Visible Light Communication Channel Research Articles

Asymmetrically Reconstructed Optical OFDM for Visible Light Communications

In conventional optical orthogonal frequency-division multiplexing (OOFDM) systems, such as pulse-amplitude-modulated discrete multitone (PAM-DMT) and asymmetrically clipped optical OFDM (ACO-OFDM), half of the time-domain transmitted signals carry meaningful information, while the other half are set at zero after asymmetric clipping. By specifically exploiting the asymmetric structure of OOFDM such as PAM-DMT and ACO-OFDM, we present a new framework for implementing OOFDM with enhanced performance in terms of both bit error rate (BER) and peak-to-average power ratio (PAPR). The proposed scheme introduces asymmetric signal reconstruction, instead of direct clipping, in the time domain before transmission. The designed asymmetric reconstruction refines the time-domain statistics of the OOFDM signals by constructively exploiting zero-padded signal positions in conventional OOFDMs. This way, the PAPR of the reconstructed OOFDM signals is effectively reduced. For receivers, to recover the received signals precisely, we further develop an optimal maximum a posteriori detection method, along with an efficient simplified method. Simulation results validate that the proposed scheme enhances the BER performance compared with conventional OOFDMs under both an ideal and a dispersive visible light communication (VLC) channel via our prototype. Specifically, under an ideal additive white Gaussian noise channel, an approximately 4-dB gain is achieved at the BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , along with a 2.5-dB lower PAPR constraint. While under the real dispersive channel, our test shows about 6-dB gain in BER performance.

Adaptive Sparse Dictionary Based Kernel Minimum Symbol Error Rate Post-Distortion for Nonlinear LEDs in Visible Light Communications

Visible light communications (VLC) has emerged as one of the prominent technologies to cater to the ever-increasing high-speed-data demand for proposed fifth-generation (5G) systems. However, two main issues affect the performance of VLC in an indoor environment: a) nonlinearity of light-emitting diode, which renders the overall system nonlinear; and b) intersymbol interference due to the propagation channel, which closes the eye diagram of the transmit constellation and, hence, causes it to be unsuitable for detection. To counter these artifacts, complex post-distortion receivers such as the Volterra-decision feedback equalizer (DFE) have been proposed to recover the transmit symbols. In this paper, the use of a reproducing kernel Hilbert space-based minimum symbol error rate equalizer is proposed that provides performance comparable to a long Volterra-DFE, with much less computational cost. Simulations have been carried over IEEE 802.15 personal area network (PAN) channels, which suggest that the proposed approach gives equivalent performance in an indoor VLC channel, as compared with Volterra-DFE with far fewer computations. An analytical expression for mean square error dynamics over these channels is also derived, and it is observed that the theoretically derived expression matches the simulation results for the considered IEEE 802.15 PAN indoor VLC channels.

Multi‐colour LED specified bipolar colour shift keying scheme for visible light communications

Colour shift keying (CSK) can achieve significant performance gains in multi-colour visible light communications (VLCs). Existing CSK constellations are designed within the positive domain which satisfies the requirement of non-negative optical intensity modulations. A scheme is proposed to incorporate spatial modulation with a CSK design by grouping LEDs. In this way, the CSK constellation is optimised within the entire real domain and hence achieves a larger minimum Euclidean distance so as to enhance the bit error rate (BER) performance. Simulation results show a noticeable performance gain by the proposed scheme compared with traditional CSK under ideal light-of-sight optical channels. Further, by experimental tests, an even more pronounced BER gain is achieved by the proposed scheme over real VLC channels.

