Practical Visible Light Communication System Research Articles

DC-Balanced Even-Dimensional CAP Modulation for Visible Light Communication

Carrierless amplitude and phase (CAP) is a high spectral efficiency (SE) modulation scheme for visible light communication (VLC) systems. For N-dimensional (N-D) CAP, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> pulse shaping filters (PSFs) should satisfy zero inter-symbol interference (ISI) and zero inter-channel interference (ICI) conditions. In a practical VLC system, direct-current-balanced (DC-balanced) CAP PSFs are required to avoid severe low-frequency distortion. In this paper, we discuss the possibility of designing perfect DC-balanced N-D PSFs based on zero ISI and zero ICI conditions. Perfect PSFs should be spectrally efficient and flattened. We first prove that it's impossible to design perfect DC-balanced PSFs if the dimension is odd, but it's possible to design perfect DC-balanced PSFs if the dimension is even. Based on our proposed theories, we use a modified minimax optimization method to design a group of perfect DC-balanced 4-D PSFs. A simulation based on the bandpass VLC channel is set up to compare the bit error rate (BER) performance of odd-dimensional and even-dimensional PSFs. The simulation shows that even-dimensional PSFs outperform odd-dimensional PSFs. Moreover, a VLC system based on a laser diode is demonstrated. The experimental results show that the BER performance of even-dimensional CAP (2-D or 4-D CAP) is much better than that of odd-dimensional CAP (3-D CAP) in the bandpass VLC system. We show that the novel 4-D CAP has the nearly identical BER performance as classical 2-D CAP and it can be a good candidate multiuser modulation scheme for bandpass VLC systems.

AI-Enabled Intelligent Visible Light Communications: Challenges, Progress, and Future

Visible light communication (VLC) is a highly promising complement to conventional wireless communication for local-area networking in future 6G. However, the extra electro-optical and photoelectric conversions in VLC systems usually introduce exceeding complexity to communication channels, in particular severe nonlinearities. Artificial intelligence (AI) techniques are investigated to overcome the unique challenges in VLC, whereas considerable obstacles are found in practical VLC systems applied with intelligent learning approaches. In this paper, we present a comprehensive study of the intelligent physical and network layer technologies for AI-empowered intelligent VLC (IVLC). We first depict a full model of the visible light channel and discuss its main challenges. The advantages and disadvantages of machine learning in VLC are discussed and analyzed by simulation. We then present a detailed overview of advances in intelligent physical layers, including optimal coding, channel emulator, MIMO, channel equalization, and optimal decision. Finally, we envision the prospects of IVLC in both the intelligent physical and network layers. This article lays out a roadmap for developing machine learning-based intelligent visible light communication in 6G.

Two-Dimensional Power Allocation for Optical MIMO-OFDM Systems Over Low-Pass Channels

Multiple-input multiple-output (MIMO) combined with orthogonal frequency-division multiplexing (OFDM) technique can dramatically increase the achievable rate of visible light communication (VLC) systems. The channel in a VLC system with light-emitting diode (LED) luminaires is generally low-pass, which highly limits the achievable rate of practical VLC systems. This aspect has largely been ignored in the analysis for MIMO-OFDM VLC systems. As wide parts of the bandwidth at higher frequencies are severely attenuated, the choice of the power loading on every frequency bin has a large impact on the achieved bit rate. Thus, in this paper, we propose a two-dimensional water-filling (2D-WF) power allocation algorithm that operates both in frequency and space domains to efficiently improve the rate achieved by MIMO-OFDM VLC systems over low-pass VLC channels. The achievable rates and optimal bandwidths of a MIMO-OFDM VLC system are derived analytically using the proposed 2D-WF power allocation algorithm and five conventional power allocation strategies, including uniform (UF) power allocation, pre-emphasis (PE) power allocation, beamforming (BF) power allocation, and two kinds of one-dimensional water-filling (1D-WF) power allocation. Our simulation results show that the achievable rate of a MIMO-OFDM VLC system in a typical indoor environment can be significantly degraded by the low-pass effect. The proposed 2D-WF power allocation outperforms all other schemes in terms of achievable bit rate.

