Visible Light Communication Channel Research Articles

Achievable rate analysis for post- and pre-equalization in DCO-OFDM VLC with limited dynamic range

In visible light communication (VLC) that uses direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM), the channel equalization can be done by dividing the channel gain of each subcarrier either at the receiver or the transmitter, termed as post- and pre-equalization, respectively. This paper aims to analyze the achievable rates of post- and pre-equalization in DCO-OFDM VLC. First, we theoretically reveal that the achievable rate of pre-equalization is greater than or equal to that of post-equalization within the same dynamic range, and the equality holds if and only if the VLC channel is flat. Then, we obtain the gap of achievable rate between post- and pre-equalization, which interestingly obeys the form of Kullback–Liebler (KL) divergence. We discuss the influence of the 3-dB bandwidth of VLC channel on the gap, and obtain the asymptotic gap as the signal-to-noise ratio (SNR) tends to infinity. Finally, the correctness of the theoretical results is verified via numerical simulations. In addition, experimental measurements are conducted in our actual VLC platform to further verify the theoretical results. The measurement results approximately match with the simulation results.

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.

Design and performance analysis of indoor cooperative IHDAF protocol-based spatial modulation-index modulation visible light communication system.

Spatial modulation (SM) has been employed in indoor visible light communication (VLC) systems because of its high data rate and low complexity transmission. However, the activated LED may have poor channel conditions in conventional SM VLC systems, which leads to significant performance degradation. In this paper, a cooperative incremental hybrid decode-amplify-forward (IHDAF) protocol-based SM-index modulation (SM-IM) VLC system is proposed. For the SM-IM scheme, the spatial information and the modulated information are jointly transmitted in one symbol via the optimal VLC channel. The system model and the theoretical outage probability of the proposed IHDAF protocol-based SM-IM VLC system are presented. Numerical results show that the outage probability of the IHDAF protocol-based SM-IM VLC system can be significantly improved compared with the traditional amplify-and-forward and decode-and-forward protocol-based SM VLC systems. Additionally, the power allocation of the proposed scheme is also studied.

Performance Analysis of Cooperative and Non-Cooperative Relaying over VLC Channels.

The line-of-sight (LoS) channel is one of the requirements for efficient data transmission in visible-light communications (VLC), but this cannot always be guaranteed in indoor applications for a variety of reasons, such as moving objects and the layout of rooms. The relay-assisted VLC system is one of the techniques that can be used to address this issue and ensures seamless connectivity. This paper investigates the performance of half-duplex (HD) conventional DF relay system and cooperative systems (i.e., selective DF (SDF) and incremental DF (IDF)) over VLC channels in terms of outage probability and energy consumption. Analytical expressions for both outage probability and the minimum energy-per-bit performance of the aforementioned relaying systems are derived. Furthermore, Monte Carlo simulations are provided throughout the paper to validate the derived expressions. The results show that exploiting SDF and IDF relaying schemes can achieve approximately 25% and 15% outage probability enhancement compared to single-hop and DF protocols, respectively. The results also demonstrate that the performance of the single-hop VLC system deteriorates when the end-to-end distances become larger. For example, when the vertical distance is 3.5m, the single-hop approach consumes 20%, 40% and 45% more energy in comparison to the DF, SDF, and IDF approaches, respectively.

Mobility management for D2D communication combining radio frequency and visible light communications bands

Combination of radio frequency (RF) and visible light communication (VLC) bands for device-to-device (D2D) communication is seen as a promising way to both increase the system capacity and cope with an overcrowded RF bands. The main concern, however, is a proper mobility management and selection of the band that is beneficial at the moment. While the VLC usually provides a much higher throughput than RF, it is also very sensitive to a signal blockage and shadowing. Therefore, throughput as well as potential sudden drops in VLC channel quality should be considered in a design of handover between VLC and RF to avoid redundant handovers. In this paper, we propose an algorithm, tailored for D2D communication, deciding whether or not it is beneficial for a user equipment to switch from VLC to RF or vice versa. If handover to RF is not beneficial at the moment despite a drop in VLC channel quality, a dwell timer waits for a specific time if VLC channel recovers. We propose an optimization of the dwell timer according to estimated throughput in RF and VLC and delay due to handover. Simulations show that the proposed algorithm increases an average throughput when compared to existing state-of-the-art algorithms while number of handovers and average interruption are still very low.