Advanced Modulation Formats in Phosphorous LED VLC Links and the Impact of Blue Filtering

A theoretical model for transmission of advanced modulation schemes [pulse amplitude modulation (PAM), carrierless amplitude-phase (CAP), and discrete multitone (DMT)] in visible light communications (VLC) link using white phosphorescent LEDs is presented. The model extends and adapts the Randel et al. , framework for advanced modulation format transmission in intensity-modulated direct-detection for the VLC channel. The main contributions are: CAP modulation, effect of transmit signal spectral shaping in PAM and CAP, analytical calculation of crest factor for CAP, and analysis of inter-symbol and cross-carrier interference due to an insufficient cyclic-prefix in DMT. Based on this model, the impact of blue filtering at the receiver is studied in the context of spectrally efficient modulation formats. The results indicate that blue filtering has a marginal effect on the performance for all the investigated modulation formats, while PAM with decision feedback equalizer shows the best performance. Without blue filtering, slightly higher data rates can be achieved, but at a price of extensive signal processing.

Layered Transmission of Space-Time Coded Signals for Image-Sensor-Based Visible Light Communications

This paper demonstrates the feasibility of layered space-time coding (STC) in an outdoor image-sensor-based (IS-based) visible light communication (VLC) system. We examined that for low-resolution IS-based VLC channel where intensity-modulated signals from two different light emitting diodes (LEDs) are detected by one pixel of an IS, STC allows us to decouple them; thus, succeeding to receive them with no errors. Consequently, STC offers extended transmission distance to pixel-resolution-limited IS-based VLC links. In the layered STC presented in this paper, additional bit streams are laid on the ${2^n\times 2^n}$ LED array for increasing the transmission rate per symbol duration for the case where the pixel resolution is improved. A prototype of a three-layered STC is built with an ${8\times 8}$ LED array, where each of the LEDs is modulated at 1 kb/s and a high-speed camera with IS operating at 1000 fps. Our experimental results validate that the two additional bit streams (layer-2 and -3), aligned in the layer-1 STC matrix pair, are extracted with no errors when the receiver comes within 155 and 55 m, respectively, from the LED array, without decreasing 210 m of the transmission distance of layer-1 bit stream.

Modelling and analysis of M ‐ary variable pulse position modulation for visible light communications

Visible light communication (VLC) is an emerging communication paradigm that supports illumination and data transmission simultaneously. To provide dimming control functionality, a range of dimmable modulation methods have been exploited. Among them, the variable pulse position modulation (VPPM) scheme, which is deployed using the IEEE 802.15.7 VLC standard is very simple and easy to implement. On the other hand, it has a limited data rate because it just uses binary PPM for data transmission. This study proposes the M -ary VPPM ( M -VPPM) scheme to increase the data rate of the VPPM scheme. To evaluate the performance of this scheme, the modelling of the transmitter, optical wireless channel and the receiver for M -VPPM are provided. In addition, the error probability according to the dimming ratio and modulation order was analysed mathematically. The simulation results showed that the analytical bounds on the error probability are tight, which provides key insights into the performance trade-off of M -VPPM in a dispersive VLC channel environment.

Design and Analysis of a Visible-Light-Communication Enhanced WiFi System

Visible light communication (VLC) has wide unlicensed bandwidth, enables communication in radiofrequency-sensitive environments, realizes energy-efficient data transmission, and has the potential to boost the capacity ofwireless access networks through spatial reuse. On the other hand, WiFi provides more coverage than VLC and does not suffer from the likelihood of blockage due to the line-of-sight requirement of VLC. In order to take the advantages of both WiFi and VLC, we propose and implement two heterogeneous systems with Internet access. One is the hybrid WiFi–VLC system, utilizing a unidirectional VLC channel as the downlink and reserving the WiFi backchannel as the uplink. The asymmetric solution resolves the optical uplink challenges and benefits from the full-duplex communication based on VLC. To further enhance the robustness and increase throughput, the other system is presented, in which we aggregate WiFi and VLC in parallel by leveraging the bonding technique in the Linux operating system. We also theoretically prove the superiority of the aggregated system in terms of average system delay. Online experiment results reveal that the hybrid system outperforms the conventional WiFi for crowded environments in terms of throughput and Web page loading time, and also demonstrate the further improved performance of the aggregated system when considering the blocking duration and the distance between the access point and the user device.