Intelligent Reflecting Surface-Aided Visible Light Communications: Potentials and Challenges

To satisfy the explosive growing traffic demands in wireless communications, visible light communication (VLC) performs as a promising technology due to its broad and license-free bandwidth. However, practical VLC systems usually confront challenges, such as blockage and high path loss. Different from traditional VLC techniques, intelligent reflecting surface (IRS)-aided VLC can achieve a remarkable performance gain.

DFT-spread OFDM with quadrature index modulation for practical VLC systems.

In this paper, we for the first time propose and investigate a novel discrete Fourier transform-spread-orthogonal frequency division multiplexing with quadrature index modulation (DFT-S-OFDM-QIM) scheme for practical visible light communication (VLC) systems. By performing subcarrier index modulation on both the in-phase and quadrature components of each subcarrier, OFDM-QIM achieves a higher spectral efficiency than conventional OFDM with index modulation (OFDM-IM). Moreover, the peak-to-average power ratio (PAPR) of OFDM-QIM can be substantially reduced by applying DFT spreading, and hence DFT-S-OFDM-QIM exhibits high tolerance against light-emitting diode (LED) nonlinearity. The superiority of the proposed DFT-S-OFDM-QIM scheme over other benchmark schemes has been successfully verified by both simulation and experimental results.

Evaluation and experimental comparisons of different photodetector receivers for visible light communication systems under typical scenarios

Positive-intrinsic-negative diode (PIN), avalanche photodiode (APD), and single photon avalanche diode (SPAD) are commonly used optoelectronic receivers with different characteristics in practical visible light communication system. To determine the most appropriate candidate, different signal and ambient light strength in the communication scenarios should be taken into account. APD and SPAD receivers have higher sensitivities than PIN, but there are still some defects that restrain their operation performance. APD receiver may generate excessive shot noise, whereas SPAD is limited by the dead time, afterpulsing, and smaller detection area. We initially analyze the noise characteristics of PIN, APD, and SPAD receivers. SNR models are established, and the main factors affecting the performance of APD and SPAD receivers are concluded. Then, the comparison experiments of three kinds of receivers in various transmission distances are conducted, respectively. Finally, the experimental results show that: short and mid distance conditions, PIN and APD are better choices, whereas SPAD performs better in long-distance communication. In addition, if a certain intensity of ambient light is introduced, APD is still better than SPAD even in long distance.

On BER analysis of nonlinear VLC systems under ambient light and imperfect/outdated CSI

Visible light communication (VLC) has emerged as a promising supplement to the existing radio frequency (RF) technology, which facilitates high speed communication for next generation communication systems. Although promising, the performance in a practical VLC system is severely impaired by ambient light interference. Furthermore, nonlinearity of light emitting diodes (LEDs) and imperfect/oudated channel state information (CSI) are other factors that degrade the achievable bit error rate (BER) performance. In this work, an analytical framework for analyzing the expression for BER of a wide class of modulation techniques is derived for an impaired VLC system considering: (a) ambient light interference, (b) LED nonlinearity, and (c) imperfect and outdated CSI. The derived analytical expressions for BER are validated through extensive computer simulations performed over typical VLC channels. Lastly, the utility of the derived analytical framework is demonstrated in the context of high data-rate VLC systems, wherein the performance of DC biased optical orthogonal frequency division multiplexing (DCO-OFDM) is quantified for VLC links impaired by the aforementioned degradations.

Dimming Techniques of Visible Light Communications for Human-Centric Illumination Networks: State-of-the-Art, Challenges, and Trends

Visible light communication (VLC), which utilizes the ubiquitous illumination network to implement high-speed communication, is rapidly emerging as one promising candidate to complement conventional radio frequency communication. In practical VLC systems, the illumination network is required to be human-centric to provide adjustable lighting for different user requirements, which limits the resource utilization of the communication aspect. Dimming control techniques, which provide compromise resource management solutions between illumination and communication, are critically essential in the implementation and commercialization of VLC. In this article, state-of-the-art dimming techniques for VLC are illustrated from a multi-dimensional perspective, including the intensity domain, the time domain, the frequency domain, and the spatial domain. We provide a comprehensive comparison of these techniques on their principles, key features, system performance, and so on. In addition, the challenges and the future research trends are presented to provide researchers with insights of future improvements and prospect of advancement in the field. Moreover, the concept of the software defined multi-dimensional dimming is also proposed for the human-centric illumination network by utilizing multi-dimensional resources according to the diversified system requirements.

