Orthogonal Frequency Division Multiplexing Visible Light Communication System Research Articles

A reduced PAPR hybrid OFDM visible light communication system

Orthogonal frequency division multiplexing (OFDM) is used by visible light communication (VLC) systems. Its main goal is to lessen inter-symbol interference's negative impacts. As a result, a lot of bandwidth and a high data rate are transmitted. Using OFDM-based VLC systems, nonetheless, have a high peak-to-average power ratio (PAPR). This research presents a new VLC system for indoor applications that reduces the PAPR while maintaining an acceptable bit error rate (BER). To achieve this, a hybrid pre-coding technique-based PAPR reduction solution is proposed; Vander-monde like matrix (VLM) combined with Gaussian matrix (GM). This technique takes advantage of both VLM and GM as a two stages for the signal to simultaneously reduce the PAPR and save the BER from damage. According to the results, the proposed solution (VLM in conjunction with GM) lowers the PAPR to 9.5 dB. Our system performs better when compared to other systems like discrete cosine transform (DCT), GM, VLM, DCT&GM, and original OFDM by 27%, 25%, 11%, 20%, and 34%, respectively.

Joint security enhancement and PAPR mitigation for OFDM-NOMA VLC systems

Security is of crucial importance to visible light communication (VLC) systems. In this paper, we propose and experimentally demonstrate a secure non-orthogonal multiple access (NOMA) VLC system with orthogonal frequency division multiplexing (OFDM) modulation, where the peak-to-average power ratio (PAPR) is also significantly reduced. Based on 2D chaotic map, the public key serves a two-fold purpose: to provide physical-layer security against eavesdropper, and to generate a set of phase-rotated candidate signals in the chaotic selected mapping (SLM) scheme where the signal with lowest PAPR is selected for transmission. The private keys further enhance the security and guarantee the privacy among the legitimate users via chaotic data scrambling. The experiment based on a direct current-biased optical OFDM (DCO-OFDM) VLC system with two legitimate users and one eavesdropper successfully verified the joint security enhancement and PAPR mitigation performance of the proposed scheme for NOMA VLC application.

Low complexity OFDM VLC system enabled by spatial summing modulation.

Commercial-off-the-shelf (COTS) devices enabled visible light communication (VLC) for Internet of things (IoT) applications has attracted extensive attentions from both academic and industrial communities, thanks to the pervasive deployments of light emitting diode (LED) lighting infrastructure. However, due to the limitation of frequency response and non-linearity of the commercial illuminating LED light consisting of multiple LED chips, the achievable data rate is far less than that provided by the experimental VLC system with a single LED with specialized devices, e.g., lens. To this end, we propose a power-of-2 arrangement scheme for LED chips to generate spatial summing modulation with low control complexity, and demonstrate its availability in an orthogonal frequency division multiplexing (OFDM) VLC system purely built upon COTS devices. It significantly differs from a conventional OFDM VLC system relying on digital-to-analog converter (DAC) and analog signal chain, which is complex and confined by LED's non-linearity, thanks to we design a novel digital-to-light converter (DLC) which can output 256 light intensities linearly and be directly controlled by the discrete digital signals generated by the OFDM modulator. An experimental demonstration with employing the QAM-OFDM modulation scheme successfully confirms the effectiveness of the proposed spatial summing VLC system, which can achieve low BERs of below the forward error correct (FEC) threshold of 3.8×10-3 for both QAM8 and QAM16 running transmission frequency of 300 kHz under a communication distance of 0.8 m. It demonstrates the promising potential for delivering a data rate at hundred kbps level with this novel spatial summing based OFDM VLC system, which is sufficient for many IoT applications.

