Towards a 100 Gb/s visible light wireless access network

Hardware implementation of a mirror array-based optical intelligent reflecting surface for VLC: prototype and experimental results

Faced with growing demands for high-speed and reliable communication systems, optical intelligent reflecting surfaces (OIRS) have recently attracted a lot of interest in visible light communication (VLC). With potential applications in a variety of scenarios, including indoor wireless communications and the Internet of Things (IoT), OIRS is expected to have a transformative impact on optical wireless communications. However, current research is predominantly theoretical, and the hardware implementation of OIRS is insufficient. Therefore, this paper introduces an OIRS prototype based on a mirror array, which is capable of adjusting the reflected lightwave by manipulating the orientation of individual OIRS units to realize an adjustable optical wireless communication environment. Additionally, a hardware platform with a configurable control system for OIRS-based VLC has been developed in this paper. Finally, experimental results demonstrate significant improvements in the amplitude of the received signal and the signal-to-noise ratio of the developed prototype, thereby verifying the enhancement of communication efficiency and the potential of practical OIRS deployment.

Fast-beam-switching optical phased array for moving objects in wireless optical communication networks

For optical wireless communication systems, mechanical beam steering struggles to timely switch between multiple users or search for moving users. Here we demonstrate a fast-beam-switching optical phased array (OPA) for agile wireless communication networks. For point-to-multi-point (P2MP) scenarios, a setup of OPA-based fast beam switching between two aligned receivers was developed. A loss-free image transmission experiment was used to demonstrate the stability of switching. Furthermore, we have developed an approach to using the fast-switching OPA to follow the trajectory of moving objects so as to help enable agile random-access switching between moving objects. These results could help offer fast switching and reconfiguration for indoor wireless optical communications.

Design of double-lattice GaN-PCSEL based on triangular and circular holes

We have theoretically designed a double-lattice photonic crystal surface-emitting laser (PCSEL) based on triangular and circular holes. In the design, porous-GaN which has the properties of lower refractive index and high quality stress-free homo-epitaxy with GaN, was first proposed to be the cladding layer for GaN-PCSEL. The finite difference-time domain (FDTD), the plane wave expansion (PWE), and the rigorous coupled-wave analysis (RCWA) method were employed in the investigation. Our simulations achieved a radiation constant of up to 50 cm-1 and a slope efficiency of more than 1 W/A while maintaining a low threshold gain. We conducted a systematic study on the effects of the filling factor, etching depth, and holes shift, on the performance of the PCSEL. The findings indicate that increasing the filling factor improves the radiation constant and slope efficiency. Asymmetric hole patterns and varying etching depths have a similar effect. The introduction of asymmetric patterns and a double lattice in the photonic crystal breaks the symmetry of electric fields in the plane, while different etching depths of the two holes break the symmetry in the vertical direction. Additionally, altering the shift of the double lattice modifies the optical feedback in the resonators, resulting in variations of cavity loss and confinement factor.

A Multicriteria Decision-Making Framework for Access Point Selection in Hybrid LiFi/WiFi Networks Using Integrated AHP-VIKOR Technique

Since light fidelity (LiFi) and wireless fidelity (WiFi) do not interfere with one another, a LiFi/WiFi hybrid network may provide superior performance to existing wireless options. With many users and constant changes, a network can easily become overloaded, leading to slowdowns and fluctuations in data transfer speeds. Access point assignment (APA) is required with the increase of users, which can negatively impact the system performance and quality-of-service (QoS) due to mobility and line-of-sight (LOS) blockage. Many variables could influence the APA process; these variables are considered as criteria, such as the network capacity, the degree of blockage, the speed of the connected user, etc. Unlike conditional APA methods, recent studies have considered treating these variables as "evaluation criteria". Considering these criteria can offer better and more accurate results, eventually enhancing the APA process and QoS. However, the variety of these criteria, the conflict amongst them, their weights (importance), and priority have not been addressed so far. Moreover, treating the criteria equally might result in inaccurate outcomes. Therefore, to solve this issue, it is essential to investigate the impact of each criterion on the APA process. In this work, a multicriteria decision-making (MCDM) problem is formulated to determine a network-level selection for each user over a period of time The decision problem is modeled as a hierarchy that fragments a problem into a hierarchy of simple and small subproblems, and the selection of the AP network among various alternatives is a considered as an MCDM problem. Based on the previous works, we are not aware of any previous research attempts using MCDM methods in the LiFi research area for network selection. Therefore, this work proposes an access point assignment framework using an MCDM approach for users in a hybrid LiFi/WiFi network. The experiment was conducted based on four phases: Five criteria were identified and evaluated with eleven APs (alternatives). The outcome of this phase was used to build the decision matrix and an MCDM was developed and built based on user mobility and blockages with various scenarios using all the criteria; The analytic hierarchy process (AHP) was employed to identify the criterion of the subjective weights of each criterion and to determine the degree of importance supported by experts' judgement. Determining the weights in the AHP process considered various investigations, including the consistency ratio (CR) and the AHP consensus indicator, which is calculated using the rank-based maximum likelihood method (RGMM) and Shannon entropy techniques. The VIekriteri-Jumsko KOmpromisno Rangiranje (VIKOR) method is adopted in the selection of the optimal AP for the proper selection of whether a LiFi or WiFi AP must serve the users. The integrated AHP-VIKOR was effective for solving the APA and was the best solution based on using weighted criteria simultaneously. Moreover, the ranking outcomes of the developed integrated AHP-VIKOR approach were evaluated using sensitivity analysis. The result of this work takes the APA for hybrid LiFi networks to a new perspective.

Hybrid NOMA/OFDMA visible light communication system with coordinated multiple point transmission

In this study, a hybrid non-orthogonal multiple access / orthogonal frequency division multiple access (NOMA/OFDMA) scheme is proposed for multi-cell multi-user visible light communication (VLC) systems. Each cell is divided into several sub-cells that are multiplexed using OFDMA. Users within the same sub-cell are multiplexed by NOMA. Thus, the hybrid NOMA/OFDMA scheme takes advantage of both NOMA and OFDMA, which not only improves the spectral efficiency of OFDMA but also avoids the co-channel interference of NOMA. Moreover, coordinated multiple-point transmission based on repeated coding is introduced to eliminate multiuser interference, which also improves the received signal-to-noise ratio of edge users. In this manner, spectrum resources are fully utilized, where the frequency reuse factor is equal to 1. Furthermore, we propose a two-dimensional power-allocation algorithm for the proposed hybrid NOMA/OFDMA VLC system. Based on the fixed power allocation strategy, power is allocated jointly among sub-cells and users within sub-cells to minimize the average symbol error rate (SER). The performance of the proposed system was investigated in detail by simulation, where the SERs were evaluated under different power ratios. Simulation results also show that the SER performance of the proposed hybrid NOMA/OFDMA VLC system is significantly improved compared to the traditional NOMA and OFDMA VLC systems in different VLC networks. Finally, the proof-of-concept experiment was set up, clearly validating the superiority of the proposed system further.