On the Effect of Multipath Reflections in Indoor Visible Light Communication Links: Channel Characterization and BER Analysis

Owing to its several advantages over other wireless schemes, visible light communication (VLC) shall be at the forefront of optical wireless communication technology. However, due to multipath reflections and spatial distribution of light-emitting diode (LED) transmitters, there is an inherent delay spread in the VLC channels. We perform a comprehensive quantitative study on the effect of several practical factors like LED semi-angle, wall reflectivity, number of reflections, number of LED panels, room size, and user locations on the channel delay parameters, namely RMS delay spread and coherence bandwidth of the channel. We present the detailed derivation of the multipath VLC channel model and incorporate the effect of inter-symbol interference in bit error rate (BER) performance of the multipath VLC system. We analyze the average BER of the system under different practical scenarios and determine the penalty in signal to noise ratio entailed by a change in the system parameters mentioned above. We conclude that it is sufficient to model up to three reflections in the VLC channel to emulate the effect of multipath propagation on the channel characterization and BER analysis of the system. The results and analyses presented herein provide critical insights into the effect of multipath reflections in indoor VLC links, particularly the BER performance and channel delay characteristics. We also provide some key recommendations for the design of practical VLC systems by outlining the data rates that can be served under different system configurations.

Nonlinear predistortion scheme based on Gaussian kernel-aided deep neural networks channel estimator for visible light communication system

A scheme for Gaussian kernel-aided deep neural networks nonlinear predistortion (GK-DNNPD), which could effectively reduce the computational complexity of the receivers, is experimentally demonstrated. Compared with lookup table (LUT) PD, the GK-DNNPD could increase the Q -factor of 8 pulse amplitude modulation visible light communication (VLC) system by 1.56 dB at 1.335 Gbps. We experimentally proved that GK-DNNPD could increase the bitrate under hard-decision forward error correction from 1.335 to 1.385 Gbps. This is the first time that GK-DNNs are utilized for PD in the field of VLC systems. Meanwhile, GK-DNNPD requires less data for training than LUT, and the space complexity of the model is lower than LUT as well, which provides GK-DNNPD with the potential to be applied in practical VLC systems.

Experimental demonstration of a real-time multi-channel uplink VLC system

We propose and experimentally demonstrate a multi-channel uplink visible light communication (VLC) system for the first time, which is based on three commercially available white light emitting diodes (LED) and only one photo-diode (PD). By employing independent Xilinx field programmable gate arrays (FPGA), we realize the real-time data transmission. The on–off keying (OOK) modulation and time division multiple access (TDMA) techniques are combined together to offer a reliable multiple uplink access for three users. The electrical idle signal is designed to deal with serious inter-symbol interference (ISI) caused by the overlap of different LED signals at the receiver side, as well as to provide a stable illuminating environment. The experiment results indicate that our scheme has a great potential for the indoor practical VLC system implementation.

LED Nonlinearity Estimation and Compensation in VLC Systems Using Probabilistic Bayesian Learning

In this paper, we propose and evaluate a novel light-emitting diode (LED) nonlinearity estimation and compensation scheme using probabilistic Bayesian learning (PBL) for spectral-efficient visible light communication (VLC) systems. The nonlinear power-current curve of the LED transmitter can be accurately estimated by exploiting PBL regression and hence the adverse effect of LED nonlinearity can be efficiently compensated. Simulation results show that, in a 80-Mbit/s orthogonal frequency division multiplexing (OFDM)-based nonlinear VLC system, comparable bit-error rate (BER) performance can be achieved by the conventional time domain averaging (TDA)-based LED nonlinearity mitigation scheme with totally 20 training symbols (TSs) and the proposed PBL-based scheme with only a single TS. Therefore, compared with the conventional TDA scheme, the proposed PBL-based scheme can substantially reduce the required training overhead and hence greatly improve the overall spectral efficiency of bandlimited VLC systems. It is also shown that the PBL-based LED nonlinearity estimation and compensation scheme is computational efficient for the implementation in practical VLC systems.

