Visible Light Communication Transmitter Research Articles

Implementation of Li-Fi based home automation system

This paper presents an Implementation of Li-Fi based home automation system prototype. This system was develop to verify the effect of LED types on VLC transmission. LED is used to transmit the input data information to the receiver LDR through visible light communication (VLC). Arduino microcontroller was used to retrieve information from the temperature sensor, light sensor, PIR sensor and MQ2 gas sensor. The output data of sensors were sent to LDR by LED. The result demonstrates that the colour types of LED has an effect on transmission distance and the voltage of LED is proportional to the distance. Red LED has better performance compared to others.

Comparison of interpolation-based sampling frequency offset compensation schemes for practical OFDM-VLC systems.

Sampling frequency offset (SFO) is an important issue in the orthogonal frequency-division multiplexing (OFDM)-based visible light communication (VLC) systems with low-cost analog-to-digital or digital-to-analog converters (ADCs/DACs). A digital interpolation or resampling filter can be used to effectively compensate the SFO. In such case, oversampling at the receiver ADC is required to mitigate the aliasing effect due to imperfect DACs and nonlinearity of visible light sources that cause extra frequency components inside/outside the OFDM signal spectrum. The oversampling factor (rate) is mainly determined by the order of the digital interpolation filter and nonlinear VLC links. The design of the OFDM-VLC receiver incorporating the digital interpolation filter is vital as it affects not only the transmission performance but also the complexity of digital signal processing (DSP). To evaluate the feasibility of the digital interpolation-based SFO compensation schemes for cost-sensitive VLC applications, in this paper, a real-time OFDM-VLC receiver incorporating the 2nd/3rd/4th order interpolation filters is experimentally demonstrated. An OFDM frame structure is designed for the synchronization including SFO estimation and compensation, in which the precision and latency of DSP are considered. On the basis of the real-time OFDM-VLC receiver, the comparison in the VLC transmission performance and DSP complexity between different interpolation-based SFO compensation schemes is discussed.

Comparison of nonlinear equalizers for high-speed visible light communication utilizing silicon substrate phosphorescent white LED.

Compared with multicolor-chip integrated white LEDs, phosphor-based white LEDs are more attractive for daily illumination due to lower cost and complexity, and thus they are preferable for future commercial use of visible light communication (VLC) systems. However, the application of phosphorescent white LEDs has a lower data rate than multicolor-chip integrated LEDs because of severe nonlinear impairments and limited bandwidth caused by the slow-responding phosphor. In this paper, for the first time we propose to employ phosphorescent white LEDs based on silicon substrate with adaptive bit-loading discrete multitone (DMT) modulation and a memoryless polynomial based nonlinear equalizer to achieve a high-speed VLC system. We also present a comprehensive comparison among nonlinear equalizers based on the Volterra series model, memory polynomial model, memoryless polynomial model and deep neural network (DNN) with experimental results utilizing a silicon substrate phosphorescent white LED, and provide detailed suggestions on how to choose the most suitable nonlinear mitigation scheme considering different practical conditions and the tradeoff between complexity and performance. Beyond 3.00 Gb/s DMT VLC transmission over 1-m indoor free space is successfully demonstrated with bit error rate (BER) under the 7% forward error correction (FEC) limit of 3.8×10-3. As far as we know, this is the highest data rate ever reported for VLC systems based on a single high-power phosphorescent white LED.

Taking Advantage of the Sum of the Light in Outphasing Technique for Visible Light Communication Transmitter

Visible light communication (VLC) takes advantage of the widespread use of the light-emitting diodes (LEDs), and by modifying the driver stage, the LEDs are capable of lighting and transmitting information. One of the main drawbacks is the low power efficiency due to the modification of the LED driver stage in order to add the communication capability. Most of the research work related to VLC is toward the communication task, whereas there is a limited work about the improvement on power efficiency. This article proposes a high-efficiency LED driver for VLC working as a transmitter based on the outphasing technique. This technique is used also in radio frequency (RF) communications and increases the efficiency of the amplifiers. The proposed transmitter is made up of two switching-mode power amplifiers that reproduce the signals required for the outphasing technique and a dc/dc converter that biases the LEDs. The proposal exploits the light and, instead of being added electrically, the signals are added in their light form, which leads to a reduction in the complexity of the design. As experimental results, a transmitter was built of two Class E amplifiers reproducing a 16-QAM modulation, achieving a signal-generation efficiency of 78% and an overall efficiency of 92%.

