Visible Light Communication Link Research Articles

Power-over-fiber-based optical wireless communication systems towards 6G

This paper reports two implementations of power-over-fiber (PoF) solutions applied to radio-over-fiber (RoF) and optical wireless communication (OWC) systems, in the context of an industrial environment. We employ a conventional 62.5-µm multimode fiber (MMF) to deliver optical power to different communication links based on RoF, free-space optics (FSO), and visible light communication (VLC) technologies aiming beyond 5G (B5G) and 6G applications. First, a 3.5-GHz 5G New Radio (5G NR) signal is transmitted throughout a 20-km single-mode optical fiber (SMF) link using RoF technology. Regarding the PoF system, a 5-W optical power is transmitted through a 100-m MMF link. A photovoltaic power converter (PPC) and a DC/DC converter are employed to convert the power from the optical to the electrical domain and adjust the voltage level, respectively, with the purpose of energizing a remote RoF module. The attainable optical and electrical power transmission efficiencies (OPTE and PTE) are 80% and 19%, respectively. Posterior, a second PoF system is implemented to power a hybrid RoF/FSO/VLC B5G system, comprising a 200-m MMF and an additional DC/DC converter. Over 10.5 W of optical power is transmitted to feed an electrical amplifier (EA) and a white LED from the VLC link. In this configuration, we achieve 78% and 18.5% of OPTE and PTE, respectively. Furthermore, a performance investigation based on the root mean square error vector magnitude (EVMRMS) metric is conducted to evaluate the signal using the implemented PoF systems and a conventional electrical power supply. In the first implementation, a throughput of 600 Mbps is achieved with 100-MHz bandwidth without performance degradation, when compared to the conventional-powered RoF system, whereas, in the second implementation, 60-Mbps throughput is achieved when employing the FSO and VLC technologies simultaneously, demonstrating the applicability and potential of the PoF technique for B5G and 6G industrial communications.

The Latency Performance Analysis and Effective Relay Selection for Visible Light Networks.

In visible light communication (VLC), the precise latency evaluation of wireless access networks and the efficient forwarding strategy of core networks are the crux for end-to-end reliability provisioning. Leveraging martingale theory, an elegant latency-bounded reliability analysis framework is studied for the VLC network. Considering the characteristic that VLC links are easy to block, the martingale of the service process is constructed. Based on the time shift features, the martingale process related to latency is proposed for the VLC system, which models the influence of entanglement between aggregate arrivals and random service on latency. A stopping time event about latency is defined. Renting the stopping time theory, a tight upper bound of the unreliability with regard to latency is derived. In the core network, a dynamic forward backhaul framework is proposed, which uses the relay selection algorithm based on back-pressure theory to improve data transmission quality. The theoretical latency-bounded reliability matches the simulation results well, which verifies the effectiveness of the proposed analysis framework, and the proposed relay selection algorithm can also improve network performance under data-intensive transmission.

NOMA for Hybrid RF/VLC Systems Using Intelligent Reflecting Surfaces

In this article, Intelligent Reflecting Surfaces (IRS) are used for Non Orthogonal Multiple Access (NOMA) in hybrid Radio Frequency (RF) Visible Light Communications (VLC). The source broadcasts the symbols of K users. The broadcasted signal by the source is reflected by IRS on RF link to reach the relay node R with the same phase. Relay node detects the symbols of K users using Interference Cancelation (IC). Then, relay nodes broadcasts the symbols of K users on VLC link. We show that NOMA using IRS for hybrid RF/VLC offers 12–18 dB gain versus conventional hybrid RF/VLC without reflectors for N=32\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$N= 32$$\\end{document},168 reflectors. The main advantage of the paper is to enhance the throughput of RF/VLC systems using IRS. The main limitation of the paper is that the results are not valid when energy harvesting is performed.

Neural-network-based end-to-end learning for adaptive optimization of two-dimensional signal generation in UVLC systems.

