corresponding to the in-plane structural stacking (100) pattern (Supplementary Fig.  18a ) 41 , which is consistent with the TEM results. As shown in Fig.  2e , the edges of the EA-400 particles are curled. The regular graphene-like aromatic system stacking with a lattice spacing of 0.21 nm, which is related to the (100) graphite-like plane, on the local surface can be clearly observed in the AC-TEM image of EA-400 (Fig.  2f ). The HAADF-STEM image of EA-400 also shows a clear edge at 0.33 nm belonging to the (002) plane of graphite (Supplementary Figs.  19 and 20 ) 3 .

a HRTEM image of EA-200 showing the blocky morphology; scale: 100 nm. b AC-TEM image of EA-200; scale: 5 Å. The morphology of EA-200 exhibits an EDTA-like atomic arrangement. c HRTEM image of EA-260; scale: 200 nm. d AC-TEM image of EA-260; scale: 2 nm. Inset: Spherical aberration magnified image. EA-260 has a triazine atomic arrangement; scale: 5 Å. e HRTEM image of EA-400; scale: 500 nm. f AC-TEM image of EA-400; scale: 2 nm. Inset: Spherical aberration magnified local image. EA-400 has a graphene-like atomic arrangement; scale: 5 Å.

Cyclic voltammetry (CV) was chosen to qualitatively determine the band configuration for various catalysts 6 . As illustrated in Supplementary Fig.  21 , the redox potential derived from the CV curve decreases as the synthesis temperature increases 27 . At high synthesis temperatures (above 350 °C), a large loss of the structure of oxygen-containing electron-deficient groups (e.g., acyl, carboxyl) leads to a decrease in the oxidizing capacity of the material. Moreover, the results shown in Fig.  3a reveal that the energy band positions of EA-200, EA-240 and EA-260 satisfy the thermodynamic conditions of the WOR and ORR. Similar results were also acquired from the XPS valence spectra of these samples (Supplementary Fig.  22 ).

a The band diagram derived from the cyclic voltammetry curves. b Transient photocurrent response curves and c Nyquist plots for the EIS spectra of EA-200, EA-240, EA-260, EA-350, and EA-400. Exact data is provided in the source data. d Transient photovoltage spectra of EA-200, EA-240, EA-260, EA-350 and EA-400. e Comparison of the intensity-time curves obtained by extracting a fixed frequency f  = 60 Hz from the wavelet transform graph of the TPV curves.

Electrochemical tests and transient photovoltage (TPV) analysis were performed to investigate the charge behavior of diverse catalysts. The transient photocurrent response (TPR) curves in Fig.  3b show that EA-260 possesses the best charge separation efficiency among all samples. At the beginning of illumination, the current signal rapidly increases and then gradually decreases to a stable state. This phenomenon indicates that the separation process between electrons and holes is extremely rapid and is accompanied by a redistribution of surface charges on the catalyst. The electrochemical impedance spectroscopy (EIS) curves in Fig.  3c show that EA-260 has the smallest semicircle radius (source data), revealing that it has the lowest charge transfer resistance. The presence of double-bond conjugated carbonyl structures reduce the band width, greatly promoting the generation of charge carriers. Simultaneously, the n-π transition of carbonyl groups promotes carrier separation. TPV was conducted to study the interfacial charge transfer behavior of these five carbon-based catalysts 42 . The results are presented in Fig.  3d and the Supplementary Table  1 , revealing that EA-260 exhibits the highest charge extraction ( A ), indicating superior surface charge separation efficiency and photogenerated charge. These findings are consistent with those obtained through EIS and TPR analyses. Furthermore, the charge decay constant ( τ ) of EA-260, which is associated with charge recombination, is much greater than that of EA-350 and EA-400, indicating a longer lifetime for surface photogenerated charges. The results are shown in the Supplementary Table  1 shows the surface effective electron numbers ( n e  =  Aτ / t max ) of the five catalysts, revealing that EA-260 has the highest n e and potential for excellent photocatalytic capacity. The electron transfer dynamics of the catalysts were further studied using fast Fourier transform (FFT) and continuous wavelet transform (CWT) methods 27 . The FFT and CWT results of the TPV relaxation curve of the EA- x catalysts are shown in Supplementary Figs.  23 – 26 . As shown in Supplementary Fig.  24 , the amount of charge at all velocities in EA-260 is greater than that in the other EA- x samples ( x  = 200, 240, 350 and 400). As shown in Fig.  3e and Supplementary Figs.  25 and 26 , the time-intensity spectra of EA- x were compared at seven increasing frequencies. When considering low-speed electrons, the interface charge transfer rate of EA-260 (t 1 ) is much faster than that of the other samples ( t 2 to t 5 ). When higher-velocity electrons are considered, the surface migration efficiency of all EA- x tends to be consistent. These results indicate that EA-260 can accelerate charge transfer and continuously generate carriers that are delivered to the reaction site owing to the presence of double-bond conjugated carbonyl structures.