Sparsity-Aware Nonlinear Equalization With Greedy Algorithms for LED-Based Visible Light Communication Systems

The nonlinearity of light emitting diode (LED) in visible light communication (VLC) systems is considered as one major limiting factor that deteriorates the systems’ performance. In this paper, the nonlinear equalization in VLC systems is deemed to be a sparse recovery problem. Three different greedy sparse recovery algorithms, namely matching pursuit (MP), orthogonal MP (OMP), and regularized OMP (ROMP) are employed in constructing the sparsity-aware nonlinear equalizer for LED-based VLC systems. By adopting these greedy algorithms, the number of kernels in the nonlinear equalizer can be significantly reduced with minor performance loss, which enables low-complexity and high-performance nonlinear equalization. The performance of the proposed sparsity-aware nonlinear equalizers is investigated experimentally in orthogonal frequency division multiplexing (OFDM) based VLC systems with commercially available red-green-blue LEDs. The results show that, with the help of greedy sparse recovery algorithms, the number of kernels in nonlinear equalization can be reduced by 32.5%∼62.5% for only 0.5-dB signal-to-noise ratio loss. The compatibility of the proposed sparsity-aware nonlinear equalizer in a ∼1 Gbit/s adaptive bit-power loading OFDM VLC system is also experimentally demonstrated. The ROMP-based nonlinear equalizer is found to be the best choice that offers the optimal balance between performance and complexity, which achieves almost the same performance as the conventional Volterra time domain nonlinear equalizer in OFDM VLC systems with approximately halved number of kernels.

Image transmission in an OFDM VLC system using symbol scrambling and chaotic Walsh-Hadamard precoding

An image secure transmission scheme combining symbol scrambling and chaotic Wahsh-Hardmard precoding is proposed in orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) system. In the proposed scheme, a two-dimensional (2D) logistic-sine-coupling map (LSCM) is employed to generate two chaotic sequences. One is used to scramble the 16 quadrature amplitude modulation (QAM) symbols, and the other is used to scramble the rows of standard Walsh-Hadamard transform (WHT). The simulation results show that the proposed scheme can not only reduce the peak-average-power ratio (PAPR) but also achieve the image secure transmission over OFDM-based VLC systems.

Investigation of DC-Biased Optical OFDM With Precoding Matrix for Visible Light Communications: Theory, Simulations, and Experiments

Orthogonal frequency-division-multiplexing (OFDM) technology is widely used in visible light communication (VLC) to achieve high data rate transmission. However, the traditional direct-current (DC)-biased optical OFDM (DCO-OFDM) VLC systems suffer from the high peak-to-average power ratio (PAPR) which causes signal clipping distortion, and, thus, performance degradation. Furthermore, severe high-frequency fading due to the limited system bandwidth results in poor bit error rate (BER) performance. Precoding matrix (PM) techniques have been proposed to enhance the performance of VLC OFDM transmission, but a little or no work has been carried out in investigating the theory of PM used in OFDM VLC systems. In this paper, we aim to reveal the theory of PM-DCO-OFDM for a VLC system. To figure out the intrinsic laws of a PM method, we investigate the principles of PAPR reduction, clipping distortion optimization, and signal-to-noise ratio (SNR) distribution equalization. Based on the analysis of PAPR, we theoretically proved the simplicity of PM as a method to reduce the possibility of high PAPR by improving the autocorrelation performance of input symbols. The clipping distortion could be improved due to the reduction of high PAPR. Moreover, the relatively uniform SNR distribution can be achieved by PM through equalizing the clipping and channel noise, which is beneficial to improve the BER performance in high-frequency constrained systems. However, the PM method used in a DCO-OFDM VLC system should consider the transmitting power, modulation format, and transmission distance as a whole to achieve the transmission performance improvement. The simulation results demonstrate the complementary cumulative distribution function of PAPR can be reduced ∼3 dB, while the performance of clipping distortion power and clipping error probability are significantly improved. Furthermore, experiment is carried out with results showing that the PM method can improve the BER performance in the case that VLC OFDM transmission has enough transmitting power, but with the low transmitting power, the PM also can damage the BER performance. The simulation and experiment results are consistent with our theoretical analysis.

Time domain reshuffling for OFDM based indoor visible light communication systems.

For orthogonal frequency division multiplexing (OFDM) based indoor visible light communication (VLC) systems, partial non-ideal transmission conditions such as insufficient guard intervals and a dispersive channel can result in severe inter-symbol crosstalk (ISC). By deriving from the inverse Fourier transform, we present a novel time domain reshuffling (TDR) concept for both DC-biased optical (DCO-) and asymmetrically clipped optical (ACO-) OFDM VLC systems. By using only simple operations in the frequency domain, potential high peaks can be relocated within each OFDM symbol to alleviate ISC. To simplify the system, we also propose an effective unified design of the TDR schemes for both DCO- and ACO-OFDM. Based on Monte-Carlo simulations, we demonstrate the statistical distribution of the signal high peak values and the complementary cumulative distribution function of the peak-to-average power ratio under different cases for comparison. Simulation results indicate improved bit error rate (BER) performance by adopting TDR to counteract ISC deterioration. For example, for binary phase shift keying at a BER of 10-3, the signal to noise ratio gains are ~1.6 dB and ~6.6 dB for DCO- and ACO-OFDM, respectively, with ISC of 1/64. We also show a reliable transmission by adopting TDR for rectangle 8-quadrature amplitude modulation with ISC of < 1/64.

Low-complexity frequency domain nonlinear compensation for OFDM based high-speed visible light communication systems with light emitting diodes.

A novel frequency domain nonlinear compensation method, FD-NC, is proposed for orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system. By tackling the memory nonlinear impairments from light emitting diodes (LEDs) in the frequency domain rather than in the time domain, the proposed method has much lower computational complexity than the conventional time domain Volterra nonlinear compensation method (TD-NC). Both theoretical derivation and experimental investigation of the proposed method in OFDM based VLC systems with four types of commercial LEDs are presented. The results of experiments show that the proposed low-complexity FD-NC method with a moderate truncation factor achieves a performance comparable to that of the TD-NC. The application of FD-NC method in the bit-power loading OFDM VLC system is also experimentally demonstrated. Compared with the linear equalization case, at a bit error rate (BER) of 3.8 × 10-3 (a), the transmission distance of a 960 Mbps VLC system can be extended from 0.7 m to 1.8 m by the FD-NC, and (b) the achievable system capacity can be enhanced by 18.7%~36.5% for transmission distance in the range of 0.5 m~2 m with the FD-NC. The complexity analysis shows that the required number of real-valued multiplications (RNRM) of the FD-NC is independent of linear or nonlinear memory length. The reduction of RNRM achieved by the FD-NC over the TD-NC becomes more profound for a larger nonlinear memory length or a smaller truncation factor.

Link Adaptation for MIMO OFDM Visible Light Communication Systems

In this paper, we investigate link adaptation for an orthogonal frequency division multiplexing (OFDM)-based multiple-input multiple-output (MIMO) visible light communication (VLC) system. The proposed adaptive OFDM VLC system supports both repetition coding (RC) and spatial multiplexing (SM) as MIMO modes and allows spatial mode switching based on channel conditions. Regarding to the instantaneous signal-to-noise ratio for both RC and SM modes, the maximum constellation size that can be supported for each MIMO mode on each subcarrier is determined. The MIMO mode that gives the highest spectral efficiency (SE) is then selected. The proposed joint MIMO mode selection and bit loading scheme maximizes the SE while satisfying a target bit error rate. Our numerical results reveal that a peak data rate up to 18.3 Gb/sec can be achieved in a $16 \times 16$ MIMO setting using light emitting diodes with cut-off frequency of 10 MHz in typical indoor environments.

Performance analysis of colour coded multiple access orthogonal frequency division multiplexing visible light communication system

Performance analysis of colour coded multiple access (CCMA) orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) System is discussed in this paper. First, characteristics of optical OFDM are studied. Second, CCMA is achieved using three laser diodes (LD): red, green and blue LDs. Finally, OFDM-modulated signals are optically modulated with red, green and blue LDs (RGB LDs). Then these three optical signals are combined together to form white light which illuminates as well as provides high-speed data transmission. The performance of the proposed CCMA OFDM VLC system is studied using constellation diagrams, signals and spectrum comparisons. The system is simulated for the data rate of 10 Gbps.

Indoor location estimation with optical-based orthogonal frequency division multiplexing communications

Visible light communication (VLC) using light-emitting diodes has been gaining increasing attention in recent years as it is appealing for a wide range of applications such as indoor positioning. Orthogonal frequency division multiplexing (OFDM) has been applied to indoor wireless optical communications in order to mitigate the effect of multipath distortion of the optical channel as well as increasing the data rate. An OFDM VLC system is proposed, which can be utilized for both communications and indoor positioning. A positioning algorithm based on power attenuation is used to estimate the receiver coordinates. We further calculate the positioning errors in all the locations of a room and compare them with those using single-carrier modulation schemes, i.e., on–off keying modulation. We demonstrate that our proposed OFDM positioning system outperforms by 74% its conventional counterpart. Finally, we investigate the impact of different system parameters on the positioning accuracy of the proposed OFDM VLC system.

The efficiency droop impact of GaN‐based LEDs on the performance of OFDM visible light communication system

AbstractThe physical mechanism in efficiency droop of GaN‐based LEDs fabricated by metal organic chemical vapor deposition (MOCVD) was investigated by experiment and calculation. On this basis, the efficiency droop impact on the performance of orthogonal frequency division multiplexing (OFDM) visible‐light communication (VLC) system was analyzed. The numerical results show that the VLC system performance decreases obviously under the impact of the LED efficiency droop. And, the performance deterioration of OFDM VLC system aggravates as FFT sampling rate increase due to signal dynamic range enlargement. Furthermore, the efficiency droop impact on the VLC performance can be alleviated by adjusting the LED operating condition. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Using specific and adaptive arrangement of grid-type pilot in channel estimation for white-lightLED-based OFDM visible light communication system

Orthogonal frequency division multiplexing (OFDM) is a promising candidate for light emitting diode (LED)-based optical wireless communication (OWC); however, precise channel estimation is required for synchronization and equalization. In this work, we study and discover that the channel response of the white-lightLED-based OWC was smooth and stable. Hence we propose and demonstrate using a specific and adaptive arrangement of grid-type pilot scheme to estimate the LED OWC channel response. Experimental results show that our scheme can achieve better transmission performance and with some transmission capacity enhancement when compared with the method using training-symbol scheme (also called block-type pilot scheme).

Wavelength Division-adaptive Interference Cancellation Applied in OFDM Visible Light Communication Systems

This paper studies the performance degradation caused by optical Doppler shift in light-emitting diode-based visible light communication systems, which integrate orthogonal frequency division multiplexing (OFDM). Through theoretical analysis and computer simulations, we can observe that frequency offset plays a significant role in the OFDM performance deterioration due to inter-carrier interference (ICI) and common phase error (CPE). Therefore, a scheme to correct the frequency offset by using wavelength division-adaptive interference cancellation (WD-AIC) was proposed. The main approach is to assign an estimated sub-carrier after every two adjacent sub-carriers to reduce the negative effects of both ICI and CPE. Finally, the simulation results declare that our proposed WD-AIC is indeed effective against the high degree of frequency offset.

Performance study of an OFDM visible light communication system based on white LED array

By introducing orthogonal frequency division multiplexing (OFDM) technology, a visible light communication (VLC) system using a 5×5 white LED array is studied in this paper. The OFDM transmitter and receiver are modeled through MATLAB/Simulink tool. The electrical-optical-electrical (EOE) response of the VLC channel, which is also the response of the detector, is derived based on Lambert’s lighting model. Then the modeling on the overall OFDM/VLC system is established by combining the above three models together. The effects of the factors which include the digital modulation, Reed-Solomon (RS) coding, pilot form, pilot ratio (PR) and communication distance on the bit error rate (BER) of the system are discussed. The results show that through the use of RS coding, block pilot, quadrate phase shift keying (QPSK) modulation and a suitable pilot ratio about 1/3, under the communication rate about 550 kbit/s, the BER can be dropped to below 10–5, and the communication distance can reach 0.9 m.