Phase-Aligned Physical-Layer Network Coding in Visible Light Communications

Deploying relay nodes in visible light communication (VLC) relieves the requirement of line-of-sight transmission and thus extends the service coverage of VLC systems. In this paper, we propose to employ physical-layer network coding (PNC) in the VLC system to increase its throughput by boosting the data transmission at the relay node. We design a unified PNC–VLC framework that includes both the physical-layer signal processing algorithm and the medium access control method. To further improve the feasibility, we propose a novel phase-aligning method to counter the phase offset among nodes in practical VLC systems. Experimental results show a throughput improvement of 88.23% by employing the PNC–VLC framework, and the link budget is reduced by 4 dB at a bit error rate of 3.8 × 10−3 by using the phase-aligning method.

Visible Light Communication System Evaluations With Integrated Hardware and Optical Parameters

Visible light communication (VLC) is an emerging nascent research area having enormous application prospects. Efficient evaluation methods are critical requirements for implementing high-performance VLC systems. Most research just focus on system analyses from optical channels, modulation methods, or communication theories. However, the basic theoretical analyses of the relationships between hardware circuit current energy and optical power are absent. This paper makes up for this deficiency. Based on a general VLC communication scenario, we theoretically analyze the transferring procedures between circuit current energy and optical power. The current energy transferring calculation model (CETCM) and CET parameters are proposed for the first time. The peak current energy response, current energy gain, threshold of optical power transferring distance, and current signal-to-noise ratio can be calculated by the CETCM and CET parameters. Experiments show that they can comprehensively reflect the communication characteristics of practical VLC system, and are quite important and valuable for VLC system evaluations, implementations, and optimizations.

New Miller Codes for Run-Length Control in Visible Light Communications

Designing run-length limited codes for visible light communication systems must account for multiple performance factors, including spectral efficiency, power efficiency, dc balance, and flicker avoidance. This paper reports a new class of enhanced Miller codes, termed eMiller codes, which are capable of achieving highly desirable performances in all of these accounts. An improved Viterbi algorithm (VA), termed $mn$ VA, is developed to help further enhance the performance of eMiller codes by preserving multiple candidate sequences at each decoding stage. This performance-enhancing algorithm introduces little complexity increase compared with the original VA. Analysis on flicker control, power spectral density, and minimum Hamming distance demonstrates the all-around wellness of these new codes. Extensive simulations are carried out to evaluate eMiller codes by themselves and in practical visible light communication (VLC) systems. It is shown that the original VA already allows eMiller codes to deliver a performance noticeably better than conventional Miller and FM0/FM1 codes (and on par with Manchester codes). This result is particularly exciting, as eMiller codes are also more spectrally efficient than Manchester codes. The $mn$ VA further allows eMiller codes to surpass Manchester codes and 4B6B codes in practical RS-coded VLC systems. Simulation results confirm the superb performance of the RS-eMiller schemes.

Noise Analysis and Modeling in Visible Light Communication Using Allan Variance

Visible Light Communication (VLC) is a rising communication technology which uses Light-Emitting Diode (LED) luminaries for high-speed data transferring; however, the optical signal is usually degraded by the noise in the practical VLC system. In this paper, Allan Variance is introduced for noise analysis and modeling in VLC for the first time, which provides an efficient method to measure different kinds of noise in the VLC systems. By applying Allan variance to analyze the signals collected from the real-world environment, both white noise and random walk are observed in the VLC systems. The observed white noise and random walk are also modeled by using the Allan variance. The noise analysis and modeling based on Allan variance provide a method to improve the performance of VLC.

Beam control for indoor FSO and dynamic dual-use VLC lighting systems

Practical VLC (Visible Light Communication) systems are expected to leverage the lighting infrastructure in order to deliver data to devices in a lighting field. These devices can be static or quasistatic (e.g., laptops or IoT devices); however, it is becoming clear that the preponderance of wireless data consumption is dominated by handheld mobile devices which will exhibit varying physical orientations and 3D dynamics. Because free-space optical and visible light communications are primarily line of sight, transmitter radiation patterns and receiver field of view are very important for predicting the data performance. Given dynamic emission characteristics, there is an opportunity to adapt to the receiver. The caveat of dynamic VLC systems is that the quality and distribution of the resulting illumination must be considered as part of the dual goal of providing high quality lighting. In this paper we investigate the impact of device orientation and mobility on static and then dynamic lighting emission under a multicell lighting model. From a source standpoint we consider the performance of beam control through angular control and beam focus for one or more sources in a lighting array. Analysis and simulation demonstrate that dynamic beam and luminaire control can increase the AP coverage range by 12.8X under a 1.67 m ceiling height. Furthermore, the use of multiple sources tracking device orientation and position can mitigate off-angle performance degradation by increasing redundancy in the number of available connections. Our proposed techniques, when applied in concert, successfully mitigate common concerns about the viability of VLC and indoor FSO (Free Space Optical Communication) methods related to signal occlusion and device dynamics.