Visible light communication channel disturbances and examination of the modulation formats

The paper experimentally investigates indoor visible light communication (VLC) channels. The disturbances come from the light bulbs and other VLC systems. The paper demonstrates that the noisiest light bulbs are the fluorescent lamps which emit high frequency disturbances. Different light bulbs are examined to make an optimal carrier choice for the transmitted signal. In the next part of the paper several modulation schemes and two kinds of detector systems are compared. The paper represents also the advantages and disadvantages of receiver constructions with and without a lens. The optimum modulation for different application is investigated.

Mobile Multi-Gigabit Visible Light Communication System in Realistic Indoor Environment

The main challenges facing high data rate visible light communication (VLC) are the low-modulation bandwidth of the current transmitters (i.e., light emitting diodes), the intersymbol interference (ISI) caused by the multipath propagation and cochannel interference (CCI) due to multiple transmitters. In this paper, for the first time, to the best of our knowledge, we propose, design, and evaluate the use of laser diodes (LDs) for communication as well as illumination. In addition, we propose an imaging receiver for a mobile VLC system to mitigate ISI. A novel delay adaptation technique is proposed to mitigate CCI, maximize the signal to noise ratio, and reduce the impact of multipath dispersion under user mobility. The proposed imaging system is able to provide data rates of 5 Gb/s in the worst-case scenario. The combination of a delay adaptation approach with an imaging receiver (DAT imaging LD-VLC system) adds a degree of freedom to the link design, which results in a VLC system that has the ability to provide higher data rates (i.e., 10 Gb/s) in the considered harsh indoor environment. The proposed technique (delay adaptation) achieves significant improvements in the VLC channel bandwidth (more than 16 GHz) over an imaging system in the worst-case scenario. The VLC channel characteristics and links were evaluated under diverse situations including an empty room and a room with very strong shadowing effects resulting from minicubicle offices.

Accuracy of the Point-Source Model of a Multi-LED Array in High-Speed Visible Light Communication Channel Characterization

During the indoor visible light communication (VLC) channel modeling procedure, the transmitter constituted by an array of light-emitting diodes (LED's) is frequently modeled as a single point source for convenience. However, how accurate this simplified treatment is remains unanswered. This paper compares the channel characteristics of both the simplified point-source model and six practical cases having various numbers of LEDs. Our numerical results show that the deviations in terms of the channel's optical path loss (OPL), as well as its bandwidth and the channel's delay spread, are steadily increased upon increasing the number of LEDs of each transmitter, until LEDs spread almost over the entire ceiling. Even in the worst case, the deviation of the OPL remains below 0.41 dBo, whereas that of the 3-dB transmission bandwidth is below 1.67 MHz. However, the deviation in terms of the root-mean-square (RMS) delay spread can reach 1.88 ns. Moreover, in terms of spatial distribution of the RMS delay spread, there is a nonnegligible difference between the simplified point-source model and different transmitter configurations.

Development and performances of standalone visible light communication terminals using imitate infrared encoding technique

By introducing the imitate infrared encoding technique and employing a white light emitting diode (WLED), a pair of standalone visible light communication (VLC) terminals, including a transmitter and a receiver, were experimentally demonstrated. The mathematic modelling and formulation were proposed on the WLED's luminous characteristics and the VLC channel response. The terminals were developed using ARM7-based embedded system, and experiments were carried out to derive their performances. A suitable illuminance of 200 lx (measured before the WLED) is preferred for obtaining a uniform bit error ratio (BER) over the distance of 0–2 m, and the measured BER is <10−4 with a communication data rate of 0.1–10 Mbps. High stability was also confirmed through measuring the BER results over 5 h at the two communication distances of 0.2 and 0.3 m and at 1 Mbps data transmission rate. Owing to the advantages of small size, low cost, and 10 Mbps data rate, this kind of terminals can be adopted in indoor short-distance wireless communication.