Adapting Techniques to Improve Efficiency in Radio Frequency Power Amplifiers for Visible Light Communications

It is well known that modern wireless communications systems need linear, wide bandwidth, efficient Radio Frequency Power Amplifiers (RFPAs). However, conventional configurations of RFPAs based on Class A, Class B, and Class AB exhibit extremely low efficiencies when they manage signals with a high Peak-to-Average Power Ratio (PAPR). Traditionally, a number of techniques have been proposed either to achieve linearity in the case of efficient Switching-Mode RFPAs or to improve the efficiency of linear RFPAs. There are two categories in the application of aforementioned techniques. First, techniques based on the use of Switching-Mode DC–DC converters with a very-fast-output response (faster than 1 µs). Second, techniques based on the interaction of several RFPAs. The current expansion of these techniques is mainly due to their application in cellphone networks, but they can also be applied in other promising wireless communications systems such as Visible Light Communication (VLC). The main contribution of this paper is to show how Envelope Tracking (ET), Envelope and Elimination (EER), Outphasing, and Doherty techniques can be helpful in developing more efficient VLC transmitters capable of reaching high bit-rates (higher than 1 Mbps) by using advance modulation schemes. Finally, two examples based on the application of the Outphasing technique and the use of a Linear-Assisted Envelope Amplifier (EA) to VLC are presented and experimentally verified.

Hardware Architectures of Visible Light Communication Transmitter and Receiver for Beacon-based Indoor Positioning Systems

High-speed applications of Visible Light Communications have been presented recently in which response times of photodiode-based VLC receivers are critical points. Typical VLC receiver routines, such as soft-decoding of run-length limited (RLL) codes and FEC codes was purely processed on embedded firmware, and potentially cause bottleneck at the receiver. To speed up the performance of receivers, ASIC-based VLC receiver could be the solution. Unfortunately, recent works on soft-decoding of RLL and FEC have shown that they are bulky and time-consuming computations. This causes hardware implementation of VLC receivers becomes heavy and unrealistic. In this paper, we introduce a compact Polar-code-based VLC receivers. in which flicker mitigation of the system can be guaranteed even without RLL codes. In particular, we utilized the centralized bit-probability distribution of a pre-scrambler and a Polar encoder to create a non-RLL flicker mitigation solution. At the receiver, a 3-bit soft-decision filter was implemented to analyze signals received from the VLC channel to extract log-likelihood ratio (LLR) values and feed them to the Polar decoder. Therefore, the proposed receiver could exploit the soft-decoding of the Polar decoder to improve the error-correction performance of the system. Due to the non-RLL characteristic, the receiver has a preeminent code-rate and a reduced complexity compared with RLL-based receivers. We present the proposed VLC receiver along with a novel very-large-scale integration (VLSI) architecture, and a synthesis of our design using FPGA/ASIC synthesis tools.

Power Allocation for Downlink Hybrid Power Line and Visible Light Communication System

Light-emitting diode (LED)-based visible light communication (VLC) is a great promising high-speed wireless access solution due to its low power consumption, no electromagnetic interference, large available bandwidth, and no authentication required. However, visible light cannot be used directly as an information source, and must be combined with other communication technologies to avoid becoming an information island. In hybrid power line and visible light communication (HPV) system, power line communication (PLC) can be used to deliver data to VLC transmitters. Multiple users can share the same time and frequency resources to provide higher spectral efficiency using non-orthogonal multiple access (NOMA) method than traditional multiple access method. Therefore, we investigate the power allocation strategy for a downlink of HPV system based on NOMA. Our goal is to maximize achievable sum rate of the HPV system while considering user fairness. To solve this optimization problem, we present an optimal power allocation (OPA) algorithm that finds optimal solution by transforming the non-convexity of the original formulated model into the convex programming. Numerical results show that the proposed OPA algorithm significantly outperforms the gain ratio power allocation (GRPA) and the fixed power allocation (FPA) algorithm in terms of the sum rate, and improves the fairness among users.

Resource Allocation in Heterogeneous Network With Visible Light Communication and D2D: A Hierarchical Game Approach

Utilizing the illuminating light-emitting diode (LED) for signal transmission, the visible light communication (VLC) offloads the congested traffic in licensed spectrum while providing users with high quality of service. However, in shaded areas, the performance of VLC is affected. In this paper, we apply Device-to-Device (D2D) technology on the VLC network, where some mobile users are able to relay the data transmission to nearby end mobile users. Considering the autonomous behaviors of mobile users as relays (RUs), cellular service provider (CSP) and VLC service provider (VLCSP), we propose a hierarchical game framework and analyze the distributive strategies for each individual. In the game, the data packet size, the price of licensed spectrum and data rates in possible data transmission routes are sequentially determined with equilibrium solutions, and each VLC transmitter (VLCT) determines the optimal data transmission route with stable matching. Simulations depict the high performance when combining VLC and D2D with the proposed strategies.