Catalytic properties of the catalysts

The photocatalytic performance of EA- x ( x  = 200, 240, 260, 280, 300, 350 and 400) was evaluated in air using a 420 nm light emitting diode (LED). As depicted in Fig.  4a and Supplementary Fig.  27 , none of the samples exhibited the production of H 2 O 2 during the initial 4 h incubation in the dark. During the subsequent 12-hour period of exposure to light, both EA-200 and EA-400 demonstrated a lack of photocatalytic activity in the production of H 2 O 2 . The EA catalysts synthesized through calcination at temperatures between 240 °C and 350 °C exhibit the production of H 2 O 2 . Among them, EA-260 possesses the greatest amount of photogenerated H 2 O 2 , reaching 31.46 ± 2.39 mM after 12 h of light irradiation (source data). A six-day continuous cycle experiment was conducted to evaluate the high-performance retention of EA-260. As depicted in Fig.  4b , the performance of EA-260 reached a H 2 O 2 production of 52.66 ± 2.07 mM after 24 h of illumination on the sixth day, indicating the stability of the catalyst (source data). The apparent quantum efficiency (AQE) versus the UV−Vis absorption diagram revealed that the AQE of EA-260 calculated from the yield of H 2 O 2 was 12.67 ± 2.02 % at 420 nm (source data), while the light absorbance increased with increasing carbonization (Fig.  4c and Supplementary Fig.  28 ). Supplementary Fig.  29 shows the photocatalytic performance test under different gas atmospheres. The rate of photocatalytic H 2 O 2 production by EA-260 under N 2 decreased to 602.2 ± 138.4 μmol g −1  h −1 , while the rate of photocatalytic H 2 O 2 production increased to 4001.46 ± 232.6 μmol g −1  h −1 in saturated O 2 condition. Thus, O 2 is the primary reactant in the H 2 O 2 production process. Using 5,5 -dimethyl- 1 -pyrroline N-oxide (DMPO) as a radical spin-trapping agent, in situ EPR was performed to investigate the radicals produced through photochemical activation of different reactants. As shown in Fig.  4d (upper panel), the signals attributed to · OH were observed, indicating that H 2 O was oxidized to · OH by photogenerated holes over EA-260. In the methanol solution, · O 2 − signals were observed in the in situ EPR spectra, indicating that O 2 was reduced by the electrons photogenerated on EA-260 (Fig.  4d , bottom). The WOR transfer electron numbers of EA-200, EA-260 and EA-400 were investigated electrochemically. As shown in Supplementary Fig.  30 , the WOR electron transfer numbers of EA-200 and EA-260 are approximately 2.14 and 2.22 (2e − WOR), respectively. However, the WOR electron transfer number of EA-400 is 3.79, suggesting a 4e − WOR route (H 2 O is converted to O 2 ) on EA-400.