Time-division Visible Light Communication Using LED Lamp Light

We introduce a new method of time-division visible light communication (VLC) using LED lamp light for the generation of synchronizing pulses. The LED lamp, driven by an AC 220-V power line, radiates light that has a 120-Hz frequency component. The pulse generator in each VLC system receives the LED lamp light and generates the synchronizing pulses that are required for time-division transmission of multiple VLC channels. The pulse period is subdivided into several time slots for VLC channels. In experiments, 120- Hz synchronizing pulses were generated using LED lamp light, and three VLC channels were transmitted independently without interfering with each other in a condition where the VLC signals overlapped in space. This configuration is useful in constructing multiple wireless sensor networks that are safe and without interference in locations where LED lamps are used for illumination.

682 Mbit/s phosphorescent white LED visible light communications utilizing analog equalized 16QAM-OFDM modulation without blue filter

In this paper, a 682Mbit/s phosphorescent white LED visible light communication (VLC) system utilizing 16-quadrature-amplitude-modulation orthogonal frequency division multiplexing (16QAM-OFDM) modulation based on analog equalizers without blue filter is firstly experimentally demonstrated. The 3-dB bandwidth of white channel VLC link is enhanced from 3 to 143MHz by using proposed optimal analog equalizers. On the basis of equalized modulation bandwidth, the maxim transmission data rate of 682Mbit/s based on 16QAM-OFDM modulation could be achieved at the distance of 1m and the resultant bit-error-ratio (BER) is 2.5×10−3. The VLC link also could be operated at illumination level of ~1000lx with data rate of 520Mbit/s and the BER is 3.76×10−3. All the BERs are below the forward-error-correction (FEC) limit 3.8×10−3. The results show that analog equalizers could be used in 16QAM-OFDM VLC system. In addition, our transmitter and receiver modules are with compact size. The proposed VLC link without blue filter provides a solution for high-speed low-complexity VLC application based on OFDM modulation.

Visible Light Communication, Networking, and Sensing: A Survey, Potential and Challenges

The solid-state lighting is revolutionizing the indoor illumination. Current incandescent and fluorescent lamps are being replaced by the LEDs at a rapid pace. Apart from extremely high energy efficiency, the LEDs have other advantages such as longer lifespan, lower heat generation, and improved color rendering without using harmful chemicals. One additional benefit of LEDs is that they are capable of switching to different light intensity at a very fast rate. This functionality has given rise to a novel communication technology (known as visible light communication-VLC) where LED luminaires can be used for high speed data transfer. This survey provides a technology overview and review of existing literature of visible light communication and sensing. This paper provides a detailed survey of 1) visible light communication system and characteristics of its various components such as transmitter and receiver; 2) physical layer properties of visible light communication channel, modulation methods, and MIMO techniques; 3) medium access techniques; 4) system design and programmable platforms; and 5) visible light sensing and application such as indoor localization, gesture recognition, screen-camera communication, and vehicular networking. We also outline important challenges that need to be addressed in order to design high-speed mobile networks using visible light communication.

Visible Light Communication Channel Models and Simulation of Coal Workface Energy Coupling

Due to their low energy consumption and small size, visible light communication systems have been widely used to eliminate communication-blind areas in coal workfaces. A workface visible light communication channel model and a mathematic model of energy coupling from the mining machine’s workspace to its footpath were established to investigate the characteristics of optical signal transmission on the workface. This paper studies the effects of coal dust (double-layer particles encapsulated by moisture) on optical signal degradation. Simulation results revealed the presence of an optimized transmitter location, which maximized the coupled energy of the two space signals due to the blocking effect of metal columns.

Modulation and Demodulation Techniques in Indoor Visible Light Communication System

Visible light communication technology is a emerging wireless and optical communication technology developed after invention and application of white LED.In this paper, we have a research based on indoor visible light communication system of lighting white LED,discuss of the visible light communication channel characteristics,detailedly analysis of the indoor visible light communication link. This paper focuses on the modulation and demodulation method applied to visible light communication. In the past visible light communication is mainly used off keying modulation,however, the data transfer speed is limited due to the influence of ISI .This paper presents a solution based on OFDM modulation and demodulation, to reduce the impact brought from inter-symbol string under high data transfer speed.Based on data analysis and computer simulation,the program presented in this paper can be used as indoor lighting and high-speed data transmission.Finally, give scheme to achieve visible light communication modulation and demodulation,provide a theoretical basis for further experiments.