Visible light communication (VLC) benefits from the underwater blue-green window and holds immense potential for underwater wireless communication. In order to address the limitations of various equipment and harsh channel conditions in the underwater visible light communication (UVLC) system, the researchers proposed to use the method of autoencoder (AE) to tap the potential of the system. However, traditional AE schemes involve replacing the transmitting and receiving components of a communication system with a large multilayer perceptron (MLP) network, and they have significant drawbacks due to their reliance on a single network structure. In this paper, a novel 2D adaptive optimization autoencoder (2D-AOAE) framework is proposed to realize adaptive modulation and demodulation of two-dimensional signals. By implementing this scheme, we experimentally achieved a transmission rate of 2.85 Gbps over a 1.2-meter underwater VLC link. Compared to the traditional 32QAM UVLC system, the 2D-AOAE scheme demonstrated a 15.4% data rate increase. Moreover, the 2D-AOAE scheme exhibited a remarkable 73% improvement when compared to the UVLC system utilizing the traditional AE scheme. This significant enhancement highlights the superior performance and capabilities of the 2D-AOAE scheme in terms of transmission rate.

Enhanced Charging/Discharging Process in Perovskite Active Light Source for High‐Speed Visible‐Light Communication

AbstractMetal halide perovskites are promising light source materials for visible light communication (VLC) due to their excellent photoelectric properties and small resistance‐capacitance time constant. However, previous reports mainly used perovskites as the passive light sources, which not only makes it susceptible to posterior excitation light sources, but also complex in the integration process. Herein, the quasi‐2D PEA2Csn‐1PbnBr3n+1 perovskite light–emitting diodes (PeLEDs) as an active light source in VLC link is demonstrated. It is found that the charging/discharging process of PeLEDs is an important factor governing the ‐3 dB bandwidth (f‐3 dB) of the VLC. To improve this, 3‐sulfopropyl methacrylate potassium salt (SMPS) molecules are introduced into perovskite to simultaneously passivate deep and shallow energy level defects at grain boundaries. Additionally, the multiple quantum wells structures of PEA2Csn‐1PbnBr3n+1 are modified to be flat. At the optimal SMPS concentration, the maximum external quantum efficiency of PeLEDs reaches 21.5%. Meanwhile, the VLC achieves 3.2 MHz f‐3 dB with data transmission of 18.6 Mbps, which is the highest f‐3 dB in PeLEDs with the same active area. Hence, it provides a versatile method to improve the performance of VLC links based on active light sources and advances toward the goal of high‐speed, energy‐efficient and secure free communication.

(Vehicle’s YB)-based LPB prediction model and anti-LPB (communication link)-reconstruction strategy in the (VLC/RF)-V2X network

The hybrid vehicle-to-everything (V2X) network based on visible light communication (VLC) and radio frequency (RF) ((VLC/RF)-V2X Network) can provide a stable communication network to vehicles. It has great development forward. However, light path blocking (LPB) is a major problem for vehicular visible light communication (V-VLC). It can cause serious communication outage because the visible light signal cannot penetrate opaque obstacles. In view of this, a (vehicle’s yielding behavior (YB))-based LPB prediction model and an anti-LPB (communication link)-reconstruction strategy in the hybrid network are firstly proposed. At first, based on vehicles’ yielding behavior, a LPB prediction model is constructed. By this model, LPB can be predicted before the VLC link is blocked. Next, based on the prediction result of the model, a strategy of restructuring communication links is present. By this strategy, communication links can be self-adaptive changed or supplemented. Then, based on the proposed model and strategy, the anti-LPB scheme is finished. By the scheme, LPB can be avoided timely, communication outages and redundant handovers can be reduced effectively. Finally, by simulations, the model’s validity and the scheme’s effectiveness have been demonstrated. Through LPB prediction and link reconstruction, the proposed scheme avoids LPB caused by user-behavior blockage at source for the first time in multiple road regions. And it has the comparative advantages of faster response time and lower handover number. Compared with other methods, the average advance time of handovers is improved by 66.94% and the average handover number of the network is reduced by 68.48%. What’s more, the path loss (PL) caused by LPBs is reduced by 92% on average.