a Photocatalytic H 2 O 2 evolution activity of EA-200, EA-240, EA-260, EA-350, and EA-400; The H 2 O 2 production rates of EA-200 and EA-400 are both zero, and the performance curves of the two samples overlap. b Photocatalytic H 2 O 2 evolution cycles of the EA-260. c AQE versus the UV-vis absorption spectrum of EA-260. Error bars represent the standard deviations of three replicate measurements in a – c . d In situ EPR spectra of the EA-260 photocatalytic system in the dark and after 30 min of visible light irradiation. DMPO- · OH: The test is performed in 200 μL of 1 mg/mL of the EA-260 in H 2 O, with the addition of 200 μL of DMPO solution at a concentration of 100 mM for · OH capture. DMPO- · O 2 − : The test was performed in 200 μL of 1 mg/mL EA-260 in methanol, with the addition of 200 μL of DMPO solution at a concentration of 100 mM for · O 2 − capture. Exact data of photocatalytic performance is provided in the source data.

To investigate the intermediate species and active sites involved in the photocatalytic H 2 O 2 production process, in situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFT spectroscopy) was performed on EA-260. As displayed in Fig.  5a, b and Supplementary Fig.  31 , the DRIFTs of EA-260 acquired through the adsorption of O 2 and water vapor for 30 minutes in the dark were subjected to background subtraction. To emphasize the spectral evolution trend, noise reduction and differential processing were adopted for in situ DRIFT. After 10 min of light exposure, the downward signal that increases with time appears at approximately 3500 cm −1 , suggesting consumption of the adsorbed H 2 O molecules, and the observation of C−O−H at 1326 cm −1 revealed the conversion of C=O into C−O−H. Additionally, the signal appeared at 1723 cm −1 attributed to the C=O from pyrazinone gradually increased along with increasing illumination (from 10 min to 70 min). Meanwhile, the increase in the downward signal of C=OH + at 1670 cm −1 confirms the pivotal sites in the photocatalytic process of accessing carbonyl groups 6 . The peaks at 1629 cm −1 , 1580 cm −1 and 1530 cm −1 are attributed to C=N, C=NH*, and N−H vibrations, respectively 34 , 43 , indicating that the N atoms may serve as potential water adsorption sites or a vibration modes caused by changes in the distribution of electrons on the C=N functional group due to changes in the surrounding electronic environment. In addition, the formation of intermediate species (*OOH and *HOOH), which occur in the two-step single-electron ORR was observed at 1223 cm −1 and 1285 cm −1 , respectively 43 . As shown in Fig.  5c , from the in situ DRIFT spectroscopy, the above information regarding the intermediates involved in the ORR and WOR was observed, as well as the reaction sites on the catalyst. Specifically, the C=O (1723 cm −1 ) and C=N (1629 cm −1 ) functional groups served as adsorption sites (3500 cm −1 ) for H 2 O molecules, leading to WOR. It is also possible to adsorb protons directly on C=O or C=N. The ORR occurred subsequent to the formation of C−O−H or C−N−H (1326 cm −1 and 1530 cm −1 ) species.

a , b In situ DRIFTS spectra of EA-260 under illumination conditions in a flow of H 2 O and O 2 for 70 min; the baseline for DRIFTS is EA-260 with 30 minutes of oxygen and water vapor in the dark, allowing the surface of EA-260 to adsorb reactants first. Local spectrum of in situ DRIFTS ( a ranging from 4000 cm −1 to 2800 cm −1 and b ranging from 1900 cm −1 to 1000 cm −1 ). The spectra of these time intervals were differentially processed (subtracting the infrared spectra measured at 5 minutes from all the infrared spectra measured at that time to highlight the changes in the infrared peaks). c Key reaction sites and intermediate information of the WOR and ORR observed in in situ DRIFTS spectra.

