MIMO-VLC System Research Articles

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.

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.

Experimental Demonstration of Compressive Sensing-Based Channel Estimation for MIMO-OFDM VLC

The combination of optical multiple inputs multiple outputs (MIMO) and orthogonal frequency division multiplexing (OFDM) is a viable option to overcome the bandwidth limitation and increase the transmission data rate in visible light communications (VLC). In MIMO-VLC systems with precoders and equalizers it is essential to know the perfect channel state information. Traditional channel estimation (CE) techniques such as least square (LS) are widely used in MIMO-VLC systems. However, the LS algorithm is subject to noise enhancement, which results in lower estimation accuracy. Besides, the pilot tones between different transmitters should be orthogonal either in time or frequency domains, which increase the overhead. Since the physical VLC channel model exhibits strong sparsity, we propose a CE method based on compressive sensing (CS) for MIMO-OFDM VLC systems. The feasibility of the proposed CS-CE method is verified by experimental demonstration of a 2×2 MIMO-OFDM VLC system. The experimental results show that, the proposed method offers improved bit error rate performance with reduced overhead compared with the LS-CE scheme.

Analysis and Demonstration of Quasi Trace Orthogonal Space Time Block Coding for Visible Light Communications

In the VLC context, pulse position modulation (PPM) and similar modulations are typically used when the overall complexity of the transmitter is required to be low or the system needs to support dimming, i.e., dynamically control the illumination level of the transmitter. However, despite the power efficiency of PPM, it is known to be bandwidth inefficient. Having known the trade-off between reliability and spectral efficiency, in this paper we propose a PPM-based space-time block coding (STBC) technique named as quasi-trace-orthogonal (QTO), derived from trace-orthogonal, to increase the spectral efficiency of PPM VLC’s by limiting the reliability loss. We provide Monte-Carlo simulations for a $4\times 4$ MIMO-VLC system and validate the results experimentally, and show that for a given signal-to-noise-ratio, the QTO-STBC based 4-PPM VLC system offers higher spectral efficiency at a cost of higher symbol error rate compared to trace-orthogonal STBC.

Capacity Maximized Linear Precoder Design for Spatial-Multiplexing MIMO VLC Systems

This paper proposes a novel linear precoder design based on the singular value decomposition (SVD) for multi-input multi-output visible light communications (MIMO VLC). We provide an analytical expression on power allocation subject to the function of maximizing the lower bound of capacity according to the optical wireless communication channel model. The expression is rather different from the radio frequency (RF) communications due to the non-negativity of the transmitted signals in VLC. Furthermore, we design an adaptive bit loading scheme for sub-streams with consideration of the tremendous gain difference among subchannels. Performances of the proposed adaptive power and bits allocation strategy are evaluated in two traditional MIMO VLC scenarios, i.e., a practical $2\times 2$ indoor scenario and a classical $4\times 4$ system that is generated according to the MIMO model. In simulations, the techniques of unipolar M -level pulse amplitude modulation ( M -PAM) and spatial-multiplexing (SMP) are employed, and the performance comparison with other SVD-based linear precoders is also given. Simulation results show that the proposed approach can effectively improve the spectral efficiency and guarantee the bit error rate (BER) performance under the same constraints of aggregate optical power budget and non-negativity included by the intensity modulation. It indicates that the strategy of uneven bits and power allocation with full consideration of the ill-conditioned characteristics of subchannels performs better than that based on equal bits transmission in VLC systems.

Novel Bit Mapping for Generalized Spatial Modulation in VLC Systems

In this letter, we propose a novel bit-to-symbol mapping for generalized spatial modulation (GSM) in visible light communication (VLC) systems. The proposed mapping first sorts the GSM symbol pairs according to their Euclidean distance (ED) and then iteratively assigns each GSM symbol to the bit sequence that satisfies the Gray codes. We also present an exhaustive sorting method and a reduced-complexity sorting method which can be used to obtain an effective mapping algorithm. Our simulation results show that the proposed ED-based mapping outperforms the conventional mapping rule for the GSM MIMO-VLC systems.