Human shadowing effect on indoor visible light communications channel characteristics

The shadowing effect caused by the human body can have a considerable influence on indoor visible light communications (VLC) channel characteristics. A shadowing ray tracing (SRT) algorithm to investigate the human shadowing channel characteristics is proposed. To validate the effectiveness of the SRT model, an extensive indoor human shadowing channel measurement has been presented. Both scenarios, the line-of-sight (LOS) and the non-line-of-sight (NLOS), have been measured. Theoretical results from the SRT model are validated against the measurement results for channel path loss. The results from measurements and simulations show that receiver rotations can mitigate human shadowing effect under the LOS scenario. Under the NLOS scenario, the VLC system becomes more sensitive to human shadowing when the distance between the LED and the receiver is large. Finally, more simulation results on the channel root mean square delay spread are also presented to analyze its relation with human shadowing distance to the receiver.

Research of LED Emission Cross-Section on Light Communication Systems

Visible light communication (VLC) is an optical wireless communication technology that uses visible light emitting diodes (LEDs) as a communication source. Since the LEDs are also used for illumination, the cross-section emission effects of the LEDs need to be analyzed as they apply to indoor VLC channels. In order to evaluate the illumination and communication performance according to the emission cross-section pattern, a simple LED model with a quasi-elliptic emission cross-section is proposed and compared to a circular LED model. The LEDs with a quasi-elliptic emission cross-section provide less fluctuation in the illumination and optical power distribution at the receiving plane. The four-transmitter VLC system is found to support at least at 30 and 33 Mb/s for circular and quasi-elliptic emission cross-section LEDs for the entire receiving plane, respectively.

Constellation Design for Channel Precompensation in Multi-Wavelength Visible Light Communications

The predicted ubiquity of light-emitting diodes (LEDs) suggests great potential for dual-use systems with visible light communications (VLC) capabilities. One class of LEDs employs multiple emitters at different wavelengths, making them appropriate for applications requiring colored output and providing multiple channels for communication. We present a design framework for optimizing the signaling constellation of VLC systems employing an arbitrary number of such LEDs, each with an arbitrary number of emitters. In particular, by considering the design of constellation-symbol locations jointly across LED emitters, the framework provides for the precompensation of linear channel distortions arising from, for example, cross-channel leakage, noise correlation, or wavelength-dependent received-signal and/or noise power, as well as the incorporation of design constraints for color-shift keying, dimming, and/or perceived color. Simulation results demonstrate the design approach and the potential performance enhancement that can be achieved for particular system scenarios. Experimental measurements using a prototype VLC system confirm such performance enhancements, providing real-world evidence of the benefit of applying the proposed framework to VLC channel precompensation.

Protocol Design and Capacity Analysis in Hybrid Network of Visible Light Communication and OFDMA Systems

Visible light communication (VLC) uses a vast unregulated and free light spectrum. It is considered to be a solution for overcoming the crowded radio spectrum for wireless communication systems. However, duplex communication, user mobility, and handover mechanisms are becoming challenging tasks in a VLC system. This paper proposes a hybrid network model of VLC and orthogonal frequency-division multiplexing access (OFDMA) in which the VLC channel is only used for downlink transmission, whereas OFDMA channels are served for uplinks in any situation or for downlinks only without VLC hotspots coverage. A novel protocol is proposed combined with access, horizontal, and vertical handover mechanisms for mobile terminal (MT) to resolve user mobility among different hotspots and OFDMA system. A new VLC network scheme and its frame format are presented to deal with the multiuser access problems in every hotspot. In addition, a new metric ρ is defined to evaluate the capacity of this hybrid network as the spatial density of interarrival time of MT requests in s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> under the assumption of the homogenous Poisson point process (HPPP) distribution of MTs. Analytical and simulation results show improvements in capacity performance of the hybrid, when compared to OFDMA system.