To investigate the influence of H 2 O on the photocatalytic production of H 2 O 2 , the photocatalytic performance of EA-260 was evaluated under various H 2 O concentrations in acetonitrile, and saturated O 2 was introduced into the testing system. As demonstrated in Fig.  6a , no H 2 O 2 was detected in pure acetonitrile after 6 h of illumination. Upon the addition of 1% H 2 O to the system, the rate of H 2 O 2 production reaches 120.96 ± 16.50 μmol g −1  h −1 (source data). With the increasing volume fraction of H₂O, the yield of H₂O₂ increases proportionally. Therefore, H 2 O is a necessary reactant for the formation of H 2 O 2 . To investigate whether the WOR affects the ORR in the photocatalytic system of EA-260, the ORR electron transfer number was examined under various conditions. As shown in Fig.  6b , the ORR electron transfer number of EA-260 tested in acetonitrile containing 0.0025 g/L H 2 O is 0.4 (single-electron ORR process). However, the calculated number of transferred electrons of the ORR is close to 2 (Fig.  6c , two-electron ORR process) in acetonitrile containing 0.1 g/L H 2 O (the detailed calculation is shown in Supplementary Note  1 ). Therefore, it can be inferred that the WOR reaction should first be performed on EA-260, and the resulting protons should be used for the ORR reaction (Supplementary Fig.  32 ).

a Catalytic performance of EA-260 in acetonitrile. The H 2 O content was controlled to test the change in the photocatalytic performance of EA-260 under the condition of trace H 2 O. Here, 15.0 mg of catalyst was added to 20 mL of acetonitrile. Different volume fractions of H 2 O (1%, 2%, 3%, 4%, and 5%) and saturated O 2 were introduced before illumination of the system, while the other conditions remained unchanged. The amount of H 2 O 2 was measured after 6 h of light exposure. b Rotating ring-disk electrode (RRDE) i-t curves of EA-260 under dark and light conditions ( λ  ≥ 420 nm) in an O 2 -saturated 0.1 M TBAP acetonitrile solution (40 mL) containing 2.5 mg/L H 2 O and c O 2 -saturated 0.1 M TBAP acetonitrile solution (40 mL) containing 100 mg/L H 2 O; d free energy diagrams of H 2 O 2 photocatalytic production though cascaded single WOR−1e − ORR pathway (See Supplementary Data  2 for details) and e the models of the molecular structure of the catalyst during the reaction; f free energy diagrams of H 2 O 2 photocatalytic production though cascaded dual WOR−2e − ORR pathway (See Supplementary Data  3 for details); and g models of the molecular structure of the catalyst during the reaction. Source data are provided as a Source Data file.