Chromaticity-Adaptive Generalized Spatial Modulation for MIMO VLC With Multi-Color LEDs

In this paper, we propose a chromaticity-adaptive generalized spatial modulation (CA-GSM) method for multiple-input multiple-output (MIMO) visible light communication (VLC) systems with quadrichromatic LEDs (QLEDs). After the MIMO-VLC system model is constructed, the CA-GSM method based on chromaticity-adaptive constellation optimization is brought forward accounting for the correlated MIMO channel with multi-color crosstalk and quadrangle chromaticity region. This work mainly investigates the optimal active QLED selection pattern in CA-GSM aiming at maximizing the minimum Euclidean distance of received multi-color constellation. Considering power, amplitude and chromaticity constraints, the optimization problem of chromaticity-adaptive constellation is transformed and solved by the linear approximation of Taylor series. Therefore, the design algorithm of CA-GSM is derived with the optimal QLED selection pattern. Simulation results verify that our proposed CA-GSM can achieve performance improvement compared to the multi-stream spatial modulation and the conventional design method without considering channel.

Neural Network-Based Joint Spatial and Temporal Equalization for MIMO-VLC System

The limited bandwidth of white light-emitting diode (LED) limits the achievable data rate in a visible light communication (VLC) system. A number of techniques, including multiple-input-multiple-output (MIMO) system, are investigated to increase the data rate. The high-speed optical MIMO system suffers from both spatial and temporal cross talks. The spatial cross-talk is often compensated by the MIMO decoding algorithm, while the temporal cross talk is mitigated using an equalizer. However, the LEDs have a non-linear transfer function and the performance of linear equalizers are limited. In this letter, we propose a joint spatial and temporal equalization using an artificial neural network (ANN) for an MIMO-VLC system. We demonstrate using a practical imaging/non-imaging optical MIMO link that the ANN-based joint equalization outperforms the joint equalization using a traditional decision feedback as ANN is able to compensate the non-linear transfer function as well as cross talk.

Improved Joint Generalized Spatial Modulations for MIMO-VLC Systems

In this letter, we propose the computationally efficient constellation design algorithms for single-mode and dual-mode joint generalized spatial modulations (JGSMs) to improve the symbol error rate (SER) performance over highly correlated indoor visible light communication channels. Our simulation results show that the proposed joint and improved dual-mode JGSMs can offer a better SER performance than the proposed single-mode JGSM (SM-JGSM) and the conventional SM-JGSM, which are validated through the analytical bounds.

Effect of multipath reflection on BER performance of indoor MIMO-VLC system

In the present work, the bit error rate (BER) performance due to multipath reflections, namely, line-of-sight (LOS), LOS plus first reflection (L-R1), LOS with first and second reflection (L-R1–R2) for the indoor MIMO-VLC system has been analysed. In the simulation, two cases have been considered. In case 1, the separation between receivers has been increased from 10 to 20 cm whereas, in case 2, the separation between LEDs has been changed from 0.2 to 0.6 m in the interval of 0.2 m. For case 1, it has been found that there is a significant effect on BER performance for higher order reflection than LOS and lower order reflection when the spatial separation between receivers is increased and hence higher order reflection should not be ignored. We have investigated impulse response, root-mean square delay, BER and bit rate performance for case 2 and it has been observed that the spatial separation between LEDs improves BER but reduces coherence bandwidth of the channel. If the coherence bandwidth becomes less than the signal bandwidth then it may result in signal distortion thereby causing inter-symbol interference. Hence, the optimal spatial separation between LEDs should be chosen for the MIMO-VLC system.

MIMO visible light communication system with block Markov superposition transmission

This study is concerned with coded multiple-input multiple-output (MIMO) visible light communication (VLC) system, where unipolar M-level pulse amplitude modulation is assumed along with MIMO transmission techniques, including repetition coding, spatial multiplexing and spatial modulation. In order to evaluate the performance of MIMO-VLC system, the authors analyse the mutual information of the MIMO techniques, based on which the block Markov superposition transmission (BMST) codes are constructed to improve the bit-error rate (BER) performance of the system. In addition, they present a genie-aided lower bound to predict the performance of the BMST-VLC system in the low-BER region. Simulation results demonstrate that the proposed scheme can achieve significant coding gain compared with Reed-Solomon-coded MIMO-VLC system adopted in IEEE standard 802.15.7.