A Survey of Hybrid Free Space Optics (FSO) Communication Networks to Achieve 5G Connectivity for Backhauling

Increased capacity, higher data rate, decreased latency, and better service quality are examples of the primary objectives or needs that must be catered to in the near future, i.e., fifth-generation (5G) and beyond. To fulfil these needs, cellular network design must be drastically improved. The 5G cellular network design, huge multiple-input multiple-output (MIMO) technology, and device-to-device communication are all highlighted in this comprehensive study. Hence, free-space optics (FSO) is a promising solution to address this field. However, FSO standalone is insufficient during turbulent weather conditions. FSO systems possess some limitations, such as being able to be disturbed by any interference between sender and receiver such as a flying bird and a tree, as it requires line-of-sight (LOS) connectivity. Moreover, it is sensitive to weather conditions; the FSO performance significantly decreases in bad weather conditions such as fog and snow; those factors deteriorate the performance of FSO. This paper conducts a systematic survey on the existing projects in the same area of research such as the hybrid FSO/Radio frequency (RF) communication system by listing each technique used for each model to achieve optimum performance in terms of data rate and Bit Error Rate (BER) to be implemented in 5G networks.

Three-Phase Handover Management and Access Point Transition Scheme for Dynamic Load Balancing in Hybrid LiFi/WiFi Networks

Since LiFi and WiFi do not interfere with one another, a LiFi/WiFi hybrid network may provide superior performance to existing wireless options. With a large number of users and constant changes, a network can easily become overloaded, leading to slowdowns and fluctuations in data transfer speeds. Handover (HO) increases significantly with an increase in users, which can negatively impact system performance and quality of service (QoS) due to connection loss and/or delay. Innovative three-phase handover management and AP transition (TPHM-APT) is proposed with the goals of maintaining a steady link with reduced HOs for all connected users, meeting high per-user data rates, and having low outage performance. The proposed scheme primarily focuses on reducing the total number of HOs, which improves reliability and keeps user densities low on individual LiFi APs, which conserves bandwidth and energy. Conventional methods of HO management and user assignment, such as those based on signal strength strategy (SSS), involve reallocating users to a different AP the moment they encounter a HO. Our technique consists of three stages that focus on the optical gain, the incidence angle of the receiver FOV, and user mobility speed for decision-making. Specifically, a data rate threshold (DRT), which is equivalent to the data rate gained from the optical gain, is used to determine whether users must be served by a LiFi or a WiFi AP. In addition, an incidence angle threshold (IAT) is identified to manage the handover process and user AP transition with the consideration of the user mobility threshold (UMT). The proposed method considers load balancing (LB) among all connected users as well. This approach is evaluated using Monte Carlo simulations with MATLAB. Mathematical expressions are derived to analyze the performance of the proposed method. Different aspects, for example, Outage Probability, HO Overhead, User density, System Average Throughput (SAT), and Average Data Rate Requirement (ADRR), are studied. Analysis shows performance gains in overall system performance in terms of system data rates, fairness, and HO rates. Simulation results show that against the standard HO scheme and traditional HO skipping and APA methods, the proposed scheme can effectively decrease HO rates, save LiFi resources, and increase user throughput. It also shows good correspondence to the analysis and reveals the associated trade-offs that occur when moving between the span of narrow to wide FOVs and vice versa (HO rates and APS). The proposed scheme achieves almost identical results for low-density and high-density systems as well, with different ADRR and HO overhead values.

In-Vehicle Visible Light Communications Data Transmission System Using Optical Fiber Distributed Light: Implementation and Experimental Evaluation

Visible light communications emerges as a promising wireless communication technology that has been found suitable for numerous indoor and outdoor applications. In this article, a new in-vehicle VLC system is designed, implemented, and experimentally evaluated. The purpose of this new system is to provide car passengers with optical wireless communications. The proposed system consists of a VLC emitter integrated into the vehicle's ambient lighting system and a mobile VLC receiver. Unlike any previous works, this article proposes a VLC emitter in which the light from a 3 W LED is distributed on a 2 square meter surface using 500 optical fibers whose main purpose is a decorative one. The proposed prototype has been implemented on a car and evaluated in relevant working conditions. The experimental evaluation of the proposed system has demonstrated the viability of the proposed concept and showed a data rate of 250 kb/s while providing a BER lower than 10^-7. As far as we know, the proposed concept is totally new in the VLC literature, opening a new area of utilization for VLC technology: using VLC with optical fiber distributed light.

Design, Implementation and Experimental Investigation of a Pedestrian Street Crossing Assistance System Based on Visible Light Communications

In urban areas, pedestrians are the road users category that is the most exposed to road accident fatalities. In this context, the present article proposes a totally new architecture, which aims to increase the safety of pedestrians on the crosswalk. The first component of the design is a pedestrian detection system, which identifies the user's presence in the region of the crosswalk and determines the future street crossing action possibility or the presence of a pedestrian engaged in street crossing. The second component of the system is the visible light communications part, which is used to transmit this information toward the approaching vehicles. The proposed architecture has been implemented at a regular scale and experimentally evaluated in outdoor conditions. The experimental results showed a 100% overall pedestrian detection rate. On the other hand, the VLC system showed a communication distance between 5 and 40 m when using a standard LED light crosswalk sign as a VLC emitter, while maintaining a bit error ratio between 10^-7 and 10^-5. These results demonstrate the fact that the VLC technology is now able to be used in real applications, making the transition from a high potential technology to a confirmed technology. As far as we know, this is the first article presenting such a pedestrian street crossing assistance system.

Effective Current Pre-Amplifiers for Visible Light Communication (VLC) Receivers

Visible light communication (VLC) is an upcoming wireless communication technology. In a VLC system, signal integrity under low illumination intensity and high transmission frequencies are of great importance. Towards this direction, the performance of the analog front end (AFE) sub-system either at the side of the transmitter or the receiver is crucial. However, little research on the AFE of the receiver is reported in the open literature. Aiming to enhance signal integrity, three pre-amplification topologies for the VLC receiver AFE are presented and compared in this paper. All three use bipolar transistors (BJT): the first consists of a single BJT, the second of a double BJT in cascade connection, and the third of a double BJT in Darlington-like connection. In order to validate the performance characteristics of the three topologies, simulation results are provided with respect to the light illumination intensity, the data transmission frequency and the power consumption. According to these simulations, the third topology is characterized by higher data transmission frequencies, lower illuminance intensity and lower power consumption per MHz of operation.

