Visible Light Communication Channel Research Articles

Secure Image Transmission in Orthogonal Frequency Division Multiplexing Visible Light Communication Systems

In this paper, a novel physical layer security based on two-level encryption is proposed for orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) systems. In the two-level encryption scheme, the bit signal, which is generated from image data, is first scrambled by a chaotic sequence. After the encrypted bit signal is mapped into 16 QAM symbols, the real and imaginary components of the resulting symbols are encrypted again by two separate chaotic sequences, respectively. A mathematical model is established to securely transmit images in OFDM-based VLC systems based on encryption. Based on the established mathematic model, the bit error rate (BER) formula to securely transmit OFDM-VLC is derived over the dispersive visible light communication (VLC) channel. A theoretical analysis shows that the encryption does not degrade the BER performance of the systems. Additionally, the validity of the BER formula obtained is verified by computer simulation. Theoretical and simulation results both show that the encryption operation has no effect on the BER of the proposed system. However, encryption is helpful for reducing the peak-to-average power ratio (PAPR) of the image-based OFDM. The simulation results show that the security of the image-based OFDM is enhanced and the PAPR is reduced.

On Optimizing VLC Networks for Downlink Multi-User Transmission: A Survey

The evolving explosion in high data rate services and applications will soon require the use of untapped, abundant unregulated spectrum of the visible light for communications to adequately meet the demands of the fifth-generation (5G) mobile technologies. Radio-frequency (RF) networks are proving to be too scarce to cover the escalation in data rate services. Visible light communication (VLC) has emerged as a great potential solution, either in replacement of, or a complement to, existing RF networks, to support the projected traffic demands. Despite the prolific advantages of VLC networks, VLC faces many challenges that must be resolved in the near future to achieve full standardization and to be integrated to future wireless systems. Here, we review the emerging research in the field of VLC networks and lay out the challenges, technological solutions, and future work predictions. Specifically, we first review the VLC channel capacity derivation and discuss the performance metrics and the associated variables. The optimization of VLC networks are also discussed, including resources and power allocation techniques, user-to-access point (AP) association and APs-to-clustered-users-association, APs coordination techniques, non-orthogonal multiple access (NOMA) VLC networks, simultaneous energy harvesting and information transmission using the visible light, and the security issues in VLC networks. Finally, we propose several open research problems to optimize the various VLC networks by maximizing either the sum rate, fairness, energy efficiency, secrecy rate, or harvested energy.

Robust Beamforming-Aided Signal Recovery for MIMO VLC System With Incomplete Channel

Visible light communication (VLC) based on light emitting diodes (LEDs) has attracted much attention because of its high data rate and energy efficiency. However, the shadowing caused by any mobile obstacles could block the light-of-sight (LoS) link which may have a dramatical effect on the indoor VLC channel state information (CSI). Instead of investigating the shadowing channel characteristics, we model the link blockages as the incomplete channel matrix with sparse missing elements and reconstruct the transmitted signal by the l 0 -minimization with additional constraint on shadowing loss. However, the indoor VLC channel is highly correlated especially in cases with small emitter separation. To further improve the accuracy performance, we design the transmit beamforming to reduce the total coherence. Simulation results illustrate the effectiveness of the proposed signal recovery algorithm with incomplete channel and further significant enhancement via the proposed beamforming design.

Effect of Wiring and Cabling Topologies on the Performance of Distributed MIMO OFDM VLC Systems

Since most indoor spaces have multiple luminaires for illumination, for visible light communication (VLC) systems, multiple-input multiple-output (MIMO) communication emerges as a natural solution to improve the data rates and/or the link reliability. The existing works on MIMO VLC systems, however, overlook the characteristics of the lighting infrastructure and the luminaire design, which might have implications for the VLC system design. A luminaire typically consists of multiple LED chips. The wiring topology refers to how the LED chips are connected within the luminaire. The cabling topology, on the other hand, refers to how the luminaires are connected to the communication access point (AP). Based on the type and length of cabling and wiring, significant delays can be introduced, which should be taken into account in channel modeling. In this paper, we adopt the non-sequential ray tracing to model the distributed MIMO VLC channels for various practical wiring and cabling topologies. Based on the developed channel models, we provide a comparative performance analysis of repetition coding (RC), spatial multiplexing (SMUX), and spatial modulation (SMOD) MIMO modes. Our results quantify the effect of wiring/cabling delays and provide insights into the optimized design of lighting infrastructure and luminaires for the support of VLC as an add-on service.