Deep Learning for Selection Between RF and VLC Bands in Device-to-Device Communication

This letter focuses on the selection between radio frequency (RF) and visible light communications (VLC) bands for users exchanging data directly with each other via device-to-device (D2D) communication. We target to maximize the energy efficiency of D2D communication while the outage is minimized. Since the VLC channel can vary quickly due to the possible changes in irradiance and incidence angles, we aim to reach a quick band selection decision in a multi-user scenario based only on the knowledge of the received power and sum interference from all D2D transmitters at the individual D2D receivers. The proposed solution is based on a deep neural network making an initial band selection decision. Then, based on the DNN’s output, a fast heuristic algorithm is proposed to further improve the band selection decision. The results show that the proposal reaches a close-to-optimal performance and outperforms the existing solutions in complexity, outage ratio, and energy efficiency.

Pilot-Aided Frame Synchronization in Optical OFDM Systems

Efficient frame synchronization is essential for data recovery in communication systems. In this study, a single pilot sequence is used to achieve both frame synchronization and peak-to-average power ratio (PAPR) reduction. The two systems considered are direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and asymmetrically clipped O-OFDM (ACO-OFDM). The pilot symbol is allocated to odd indexed subcarriers only. Thus, the synchronization algorithm leverages the mirror symmetric property of the pilot symbol within a frame to detect the start of the pilot signal at the receiver. This scheme has low complexity and gives precise frame synchronization at signal-to-noise ratios as low as 4 dB in an indoor visible light communication (VLC) channel.

Outage of Cognitive Electric Vehicle Networks Over Mixed RF/VLC Channels With Signal-Dependent Noise and Imperfect CSI

In this paper, we propose a novel jamming-robust communication technique for the outdoor cognitive EV-enabled network over mixed radio-frequency (RF)/visible light communication (VLC) channels with signal-dependent noise. One EV acts as a relaying node to allow an aggregator to reach the jammed EV and, at the same time, operates in both RF and VLC spectrum bands while satisfying interference constraints imposed by the primary network entities. We derive an exact closed-form analytical expression for the outage probability and also provide its asymptotic analysis while considering various channel state information quality scenarios. Finally, simulation results validate the accuracy of our analysis.

On the average secrecy capacity for indoor visible light communication systems

For visible light communication (VLC), the light signals are transmitted without optical fibers or any sort of wave-guiding. Due to the inherent broadcast nature, physical-layer security emerges as a promising method to protect information delivery from eavesdropping. As for the secrecy capacity of VLC channel, there exist two features. In one way, the limited optical power makes the common capacity expressions in radio-frequency (RF) communication unapplicable for VLC. In another way, several correlated geometrical parameters directly alters the Lambertian model of indoor VLC channel, which gives the secrecy capacity more meanings. However, the issue considering both aspects has not been studied recently. In this paper, from the practical scenarios, we extract a typical geometrical model to reveal the mobility principles of the legitimate receiver and the eavesdroppers. Then, we character two typical distributions of the geometrical parameter. Correspondingly, we derive the upper and lower bounds on the average secrecy capacity, which have the closed forms. Finally, simulation results show that our upper and lower bounds are tight at high optical signal-to-noise rates (OSNRs). Moreover, the geometrical features of VLC systems and distribution parameters of the receiver mobility are effectively reveal by the bounds.

Achieving Channel Capacity of Visible Light Communication

In this article, we propose an efficient inexact gradient descent method to obtain the channel capacity-achieving input distribution of visible light communication (VLC). Under both a peak and an average optical power constraints, finding the channel capacity of VLC is formulated as a mixed continuous-discrete optimization problem without an analytical expression of the objective function. To solve this challenging problem, we first adopt the numerical integration method to approximate the objective function and its gradient. Here, we prove that the gaps between the original functions and the approximations can be arbitrarily small. Then, based on the approximated functions, we describe the method, and theoretically show that the obtained solution sequence converges to the channel capacity-achieving discrete distribution of VLC. We also provide simulation results to verify the effectiveness and optimality of the proposed method. More importantly, the simulations numerically reveal that on-off keying (OOK) modulation achieves the capacity of VLC channel at low signal-to-noise ratio (SNR), while pulse amplitude modulation (PAM) achieves the capacity at high SNR.

Visible light communication channel with a smartphone screen as a transmitter and color shift keying modulation for access control applications

Smartphones or tablets are a new kind of mobile device that allows developers the optical wireless communication. This enables a low-cost implementation interface with a visible light communication system. The present work aims to implement the access control system based on visible light communication channel with a smartphone screen as a transmitter and color shift keying modulation. The proposed system runs on the Samsung Galaxy notebook tablet to control the light colors emitted by the screen. We used Android Studio to develop the application for access control system. In this scheme, the optical response of the Galaxy screen to 4 color shift keying constellation design based on standard color space is also discussed. The proposed system is validated by experimental tests for access control applications. The results obtained suggest that a bit rate of 100 b/s can be achieved with a 4 color shift keying constellation format. It was concluded that the main advantage of using the Galaxy screen as a transmitter in a visible light communication lies in the low complexity to generate color shift keying modulation and its application on access control system. This can generate opportunities for this method to become realistic on mobile consumer electronics.