Investigation on GaN-Based Membrane Photonic Crystal Surface Emitting Lasers

A GaN-based blue photonic crystal surface emitting laser (PCSEL) featured with membrane configuration was proposed and theoretically investigated. The membrane dimension, photonic crystal (PhC) material, lattice constant and thickness were studied by RCWA (Rigorous Coupled Wave Analysis), FDTD (Finite Difference Time Domain) simulations with the confinement factor and gain threshold as indicators. The membrane PCSEL's confinement factor of active media is of 13~14% which is attributed to multi-pairs of quantum wells and efficient confinement of the mode in the membrane cavity with air claddings. The excellent confinement factor and larger Q factor of resonance mutually contribute to the lower gain threshold of the design (below 400 cm-1 for GaN-PhC with 100 nm thick top and bottom GaN layer, 40 nm hole radius and 40 nm depth). The PhC confinement factor exceeds 13% and 6% for TiO2-PhC with 80 nm and 60 nm PhC thickness and 20 nm and 40 nm distance between PhC and active media, respectively. It is around two times larger than that of GaN-PhC, which is attributed to the higher refractive index of TiO2 that pulls field distribution to the PhC layer.

Effective interference mitigation scheme for multi-LED-based mobile optical camera communication

Recently, optical camera communication (OCC) based on light-emitting diodes (LEDs) and complementary metal oxide semiconductor (CMOS) image sensors has received extensive attention due to its low cost and high flexibility. However, considering that the location of the user with a CMOS image sensor changes frequently, multi-LEDs are required to achieve seamless communication. In the paper, to mitigate random interchannel interference in overlapping areas of LED radiation, an effective identification recognition and sorting-based column selection are proposed. Meanwhile, to alleviate the distortion of bright and dark stripes caused by overlapping exposure time and user movement, an improved sampling scheme based on length estimation is proposed and experimentally demonstrated in a multi-LED-based mobile OCC system. Moreover, by varying the overlap rate of the adjacent LEDs' radiation area and the moving speed of the user, respectively, the performance of the system is investigated. The experimental results show that, the bit error rate (BER) performance of the system decreases slightly with the increasing of moving speed, but it is lower than the hard-decision forward error code rate limit. In addition, at the overlap rate of the LED radiation area of 4.3% and the moving speed of 80 cm/s, the BER is close to 10^-6 using the proposed scheme.

A Review on LiFi Network Research: Open Issues, Applications and Future Directions

This paper extensively reviews and analyses Light Fidelity (LiFi), a new technology that uses light to transmit data as a high-speed wireless connection system from a wide spectrum of domains. An in-depth analysis and classifications of pertinent research areas for LiFi networks are presented in this paper. The various aspects constituting this paper include a detailed literature review, proposed classifications, and statistics, which further is deliberated to encompass applications, system architecture, system components, advantages, and disadvantages. LiFi and other technologies are compared, multi-user access techniques used in LiFi networks are investigated and open issues are addressed in detail. The paper is concluded with a comprehensive taxonomy of literature comparison that has served as the basis of the proposed open issues and research trends.

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

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

Hybrid POF/VLC Links Based on a Single LED for Indoor Communications

A hybrid fiber/wireless link based on a single visible LED and free of opto-electronic intermediate conversion stages has been demonstrated for indoor communications. This paper shows the main guidelines for proper coupling in fiber/air/detector interfaces. Experimental demonstration has validated the design results with very good agreement between geometrical optics simulation and received optical power measurements. Different signal bandwidths and modulation formats, i.e., QPSK, 16-QAM, and 64-QAM, have been transmitted over 1.5 m polymer optical fiber (POF) and 1.5 m free-space optics (FSO). Throughputs up to 294 Mb/s using a 64-QAM signal have been demonstrated using a commercial LED, which paves the way for massive deployment in industrial applications.

Evaluation of Misalignment Effect in Vehicle-to-Vehicle Visible Light Communications: Experimental Demonstration of a 75 Meters Link

The use of visible light communications technology in communication-based vehicle applications is gaining more and more interest as the research community is constantly overcoming challenge after challenge. In this context, this article addresses the issues associated with the use of Visible Light Communications (VLC) technology in Vehicle-to-Vehicle (V2V) communications, while focusing on two crucial issues. On the one hand, it aims to investigate the achievable communication distance in V2V applications while addressing the least favorable case, namely the one when a standard vehicle rear lighting system is used as a VLC emitter. On the other hand, this article investigates another highly unfavorable use case scenario, i.e., the case when two vehicles are located on adjacent lanes, rather than on the same lane. In order to evaluate the compatibility of the VLC technology with the usage in inter-vehicle communication, a VLC prototype is intensively evaluated in outdoor conditions. The experimental results show a record V2V VLC distance of 75 m, while providing a Bit Error Ratio (BER) of 10⁻⁷–10⁻⁶. The results also show that the VLC technology is able to provide V2V connectivity even in a situation where the vehicles are located on adjacent lanes, without a major impact on the link performances. Nevertheless, this situation generates an initial no-coverage zone, which is determined by the VLC receiver reception angle, whereas in some cases, vehicle misalignment can generate a BER increase that can go up to two orders of magnitude.

Beam steering in a narrow-beam phosphor down-converted white light visible light communication link using transmitter lens decentering

Indoor visible light communication (VLC) systems with narrow beams can achieve practical few-meters-long wireless optical links. Such links can operate at low power levels and high data rate for supporting point-to-point or multipoint communication. The narrow-beam VLC links can, however, benefit from beam steering to support mobility of user equipment and cater to multiple users. Simple beam-steering techniques with minimal changes to the existing optical hardware are required to enable widespread adoption of beam steering in VLC links. We study the performance of a simple transmitter-lens-decenter-based beam-steering scheme in a VLC link, utilizing a phosphor down-converted blue laser transmitter. The beam-steering angle and hence the receiver coverage depend on the transmitter lens decenter and the choice of the transmitter and receiver lens's focal lengths. Optical ray tracing is used to quantify the collection efficiency achievable with beam steering, choose a suitable receiver lens, and understand the role of off-axis aberration in the system performance. In our experimental implementation, the transmitter lens decentering technique results in a maximum steering angle of 7.1°. This corresponds to a receiver coverage of 30 cm per cm of transmitter lens decenter for a fixed link length of 300 cm. The measured on-axis white light color coordinates of (0.286, 0.253) is found to shift toward warmer white colors with beam steering. The on-axis illuminance level of ∼19lux decreases slightly with beam steering and is found to be below the maximum permissible exposure limit for indoor illumination. We also quantify the data communication performance as a function of beam steering using on-off modulated data. Bit-error rates below the forward error correction limit are obtained for receiver coverage diameter of 75 cm and 60 cm for 1.25 Gbps and 1.5 Gbps data rates, respectively.