Artificial-Noise-Aided Precoding Design for Multi-User Visible Light Communication Channels

Recent developments in visible light communications (VLC) have focused on enhancing its security and privacy. This paper studies the physical layer security in VLC networks with multiple users when there is a single wiretap eavesdropper. In particular, our aim is to design the optimal artificial noise (AN)-aided precoding scheme to improve the secrecy performance in terms of legitimate users and eavesdropper’s signal-to-interference-plus-noise ratios (SINRs). The purpose of our AN-aided precoding design is to ensure a fairness of the legitimate users’ SINRs while impairing the quality of eavesdropper’s channel as much as possible. Depending on the availability of the eavesdropper’s channel state information (CSI) at the transmitters, there are generally two design strategies. When the eavesdropper’s CSI is unknown to the transmitters (i.e., passive eavesdropper), the AN is constructed to be orthogonal to the users’ aggregate channel matrix. In case the eavesdropper’s CSI is available (i.e., active eavesdropper), the AN design strategy is to keep the SINR of the eavesdropper below a certain predefined threshold. Aside from the general design, we also study a specific design with the zero-forcing (ZF) technique and compare its performance with that of the general design. In both designs, numerical results show that significant gaps between users’ and eavesdropper’s SINR can always be achieved, thus guaranteeing a high secrecy performance. It is also observed that while the general design outperforms the ZF one in terms of the users’ SINRs, it does not result in lower eavesdropper’s SINRs compared to the ZF design, especially in the low transmit power region.

Evaluating the risk of eavesdropping a visible light communication channel

Visible light communication (VLC) technologies are becoming a method to communicate with a wide range of devices. One of VLC's biggest proposed advantages is the inherent security that this technology provides. The concept of securing on VLC is based on its indoor use since optical radiation is confined by walls. However, doors and windows are usually present in any inhabited structure through which the optical signal can leak and be detected by external devices. In this study, the signal leaked through such a window and the risk of eavesdropping the VLC-based channel may have is studied. In this way, a risk evaluation parameter, which includes different variables, is proposed and analysed.

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.

A 2-D Non-Stationary GBSM for Vehicular Visible Light Communication Channels

In this paper, a new non-stationary regular-shaped geometry-based stochastic model (RS-GBSM) is proposed for vehicular visible light communications (VVLC) channels. The proposed model utilizes a combined two-ring model and a confocal ellipse model, in which the received optical power is constructed as a sum of single-bounced (SB) and double-bounced (DB) components, in addition to the line-of-sight (LoS) component. Using the proposed RS-GBSM, the channel impulse response is generated and utilized to investigate VVLC channel characteristics, such as channel gain and root-mean-square (RMS) delay spread. The received optical power is computed considering the distance between the optical transmitter (Tx) and the optical receiver (Rx). Moreover, the impact of the Rx height on the received power is considered for the LoS scenario. The results show that the LoS power highly depends on the distance, Rx height, and the optical source pattern. For the SB components, it is confirmed that the channel gain in dB and the RMS delay spread follow Gaussian distributions. Finally, the results indicate that the detected optical power from the DB components is low enough to be overlooked.