Optical Intelligent Reflecting Surface for Cascaded FSO-VLC Communication System

For optical wireless communication, laser-based free-space optical communication (FSO) has high energy utilization efficiency and speed, but is easily blocked and cannot support multiple users. Visible light communication (VLC) based on light-emitting diode (LED) can support multiple mobile users, but has low energy efficiency in long-distance communication. Aiming at the above problems, this paper proposes an OIRS-assisted cascaded FSO-VLC system. In this system, we adopt a FSO link with OIRSs at the front end. Multiple OIRSs are utilized to screen the optimal channel, thereby improving the flexibility and robustness of the system. The VLC link is used in the back end to support multi-user mobile communication. By jointly considering atmospheric turbulence, pointing error, OIRS's physical model and VLC geometric model, the closed-form solutions of the system's precise outage probability, asymptotic outage probability and BER have been derived. It is shown through numerical results that the derivation results are accurate. We can observe that both the increase of VLC source's coverage radius in VLC link and the decrease of pointing error in FSO link can effectively improve the system performance. Based on theoretical analysis and simulation results, the effect of each parameter and system design have been discussed.

On security performance analysis of IRS-aided VLC/RF hybrid system

Intelligent reflecting surface (IRS) has been introduced into wireless communication to improve physical layer security (PLS) due to its ability of reconfiguring propagation environments. However, most current research on IRS-aided PLS is for standalone radio frequency (RF) or visible light communication (VLC), and few has focused on IRS-aided VLC/RF hybrid systems combining the wide coverage of RF and high speed of VLC. We study the PLS of an IRS-aided dual-hop VLC/RF hybrid system, where the first VLC link is to transmit optical signal, and the second RF link assisted by IRS is to extend communication region inaccessible to visible light, and the two hops are connected by a relay in decode-and-forward or amplify-and-forward mode. Two eavesdropping scenarios are considered: eavesdropping from IRS and eavesdropping from relay. Then, the closed-form expressions of secrecy outage probability and strictly positive secrecy capacity probability are derived for four combinations of eavesdropping scenarios and relay modes, and corresponding asymptotic solutions are obtained. Finally, simulations are carried out to verify the superiority of the proposed system over a conventional hybrid system without IRS, and the proposed system shows different security performance for various scenarios and relay modes, and IRS has a positive effect on improving PLS.

KRLS post-distorter with adaptive kernel width for visible light communications.

As a viable supplement to the fifth generation wireless communication, visible light communications (VLC) with affluent spectrum resources can cater to the ever-increasing high speed data transmission demand. However, the nonlinear characteristics of light emitting diode (LED) can distort the transmitted signal in the VLC link, which damages the communication quality. To mitigate the nonlinear impairments, a reproducing kernel Hilbert space post-distortion scheme is proposed in this paper, which is based on kernel recursive least squares (KRLS) with adaptive kernel width. In this kernel based method, the kernel width will affect the approximation ability of the model. Therefore, in the recursive process of KRLS, Gauss-Newton (GN) algorithm is adopted to update the kernel width. In addition, combined with the enhanced novelty criterion (ENC), the KRLS-GN post-distorter learns the sparse dictionary adaptively according to the input data, which is beneficial to complete the linearization under the limited memory budget constraints. The performance of the proposed KRLS-GN-ENC scheme is verified by simulations, and the results show that KRLS-GN-ENC can achieve a significant improvement over KRLS-ENC. Compared with the schemes based on classical polynomial filtering, KRLS-GN-ENC exhibits better nonlinear compensation performance and faster convergence speed.

AL-aided AMC in a multi-user white-light OFDMA VLC system over a light-diffusing fiber loop.