On Improving 5G Internet of Radio Light Security Based on LED Fingerprint Identification Method

In this paper, a novel device identification method is proposed to improve the security of Visible Light Communication (VLC) in 5G networks. This method extracts the fingerprints of Light-Emitting Diodes (LEDs) to identify the devices accessing the 5G network. The extraction and identification mechanisms have been investigated from the theoretical perspective as well as verified experimentally. Moreover, a demonstration in a practical indoor VLC-based 5G network has been carried out to evaluate the feasibility and accuracy of this approach. The fingerprints of four identical white LEDs were extracted successfully from the received 5G NR (New Radio) signals. To perform identification, four types of machine-learning-based classifiers were employed and the resulting accuracy was up to 97.1%.

Deep learning-based detection scheme for visible light communication with generalized spatial modulation

In this paper, a deep learning-based detection scheme is proposed for the visible light communication (VLC) systems using generalized spatial modulation (GenSM). In the proposed detection scheme, a deep neural network consisting of several neural layers is applied to detect the received signals. By integrating the signal processing modules of the conventional detection schemes into one deep neural network, the proposed scheme is able to extract the information bits from the received signals efficiently. After offline training, the proposed detection scheme can serve as a promising detection method for the VLC system with GenSM. Simulation results validate that the proposed detection scheme is capable of achieving superior detection error performance than conventional detection schemes at acceptable complexity.

Advanced Progress of Optical Wireless Technologies for Power Industry: An Overview

Optical wireless communications have attracted widespread attention in the traditional power industry because of the advantages of large spectrum resources, strong confidentiality, and freedom from traditional electromagnetic interference. This paper mainly summarizes the major classification and frontier development of power industry optical wireless technologies, including the indoor and outdoor channel characteristics of power industry optical wireless communication system, modulation scheme, the performance of hybrid power line, and indoor wireless optical communications system. Furthermore, this article compares domestic and foreign experiments, analyzes parameters for instance transmission rate, and reviews different application scenarios such as power wireless optical positioning and monitoring. In addition, in view of the shortcomings of traditional power technology, optical wireless power transfer technology is proposed and combined with unmanned aerial vehicles to achieve remote communication. At last, the main challenges and possible solutions faced by power industry wireless optical technologies are proposed.

Constellation optimization under the ergodic VLC channel based on generalized spatial modulation

When the generalized spatial modulation (GSM) is applied to visible light communication (VLC), radio frequency (RF) technologies, such as constellation designs, should be modified for the ergodic VLC channel, which is different from that in typical RF environments. In this paper, constellations of amplitude-phase modulation (APM) symbols are optimized for GSM-based VLC systems in which optical signals must be real-valued and non-negative, while the ergodic VLC channel and the random characteristic of receiver locations are also considered. The algorithm of statistically convergent gradient descent (SCGD) is adopted for the maximization of coded modulation average mutual information (CM-AMI), and superior performances of optimal constellations are verified by numerical simulations.

Noise-Adaptive Visible Light Communications Receiver for Automotive Applications: A Step Toward Self-Awareness

Visible light communications are considered as a promising solution for inter-vehicle communications, which in turn can significantly enhance the traffic safety and efficiency. However, the vehicular visible light communications (VLC) channel is highly dynamic, very unpredictable, and subject to many noise sources. Enhancing VLC systems with self-aware capabilities would maximize the communication performances and efficiency, whatever the environmental conditions. Within this context, this letter proposes a novel signal to noise ratio (SNR)-adaptive visible light communication receiver architecture aimed for automotive applications. The novelty of this letter comes from an open loop signal processing technique in which the signal treatment complexity is established based on a real-time SNR analysis. So, the receiver evaluates the SNR, and based on this assessment, it reconfigures its structural design in order to ensure a proper signal treatment, while providing an optimal tradeoff between communication performances and computational resources usage. This approach based on software reconfiguration has the potential to provide the system with enhanced flexibility and enables its usage in resource sharing application. As far as we know, this approach has not been considered in vehicular VLC systems. The performances of the proposed architecture are demonstrated by simulations, which confirm the SNR-adaptive capacity and the optimized performances.

Noise-Adaptive Visible Light Communications Receiver for Automotive Applications: A Step Toward Self-Awareness

Visible light communications are considered as a promising solution for inter-vehicle communications, which in turn can significantly enhance the traffic safety and efficiency. However, the vehicular visible light communications (VLC) channel is highly dynamic, very unpredictable, and subject to many noise sources. Enhancing VLC systems with self-aware capabilities would maximize the communication performances and efficiency, whatever the environmental conditions. Within this context, this letter proposes a novel signal to noise ratio (SNR)-adaptive visible light communication receiver architecture aimed for automotive applications. The novelty of this letter comes from an open loop signal processing technique in which the signal treatment complexity is established based on a real-time SNR analysis. So, the receiver evaluates the SNR, and based on this assessment, it reconfigures its structural design in order to ensure a proper signal treatment, while providing an optimal tradeoff between communication performances and computational resources usage. This approach based on software reconfiguration has the potential to provide the system with enhanced flexibility and enables its usage in resource sharing application. As far as we know, this approach has not been considered in vehicular VLC systems. The performances of the proposed architecture are demonstrated by simulations, which confirm the SNR-adaptive capacity and the optimized performances.

Blue-laser-diode-based high CRI lighting and high-speed visible light communication using narrowband green-/red-emitting composite phosphor film

We experimentally demonstrate high-speed visible light communication (VLC) and high-quality solid-state lighting (SSL) using polymethyl-methacrylate-doped phosphor film based on cesium lead bromide quantum dot (CsPbBr₃-QD) and potassium fluorosilicate K₂SiF₆:Mn⁴⁺, which is excited by a blue gallium nitride laser diode. A 1.6 Gbps data rate is achieved by employing a non-return-to-zero on-off keying modulation scheme. The measured bit error rate of 2.7×10^-3 adheres to the standard threshold (3.8×10^-3) of forward error correction. Moreover, the generated white-light source has a high color rendering index of 93.8 and a correlated color temperature of 4435 K, and it exhibits a Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinate at (0.3556, 0.3520), which is close to the ideal CIE value of white light (0.3333, 0.3333). This work opens up exciting possibilities for future high-speed indoor VLC and high-quality SSL.