To verify the detailed reaction mechanism of H 2 O 2 production on the asymmetric units of EA-260, density functional theory (DFT) calculations were conducted. In the first step, the H + adsorption energies on the N and O atoms of EA-260 were calculated to determine the possible active sites (H 2 O adsorption sites). As shown in Supplementary Fig.  33 , H + adsorption on the N atom of EA-260 is calculated at 2.21 eV, higher than that of O sites (1.26 eV with an O−H distance of 0.98 Å) (see Supplementary Fig.  33 and Supplementary Data  1 ). Thus, the WOR mainly occurs on the O atom of EA-260. Based on the experimental results discussed above, DFT calculations of the two proposed reaction mechanisms were conducted (Fig.  6 d– g ). As shown in the free energy diagram (Fig.  6 d, e ), the procedure comprises one electron (1e − ) transfer mechanism featuring a single ·OH intermediate and H + , followed by an end-on (Pauling type) O 2 adsorption configuration and a two-step 1e − ORR (Supplementary Data  2 ) 44 . Specifically, C=O groups have the ability to undergo n-π transitions when exposed to light, leading to the transfer of electrons from non-bonding σ orbitals to the π anti-bonding orbitals, resulting in the formation of a triplet biradical. Simultaneously conjugation stabilizes the existence of free radicals (Supplementary Figs.  34 and 35 ) 45 , 46 , 47 . Subsequently, the H 2 O molecule is adsorbed onto the O atom of EA-260 (the Δ G of H 2 O adsorption is calculated to be 0.20 eV), leading to a single-electron WOR with an energy barrier of 1.06 eV (source data), which is the rate-determining step (RDS). The WOR process in this stage is in accordance with the previously discussed first-order reaction (Fig.  6a ). Then O 2 is adsorbed on the H + generated from the WOR in the end-on (Pauling type) O 2 adsorption configuration (Fig.  6e and Supplementary Fig.  36 ) 44 , 48 . O 2 is first reduced to ·OOH, with a calculated energy barrier of −1.15 eV, and then further reduced to HOOH, with a calculated energy barrier of −1.66 eV. This result is consistent with the 1e − ORR process observed in our experiments (Fig.  6b ). When considering a high H 2 O content, the second mechanism was illustrated in Fig.  6 f, g . As shown in Fig.  6g , two molecules of H 2 O are adsorbed on the O atoms of two adjacent units of EA-260. The adsorption energy of the first H 2 O molecule is calculated as 0.20 eV, and that of the second H 2 O molecule is calculated as 0.03 eV (source data). After WOR (Δ G  = 1.96 eV), two H + remain on the oxygen. O 2 is adsorbed onto the H + generated from the WOR on EA-260, and the side-on (Yeager-type) configuration prevents the cleavage of the O=O bond and facilitates the one-step 2e − ORR (Fig.  6g and Supplementary Data  3 ) 44 , 48 . Sufficient supply of H + (at a high H 2 O concentration) promotes the one-step 2e − ORR with an energy barrier of −3.68 eV, which verifies the experimental results (Fig.  6c ). The presence of a double-bond conjugated carbonyl group on EA-260 serves as both a water adsorption site and facilitates the generation of protons through the oxidation of water molecules adsorbed under illumination. These protons are safeguarded by the conjugated structure of the molecule. Finally, we evaluated O 2 adsorption and direct activation during the initial stage of the ORR, and performed DFT calculations of the ORR on the C atom of EA-260 (Supplementary Data  4 ). As shown in Supplementary Fig.  37 , when adsorbed on the C atom of EA-260, the side-on (Yeager-type) O 2 adsorption configuration tends to break the O=O bond, hindering the 2e − ORR 44 , 48 . This is consistent with experimental results indicating that the catalytic system produces no H 2 O 2 without water (Fig.  6a ). To confirm the proposed reaction mechanism, Fukui function calculations were performed to evaluate the activity of each point reaction. As shown in Supplementary Fig.  38 and Supplementary Table  3 , in the simulated monomer structure of EA-260, the electronegativity on the O 2 and O 3 atoms is relatively higher ( E + (O2)  = 0.053 e eV, E + (O₃)  = 0.03965 e eV), and the CCD values also indicate that the O 2 and O 3 atoms are more likely to undergo nucleophilic reactions and be reduced (CCD (O₂)  = 0.0396 e, CCD (O₂)  = 0.0286 e). Hence, in the photocatalytic system of EA-260, the predominant mechanism is the two-step 1e − ORR process under low H 2 O content conditions, whereas under high H 2 O content conditions, a one-step, 2e − ORR process is favored.

In the next study, we further confirmed that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts for H 2 O 2 photoproduction. Five small-molecule organics were selected as model catalysts (Fig.  7a–e ), and their catalytic activities for the photocatalytic production of H 2 O 2 were examined under the same conditions. As shown in Fig.  7f , the production of H 2 O 2 in cyclohexanone and 2 -phenyl- 1, 4 -benzoquinone was detected under light (H 2 O 2 formation rates of 502 ± 67.6 and 136 ± 34.1 μmol g −1  h −1 for cyclohexanone and 2 -phenyl- 1, 4 -benzoquinone catalysts, respectively), but no H 2 O 2 was detected in the inositol, 5 -phenyl- 1, 3 -cyclohexadidione and 1, 4 -cyclohexadidione catalytic systems (source data). Hence, it can be deduced that small molecules containing double-bond conjugated carbonyl structures possess the capability to generate H 2 O 2 through photocatalysis.

Five selected small molecules used as model catalysts: a Cyclohexanone octahydrate, b 1, 4 -cyclohexanedione, c 2 -phenyl- 1, 4 -benzoquinone, d 5 -phenyl- 1, 3 -cyclohexanedione, and e inositol. All materials were used directly without further processing. f The H 2 O 2 evolution rate over these five model materials ( a – e ) under visible light (λ ≥ 420 nm). Error bars represent the standard deviations of three replicate measurements. Source data are provided as a Source Data file.