To develop an adaptive modulation scheme for flexible high-speed multi-user visible light communication (VLC), automatic modulation classification (AMC) is adopted for monitoring the modulation formats of different subcarrier groups. An AMC scheme based on a joint convolutional neural network (CNN), active learning (AL), and data augmentation (DA) is demonstrated over an orthogonal frequency division multiplexing access (OFDMA) VLC system. The configuration of the diffuse white-light VLC system is combined with a pair integrated transceiver module, a light-diffusing fiber (LDF), and a wireless channel, which can provide white-light illumination and ubiquitous access. Within the forward error correction (FEC) threshold, the data rates of the white-light VLC links can reach 325.5 Mbps with a bit error rate (BER) of 2.163 × 10-3. An experiment with two-user access via the proposed VLC link with an unequal bandwidth allocation was demonstrated. The performance of the AL-aided CNN AMC scheme also shows a classification accuracy rate of 95.48% for the constellation diagrams of different subcarriers of the OFDMA signal over 240 training samples and faster convergence than a CNN-based AMC.

Machine-Learning-Based Indoor Localization under Shadowing Condition for P-NOMA VLC Systems.

The localization of agents for collaborative tasks is crucial to maintain the quality of the communication link for successful data transmission between the base station and agents. Power-domain Non-Orthogonal Multiple Access (P-NOMA) is an emerging multiplexing technique that enables the base station to accumulate signals for different agents using the same time-frequency channel. The environment information such as distance from the base station is required at the base station to calculate communication channel gains and allocate suitable signal power to each agent. The accurate estimate of the position for power allocation of P-NOMA in a dynamic environment is challenging due to the changing location of the end-agent and shadowing. In this paper, we take advantage of the two-way Visible Light Communication (VLC) link to (1) estimate the position of the end-agent in a real-time indoor environment based on the signal power received at the base station using machine learning algorithms and (2) allocate resources using the Simplified Gain Ratio Power Allocation (S-GRPA) scheme with the look-up table method. In addition, we use the Euclidean Distance Matrix (EDM) to estimate the location of the end-agent whose signal was lost due to shadowing. The simulation results show that the machine learning algorithm is able to provide an accuracy of 0.19 m and allocate power to the agent.

Visible Light Communications for Unmanned Aerial Vehicle: Channel Modeling and Experimental Validation

Unmanned aerial vehicles (UAVs) have been widely applied for their high mobility and low cost, and the visible light communication (VLC) aided UAV can combine the flexibility of UAV and the high transmission rate of VLC. However, the VLC links of a UAV are inevitably affected by the jittering effects, which were rarely characterized. In this letter, the channel between a light emitting diode (LED) and a hovering UAV is modeled considering the random orientation error of the UAV. Closed-form expressions are derived for the probability density function (PDF) and the cumulative distribution function (CDF) of the orientation error, which also leads to closed-form asymptotic PDF and CDF expressions of the UAV’s VLC channel gain. Simulation and experimental results are demonstrated to verify the theoretical derivations. The work lays a theoretical foundation for VLC-aided UAVs, providing insightful suggestions for the system design.

Visible-light communication using thermally evaporated CsPbBr3 perovskite thin films

Lead halide perovskites are widely used in optoelectronic applications owing to their promising photophysical properties, but developing components of the order of nanoscale remains challenging. In this work, the authors fabricated cesium lead bromide (CsPbBr3) thin films of varying thicknesses and investigated their visible-light communication (VLC) performance. The thickness of the CsPbBr3 thin films was precisely controlled by using a single-beam thermal evaporation technique, and their morphology was analyzed through scanning electron microscopy and x-ray diffraction. Thicker films were found to have a homogeneous surface, with gain boundaries of increasing size and fewer surface trap states than the thinner films. Furthermore, we identified the thickness-dependent photoluminescence (PL) property of the CsPbBr3 thin films based on steady-state PL measurements and verified it by using time-correlated single-photon counting as well as femtosecond upconversion measurements. Films with thicknesses of 5, 10, and 20 nm, with enhanced surface homogeneity and purity, were used in a VLC link as color-converting fluorescent components. The 20-nm-thick CsPbBr3 film delivered the best performance because it had the highest PL intensity and the most suitable morphology, with a –3-dB bandwidth of 30.7 MHz and a net data rate of 330 Mb/s. These results reflect a facile and well-controlled approach to fabricating such films that can be used for high-power, high-speed, and large-area transmission and detection of visible-light signals.