Design and Intensive Experimental Evaluation of an Enhanced Visible Light Communication System for Automotive Applications

As the interest toward communication-based vehicle safety applications is increasing, the development of secure wireless communication techniques has become an important research area. In this context, the article addresses issues that are related to the use of the visible light communication (VLC) technology in vehicular applications. Thus, it provides an extensive presentation concerning the main challenges and issues that are associated to vehicular VLC applications and of some of the existing VLC solutions. Moreover, the article presents the aspects related to the design and intensive experimental evaluation of a new automotive VLC system. The experimental evaluation performed in indoor and outdoor conditions shows that the proposed system can achieve communication distances up to 50 m and bit error ratio (BER) lower than 10-6, while being exposed to optical and weather perturbations. This article provides important evidence concerning the snowfall effect on middle to long range outdoor VLC, as the proposed VLC system was also evaluated in snowfall conditions. Accordingly, the experimental evaluation showed that snowfall and heavy gust could increase bit error rate by up to 10,000 times. Even so, this article provides encouraging evidence that VLC systems will soon be able to reliably support V2X communications.

Some practical constraints and solutions for optical camera communication

Mobile wireless communication heavily relies on the radio frequency to convey message and data. However, its limited spectrum can hardly meet the demands for the future high data rate applications. Optical wireless communication, in particular visible light communication, opens up vast optical spectrum for communication, and meanwhile can retrofit the light sources as the communication transmitters in the existing working or living environments. In conjunction with the ubiquitous cameras in hand-held consumer electronics such as smartphones and pads, optical camera communication (OCC) further takes advantages of image sensors as the communication receivers and realizes low-cost communication systems. This article first provides an overview of OCC systems. It then addresses some practical constraints, ranging from sensor low frame rate and instability, rolling shutter readout, to visual qualities of displayed images and videos, and link blockage between the transmitter and receiver. Accordingly, it introduces existing and new solutions to deal with those constraints by data modulation, newly developed camera structures, post-processing of sensed signals and non-line of sight OCC as a new form. In particular, indirect paths by either the indoor surface reflection or the outdoor atmospheric scattering are explored for link connectivity under blockage. Finally, some future research directions are suggested. This article is part of the theme issue 'Optical wireless communication'.

The Role of Optical Wireless Communication Technologies in 5G/6G and IoT Solutions: Prospects, Directions, and Challenges

The upcoming fifth- and sixth-generation (5G and 6G, respectively) communication systems are expected to deal with enormous advances compared to the existing fourth-generation communication system. The few important and common issues related to the service quality of 5G and 6G communication systems are high capacity, massive connectivity, low latency, high security, low-energy consumption, high quality of experience, and reliable connectivity. Of course, 6G communication will provide several-fold improved performances compared to the 5G communication regarding these issues. The Internet of Things (IoT) based on the tactile internet will also be an essential part of 5G-and-beyond (5GB) (e.g., 5G and 6G) communication systems. Accordingly, 5GB wireless networks will face numerous challenges in supporting the extensive verities of heterogeneous traffic and in satisfying the mentioned service-quality-related parameters. Optical wireless communication (OWC), along with many other wireless technologies, is a promising candidate for serving the demands of 5GB communication systems. This review paper clearly presents how OWC technologies, such as visible light communication, light fidelity, optical camera communication, and free space optics communication, will be an effective solution for successful deployment of 5G/6G and IoT systems.

Design and Implementation of a Multi-Colour Visible Light Communication System Based on a Light-to-Frequency Receiver

Colour-shift keying (CSK) is a visible light communication (VLC) modulation scheme used in the existing IEEE 802.15.7 standard. In CSK, information is transmitted by changing the light intensities of the RGB LEDs. In this work, a low-complexity VLC system is proposed using CSK modulation and a novel receiver based on a light-to-frequency (LTF) converter. At the receiver, CSK symbols are interpreted and decoded in terms of frequencies, which are processed by a counter module of a generic microcontroller, thus avoiding the use of analog-to-digital converters (ADCs), which results in a low-cost VLC system. The main contributions of this work are summarized in the following key points: (1) A low-complexity receiver for CSK modulation is introduced; (2) A particle swarm optimization (PSO) algorithm for CSK constellation design is suggested considering the restrictions of the LTF based receiver; (3) Experimental and theoretical validation is perfomed for the proposed multi-colour VLC system. The results show that this system can provide a transmission speed of 100 kbps using a 4-CSK-LTF constellation for a symbol error rate (SER) of 10 − 4 and a signal to noise ratio (SNR) around 35 dB. These results suggest that the analysed system could find applications on those scenarios where low transmission speeds and ease of deployment are the goals.

A Wide-Area Coverage 35 Gb/s Visible Light Communications Link for Indoor Wireless Applications

Visible Light Communications (VLC) can provide both illumination and communications and offers a means to alleviate the predicted spectrum crunch for radio-frequency wireless communications. In this paper, we report a laser diode based white-light communications link that operates over a wide area and supports high data rates. The proposed system is a four-colour multiplexed high-speed VLC system that uses a microelectromechanical system (MEMS) mirror-based beam-steering. The system operates at record data-rates of more than 35 Gb/s (Bit Error Rate(BER) < 3.8 × 10⁻³) with a coverage area of 39 m² at a link distance of 4 m. To the best of our knowledge this is the fastest VLC demonstration reported thus far. The paper also addresses issues of eye-safety, showing data rates of  more than 10 Gb/s are feasible.

Enabling user mobility for optical camera communication using mobile phone

We propose and experimentally demonstrate an optical camera communication (OCC) supporting user mobility. A mobile test platform is designed to emulate user mobility. In the mobile scenario, dynamic column matrix selection algorithm is proposed to select an appropriate column matrix with high extinction-ratio (ER) while avoiding the blooming effect. The mobile phone is placed on the moving track to receive the visible light at a vertical distance of 60 cm. By varying the moving speed at 20, 40, 60, 80, and 100 cm/s and lateral distance at 50 and 70 cm respectively, the system performance using the proposed algorithm is investigated. The experimental results show that with the increase of lateral distance (far from the light source) and user moving speed, the system performance gets degraded. Moreover, it demonstrates that the mobile system can achieve a throughput of 4.08 kbps under a low illuminance of 275 lx.