On-chip GaN-based dual-color micro-LED arrays and their application in visible light communication.

Integrated multi-color micron-sized light emitting diode (micro-LED) arrays have been demonstrated in recent years for display applications; however, their potential as visible light communication (VLC) transmitters is yet to be fully explored. In this work, we report on the fabrication and characterization of on-chip dual-color micro-LED arrays and their application in VLC. For this purpose, blue-green and blue-violet micro-LED arrays were fabricated by transfer printing blue-emitting micro-LEDs onto the substrate of green and violet micro-LEDs, respectively. The potential of these dual-color micro-LED arrays as VLC transmitters is demonstrated with respective error-free data rates of 1.79 and 3.35 Gbps, achieved by the blue-green and blue-violet devices in a dual wavelength multiplexing scheme.

An optical backhaul solution for LiFi-based access networks

The light fidelity (Li-Fi) technology is being considered as one of the most promising solutions that can achieve a dramatic increase in communication capacity and a significant reduction in transmission latency especially when it is deployed in the indoor scenario. This technique can enable fully networking capabilities by exploiting the high speed transmission of visible light communication (VLC) technique and supporting massive deployment of LiFi access points to create a network of attocells. In this paper, we investigate the design of an optical backhaul network architecture that can provide efficient interconnection for LiFi access points. Specifically, we propose a LiFi access-point structure implementing orthogonal frequency division multiplexing (OFDM) and optical encoding technique to provide multi-users access and allow all-optical processing and transmission in the backhaul network. A tunable optical encoding/decoding technique based on delaying optical pulses in a vector of optical delay line (ODLs) loops is designed to allow efficient mapping of the OFDM based access and data forwarding in the optical backhaul network. In this encoding scheme, optical codewords can be easily modified by dynamically changing the number of rounds that must be performed by the optical pulse in the ODL. This feature was exploited in the backhaul architecture to enable appropriate mobility management schemes and allow seamless handover between attocells. Performance evaluation using simulation was performed to show that the proposed optical backhaul solution can achieve high transmission capacity with reduced latency.

Performance of hybrid cellular-VLC link for indoor environments under dynamic user movement

Visible light communications (VLC) is an emerging technology that complements the existing cellular network in order to provide high throughput, secure and power efficient indoor communication system. In this paper, we analyze the performance of hybrid cellular-VLC downlink where the outdoor coverage is provided via cellular network, and the indoor coverage is provided through a VLC system. In the proposed framework, VLC access point (AP) helps to communicate the information between the base station (BTS) and user by acting as a decode-and-forward (DF) relay. The relay decodes the received signal from the base station and then uses the decoded signal to modulate the intensity of the optical transmitter. Multiple light emitting diode (LED) transmitters with rectangular deployment are considered for a VLC transmitter. The channel model for VLC link has been modeled as an integrating sphere VLC channel which includes both line-of-sight (LOS) and non-line-of-sight (NLOS) links. Further, the movement and density of people have also been considered while deriving the closed-form expressions for the outage probability and bit-error-rate (BER) for the hybrid cellular-VLC downlink. In addition, the proposed work also utilizes optimal Lambertian order (OLO) to achieve increased average received optical power through LOS path and uniform delay spread in an indoor environment. It has been shown that by using the proposed hybrid cellular-VLC downlink, it is possible to achieve significant amount of power saving (approximately 51%) at BTS as compared to direct cellular link. Finally, the dimming range is calculated for the proposed system under dimming constraint. Results show that dimming range up to 70% can be achieved in the hybrid cellular-VLC link.

1.7 to 2.3 Gbps OOK LED VLC Transmission Based on 4 × 4 Color-Polarization-Multiplexing at Extremely Low Illumination

To achieve the high traffic rate of a GaN LED-based visible light communication (VLC), a 4 × 4 color-polarization-multiplexing system is demonstrated by using two blue and two green LEDs. In the measurement, an available bandwidth of blue and green LEDs can be greater than 465 MHz for VLC modulation traffic by using the suitable analogy front end circuit. Hence, the proposed VLC system can deliver 1.7 to 2.3 Gbit/s on – off keying traffic rates in the free space transmission length of 1–4 m under the extremely low illumination of 6.9–136.1 lux for the indoor application. Moreover, the measured BER of each LED is less than the forward error correction target of 3.8 × 10−3 to maintain the better signal performance.