R-AP's position-domain model for an RO-receiver and AP placement strategy for an uninterrupted link between the robotic arm and AP in the VLC network.

In the visible light communication (VLC) network, the optical link between the robotic arm and the access point (AP) is easily interrupted due to the random orientation of the receiver on the robotic arm. First, based on the VLC channel model, a position-domain model of reliable AP (R-AP) for a random-orientation receiver (RO-receiver) is proposed. The channel gain of the VLC link between the receiver and the R-AP is non-zero. The tilt-angle range of the RO-receiver is [0,π]. Given the field of view (FOV) angle and the orientation of the receiver, the R-AP's position domain of the receiver can be obtained by this model. Then, based on the R-AP's position-domain model for the RO-receiver, a novel AP placement strategy is proposed. Under this AP placement strategy, the number of R-APs for the RO-receiver is not less than 1, thereby effectively avoiding link interruption caused by the random orientation of receivers. Finally, it is proved by the Monte Carlo method that, under the AP placement strategy proposed in this paper, the VLC link of the receiver on the robotic arm can remain uninterrupted during the movement of the robotic arm.

Effect of Sea Waves on Vertical Underwater Visible Light Communication Links

Underwater visible light communication (VLC) has been proposed to deal with emerging high bandwidth underwater applications. Initial research works on underwater VLC are based on the assumption that both transmitter and receiver are submerged, creating a horizontal link. In most of the vertical communication links, one of the transceiver nodes takes the form of a buoy and requires taking into the effect of the sea surface, which is inherently unsteady due to wind and waves. In this article, we consider a vertical underwater VLC link where the transmitter is in the form of a buoy at the sea surface, and the receiver is a submerged node at a certain depth. We assume sinusoidal waves and consider the fact that the buoy will fluctuate and oscillate, during drifting up and down, around its vertical axis. This effectively results in a 3-D displacement at the suspended transmitter. Building upon these assumptions of practical relevance, we propose an aggregate channel model, which includes a random path loss due to periodic changes of the transmission distance and a fading term induced by pointing errors with periodic changes of relative movement. Based on the proposed statistical model, we derive closed-form expressions for the exact and asymptotic bit error ratio and investigate the achievable diversity orders. We further present numerical results to confirm the analytical findings.

Optimal Adaptive Waveform Design Utilizing an End-to-End Learning-Based Pre-Equalization Neural Network in an UVLC System

Visible light communication (VLC) has emerged as a promising communication method in the special field of 6G, such as intersatellite communication (ISC) and underwater wireless optical communication (UWOC). Green light-emitting diode (LED)-based underwater visible light communication (VLC) is considered a potential candidate for UWOC of mobile autonomous underwater vehicle (AUV) and remotely operated underwater vehicle, particularly in coastal and harbor environments. However, due to bandwidth limitations and variable underwater channels, implementing high-speed underwater VLC remains a challenge. In this paper, a neural network-based auto equalization model (NNAEM) using end-to-end learning is proposed to achieve a high-speed underwater VLC link with a bandwidth-limited green LED. The entire NNAEM includes a neural network-based channel model, a pre-equalization neural network, and a post-equalization neural network. Based on the data-driven channel model, our auto equalization (AE) method can dynamically pre-equalize the discrete multitone (DMT) modulated signal in different bandwidth-limited cases and various nonlinearity cases. An underwater VLC link with a data rate of 3.451 Gbps and a transmission distance of 1.2 m is experimentally demonstrated by employing the proposed NNAEM under the hard decision-forward error correction (HD-FEC) threshold of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . The data rate improvement is up to 30.4% compared to other advanced digital signal processing (DSP) methods. The scenario adaptation and robustness of the NNAEM are also verified in the experiments, which demonstrate that the NNAEM can serve an important role in underwater high-speed VLC transmission.