Laser-based white-light source for high-speed underwater wireless optical communication and high-efficiency underwater solid-state lighting

White light generated by mixing the red, green, and blue laser diodes (RGB LDs) for simultaneous high-speed underwater wireless optical communication (UWOC) and high-efficiency underwater solid-state lighting (SSL) was proposed and demonstrated experimentally for the first time. The allowable maximum real-time data transmission rates of 3.2 Gbps, 3.4 Gbps, and 3.1 Gbps for RGB LDs with corresponding BERs of 3.6 × 10^-3, 3.5 × 10^-3 and 3.7 × 10^-3 were obtained at a 2.3 m underwater transmission distance using an on-off keying (OOK) modulation scheme, respectively. And the corresponding UWOC aggregate data rate of 9.7 Gbps was achieved based on RGB LDs-based wavelength-division multiplexing (WDM) UWOC. Moreover, UWOC and underwater SSL by using RGB LDs mixed white light were investigated at different scenarios over an underwater link of 2.3 m. The RGB LDs mixed white light-based UWOC system without optical diffusers yielded a maximum allowable data rate of 8.7 Gbps with Commission International de l'Eclairage coordinates (CIE) of (0.3154, 0.3354), a correlated color temperature of 6322 K, a color rendering index of 69.3 and a corresponding illuminance of 7084 lux. Furthermore, optical diffusers were employed to provide large-area underwater SSL. The LDs mixed white light-based UWOC system with line and circle optical diffusers implemented data rates of 5.9 Gbps and 6.6 Gbps with CIE coordinates of (0.3183, 0.3269) and (0.3298, 0.3390), respectively. This work suggests the potential of LDs for applications in high-efficiency underwater white-light SSL and high-speed UWOC.

High-speed indoor optical wireless communication system employing a silicon integrated photonic circuit

Beam-steering-based optical wireless technologies are being widely investigated due to the capability of providing high-speed wireless connectivity in indoor applications. However, high-speed indoor optical wireless systems are traditionally realized with discrete bulky components, significantly limiting their practical applications. In this Letter, we demonstrate an infrared optical wireless communication system employing a miniaturized silicon integrated photonic circuit for beam steering for the first time. Experimental results show that up to 12.5 Gb/s optical wireless communication can be achieved with error-free performance over a free-space range of 140 cm, and limited mobility of users can be realized. The experimental results of this Letter open the way for realizing integrated high-speed optical wireless communications.

Room-temperature continuous-wave electrically pumped InGaN/GaN quantum well blue laser diode directly grown on Si

Current laser-based display and lighting applications are invariably using blue laser diodes (LDs) grown on free-standing GaN substrates, which are costly and smaller in size compared with other substrate materials.^1-3 Utilizing less expensive and large-diameter Si substrates for hetero-epitaxial growth of indium gallium nitride/gallium nitride (InGaN/GaN) multiple quantum well (MQW) structure can substantially reduce the cost of blue LDs and boost their applications. To obtain a high crystalline quality crack-free GaN thin film on Si for the subsequent growth of a blue laser structure, a hand-shaking structure was formed by inserting Al-composition step down-graded AlN/Al_xGa_1-xN buffer layers between GaN and Si substrate. Thermal degradation in InGaN/GaN blue MQWs was successfully suppressed with indium-rich clusters eliminated by introducing hydrogen during the growth of GaN quantum barriers (QBs) and lowering the growth temperature for the p-type AlGaN/GaN superlattice optical cladding layer. A continuous-wave (CW) electrically pumped InGaN/GaN quantum well (QW) blue (450 nm) LD grown on Si was successfully demonstrated at room temperature (RT) with a threshold current density of 7.8 kA/cm².

Performance of heterodyne differential phase-shift-keying underwater wireless optical communication systems in gamma-gamma-distributed turbulence

The analytical expressions for the average bit error rate and the outage probability of a heterodyne differential phase-shift-keying underwater wireless optical communication (UWOC) system are derived with proper consideration of all of the channel-degrading effects, including absorption, scattering, and turbulence-induced fading. The scintillation index of a spherical wave is evaluated in order to quantify the underwater system performance in a strong turbulence regime. The spherical wave propagating through the strong underwater turbulence environment is modeled as gamma-gamma distribution. Then, the system performance is simulated for various variations of the underwater turbulence, i.e., the rate of dissipation of kinetic energy per unit mass of fluid, the ratio of temperature to salinity contributions to the refractive index spectrum, and the UWOC system link length. The results show that the analytical expressions for describing the system performance are valid.

Semipolar InGaN quantum-well laser diode with integrated amplifier for visible light communications

GaN-based semiconductor optical amplifier (SOA) and its integration with laser diode (LD) is an essential building block yet to be demonstrated for III-nitride photonic integrated circuits (PICs) at visible wavelength. This paper presents the InGaN/GaN quantum well (QW) based dual-section LD consisting of integrated amplifier and laser gain regions fabricated on a semipolar GaN substrate. The threshold current in the laser gain region was favorably reduced from 229mA to 135mA at SOA driving voltages, V_SOA, of 0V and 6.25V, respectively. The amplification effect was measured based on a large gain of 5.7 dB at V_SOA = 6.25V from the increased optical output power of 8.2 mW to 30.5 mW. Such integrated amplifier can be modulated to achieve Gbps data communication using on-off keying technique. The monolithically integrated amplifier-LD paves the way towards the III-nitride on-chip photonic system, providing a compact, low-cost, and multi-functional solution for applications such as smart lighting and visible light communications.

High-Bandwidth White-Light System Combining a Micro-LED with Perovskite Quantum Dots for Visible Light Communication

This work proposes a high-bandwidth white-light system consisting of a blue gallium nitride (GaN) micro-LED (μLED) exciting yellow-emitting CsPbBr_1.8I_1.2 perovskite quantum dots (YQDs) for high-speed real-time visible light communication (VLC). The packaged 80 μm × 80 μm blue-emitting μLED has a modulation bandwidth of ∼160 MHz and a peak emission wavelength of ∼445 nm. The achievable bandwidth of the white-light system is up to 85 MHz in the absence of filters and equalization technology. Meanwhile, the bandwidth of the YQDs as a color converter is as high as 73 MHz with the blue GaN μLED as the pump source. A maximum data rate of 300 Mbps can be achieved by taking advantage of the high bandwidth of the white-light system using the non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. The resultant bit-error rate is 2.0 × 10^-3, well beneath the forward error correction criterion of 3.8 × 10^-3 required for error-free data transmission. In addition, the YQDs which we proposed as a color converter possess high stability for VLC. After half a year, the achievable bandwidths of the white-light system and the YQDs are still up to 83 and 70 MHz, respectively. This study provides the direction of developing high-bandwidth white-light system for both high-efficiency solid-state lighting and high-speed VLC.