Investigation of frequency dependent nonlinearity on pulse amplitude modulation in bandlimited visible light communications

In this work, the nonlinear behavior on pulse-amplitude-modulation (PAM) and their impact on the performance of RGB light-emitting-diodes (LED) based visible light communications (VLC) is presented. Particularly, how the limited source bandwidth affects the nonlinearity of fully modulated RGB LEDs is investigated. It is found that when symbol-rate is higher than the channel bandwidth the nonlinearity gradually diminishes. It causes the symbol-rate dependent nonlinearity. When applying PAM in VLC links, a pre-distortion method is often applied to compensate for the nonlinearity. Due to this symbol-rate dependency, however, the conventional pre-distortion method has a limitation on linearizing the non-linear VLC channel, especially in the case of applying various symbol-rates according to the channel condition. Although it can be adaptively compensated by nonlinear equalizers, their complexity is considerably high. In this work, the characteristics of multi-level PAM scheme in a band-limited nonlinear channel is presented. Also, the feasibility of a low complex but efficient pre-distortion method derived from monotonically decreasing characteristics of the frequency dependent nonlinearity is investigated.

Outage Analysis of Cognitive Electric Vehicular Networks Over Mixed RF/VLC Channels

Modern transportation infrastructures are considered as one of the main sources of the greenhouse gases emitted into the atmosphere. This situation requires the decision-making players to enact the mass use of electric vehicles (EVs) which, in turn, highly demand novel secure communication technologies robust to various cyber-attacks. Therefore, in this paper, we propose a novel jamming-robust communication technique for different outdoor cognitive EV-enabled network cases over mixed radio-frequency (RF)/visible light communication (VLC) channels. One EV acts as a relaying node to allow an aggregator to reach the jammed EV and, at the same time, operates in both RF and VLC spectrum bands while satisfying interference constraints imposed by the primary network entities. We derive exact closed-form analytical expressions for the outage probability and also provide their asymptotic analysis while considering various channel state information quality scenarios. Moreover, we quantify the outage reduction achievable by deploying such mixed VLC/RF channels. Finally, analytical and simulation results validate the accuracy of our analysis.

Statistical Modeling and Analysis on the Confidentiality of Indoor VLC Systems

While visible light communication (VLC) is expected to have a wide range of applications in the near future, the security vulnerabilities of this technology have not been well understood so far. In particular, due to the extremely short wavelength of visible light, the VLC channel presents several unique characteristics than its radio frequency counterparts, which impose new features on the VLC security. Taking a physical-layer security perspective, this paper studies the intrinsic secrecy capacity of VLC as induced by its special channel characteristics. Different from existing models that only consider the specular reflection in the VLC channel, a modified Monte Carlo ray tracing model is proposed to account for both the specular and the diffusive reflections, which is unique to VLC. A deep neural network model is also proposed to describe the spatial VLC channel response based on a limited number of channel response samples calculated from the ray tracing model. Based on these models the upper and the lower bounds of the VLC secrecy capacity are derived, which allow us to evaluate the VLC communication confidentiality against a comprehensive set of factors, including the locations of the transmitter, receiver, and eavesdropper, the VLC channel bandwidth, the ratio between the specular and diffusive reflections, and the reflection coefficient. Our results reveal that due to the different types of reflections, the VLC system becomes more vulnerable at specific locations where strong reflections exist.

Generalized quadrature spatial modulation techniques for VLC

In this paper, generalized quadrature spatial modulation (QSM) techniques are proposed for multiple-input single-output (MISO) visible light communication (VLC) systems using quadrature spatial modulation subcarrier intensity modulation (QSM-SIM). Additionally, realistic channel condition including both line-of-sight (LOS) and non-LOS (NLOS) components up to the third reflection is also considered and channel coherence bandwidths are calculated for varying number of LEDs. Since the indoor MISO VLC channel has strong correlation between coefficients, particle swarm optimization (PSO) and approximated maximum minimum Euclidean distance (AMMD) based precoding on the transmitter side are studied to reduce the channel correlation under illumination constraints. As a performance metric, bit error rate (BER) performance of considered modulation techniques is analyzed for different MISO schemes. The upper bound BER performances are also derived analytically in order to validate the simulation results. According to the results, we reached three important conclusions. First one is that variable length generalized QSM (VGQSM) by having few LED arrays on the transmitter side performs the best BER results compared to other schemes that provide the same spectral efficiency (SE) and the second is that precoding is essential for indoor MISO VLC systems to reduce the channel correlation. Finally, an increase in the number of LED arrays results in a decrease in channel coherence bandwidth.