Recent Advances in Optical Injection Locking for Visible Light Communication Applications

The introduction of visible light communication (VLC) technology could increase the capacity of existing wireless communication systems towards 6G networks. In practice, VLC can make good use of lighting system infrastructures to transmit data using light fidelity (Li-Fi). The use of semiconductor light sources, including light-emitting diodes (LEDs) and laser diodes (LDs) are essential to VLC technology because these devices are energy-efficient and have long lifespans. To achieve high-speed VLC links, various technologies have been utilized, including injection locking. Optical injection locking (OIL) is an optical frequency and phase synchronization technique that has been implemented in semiconductor laser systems for performance enhancement. High-performance optoelectronic devices with narrow linewidth, wide tunable emission, large modulation bandwidth and high data transmission rates are desired for advanced VLC. Thus, the features of OIL could be promising for building high-performance VLC systems. In this paper, we present a comprehensive review of the implementation of the injection-locking technique in optical communication systems. The enhancement of characteristics through OIL is elucidated. The applications of OIL in VLC systems are discussed. The prospects of OIL for future VLC systems are evaluated.

Comparing the Performance of OFDM and OCDM-based Visible Light Communications: Numerical and Experimental Analysis

Numerical and experimental analysis of visible light communication (VLC) systems based on orthogonal chirp division multiplexing (OCDM) and orthogonal frequency division multiplexing (OFDM) are presented in this paper. Unlike most of the related publications, in this work we numerically compared OCDM and OFDM based VLC systems with the same equalization process, i.e., employing an one tap equalizer (OTE) in both systems. Simulation results in line-of-sight VLC channels show that both systems have the same performance when baseband multicarrier signals are used. The impact of the optical modulation index and the bias current of a previously characterized light-emitting diode were also evaluated. Nevertheless, the experimental demonstrations, in which bandpass signals were used due to the bandpass characteristic of the exploited setup, show that OFDM-based system outperforms the VLC links with OCDM signals, when the OTE equalizer is used in both systems. The performance of the OCDM-based system was improved by using a zero forcing frequency domain equalizer.

Asymmetric hybrid full-duplex POF-based VLC transmission links

This paper presents an asymmetric full-duplex solution based on visible light communications (VLC) for indoor wired and wireless hybrid signal transmission. The signal is distributed by a polymer optical fiber (POF) link and passively radiated. The proposed system includes two VLC links: a low-speed downlink and a high-speed uplink using an image sensor (IS) and photodiode detection (PD) based receivers. PD-based system communication performance is evaluated by means of error vector magnitude (EVM) for different modulation orders quadrature phase shift keying (QPSK), 16- and 64-quadrature amplitude modulation (QAM), while the IS-based system is characterized in terms of the success of reception (SoR). High-speed data transmission up to 160 Mbps with a 16-QAM modulation scheme and low-speed data transmission up to 500 bps with 98.6% of SoR using a non-return zero on–off keying (NRZ-OOK) scheme over a 1.5 m optical wireless link are demonstrated. Finally, simulations of the wireless channel are performed to evaluate the misalignment tolerance of the system; and, to estimate the uplink performance at longer distances.

Demonstration of a hybrid A-RoF/VLC system for beyond 5G applications.

This work reports the implementation of a fiber-optics and visible light communication (VLC)-based flexible optical distribution network for beyond the fifth generation of mobile networks (B5G) applications. The proposed hybrid architecture is composed of a 12.5-km single-mode fiber fronthaul based on the analog radio-over-fiber (A-RoF) technology, followed by a red, green, and blue (RGB)-based VLC link of 1.2 m. As a proof of concept, we experimentally demonstrate a successful deployment of a 5G hybrid A-RoF/VLC system without using pre-/post-equalization, digital pre-distortion, or individual filters for each color, by means of using a dichroic cube filter at the receiver side. The system performance is evaluated in terms of the root mean square error vector magnitude (E V M R M S ), in accordance with the 3rd Generation Partnership Project requirements, and as a function of the light-emitting diodes' injected electrical power and signal bandwidth.