CeilingTalk: Lightweight Indoor Broadcast Through LED-Camera Communication

Although Visible Light Communication (VLC) is gaining increasing attention in research, developing a practical VLC system to harness its immediate benefits using Commercial Off-The-Shelf (COTS) devices is still an open issue. To this end, we develop and deploy CeilingTalk as a lightweight wireless broadcast system using COTS LED luminaries as transmitters and smartphone cameras as receivers so that it can be fully hosted in a smartphone and is feasible for all possible indoor environments. CeilingTalk innovates in both encoding and decoding to achieve an adequate throughput for realistic applications. On one hand, it employs Raptor coding to allow multiple LED luminaries to transmit collaboratively so as to benefit both throughput and reliability. On the other hand, it involves a lightweight decoding scheme to handle the asynchrony (both spatial and temporal) in transmissions. Moreover, we analyze the impact of various parameters on the performance of CeilingTalk, in order to derive a model for such VLC systems enabled by COTS devices and hence provide general guidance for future VLC deployments in larger scales. Finally, we conduct extensive field experiments to validate the effectiveness of our LED-camera VLC model, as well as to demonstrate the promising performance of CeilingTalk: up to 1.0 kb/s at a distance of 5 m.

Multiuser MISO Transceiver Design for Indoor Downlink Visible Light Communication Under Per-LED Optical Power Constraints

Light-emitting diode (LED)-based visible light communication (VLC), combining illumination and communication, is a promising technique for providing high-speed, low-cost indoor wireless services. In indoor environments, multiple LEDs routinely used as lighting sources may also be concomitantly invoked to support wireless services for multiple users, thus forming a multiuser multiple-input-single-output (MU-MISO) system. Since the user terminals detect all the light rays impinging from multiple LEDs, inter-user interference may severely degrade the attainable system performance. Hence, we conceive a transceiver design for indoor VLC MU-MISO systems to suppress the multiuser interference (MUI). In contrast to classic radio-frequency (RF) communication, in VLC, the signals transmitted from the LEDs are restricted by optical constraints, such as the real-valued nonnegativity of the optical signal, the maximum permissible optical intensity, and the constant brightness requirements of the LEDs. Given these practical constraints, we design the optimal transceiver relying on the objective function of minimizing the maximum mean square error (MMSE) between the legitimate transmitted and received signals of the users and show that it can be readily found by solving a convex second-order cone program. Then, we also propose a simplified transceiver design by incorporating zero-forcing (ZF) transmit precoding (TPC) and show that the TPC coefficients can be efficiently found by solving a linear program. The performance of both the optimal and the simplified transceiver is characterized by comprehensive numerical results under diverse practical VLC system setups.

Overlapping PPM for band-limited visible light communication and dimming

Abstract The synthesis of visible light communication (VLC) and lighting state control necessitates data-light modulation that can accommodate intensity control. A number of techniques that enable both optical wireless data transmission and intensity control of light-emitting diodes (LEDs) have been proposed as a response to this need. Relevant schemes leverage amplitude modulation (AM)/continuous current reduction (CCR) and/or pulse-width modulation (PWM) for dimming capability. Two-level schemes related to PWM, such as on-off keying with compensation time (OOK + CT), variable pulse position modulation (VPPM), and multiple pulse position modulation (MPPM), are most commonly investigated. In this paper, we survey and compare OOK + CT, VPPM and MPPM. Moreover, we propose a novel approach towards dimming and data transmission through the variation of codeword weights in overlapping pulse-position modulation (OPPM). The proposed approach has comparatively high spectral efficiency. Using realistic constraints of a practical VLC system, analysis reveals that OPPM can increase data rates by more than 20Mbps over expected performance of related, two-level schemes, when using LEDs suitable for lighting that have relatively low modulation bandwidths.