1250 Mbit/s OOK Wireless White-Light VLC Transmission Based on Phosphor Laser Diode

In this work, for the first time, we demonstrate a 1250 Mbit/s on-off keying white-light visible light communication (VLC) transmission in a free space transmission length of 1 m by utilizing a yellow-phosphor laser diode (LD). Here, the blue optical filter, which is used to enhance the data rate by removing the yellow color component, is not required to. Besides, the wireless transmission lengths, illuminations, and modulation traffic rates of the proposed LD-based VLC system are analyzed and discussed.

Reproducing Multicarrier Modulation Schemes for Visible Light Communication With the Ripple Modulation Technique

The ripple modulation (RM) is a technique that provides a variable output voltage by using the output voltage ripple of a switching-mode power converter. It was proposed to reproduce the communication signal of a visible light communication transmitter, thus avoiding the use of power inefficient linear power amplifiers. However, the RM has a major drawback: it has not been reported how to reproduce multicarrier modulation (MCM) schemes, which are the preferred modulation schemes due to their high performance in wireless communications. In this paper, it is demonstrated that the RM can be used to reproduce MCM schemes by considering the envelope and the instantaneous phase of the communication signal. Moreover, the control system of the power stage is described in detail, explaining how to modulate both the width and the phase of the gate signals, the calculation of the phase shift that is required for tracking the envelope, etc.

Modeling and Analysis of Transmitter Performance in Visible Light Communications

This paper addresses the power penalty in a visible light communication (VLC) emitter that re-uses illumination light emitting diodes (LEDs) for high-speed communications. We quantify the effect of modulation depth for two widely explored modulation schemes in VLC, namely orthogonal frequency division multiplexing (OFDM) and pulse amplitude modulation (PAM). Extra power is dissipated in the LED and in the modulator due to such modulation. Two popular high-speed VLC transmitter topologies, namely a Bias Tee and a Serial FET, are compared in terms of efficiency, extra energy per symbol/bit and complexity. This paper presents a theoretical model and compares it to simulation results. We show that PAM outperforms OFDM in terms of better bit error rate, larger dynamic range for modulation depth, less extra power loss, thus higher power efficiency for communication over an additive white Gaussian noise channel.

Power-Efficient VLC Transmitter Based on Pulse-Width Modulated DC–DC Converters and the Split of the Power

Visible light communication (VLC) has gained relevance during the last few years. It consists in using high-brightness LEDs (HB-LEDs) both for lighting and transmitting information by changing the light intensity rapidly. However, there are some bottlenecks that are slowing down the deployment of this technology. One of the most important problems is that the HB-LED drivers proposed for addressing high data rates in VLC achieve poor power efficiency. Since these HB-LED drivers must be able to reproduce fast current waveforms, the use of linear power amplifiers has been adopted, which clearly damages the power efficiency of HB-LED lighting. In order to alleviate this problem, an HB-LED driver made up of two DC–DC power converters is presented in this work. One of them is responsible for performing the communication functionality by operating at high switching frequency (10 MHz), whereas the second converter fulfills the illumination functionality by ensuring a certain biasing point. The split of the power allows us to minimize the power delivered by the fast-response DC–DC power converter, which suffers from high switching losses. Thus, the overall efficiency can be maximized for each particular communication scenario and for scenarios with changing conditions (i.e., mobile transmitter and/or receiver, presence of mobile obstacles, etc.). In this sense, how the lighting level and the communication signal power affect both the power efficiency and the communication efficiency is deeply analyzed in the experimental section. The implemented prototype achieves an overall efficiency around 90%. In addition, the proposed VLC transmitter is able to reproduce a wide range of digital modulation schemes, including the preferred one for wireless communications: orthogonal frequency division multiplexing.