Iterative Combinatorial Symbol Design for Spatial Modulations in MIMO VLC Systems

This letter presents an iterative combinatorial symbol design algorithm for the generalized spatial modulation (GSM) in visible light communication systems. An approach that is fundamentally different from the available low complexity design methods is adopted by jointly considering both spatial and intensity bits in the symbol design. The proposed method is compared against existing symbol design methods for both photodiode array and camera receivers in terms of error performance and implementation complexity. The algorithm is shown to outperform several GSM symbol design methods, providing many decibels of performance gain. The proposed algorithm is also applicable to generalized space-shift keying and spatial modulation.

On the Performance of MIMO-NOMA-Based Visible Light Communication Systems

In this letter, we apply the non-orthogonal multiple access (NOMA) technique to improve the achievable sum rate of multiple-input multiple-output (MIMO)-based multi-user visible light communication (VLC) systems. To ensure efficient and low-complexity power allocation in indoor MIMO-NOMA-based VLC systems, a normalized gain difference power allocation (NGDPA) method is first proposed by exploiting users’ channel conditions. We investigate the performance of an indoor $2\times 2$ MIMO-NOMA-based multi-user VLC system through numerical simulations. The obtained results show that the achievable sum rate of the $2\times 2$ MIMO-VLC system can be significantly improved by employing NOMA with the proposed NGDPA method. It is demonstrated that NOMA with NGDPA achieves a sum rate improvement of up to 29.1% compared with NOMA with the gain ratio power allocation method in the $2\times 2$ MIMO-VLC system with three users.

Secure MIMO Visible Light Communication System Based on User's Location and Encryption

Visible light communication systems are rapidly growing research areas with wide applications ranging from illumination and data communication. To achieve high data rates in such systems, a number of techniques have been employed such as equalization of transmission signals, deployment of complex data modulation and the use of multiple input multiple output (MIMO) systems. However, security in wireless telecommunication systems is a common concern. This paper proposes a secure MIMO-VLC system that relies on the position of the user by incorporating a new modified version of the Rivest–Shamir–Adleman (RSA) technique for encrypting the transmitted data in the media access control MAC layer. The performance of the positioning method is evaluated showing a positioning accuracy of less than 5 cm for a signal-to-noise ratio of 15 dB. Furthermore, the ability of the proposed system to control the size of the encrypted cell, depending on the application environment, is demonstrated.

A Novel Mirror-Aided Non-Imaging Receiver for Indoor $2\times 2$ MIMO-Visible Light Communication Systems

Indoor visible light communication (VLC) systems are now possible because of advances in light emitting diode and laser diode technologies. These lighting technologies provide the foundation for multiple-input multiple-output (MIMO) data transmission through visible light. However, the channel matrix can be strongly correlated in indoor MIMO-VLC systems, preventing parallel data streams from being decoded. Here, in $2\times 2$ MIMO-VLC systems, we describe a mirror diversity receiver (MDR) design that reduces the channel correlation by both blocking the reception of light from one specific direction and improving the channel gain from light from another direction by utilizing a double-sided mirror deployed between the receiver’s photodetectors. We report on the channel capacity of the MDR system and the optimal height of its mirrors in terms of maximum channel capacity. We also derived analytic results on the effect of rotation on MDR’s performance. Based on numerical and experimental results, we show that the double-sided mirror has both constructive and destructive effects on the channel matrix. Our design can be used with previously described non-imaging systems to improve the performance of indoor VLC systems.