In this work, a series of metal-free photocatalysts were prepared through a one-step thermal polymerization of EDTA. Among them, EA-260 exhibited the highest photocatalytic performance for producing H 2 O 2 at a rate of 2884.7 μmol g −1  h −1 under visible light without any sacrificial agent. Structural analysis, in situ characterization and DFT calculations confirmed that the double-bond conjugated carbonyl groups in the catalyst of EA-260 serve as the catalytic active sites for both H 2 O adsorption and the cascaded WOR−ORR process. The activity of five small-molecule organics as model catalysts for H 2 O 2 production was investigated, confirming that this type of double-bond conjugated carbonyl structure is the primary and universal characteristic of the active site of metal-free photocatalysts. The main aim of this work was to analyze the structure of small-molecule polymer materials through spectra and to identify specific reaction sites and pathways through a structural perspective in actual photocatalytic reaction systems and theoretical calculations. Moreover, the key structural fragment for the photocatalytic production of H 2 O 2 , the double-bond conjugated carbonyl group, was verified. This research is crucial for elucidating how carbon-based materials provide reaction sites and regulate the microenvironment of local reactions in certain photocatalytic reactions, which is pivotal for comprehending the influence of nonmetallic material structures on photocatalytic reactions 45 . In actual catalytic reaction settings, multiple reaction conditions or factors may yield entirely disparate elucidations and material designs 45 , 49 . The findings of this work have the potential to offer perspective on understanding particular reactions and material design from a microscopic vantage point.

All commercially available chemicals were not further purified for use, unless otherwise specified. Ethylenediaminetetraacetic acid (99.9%) (EDTA) was purchased from Amresco. KMnO 4 titrant (0.02 M) was purchased from Fude Biological Technology. 5 -phenyl- 1, 3 -cyclohexanedione and Acetonitrile was purchased from Energy Chemical. Cyclohexanone octahydrate was purchased from J&K Scientific. 1, 4 -cyclohexanedione was procured from Aladdin. 2 -phenyl- 1, 4 -benzoquinone was bought from TCI. Inositol was purchased from Shanghai Yuanye.

Synthesis of EA-x

EDTA (1 g) was placed in a 50 mL ceramic crucible covered with a lid and sealed with high-temperature adhesive. Next, the samples were placed it in a muffle furnace and heated to a specific temperature (i.e., x  °C, where x is equal to 200, 240, 260, 280, 300, 350, and 400.) at a rate of 1 °C/min and maintained incubated at x  °C for 30 min (the process was completed under an air atmosphere). After naturally cooling to room temperature, the resulting powder was ground to obtain powder. The powder was washed with deionized water and ethanol for 3 times. Finally, the obtained solid was placed in an oven at 70 °C for 24 h and denoted as EA- x (where x corresponds to the temperature during the thermal polymerization).

Instruments

Transmission electron microscopy (TEM) and high−resolution TEM (HRTEM) images were observed by FEI Tecnai F20 transmission electron microscope operating at 200 kV. Spherical Aberration Corrected Transmission Electron Microscope (AC-TEM) was conducted by JEM-ARM300F. The structure of samples was characterized by X−ray powder diffraction (XRD) by using an X’PertProMPD (Holland) D/max−γA X−ray diffractometer with Cu Kα radiation (k = 0.154 nm). X−ray photoelectron spectra (XPS) were obtained by using a Thermo Fisher Nexsa X−ray photoelectron spectrometer with a monochromatised Al Ka X−ray source. Aurora M90 ICP−MS was used to test the element content of Co. X−ray absorption near edge structure (XANES) data were collected X-ray absorption spectroscopy (XAS) experiments were performed at Beamlines MCD-A and MCD-B (Soochow Beamline for Energy Materials) at NSRL. Hyperion spectrometer (Bruker, Germany) was employed to determine Fourier transform infrared (FTIR) spectrum over the scan range of 400–4000 cm −1 . In situ FTIR spectrum ranging from 800–4000 cm −1 was conducted with Thermo IS 50. The UV-Vis adsorption spectrum at room temperature was acquired using a UV/visible/NIR spectrophotometer (lambda750, Perkinelmer) with a wavelength range of 300–800 nm. The thermogravimetric test and mass spectrum was obtained on RIGAKU, thermo plus EVO2/ thermo mass photo. Electron spin-resonance spectroscopy (ESR) measurements were performed on Bruker EMXplus-6/1 to analyzed H 2 O 2 evolution process. Electrochemical measurements were conducted on the CHI 760 C workstation (CH Instrument, Shanghai, China). Gel Permeation Chromatography (GPC) was carried out on PL-GPC50. TOF-SIMS was performed on TOF-SIMS 5 iontof. Bruker Avance Neo 400WB was used for the 13 C Nuclear Magnetic Resonance Spectra (13 C NMR). Bruker 400 MHz was applied for the H Nuclear Magnetic Resonance Spectra. (1H NMR).