3.2 Gigabit-per-second Visible Light Communication Link with InGaN/GaN MQW Micro-photodetector

This paper presents the first demonstration of InGaN multiple quantum well (MQW) based micro-photodetectors (µPD) used as the optical receiver in orthogonal frequency-division multiplexing (OFDM) modulated visible communication system (VLC). The 80-µm diameter µPD exhibits a wavelength-selective responsivity in the near-UV to violet regime (374 nm - 408 nm) under a low reverse bias of -3 V. The modulation scheme of 16-quadrature amplitude modulation (16-QAM) OFDM enables the use of frequency response beyond -3 dB cutoff bandwidth of µPD. A record high data rate of 3.2 Gigabit per second (Gpbs) was achieved as a result, which provides the proof-of-concept verification of a viable high speed VLC link.

Efficient signal design and optimal power allocation for visible light communication attocell systems

In this paper, we investigate visible light communication (VLC) attocell systems in which any two neighboring attocells overlap. An efficient signal design for VLC called time superposition reuse (TSR) is proposed to mitigate interference and improve spectral efficiency. In the scheme, two neighboring cells are allocated with two time slots that have superposition in the time domain. By adjusting superposition between time slots, the system can achieve a flexible spectral efficiency and system performance. Further, we develop an optimal power allocation strategy for TSR in the system. The strategy is given according to the position of the user and the level of superposition, and then the corresponding optimal Euclidean distance is derived. In addition, we analyze the system performance and prove that the optimal Euclidean distance is an increasing function of the delay between signals and a decreasing function of the sampling period. Simulation results demonstrate that the optimal power allocation has a better performance than uniform power allocation and TSR outperforms time division multiple access significantly for the user in VLC attocell systems.

71-Mbit/s ultraviolet-B LED communication link based on 8-QAM-OFDM modulation

A demonstration of ultraviolet-B (UVB) communication link is implemented utilizing quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM). The demonstration is based on a 294-nm UVB-light-emitting-diode (UVB-LED) with a full-width at half-maximum (FWHM) of 9 nm and light output power of 190 μW, at 7 V, with a special silica gel lens on top of it. A -3-dB bandwidth of 29 MHz was measured and a high-speed near-solar-blind communication link with a data rate of 71 Mbit/s was achieved using 8-QAM-OFDM at perfect alignment. 23.6 Mbit/s using 2-QAM-OFDM when the angle subtended by the pointing directions of the UVB-LED and photodetector (PD) is 12 degrees, thus establishing a diffuse-line-of-sight (LOS) link. The measured bit-error rate (BER) of 2.8 ×10^-4 and 2.4 ×10^-4, respectively, are well below the forward error correction (FEC) criterion of 3.8 ×10^-3. The demonstrated high data-rate OFDM-based UVB communication link paves the way for realizing high-speed non-line-of-sight free-space optical communications.

Violet Laser Diode Enables Lighting Communication

Violet laser diode (VLD) based white-light source with high color rendering index (CRI) for lighting communication is implemented by covering with Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) or Lu₃Al₅O₁₂:Ce³⁺/CaAlSiN₃:Eu²⁺ (LuAG:Ce/CASN:Eu) phosphorous diffuser plates. After passing the beam of VLD biased at 70 mA (~2I _th ) through the YAG:Ce phosphorous diffuser, a daylight with a correlated color temperature (CCT) of 5068 K and a CRI of 65 is acquired to provide a forward error correction (FEC) certified data rate of 4.4 Gbit/s. By using the VLD biased at 122 mA (~3.5I _th ) to excite the LuAG:Ce/CASN:Eu phosphorous diffuser with 0.85-mm thickness, a warm white-light source with a CCT of 2700 K and a CRI of 87.9 is obtained at a cost of decreasing transmission capacity to 2.4 Gbit/s. Thinning the phosphor thickness to 0.75 mm effectively reduces the required bias current by 32 mA to achieve the same CCT for the delivered white light, which offers an enlarged CRI of 89.1 and an increased data rate of 4.4 Gbit/s. Further enlarging the bias current to 105 mA remains the white-light transmission capacity at 4.4 Gbit/s but reveals an increased CCT of 3023 K and an upgraded CRI of 91.5.

Nanopatterned luminescent concentrators for visible light communications

Visible light communication (VLC) is a promising candidate for high-speed wireless communication with numerous unlicensed spectrum. To achieve high-speed data communication, it requires intense light signals concentrated on a tiny fast photodiode. The common way of using focusing optics reduces the field of view (FoV) of the photodiode due to the conservation of étendue. Luminescent solar concentrators (LSC) provide a solution to enhance the signals without affecting the FoV. In this paper we demonstrate nanopatterned LSCs fabricated on flexible plastics that achieve a doubling of optical gain compared to its traditional rectangular counterparts. These LSCs can free VLC detectors from complex active pointing and tracking systems, making them compatible with smart mobile terminals in a simple fashion.

10-m 9.51-Gb/s RGB laser diodes-based WDM underwater wireless optical communication

The availability of the underwater wireless optical communication (UWOC) based on red (R), green (G) and blue (B) lights makes the realization of the RGB wavelength division multiplexing (WDM) UWOC system possible. By properly mixing RGB lights to form white light, the WDM UWOC system has prominent potentiality for simultaneous underwater illumination and high-speed communication. In this work, for the first time, we experimentally demonstrate a 9.51-Gb/s WDM UWOC system using a red-emitting laser diode (LD), a single-mode pigtailed green-emitting LD and a multi-mode pigtailed blue-emitting LD. By employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation in the demonstration, the red-light, the green-light and the blue-light LDs successfully transmit signals with the data rates of 4.17 Gb/s, 4.17 Gb/s and 1.17 Gb/s, respectively, over a 10-m underwater channel. The corresponding bit error rates (BERs) are 2.2 × 10^-3, 2.0 × 10^-3 and 2.3 × 10^-3, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10^-3.