Rotated Color Shift Keying for Visible Light Communications With Signal-Dependent Noise

This letter proposes three distinct modulation schemes belonging to the class of rotated color-shift keying (CSK) to improve the performance of regular CSK over a tricolor visible light communications (VLC) channel subject to signal-dependent noise (SDN). In such channels, regular CSK schemes suffer from a severe constellation distortion. The proposed rotated CSK schemes termed respectively separative whitening-like rotation (SWR), multiplicative whitening-like rotation (MWR), and additive whitening-like rotation (AWR), provide error rate gains relative to the regular CSK. The proposed modulations are carried out by employing practical lighting constraints, including a relaxed color constraint by the MacAdam ellipses with appropriate steps.

Least Minimum Symbol Error Rate Based Post-Distortion for VLC Using Random Fourier Features

Visible light communications (VLC) has emerged as a viable supplement for high speed next-generation communication systems. However, performance of a VLC link is limited by the nonlinear characteristics of light emitting diodes (LEDs), ambient light, and inter-symbol interference (ISI). In this letter, the paradigm of least-squares based post-distortion over reproducing kernel Hilbert spaces (RKHS) is considered for a visible light communication (VLC) link. The performance-limiting impairments (viz. LED nonlinearity, and ISI), are mitigated by random Fourier features (RFF) based least minimum symbol error-rate post-distorter in RKHS, which facilitates post-distortion under finite memory-budget constrained scenarios. Performance of the proposed algorithm is tested over IEEE 802.15r1 PAN VLC channels, and the proposed algorithm is observed to converge significantly faster, whilst supporting higher data rates as compared to the existing RKHS based post-distorters.

Photo detector-based indoor positioning systems variants: A new look

Indoor positioning system (IPS) coupling with photo-detectors at the receiving end has gained much attention from the consumers. The accuracy level using the global positioning system (GPS) is not up-to-the-mark in the indoor environment. Meanwhile, visible light communication (VLC) employing the optical transmitter, e.g., light-emitting diodes (LEDs), is considered a promising candidate nowadays because of its enormous advantages. To better understand the VLC channel model, a comprehensive review is presented. An investigation of the LED-based IPS, employed for the position calculation using various techniques, is intensely discussed. The types of different photodetectors-based IPSs are elaborated along with the metrics. A comparison of various IPSs is developed to select the optimum one better. A new look of the IPS is proposed with extended features and better accuracy. A conclusion is drawn in this field for researchers to enhance VLC-based systems towards future research.

Seamless rate adaptation for indoor visible light communication without CSI at the transmitter

Visible light communication (VLC), which uses LEDs for both lighting and communication purposes, has recently emerged as an alternative or support system to radio frequency (RF) communication. VLC channel between LEDs and photo-diodes is highly variable and pretty much depends on optical line of sight (LOS) and field of view (FOV). Therefore, received signal strength may vary dramatically, especially for mobile receivers. To ensure spectral efficient communication, it is essential to use rate adaptation methods. In this study, we propose rateless codes based rate adaptation method that adjusts transmission rate at receiver side and does not require channel state information (CSI) at the transmitter side. The spectral efficiency of the proposed method is analyzed and compared with adaptive modulation and coding (AMC) rate adaptation method widely used in literature. For analyses, we consider a realistic communication system model by using commercially available LEDs and also fulfilling illumination requirements. In order to minimize the multi-path effects, optical orthogonal frequency-division multiplexing (OFDM) is utilized. Numerical results indicate that proposed rate adaptive method can achieve seamless rate adaptation and has higher spectral efficiency compared to AMC in the presence of significant signal quality variations caused by mobility.

Q-Learning Based Two-Timescale Power Allocation for Multi-Homing Hybrid RF/VLC Networks

This letter investigates hybrid networks composed of a radio frequency (RF) access point (AP) and multiple visible light communication (VLC) APs. We consider mobile multi-homing users that can aggregate resources from both RF and VLC APs. In hybrid RF/VLC networks, RF channel gains vary faster than VLC channels due to small scale fading. By leveraging multi-agent Q-learning to interact with the dynamics of wireless environments, we develop an online two-timescale power allocation strategy that optimizes the transmit powers at the RF and VLC APs to ensure quality-of-service satisfaction. Simulation results demonstrate the effectiveness of the proposed Q-learning based strategy.