Performance analysis of adaptive OFDM modulation scheme in VLC vehicular communication network in realistic noise environment

Optical wireless communications (OWC) has emerged as a strong candidate for wireless communications, due to the capacity limitation in the radio frequency (RF) spectrum. Especially visible light communication (VLC) has great potential for short-range outdoor vehicular communications, as vehicle LED lights also transmit data. However, outdoor VLC channels vary fast and, experience multipath scattering and reflection resulting in time domain dispersion. Outdoor VLC links are also subjected to high levels of ambient noise, especially from the sun. Orthogonal frequency-division multiplexing (OFDM), which has proven robustness to multi path fading and noise effects in RF links can also be deployed in VLC links. In this paper, optical OFDM (O-OFDM) along with adaptive modulation scheme is investigated in VLC for vehicle to vehicle (V2V) communications. A (2×2) multiple input multiple output (MIMO) channel, with multiple polarimetric bidirectional reflections and realistic sunlight interference is considered. Two schemes of O-OFDM; direct current biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are investigated. Simulation results of the proposed model show increase in data rates up to 50 Mbps along with reduced bit error rate (BER) under both line of sight (LOS) and non-LOS and high noise conditions.

A Physical-Layer Secure Coding Scheme for Indoor Visible Light Communication Based on Polar Codes

Visible light communication (VLC) can provide short-range optical wireless communication together with illumination using LED lightings. Since VLC channel is an open wireless channel, physical-layer security is desirable in order to hide secret information from unauthorized receivers, without reliance on upper-layer cryptographic techniques. In this paper, a secure coding scheme based on polar codes is proposed to simultaneously achieve physical-layer security and transmission reliability for indoor VLC systems under Wyner's wiretap model. An indoor VLC degraded wiretap channel model and an indoor VLC undegraded wiretap model are analyzed to estimate the corresponding secrecy capacity with typical indoor VLC scenarios considering the requirements of illumination and reliable transmission. Then, a physical-layer secure coding scheme and an enhanced scheme are designed to provide securely reliable communication without sacrificing the illumination functionality. Numerical results show that for VLC degraded wiretap channel, the proposed coding scheme can reduce the bit error rate (BER) of main channel to nearly 0 and increase that of wiretap channel to 0.5. Therefore, the mutual information between the sender and the eavesdropper and the BER of the legitimate receiver are both decreased to nearly 0. Both the security and reliability of VLC are guaranteed by the proposed coding scheme. And the practical secrecy rate reaches 85.96% at some positions when codeword length is 2 048 bits. Furthermore, an enhanced coding scheme with a feedback channel is proposed to handle the case that the wiretap channel is not degraded with respect to the main channel. With the feedback channel, the wiretap channel can be equivalently degraded and the security can be increased gradually with the increment of the degradation level of the wiretap channel. Meanwhile, the secrecy capacities can be asymptotically achieved for indoor VLC systems.

Gaussian kernel-aided deep neural network equalizer utilized in underwater PAM8 visible light communication system.

In this paper, we demonstrate a novel Gaussian kernel-aided deep neural network (GK-DNN) equalizer that can effectively compensate for the high nonlinear distortion of underwater PAM8 visible light communication (VLC) channels. The application of a Gaussian kernel can reduce the necessary training iterations to 47.06%, enabling it to outperform the traditional DNN equalizer. At the same time, a novel design strategy with respect to the structure of the GK-DNN equalizer is proposed, which can effectively save computing resources and reduce the data volume of the necessary training data set. By using the GK-DNN equalizer, a 1.5 Gbps PAM8 VLC system over 1.2-m underwater transmission is successfully demonstrated.

A general channel model for visible light communications in underground mines

In underground mines, visible light communication (VLC) system is a promising method to realize effective communication, which supports communication and illumination at the same time. Therefore, adequate research of underlying physical propagation phenomenon should be carried out to realize VLC system in underground mines. To design VLC system and evaluate its performance, accurate and efficient channel models, including large-scale fading and scattering characteristics, are needed to be established. However, the characteristics of the underlying VLC channels about fading and scattering have not been sufficiently investigated yet. In this paper, a path loss channel model, based on the recursive model, is proposed precisely. Its path loss exponent is determined by three different trajectories, which is studied in the mining roadway and working face environment. Besides, the shadowing effect for VLC has been modelled by a Bimodal Gaussian distribution in underground mines. Considering the number of transmitters in line-of-sight (LoS) as well as non-line-of-sight (NLoS) scenarios, our simulation illustrates the fact that, as the curve fitting technique is employed, the path loss displays a linear behavior in log-domain. The path loss expression is derived, it is related to the distance. Finally, root mean square (RMS) delay spread and Mie scattering in underground mines are analyzed.