Gigabit-per-second white light-based visible light communication using near-ultraviolet laser diode and red-, green-, and blue-emitting phosphors

Data communication based on white light generated using a near-ultraviolet (NUV) laser diode (LD) pumping red-, green-, and blue-emitting (RGB) phosphors was demonstrated for the first time. A III-nitride laser diode (LD) on a semipolar (2021¯)  substrate emitting at 410 nm was used for the transmitter. The measured modulation bandwidth of the LD was 1 GHz, which was limited by the avalanche photodetector. The emission from the NUV LD and the RGB phosphor combination measured a color rendering index (CRI) of 79 and correlated color temperature (CCT) of 4050 K, indicating promise of this approach for creating high quality white lighting. Using this configuration, data was successfully transmitted at a rate of more than 1 Gbps. This NUV laser-based system is expected to have lower background noise from sunlight at the LD emission wavelength than a system that uses a blue LD due to the rapid fall off in intensity of the solar spectrum in the NUV spectral region.

Multilevel modulation scheme using the overlapping of two light sources for visible light communication with mobile phone camera

Visible light communication (VLC) with light emitting diodes (LEDs) is an emerging technology for 5G wireless communications. Recently, using complementary metal-oxide-semiconductor (CMOS) image sensor as VLC receiver is developed owing to its flexibility and low-cost. However, two illumination levels such as on-off keying (OOK) signal are used. To improve the system throughput and reduce complexity of the hardware design, in this paper, we propose and experimentally demonstrate a multilevel modulation scheme for VLC system utilizing the overlapping of two light sources for the first time, and the two light sources are modulated by an OOK and a Manchester signal respectively. At the receiver, a CMOS camera can demodulate the Manchester and the OOK signal simultaneously. Meanwhile, a low-pass filter (LPF) is used to enhance the system performance. The experimental results demonstrate that the proposed multilevel modulation scheme can achieve a net data rate of 4.32 kbit/s.

Tricolor R/G/B Laser Diode Based Eye-Safe White Lighting Communication Beyond 8 Gbit/s

White light generation by mixing red, green, and blue laser diodes (RGB LDs) was demonstrated with Commission International de l'Eclairage coordinates of (0.2928, 0.2981), a correlated color temperature of 8382 K, and a color rendering index of 54.4 to provide a maximal illuminance of 7540 lux. All the white lights generated using RGB LDs were set within the risk group-1 criterion to avoid the blue-light hazard to human eyes. In addition, the RGB-LD mixed white light was diffused using a frosted glass to avoid optical aberration and to improve the performance of the lighting source. In addition, visible light communication (VLC) by using RGB-LD mixed white-light carriers and a point-to-point scheme over 1 m was performed in the directly modulated 16-QAM OFDM data format. In back-to-back transmission, the maximal allowable data rate at 10.8, 10.4, and 8 Gbps was determined for R, G, and B LDs, respectively. Moreover, the RGB-LD mixed white light-based indoor wavelength-division multiplexing (WDM)-VLC system yielded a total allowable transmission data rate of 8.8 Gbps over 0.5 m in free space. Such a high-speed RGB-LD mixed WDM-VLC system without any channel interference can be used to simultaneously provide data transmission and white lighting in an indoor environment.

Blue Laser Diode Enables Underwater Communication at 12.4 Gbps

To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (LD) directly modulated by pre-leveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of -0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m.

20-meter underwater wireless optical communication link with 1.5 Gbps data rate

The video streaming, data transmission, and remote control in underwater call for high speed (Gbps) communication link with a long channel length (~10 meters). We present a compact and low power consumption underwater wireless optical communication (UWOC) system utilizing a 450-nm laser diode (LD) and a Si avalanche photodetector. With the LD operating at a driving current of 80 mA with an optical power of 51.3 mW, we demonstrated a high-speed UWOC link offering a data rate up to 2 Gbps over a 12-meter-long, and 1.5 Gbps over a record 20-meter-long underwater channel. The measured bit-error rate (BER) are 2.8 × 10^-5, and 3.0 × 10^-3, respectively, which pass well the forward error correction (FEC) criterion.

Ultrabroad linewidth orange-emitting nanowires LED for high CRI laser-based white lighting and gigahertz communications

Group-III-nitride laser diode (LD)-based solid-state lighting device has been demonstrated to be droop-free compared to light-emitting diodes (LEDs), and highly energy-efficient compared to that of the traditional incandescent and fluorescent white light systems. The YAG:Ce³⁺ phosphor used in LD-based solid-state lighting, however, is associated with rapid degradation issue. An alternate phosphor/LD architecture, which is capable of sustaining high temperature, high power density, while still intensity- and bandwidth-tunable for high color-quality remained unexplored. In this paper, we present for the first time, the proof-of-concept of the generation of high-quality white light using an InGaN-based orange nanowires (NWs) LED grown on silicon, in conjunction with a blue LD, and in place of the compound-phosphor. By changing the relative intensities of the ultrabroad linewidth orange and narrow-linewidth blue components, our LED/LD device architecture achieved correlated color temperature (CCT) ranging from 3000 K to above 6000K with color rendering index (CRI) values reaching 83.1, a value unsurpassed by the YAG-phosphor/blue-LD counterpart. The white-light wireless communications was implemented using the blue LD through on-off keying (OOK) modulation to obtain a data rate of 1.06 Gbps. We therefore achieved the best of both worlds when orange-emitting NWs LED are utilized as "active-phosphor", while blue LD is used for both color mixing and optical wireless communications.

Augmenting the spectral efficiency of enhanced PAM-DMT-based optical wireless communications

The energy efficiency of pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) decreases as the modulation order of M-PAM modulation increases. Enhanced PAM-DMT (ePAM-DMT) was proposed as a solution to the reduced energy efficiency of PAM-DMT. This was achieved by allowing multiple streams of PAM-DMT to be superimposed and successively demodulated at the receiver side. In order to maintain a distortion-free unipolar ePAM-DMT system, the multiple time-domain PAM-DMT streams are required to be aligned. However, aligning the antisymmetry in ePAM-DMT is complex and results in efficiency losses. In this paper, a novel simplified method to apply the superposition modulation on M-PAM modulated discrete multitone (DMT) is introduced. Contrary to ePAM-DMT, the signal generation of the proposed system, termed augmented spectral efficiency discrete multitone (ASE-DMT), occurs in the frequency domain. This results in an improved spectral and energy efficiency. The analytical bit error rate (BER) performance bound of the proposed system is derived and compared with Monte-Carlo simulations. The system performance is shown to offer significant electrical and optical energy savings compared with ePAM-DMT and DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM).