Multiple-input Multiple-output Visible Light Communication Research Articles

Flipped superposed constellation design for MIMO visible-light communication systems

Multiple-input multiple-output (MIMO) visible-light communication (VLC) systems based on the concept of superposed constellation have attracted increasing attention because multiplexing gains can be achieved even in a highly correlated MIMO channel. In this paper, a novel superposed constellation scheme is proposed for 2×2 MIMO VLC systems, where a 2n-order quadrature amplitude modulation (QAM) signal is obtained by superposing 4QAM and processed 2n-2-order QAM signals. Based on the original 2n-2-order QAM signal, the processed signal is generated by adding an optimal offset, power normalization, and flipping according to the value of the 4QAM signal. Thus, the required power ratio between the two superposed signals is equal to 1, which not only avoids the power competition in the receiver, but also reduces the risk of the nonlinear distortion of the light-emitting diode at the transmitter. Moreover, benefitting from the flipped superposition method, two additional performance gains are provided. First, thorough Gray coding can be achieved. Second, the received power can be improved because the two transmitted signals are correlated. An algorithm for solving the optimal offset is proposed based on the target of the equal power ratio, and the expression of the received power is derived as well. Then, the performance of the proposed scheme is investigated using detailed simulations in an additive white Gaussian noise channel, where the superposed 64QAM constellation is assumed as an example. Further, we perform an experimental demonstration and examine the performance of the proposed system when a practical optical channel is considered, where nonideal impacts, such as nonlinearity and power competition, may occur. The experimental results confirm that the proposed scheme achieves a lower bit error rate (BER) and a larger dynamic range of driving peak-to-peak voltage compared with the existing superposed constellation schemes. Considering the 7% pre-forward error correction BER threshold of 3.8 × 10-3, the proposed superposed 64QAM constellation system can achieve a maximum transmission rate of 3 Gb/s.

A Reliable Singular Value Decomposition and Geometric Mean Decomposition Based Precoding Scheme for MU-MIMO-VLC System

In this paper, in order to improve the reliability of Multi-User Multiple-Input Multiple-Output Visible Light Communication (MU-MIMO-VLC) system, we propose a precoding scheme that based on Singular Value Decomposition (SVD) and Geometric Mean Decomposition (GMD) assisted Orthogonal Transformation (OT). In this design, we use SVD to conduct the decomposition of the user’s complementary channel matrix. Then, the equivalent channel matrix is obtained by multiplying the user’s channel matrix and the decomposed component. After that, the GMD is carried out to decompose the equivalent channel matrix. Finally, the precoding matrix of the proposed scheme is obtained. We then derive the theoretical Symbol Error Rate (SER) of the proposed scheme. The simulation results are provided to validate the effectiveness of the proposed scheme, and to demonstrate that the reliability of the proposed design is superior to benchmark schemes.

Performance analysis of angular diversity receiver based MIMO–VLC system for imperfect CSI

Visible light communication (VLC) has emerged as a promising technology for 5G and beyond communication networks in the last few years. In this paper, a multiple-input multiple-output (MIMO) VLC system using angular diversity receiver is proposed. In particular, the repetition coding is adopted at the transmitter and receiver diversity schemes, i.e. maximum-ratio combining (MRC), equal gain combining (EGC) and selection best combining (SBC), are adopted at the receiver for further performance enhancement. The closed-form expressions for average error probability of the proposed system are derived in the presence of imperfect channel state information (CSI). This research investigates that the increment in SNR is not sufficient to offset the effect of channel estimation error especially at high estimation error. The error performance analysis of the proposed MIMO–VLC system with and without estimation error is presented for different receiver locations and semi-half angles of the transmitting light emitting diodes (LEDs). Analytical results are verified with the simulation results.

Magnitude and Wrap-Phase OFDM for MIMO Visible Light Communication Systems

Generalized light emitting diode (LED) index modulation (GLIM) has been proposed as an effective multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) scheme for visible light communications (VLC) systems. In this letter, we propose a magnitude and wrap-phase OFDM (MW-OFDM) scheme for MIMO VLC systems. The proposed MW-OFDM system relies on the conversion of complex signals into polar form with the magnitudes and wrap-phases to decrease the restriction on the number of LEDs compared to the conventional GLIM-OFDM. Furthermore, maximum likelihood and best linear unbiased estimators of the proposed scheme are derived. The proposed MW-OFDM scheme improves the average bit error rate and provides a significant reduction in the decoding complexity, compared to the GLIM-OFDM. Moreover, a half number of LEDs are required for the proposed scheme to deliver the same spectral efficiency in a comparison with the GLIM-OFDM. Numerical results are presented over MIMO VLC channels to elaborate the superiority of our proposed scheme.

Data transmission scheme based on the DC-QOSTBC in indoor MIMO-VLC systems.

We propose a data transmission scheme based on the dimming coding and quasi-orthogonal space-time block coding (QOSTBC) to improve the performance of indoor multiple-input multiple-output visible-light communication (MIMO-VLC) systems. After channel coding, the dimming coding was first applied to encode the input information bits into dimming bits. Then these encoded dimming bits were mapped to different symbols by head pulse position modulation. Finally, the symbol matrix was constructed on the basis of QOSTBC; then the light-emitting diodes would be activated to transmit the optical signals carrying the information according to the symbol matrix. The Monte Carlo simulations demonstrated the superiority of the proposed scheme in channel capacity and bit error ratio (BER). The channel capacity was a good approximation to the actual one, even though the full diversity gain could not be completely achieved. The BER could achieve 0.4×10-6 when the number of photodetectors was two.

Performance enhancement for a non-orthogonal multiple access system using 4 × 4 multiple-input multiple-output visible-light communication

A multi-user visible-light communication (VLC) system based on 4 × 4 multiple-input multiple-output (MIMO) was investigated. The system consists of four light-emitting diodes (as a transmitter) and four photodetectors (as a receiver). The proposed system was simulated for two, three, and four users. The objective of this study is to optimize the system power consumption and to utilize the sum rate on the system coverage area. By expanding the system from a 2 × 2 MIMO-VLC to a 4 × 4 MIMO-VLC, the results demonstrate that the total sum rate of the system can be improved. The findings reveal that the 4 × 4 MIMO-VLC achieves a maximum sum-rate enhancement of 143% compared with the 2 × 2 MIMO-VLC. The results also show that the efficiency of 4 × 4 MIMO-VLC system with non-orthogonal multiple access (NOMA) and gain ratio power allocation (GRPA) was increased by around 181% compared to 2 × 2 MIMO-NOMA-based VLC with GRPA in combination. For the 4 × 4 MIMO-NOMA-VLC device with GRPA, the sum bit rate for two, three, and four users was increased by 40%, 33%, and 24%, respectively. Through this study, it can be concluded that GRPA scheme plays an important role in utilizing MIMO-VLC system sum rate of the MIMO-VLC system.

Optical Power Distributions for 4×4 MIMO Visible Light Communications in Indoor Environment

Visible Light Communication (VLC) has emerged as a powerful technique for wireless communication systems. Providing high data rate and increasing capacity are the major problems in VLC. Recent evidence suggests that Multiple Input Multiple Output (MIMO) technique can offers improved data rates and increased link range. This paper describes the design and implementation of visible light communication system in indoor environment exploring the benefits of MIMO. The specific objective of this research was to implement a 4×4 Multiple Input (LEDs) Multiple Output (photodetectors)-VLC communication system, where a 16 white power LEDs in four arrays are setting up at transmitter and four RX modules are setting up at receiver side without the need for additional power or bandwidth as well as analyze a mathematical model for a VLC- 4×4 MIMO with different modes such as the suitable angles to cover the entire room. The research designs practically an electronic circuit for the transmitter and receiver with inexpensive components. The power distribution of different propagation modes is calculated for Single Input Single Output (SISO) and MIMO channels in typical room dimensions. The results in this paper indicate that the power is distributed uniformly for entire room when implemented VLC-MIMO as compared to VLC-SISO. Furthermore, a 4×4 MIMO implementing VLC is compared in term of BER vs. SINR with SISO communication system for both Line of Sight (LOS) and Non Line of Sight (NLOS) propagation modes. Comparing the two results of LOS and NLOS, it can be seen that a 4×4 MIMO implementing VLC for LOS perform better than the same system with NLOS due to decreasing in the received power resulted from the multipath effect.

Spatial Multiplexing MIMO Visible Light Communications With Densely Distributed LEDs and PDs

Multiple-input multiple-output visible light communications (MIMO-VLC) have been considered as an advanced technique owing to the enhancement of communication capacity and reliability. However, the performance of MIMO-VLC highly depends on channel correlation, which means that the system performs worse when the channels are more highly correlated. Some studies have developed techniques to alleviate channel correlation and improve the performance of spatial multiplexing MIMO-VLC systems. However, these works have not considered situations where all LEDs and PDs are integrated into a compact structure. In this paper, a single-user spatial multiplexing MIMO-VLC system with densely distributed LEDs (light emitting diodes) and PDs (photodiodes) is proposed. Designs of optimal signal vectors and system structures are introduced, to fulfill multiplexing gains when LEDs and PDs are set close to each other respectively. Numerical simulations are carried out and the results illustrate that the proposed system can realize multiplexing gains over the system where LEDs and PDs are distributed densely, and also outperforms the traditional repetition coding (RC) technique.

BER performance analysis of OFDM‐based integrated PLC and MIMO‐VLC system

In the present paper, the bit error rate (BER) performance of hybrid power line communication (PLC) and multiple-input multiple-output (MIMO) visible light communication (VLC) system (HPMV) has been investigated using line-of-sight (LOS) and LOS plus first reflection (L-R1) signals for room size of 5 m × 5 m × 3 m (configuration A) and 7.5 m × 7.5 m × 3.5 m (configuration B). In the work, Rayleigh fading and Barry channel models have been considered for PLC and VLC channels respectively. The BER has been analysed by varying separations between LEDs as 0.2 m, 0.6m and 1m for both configurations. The results show that the BER for LOS and L-R1 signals decreases upto optimal separation of 0.6 m for configuration A. However, the result indicates that the required optimal separation for configuration B is more than configuration A. The channel frequency response of MIMO-VLC and HPMV system has been inspected and it supports the results of BER simulation. It has been found that, with increase in spatial separation between LEDs, the deep notches and attenuation in the frequency response of MIMO-VLC system decrease. The overall investigation shows that even though OFDM is resilient to frequency selective fading, an optimum separation between LEDs must be chosen according to the room size for the HPMV system.

Deep Learning-Based Collaborative Constellation Design for Visible Light Communication

In multiple-input multiple-output (MIMO) visible light communication (VLC) systems, collaborative constellation design (CC) is effective in improving performance while significantly reducing the total optical power. This letter presents the first attempt to exploit deep learning (DL) in the signal design for MIMO VLC systems using CC, namely CCNet. More specifically, we present a deep neural network with fully connected layers to design an optimal constellation which can be derived conventionally at a high computational cost using the convex optimization solving procedure. Moreover, to enhance the performance of the proposed CCNet, instead of employing only the ordinary channel state information (CSI), the input is efficiently preprocessed before entering the network. By employing training samples collected in simulations, CCNet is first trained offline with the objective of minimizing the mean square error (MSE), and the online CC designing process can then be effectively deployed using the trained model. Simulation results show that CCNet can achieve a near-optimal MSE with a remarkably lower running time than the previous CC designing scheme.

Low-complexity MIMO demultiplexing scheme based on geometric vector extraction in visible light communication system

As one of the main optical wireless communication technologies, visible light communication (VLC) is considered a promising supplement in 5G/6G and the internet of things (IoT) networks. The multiple-input–multiple-output (MIMO) multiplexing scheme can significantly enhance a VLC system’s performance. To handle the blind MIMO demultiplexing, geometric vector extraction (GVE) algorithms are used in a mixture space (MS), which has not been presented in the extant literature. In a 2 × 2 VLC MIMO scenario, the singular value decomposition of an arbitrary MIMO channel matrix is introduced to discuss the one-to-one relationship between channel-matrix elements and the outline-shape conversion starting from the transmitted rectangular-shaped points in the MS. By extracting the vector features of the adjacent sides of the received parallelogram-shaped constellation points, we can obtain the elements of an unknown channel matrix and recover the superposed signals transmitted by two light-emitting diodes (LED). To obtain a vector from these constellation points, clustering is required to obtain the scattered points’ centroids in advance. To avoid falling into a local minimum, we present a clustering algorithm with an initial-position-expansion pre-processing stage to place the initial centroids with the greatest accumulated distance. To enhance the computational efficiency, we enhance the clustering algorithm via a single iteration to obtain the centroids, which we named the GVE-for-iteration-once (GVE–IO) algorithm. The enhanced algorithm has an equivalent or better MIMO demultiplexing performance and quick convergence performance compared to the fast independent component analysis (FastICA) algorithm. Moreover, the GVE–IO algorithm’s computational complexity and measured running time is reduced to 29% and 23%, respectively, of the FastICA algorithm’ time costs. Finally, the experimental investigation shows that the proposed scheme exhibits a significantly reduced computational complexity in all discussed voltage peak-to-peak (Vpp) cases of LED and a Q-value enhancement in two-thirds of the Vpp combination regions for 50 input points.

Effect of LED Radiation Pattern on BER Performance in Indoor Multipath MIMO-VLC System

The present paper investigates the impact of three radiation patterns of LED namely Lambertian, Batwing, Elliptical on the BER performance of 4 × 4 indoor multiple-input multiple-output (MIMO) visible light communication (VLC) system. The BER simulation has been done using LOS and LOS plus first reflection (L-R1) signals for ceiling-mounted and wall-mounted LEDs. For the case of ceiling-mounted LEDs, impulse response and BER have been computed using those radiation patterns at the centre and corner of a room (5 m × 5 m × 3 m). It has been found that LED having Elliptical radiation profile is more suitable for LOS signal when receiver is placed at the centre position whereas, Batwing radiation is suitable for L-R1 signal, both at the centre and corner positions. In the case of wall-mounted LEDs, impulse response and BER have been determined at the centre of the room using LOS and L-R1 signals. The results show that Batwing has least BER using LOS signal due to its wide radiation beam, but for L-R1 signal, the Elliptical profile having its narrow radiation beam has provided better BER performance. The present study shows that the BER in MIMO-VLC system not only depends on the transmitter and receiver position but also on the radiation patterns of LED and higher order reflection signals.

Orientation-induced link-blocked receiver for MIMO visible light communication.

In this paper, we propose an orientation-induced link-blocked receiver (OLBR) to simultaneously reduce the correlation of the multiple-input multiple-output (MIMO) channels and avoid disruptive transmission in visible light communication (VLC) system. The idea of this paper is to combine the adjustable normal vector ideology of an angle diversity receiver with the channel correlation reduction of a link-blocked receiver. The proposed OLBR is designed to intentionally block specific links while enhancing other links between light-emitting diodes (LEDs) and photodiodes (PDs). Besides, the proposed receiver can perform well in many locations in the room whereas conventional receivers fail to receive signals from at least one LED due to unwanted line-of-sight (LOS) link blocking. Moreover, an algorithm is proposed to iteratively solve the optimization problem, taking the normal vectors of the PDs into consideration. The simulation results show that the proposed OLBR can achieve a performance gain in comparison with previous receiver design schemes for different scenarios, especially in the locations that are far from LEDs.

Superposed 32QAM Constellation Design for 2 × 2 Spatial Multiplexing MIMO VLC Systems

Spatial multiplexing multiple-input multiple-output (MIMO) visible light communication (VLC) has become a potential candidate for next-generation wireless communication networks. In this paper, a novel superposed odd-order 32QAM constellation scheme is proposed in 2 × 2 MIMO VLC systems. Two independent signals generated from two light emitting diode (LED)-based transmitters with a four-quadrature amplitude modulation (4QAM) and geometrically shaped square 8QAM, respectively, are transmitted over free space and spatially multiplexed in the receiver to obtain a distinctly square-shaped 32QAM signal. The high channel correlation causes failure of MIMO detection in traditional spatial multiplexing (SMP) systems owing to the rank defect of the channel matrix. However, MIMO detection can always be realized by a simple constellation de-mapping based on the concept of superposed constellations, regardless of the extent of channel correlation. Considering the non-linearity of an LED, the square-shaped 8QAM constellation is introduced to reduce the peak to average power ratio (PAPR) at the transmitter, which also benefits from its maximum minimum Euclidean distance (MED) at the receiver compared to those of traditional 8QAM constellations. Moreover, the equal-gain combining (EGC) algorithm is employed to gain the receive diversity. The proposed system is experimentally studied in detail, where the system performance is evaluated under the conditions of different driving peak-to-peak voltages (Vpps) and channel gains. The experimental demonstration also shows that the dynamic range of the driving Vpp can be improved from 0.3 V to 0.6 V when compared with that of the traditional non-geometrically-shaped 8QAM, clearly validating the superiority of the proposed system.

High-gain and volume-efficient concave–convex lens for multiple-input multiple-output visible light communication system

We have proposed a concave–convex lens for the multiple-input multiple-output (MIMO) visible light communication (VLC) system, which provides more received power than conventional lenses. Ray trajectories using a hemispherical lens, compound parabolic concentrator (CPC), dielectric totally internally reflecting concentrator, and the proposed lens have been simulated in COMSOL software, and it has been shown that the proposed lens provides better convergence at the exit aperture. Also, the images of individual light-emitting diodes (LEDs) are distinguishable, and hence it can support a higher diversity order for the MIMO system. It has been found that our designed lens provides 1.081 times more received power than the hemispherical lens. We have investigated that the average received power using a concave–convex lens is 2.3593 and 1.3737 times more than the received power through bare (uniform refractive index) CPC and gradient-index-based CPC lenses, respectively. We have compared the channel gain using concave–convex and hemispherical lenses considering different values of incidence and half power semiangle of LED. The concave–convex lens has more gain and better volume efficiency; therefore, it can be considered as an appropriate choice for a compact receiver system in an MIMO-VLC system.

Enhanced Multi-Level Multi-Pulse Modulation for MIMO Visible Light Communication

Any realistic indoor multiple-input multiple-output (MIMO) visible light communication (VLC) system, besides providing reliable high rate transmission, should also support flicker-free, dimming capable, and uniform illumination. In this article, we propose a multi-level multi-pulse modulation for a MIMO VLC system to provide a high data rate with flicker-free uniform illumination. A low complexity detector and a searching algorithm to design the symbol set for transmission are also discussed. Simulation results demonstrate that the proposed detector can achieve a remarkable reduction in computational complexity without any performance degradation as compared with the conventional maximum likelihood detector. Moreover, the proposed symbols design algorithm can help to reduce the symbol error rate (SER), especially in high correlation MIMO VLC channels.

Space Division Multiple Access With Distributed User Grouping for Multi-User MIMO-VLC Systems

To support multiple users in an indoor multiple-input multiple-output visible light communication (MIMO-VLC) system adopting spatial multiplexing, orthogonal frequency division multiple access (OFDMA) is usually adopted, where the overall modulation bandwidth is shared by all the users. In this paper, by fully exploiting the spatial distributions of light-emitting diode (LED) transmitters in the ceiling and end users around the receiving plane, we propose a space division multiple access (SDMA) technique for indoor spatial multiplexing-based MIMO-VLC systems. When applying SDMA, users within the MIMO-VLC system are divided into different user groups (UGs) based on their spatial locations with respect to different LED transmitters. Specifically, each UG can use the overall modulation bandwidth of the system. For efficient implementation of SDMA, two distributed user grouping (DUG) approaches are proposed, including basic DUG and transmit diversity-enhanced DUG (TD-DUG), and a two-step resource allocation algorithm is further designed. The achievable rates of the MIMO-VLC system employing SDMA with both basic DUG and TD-DUG are derived accordingly. To verify the superiority of SDMA over conventional OFDMA, detailed analytical and simulation results are presented. Moreover, a proof-of-concept experiment is conducted to demonstrate the advantage of SDMA in a practical spatial multiplexing-based MIMO-VLC system.

Energy Efficiency Optimization for MIMO Visible Light Communication Systems

Multiple-input multiple-output (MIMO) visible light communication (VLC) systems can support high data rates for indoor communications. In this letter, we focus on maximizing the energy efficiency (EE) of MIMO VLC systems, where the non-negativeness of the transmit signal, the optical and electrical power constraints, and the data rate and bit error rate (BER) requirements are taken into account. The formulated problem is a difficult nonlinear fractional program. We exploit the Dinkelbach’s algorithm and Lagrangian analysis to obtain a semi-closed form solution. The performance advantage of the proposed design is verified by simulation results.

MBER Combining for MIMO VLC With User Mobility and Imperfect CSI

Visible light communication (VLC) has emerged as a green, low-cost, and secure supplement for existing radio frequency (RF) based systems. Although promising, the performance of VLC based systems is limited by several impairments, which degrade the achievable bit error-rate (BER) performance, viz. variations in channel-gain due to user-mobility, and channel-estimation error. Hence, in this letter, the bit error rate (BER) of maximal ratio combining is quantified over a multiple-input multiple-output (MIMO) VLC system, while considering the aforementioned impairments. Next, using the derived expressions, exact and asymptotic combiners based on the minimum bit error-rate (MBER) criterion are formulated for MIMO VLC systems impaired by user-mobility, and channel-estimation error. The accuracy of the contributed expressions for BER, and the performance of the proposed MBER combiner (MBERC) are corroborated by simulations performed over realistic MIMO VLC channels. Additionally, it is found that the proposed MBERC delivers improved BER performance in presence of channel-estimation error and user-mobility, which makes the proposed methodology viable.

FFDNet-Based Channel Estimation for Massive MIMO Visible Light Communication Systems

Channel estimation is of crucial importance in massive multiple-input multiple-output (m-MIMO) visible light communication (VLC) systems. In order to tackle this problem, a fast and flexible denoising convolutional neural network (FFDNet)-based channel estimation scheme for m-MIMO VLC systems was proposed. The channel matrix of the m-MIMO VLC channel is identified as a two-dimensional natural image since the channel has the characteristic of sparsity. A deep learning-enabled image denoising network FFDNet is exploited to learn from a large number of training data and to estimate the m-MIMO VLC channel. Simulation results demonstrate that our proposed channel estimation based on the FFDNet significantly outperforms the benchmark scheme based on minimum mean square error.