VLC and D2D Heterogeneous Network Optimization: A Reinforcement Learning Approach Based on Equilibrium Problems With Equilibrium Constraints

The radio frequency spectrum crunch has triggered the harnessing of other sources of bandwidth, for which visible light is a promising candidate. Even though visible light communication (VLC) ensures high capacity, coverage is limited. This necessitates the integration of VLC and device-to-device (D2D) technologies into heterogeneous networks. In particular, mobile users which are accessible by the VLC transmitters can relay data to mobile users which are not, by means of D2D communication. However, due to the distributed behaviors of mobile users, determining optimal data transmission routes from VLC transmitters to end mobile devices is a major challenge. In this paper, we propose a reinforcement learning (RL)-based approach to determine multi-hop data transmission routes in an indoor VLC-D2D heterogeneous network. We obtain the rewards for the RL-based method dynamically, by formulating the interactions between the mobile users relaying the data as an equilibrium problem with equilibrium constraints and using alternating direction method of multipliers to solve it. The proposed technique can achieve optimal data transmission routes in a distributed manner. The simulation results demonstrate the effectiveness of the proposed approach, showing that transmission routes with low delays and high capacities can be achieved through the learning algorithm.

Comparing Gausian and exact models of malicious interference in VLC systems

Visible Light Communication (VLC) is a technique for high-speed, low-cost wireless data transmission based on LED luminaries. Wireless LAN environments are a major application of VLC. In these environments, VLC is used in place of traditional systems such as Wi-Fi. Because of the physical characteristics of visible light, VLC is considered to be superior to traditional radio-based communication in terms of security. However, as in all wireless systems, the security of VLC with respect to eavesdropping, signal jamming and modification must be analyzed. This paper focuses on the aspect of jamming in VLC networks. In environments where multiple VLC transmitters are used, there is the possibility that one or more transmitters will be hostile (or rogue). This leads to communication disruption, and in some cases, the hijacking of the legitimate data stream. In this paper we present the theoretical system model that is used in simulations to evaluate various rogue transmission scenarios in a typical indoor environment. The typical approach used so far in jamming analysis assumes that all disruptive transmissions may be modeled as Gaussian noise, but this assumption may be too simplistic. We analyze and compare two models of VLC jamming: the simplified Gaussian and the exact model, where the full characteristics of the interfering signal are taken into account. Our aim is to determine which methodology is adequate for studying signal jamming in VLC systems.

LiDAL: Light Detection and Localization

In this paper, we present the first indoor light-based detection and localization system that builds on concepts from radio detection and ranging (radar) making use of the expected growth in the use and adoption of visible light communication (VLC), which can provide the infrastructure for our Light Detection and Localization (LiDAL) system. Our system enables active detection, counting, and localization of people, in addition to being fully compatible with the existing VLC systems. In order to detect human (targets), LiDAL uses the visible light spectrum. It sends pulses using a VLC transmitter and analyses the reflected signal collected by a photodetector receiver. Although we examine the use of the visible spectrum here, LiDAL can be used in the infrared spectrum and other parts of the light spectrum. We introduce LiDAL with different transmitter-receiver configurations and optimum and sub-optimum detectors considering the fluctuation of the received reflected signal from the target in the presence of Gaussian noise. We design an efficient multiple input multiple output (MIMO) LiDAL system with a wide field of view (FOV) single photodetector receiver, and also design a multiple input single output (MISO) LiDAL system with an imaging receiver to eliminate the ambiguity in target detection and localization. We develop models for the human body and its reflections and consider the impact of the color and texture of the cloth used as well as the impact of target mobility. A number of detection and localization methods are developed for our LiDAL system, including cross correlation and a background subtraction method. These methods are considered to distinguish a mobile target from the ambient reflections due to background obstacles (furniture) in a realistic indoor environment.

Design and Implementation of a Long Range Indoor VLC System using PWM

Visible Light Communications (VLC) is currently considered one of the most promising Optical Wireless Communications (OWC) for commercial applications, due to the widespread deployment of Light Emitting Diodes (LEDs) for energy efficiency, durability and low cost. With the ability to provide several THz of bandwidth, VLC is expected to co-exist with legacy and future Radio Frequency (RF) media as a reliable solution to the rapid demand of high-speed wireless communication. VLC is challenged by two main drawbacks: ambient light interference and the transmission range. To overcome these drawbacks, we propose a tuned VLC Analog Front-End (AFE) using simple and low-cost electronic circuity. The proposed VLC receiver architecture consists of a transimpedance amplifier, a fourth order Multiple Feed Back (MFB) band pass filter and a comparator. Experimental results demonstrated that by employing the proposed front-end the VLC transmission range could be extended up to 4.7 m by using Pulse Width Modulation (PWM). Moreover, the proposed system showed robustness against ambient light interference under the indoor scenario.