Adaptive Precoding-Based Detection Algorithm for Massive MIMO Visible Light Communication

Visible light communication (VLC) has emerged as a promising, green, and interference-free supplement to existing radio frequency-based communication systems. To enhance the capacity of existing VLC-based systems, using massive arrays of light emitting diodes (LEDs) at the transmitter and photodiodes at the receiver has been proposed recently, resulting in a massive multiple-input multiple-output (m-MIMO) VLC system. However, by increasing LED array pitch in m-MIMO, the VLC channel matrices are generally ill conditioned which makes them sensitive to small perturbations and degrades the overall bit error rate (BER) performance. To reduce the condition number of the overall channel matrix, a novel precoder using an exponent-based singular value decomposition is proposed for m-MIMO VLC system. Furthermore, a joint optimization problem is formulated to optimize the exponent and to detect symbols iteratively using the minimum symbol error rate criterion. Simulations show that the proposed adaptive precoding technique exhibits superior BER performance over existing techniques. Analytical upper bounds for BER are also derived and validated by simulations.

A generalized multi‐wavelength propagation model for VLC indoor channels using Monte Carlo simulation

AbstractIn this paper, we introduce the Matrix‐MMC propagation model for indoor wireless optical channels. This proposed method is evolved from the modified Monte Carlo (MMC)–based ray tracing algorithm, but in addition to the properties of MMC, it has the capability of capturing reflection phenomena implying changes on wavelength (eg, fluorescence, phosphorescence, or iridescence) with a simplified analytical expression. The model provides unprecedented information about the mechanism of surface reflection in a visible light communication channel, which influences the propagation of the light waves, that depends on wavelength.

Interference mitigation and capacity enhancement using constraint field of view ADR in downlink VLC channel

The continued increase in several mobile applications forces to replace existing limited spectrum indoor radio-frequency wireless connections with high-speed ones. Visible light communication (VLC) is a promising license free indoor wireless technology that could offer high-speed connections. However, both co-channel interference (CCI) from more neighbour transmitters and inter-symbol interferences (ISI) from multipath reflections limit the performance of VLC downlink channel. In this study, a constraint field of view angular diversity receiver (CFOV-ADR) is proposed to mitigate these limitations. By optimising the photodetector's (PD) field of view (FOV) angle, the line of sight CCI could be totally eliminated and the ISI could be significantly reduced. The optimal range for FOV angle is calculated in a typical indoor scenario. Furthermore, the zero-forcing (ZF) algorithm is applied to the conventional ADR (ADR-ZF) which can significantly eliminate the ISI components. The received optical signal-to-interference-plus-noise ratio (SINR) is tested at various room positions. The simulation results show that the proposed CFOV-ADR can achieve higher SINR performance than single receiver, conventional ADR, and ADR-ZF at all positions and orientations.

Performance analysis of gain ratio power allocation strategies for non-orthogonal multiple access in indoor visible light communication networks

Non-orthogonal multiple access (NOMA) is a promising method for enhancing the throughput in visible light communication (VLC) networks. In NOMA, signal power domain control, called gain ratio power allocation (GRPA), can significantly improve the user sum rate with full-time frequency resource utilization. In indoor NOMA-VLC networks, the scenario in which users are covered by VLC illuminants on the ceiling is typical. First, this paper proposes a novel GRPA strategy for a single VLC cell. Second, due to the difficulty of direct comparisons between our and previously reported GRPA strategies, this paper presents an alternative lower bound for comparability. Third, in two- and three- user cases, this paper analytically demonstrates that our NOMA-VLC GRPA strategy outperforms the aforementioned strategy, when successive interference cancellation (SIC) is used. Moreover, in the multi-user case, experimental results show that the average user data rate (AUDR) of multi-user cases under our strategy is better than that under the previous strategy. Finally, our paper provides both analytic and numerical results to prove that VLC single-cell system throughput under our strategy is better than that under the previous strategy. The proposed alternative lower bound is also proved to fit the original NOMA-VLC GRPA target asymptotically based on indoor VLC channels. Furthermore, to achieve the same AUDR, our strategy supports worse received conditions than does the previous strategy.

Coded color shift keying with frequency domain equalization for low complexity energy efficient indoor visible light communications

The adaptive tri-chromatic color shift keying (CSK) modulation standardized in IEEE 802.15.7 and the advanced quad-chromatic CSK are unable to utilize their entire spectral efficiency range over dispersive indoor VLC channels and incur large power penalties without the use of channel equalization and forward error correction. This degrades the energy efficiency and limits the throughput capability of CSK schemes to approximately half of the data-rates specified in the IEEE standard for multimedia services in wireless personal area networks. To comply with the desire for low latency and minimal implementation complexity in the CSK standardization framework, we consider an industry standard binary convolutional code and frequency domain equalization (FDE) which provide one-shot data packet processing. Our results show that when operating over hybrid indoor visible light communication (VLC) links, the FDE based rate-adaptive coded modulation CSK scheme achieves a significant 11 dB SNR gain over an uncoded, unequalized system.

Experimental Study and Application of Response Mask Invariant Characteristic for Generalized Visible Light Communication Channel

In this paper, we describe a generalized visible light communication (VLC) channel, which consists of the frequency responses of the light emitting diode (LED), the wireless optical channel, and the photo detector (PD). Based on the theoretical analysis of the generalized VLC channel, we found the response mask invariant characteristic (RMIC) under the condition of strong line of sight components, which means the shape of frequency response of generalized VLC channel is time-independent and invariant at different distances of the transceivers, different emitting angles of the LED and different incident angles of the PD. Depending on the RMIC, the shapes of received spectrum and signal-to-noise rate are also invariant. The experimental platform was set up to verify the proposed RMIC. Furthermore, a predefined VLC system with a new channel feedback method is described, both the complexity and time cost of feedback can greatly reduce through the proposed channel feedback method.

Enhanced Multiple Pulse Position Modulation Aided Reverse Polarity Optical OFDM System With Extended Dimming Control

In this paper, we propose a multiple pulse position modulation (MPPM) aided reverse polarity optical orthogonal frequency division multiplexing scheme that aims to enhance the spectral efficiency (SE) for dimmable visible light communication (VLC) systems. In this scheme, we exploit the MPPM patterns to transmit the extra information bits without the increase of the signal bandwidth and, thus, improving the SE. Specifically, for each frame, the binary information stream is divided into two parallel groups, where the first group is used to select the patterns of MPPM symbols, while the other is modulated by optical OFDM. Then, the time-domain OFDM signal is accommodated by both the “on” and “off” slots of MPPM symbols for transmissions over the VLC channel. With the aid of a forward error correction aided decision feedback approach, the system performance can be further improved. Theoretical analyses on the dimming level, the average power consumption and the frame error probability are provided and verified by simulation results. It is shown that the proposed system with extended dimming control can achieve the highest effective spectral efficiency in most scenarios without substantially increasing the computational complexity, while still maintaining a power consumption level similar to those of the reference systems.

Uncoordinated chaotic channel scrambling scheme for multiple‐input, multiple‐output‐based VLC system

Due to the broadcasting property of visible light communication (VLC) channels, VLC systems are under the threat of eavesdropping and malicious attack. In this study, the authors present an uncoordinated chaotic channel scrambling scheme to enhance the physical layer security for a multiple-input, multiple-output based VLC system. With the authors design, the legitimate receiver can self-descramble the data with the location information shared between the transceivers, and the eavesdroppers cannot retrieve the information without the secret key. The authors firstly embed the location information into the chaotic channel scrambling matrix, and then apply the non-negative orthogonal factorisation method to improve both the reliability and the security. Notably, with the aid of the shared location information, the received data can be self-descrambled and thus the uncoordinated channel scrambling could be realised. The theoretical security performances including the information leakage and the secrecy capacity are analysed and the simulation results demonstrate that the proposed scheme achieves higher security and more reliable performances compared with the counterpart schemes.