Photocatalytic performance measurements

The experiments of photocatalytic H 2 O 2 performance was evaluated by a multichannel photocatalytic reaction system (CELLAB200E7, 80 mW/cm 2 ). Unless otherwise specified, 100 mg of photocatalyst was evenly dispersed in 110 mL of ultrapure water evenly by ultrasonication without any introduction of a sacrificial agent or cocatalyst. The photocatalytic reaction was illuminated by visible light (420 nm ≤ λ ≤ 700 nm) under continuous stirring. The suspension was centrifuged and filtered with a 0.22 μm disposable syringe filter to remove the dispersed catalyst. The evolution rate of H 2 O 2 was gauged by potassium permanganate (KMnO 4 ) titration 27 . Specifically, 5 mL of extraction liquor supplemented with the adjunction of 3 mL of H 2 SO 4 (3 mol/L) was titrated with 0.02 mol/L KMnO 4 . When the final concentration of the KMnO 4 standard solution dropped, the color of the solution changed suddenly and the variation remained for half a minute. At this juncture, the total amount of H 2 O 2 was calculated by the consumption of KMnO 4 (Subtract the titration of the solution was subtracted before irradiation to exclude the error caused by the reaction of other substances in the solution with KMnO 4 ).

Data availability

Source data are provided as a Source Data file.  Source data are provided in this paper.

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Acknowledgements

This work is supported by the National Key R&D Program of China (2020YFA0406104, 2020YFA0406101), the Natural Science Foundation of Jiangsu Province (BK20220028), Innovative Research Group Project of the National Natural Science Foundation of China (51821002), National Natural Science Foundation of China (52272043, 52271223, 52202107, 52201269), The Science and Technology Development Fund, Macau SAR (0009/2022/ITP), Collaborative Innovation Center of Suzhou Nano Science & Technology, and the 111 Project. We also acknowledge the support from Suzhou Key Laboratory of Functional Nano & Soft Materials and Beamlines MCD-A and MCD-B (Soochow Beamline for Energy Materials) at NSRL.

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Tiwei He, Hongchao Tang, Jie Wu, Jiaxuan Wang, Mengling Zhang, Cheng Lu, Hui Huang, Jun Zhong, Tao Cheng, Yang Liu & Zhenhui Kang

Macao Institute of Materials Science and Engineering (MIMSE), MUST-SUDA Joint Research Center for Advanced Functional Materials, Macau University of Science and Technology, Taipa, 999078, Macao, China

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T.H. collected the SEM, TEM, HRTEM, and STEM images. H.T. conducted the DFT simulation. T.H., Ji.W., J.Wa., and M.Z. conceived the idea and designed the experiments. T.H. and C.L. performed the EXAFS test and analyzed the data. T.H. and H. H. discussed the results and commented on the paper. T.H. and J.Z. created figures. T.C., Y.L., and Z.K. supervised the whole project and were involved in manuscript preparation and revision.

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Correspondence to Tao Cheng , Yang Liu or Zhenhui Kang .

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He, T., Tang, H., Wu, J. et al. A metal-free cascaded process for efficient H 2 O 2 photoproduction using conjugated carbonyl sites. Nat Commun 15 , 7833 (2024). https://doi.org/10.1038/s41467-024-52162-3

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DOI : https://doi.org/10.1038/s41467-024-52162-3

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