On the Coverage of Multiple-Input Multiple-Output Visible Light Communications [Invited

Visible light communication (VLC) using multiple-input multiple-output (MIMO) has great potential for future high-data-rate indoor wireless communications. Owing to the dual function of white light-emitting diodes (LEDs), i.e., illumination and communication functions, both the illumination and communication coverages need to be evaluated when implementing an indoor MIMO-VLC system. In this paper, we present a comprehensive investigation of both the illumination and communication coverages of an indoor MIMO-VLC system. Two techniques are applied in the MIMO-VLC system for communication coverage improvement, including a non-Hermitian symmetry orthogonal frequency division multiplexing (NHS-OFDM) modulation scheme and an imaging angle diversity receiver. A general indoor imaging MIMO-VLC system using NHS-OFDM is described and the analytical bit error rate expression of the system is derived. The impact of LED pairing on the communication coverage of an indoor imaging MIMO-VLC system based on NHS-OFDM is also analyzed.

Channel-Adaptive Space-Collaborative Constellation Design for MIMO VLC With Fast Maximum Likelihood Detection

Multi-input multi-output (MIMO) holds great potential in the high-speed transmission of visible light communication (VLC). However, as for the current MIMO-VLC techniques, the error performance is improved by wasting spatial resources, which just drops the core advantage within an MIMO structure. Moreover, they cannot change over the channel, which greatly limits their application range. In this paper, a general design criterion of the channel-adaptive space-collaborative constellation (CASCC) is established for MIMO-VLC systems. Specifically, we not only consider the channel into our criterion, but also build relations among the light-emitting diodes to exploit the spatial resources instead of wasting. Under the criterion, a regular CASCC is particularly designed for $2\times 2$ MIMO-VLC in terms of the basic four-point constellation and the high-order constellation. Correspondingly, a fast maximum likelihood (ML) detection algorithm for CASCC is proposed. Simulation results show that our proposed CASCC obtains better error performance and wider application range compared with the conventional MIMO-VLC schemes. Moreover, the fast ML algorithm reduces the computational complexity without any performance loss.

Non-Hermitian Symmetry Orthogonal Frequency Division Multiplexing for Multiple-Input Multiple-Output Visible Light Communications

Multiple-input multiple-output (MIMO) is a natural and effective way to increase the capacity of white light-emitting diode (LED) based visible light communication (VLC) systems. Orthogonal frequency division multiplexing (OFDM) using high-order modulation is another widely used technique in VLC systems. Due to the intensity modulation and direct detection nature of VLC systems, Hermitian symmetry is usually imposed in OFDM so as to obtain a real-valued signal. In this paper, we investigate a non-Hermitian symmetry OFDM (NHS-OFDM) scheme for MIMO-VLC systems. By transmitting the real and imaginary parts of a complex-valued OFDM signal via a pair of white LEDs, NHS-OFDM circumvents the constraint of Hermitian symmetry. We evaluate the performance of an indoor 2×2 MIMO-VLC system using conventional Hermitian symmetry-based OFDM (HS-OFDM) and NHS-OFDM, where both a non-imaging receiver and an imaging receiver are considered. Analytical results show that the system using NHS-OFDM achieves superior bit error rate (BER) performance to that using HS-OFDM, with lower or nearly the same computational complexity. The superior BER performance of NHS-OFDM-based MIMO-VLC is further verified by experiments. The experimental results demonstrate that, in a 400 Mb/s2×2 MIMO-VLC system with an imaging receiver, NHS-OFDM improves the communication coverage by about 30% compared with conventional HS-OFDM for a target BER of 3.8×10−3.

Optimal Constellation Design for Indoor 2x2 MIMO Visible Light Communications

Owing to the line-of-sight characteristics in indoor visible light communication (VLC) systems, different geometrical locations of transmitters and receivers provide distinct channel correlations, thus bringing various impacts on the conventional multiple-input multiple-output (MIMO) techniques. In this letter, by taking full advantage of the channel state information (CSI) available at both the transmitter and the receiver, we present an optimal constellation for the 2 × 2 MIMO-VLC system, where the signal vectors are selected to maximize the minimum Euclidean distance under a total optical power constraint. Simulation results verify both the accuracy and the validity of our analytical derivation, and show its significant error performance gains over repetition coding (RC) and space shift keying (SSK) for mobile receivers, respectively.

MIMO-VLC에서 디밍제어에 따른 변조방식들의 성능 비교

MIMO-VLC에서 디밍제어에 따른 변조방식들의 성능 비교