Visible Light Communication Channel Research Articles

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.

OLED Panel Radiation Pattern and Its Impact on VLC Channel Characteristics

Organic light-emitting diodes (OLEDs) show several attractive features superior to traditional semiconductor LEDs for visible light communication (VLC). This paper experimentally tests and models the radiation pattern of a bent OLED panel for the first time. Different from a Lambertian radiation pattern, we propose an improved analytic mixed Gaussian model to describe the rotational radiation asymmetry, whose parameter values are found by applying an expectation-maximization algorithm for curve fitting with measurements. The OLED radiation pattern is observed to show strong directionality along the bent axes. Its impact on VLC channel characteristics is further studied. Compared with the Lambertian source, the OLED source is more flexible in radiation pattern control and shows advantages in smaller root mean square delay spread and optical path loss.

Synchronization in Visible Light Communication for Smart Cities

The rapid deployment of light emitting diodes and the prominence of facilitating simultaneous illumination and communication have driven Visible Light Communication (VLC) to gain enormous prevalence in the recent times. This expedited VLC to become amalgamated with many smart city applications in particular, with the lighting infrastructure to enable seamless connectivity. The underlying challenge of this technology is to assure high data rate communication while fulfilling illumination requirements in smart cities. In this regard, optical orthogonal frequency division multiplexing (OOFDM) compatible with intensity modulated/direct detection (IM/DD) system can be exploited for VLC. Where, IM and DD is one approach of envisioning VLC. DC biased Optical OFDM (DCO-OFDM), the earliest variant of OFDM for VLC is employed in this paper. It is apparent that, the advantages of OOFDM cannot be utilized if its orthogonality is destroyed by any means. The striking difference between RF-based OFDM and OOFDM is that, OOFDM necessitates the transmitted time-domain signal to be both real and unipolar. Hence, the synchronization techniques existing for RF cannot be directly applied for VLC and consequently must be revisited. This paper analyzes the effects of carrier frequency offset (CFO) and symbol time offset (STO) in DCO-OFDM over VLC channel. Here, mathematical analysis is performed showing the effects of CFO and STO in OOFDM. Synchronization algorithms, such as Moose, Classen and Training Symbol assisted are revisited. Detailed analysis of Cramer Rao lower bound is carried out considering the joint parameter estimation of both CFO and channel estimation error.

Transmit Power Optimization for a Hybrid PLC/VLC/RF Communication System

The number of indoor smart devices is rapidly growing which requires ubiquitous wireless connectivity for data communications. The limited available radio frequency (RF) spectrum motivates finding alternate wireless solutions. Visible light communications (VLC) are a wireless technology which have shown a high potential to offer wireless data communications in both indoor and outdoor scenarios. Power-line communications (PLC) technology is employed in VLC networks for data transfer between VLC sources due to the ubiquity and the infrastructure availability of power-line networks. As a result, the hybrid wireline/wireless technology of PLC/VLC can be exploited in a complementary fashion to the existing indoor RF networks for improving wireless connectivity. In this paper, we investigate the power allocation problem of a hybrid system composed of a cascaded PLC/VLC link in parallel to an RF wireless link. We formulate and analyze the problem of minimizing the total transmission power consumption under a defined quality of service constraint. Our results show that the proposed hybrid system, under different realizations of PLC, VLC, and RF channel models, provides considerable performance improvement compared to conventional RF systems.

Efficient Analytical Calculation of Non-Line-of-Sight Channel Impulse Response in Visible Light Communications

This study provides an analytical method to calculate the non-line-of-sight (NLoS) channel impulse response (CIR) in visible light communication (VLC) systems based on intensity modulation and direct detection (IM/DD). In this method, the NLoS channel is decomposed into a number of components with different propagation categories. These propagation categories are defined according to the number of reflections and the reflective surfaces that the light undergoes. The CIR corresponding to each light propagation category is analysed and the overall NLoS CIR is approximated by the combination of the calculated CIR components in different propagation categories. The proposed method has the major advantage of offering accurate results with very low computational complexity. Typically, a NLoS CIR with a time resolution of 0.1 ns can be generated within a second in MATLAB. Furthermore, the analytical results derived herein could be used as an analytical tool for the VLC channel characterisation study in future research.

Hermitian symmetry free optical-single-carrier frequency division multiple access for visible light communication

Optical-orthogonal frequency division multiplexing (O-OFDM) is an effective scheme for visible light communications (VLC), offering a candid extension to multiple access (MA) scenarios, i.e., O-OFDMA. However, O-OFDMA exhibits high peak-to-average power ratio (PAPR), which exacerbates the non-linear distortions from the light emitting diode (LED). To overcome high PAPR while sustaining MA, optical-single-carrier frequency-division multiple access (O-SCFDMA) is used. For both O-OFDMA and O-SCFDMA, Hermitian symmetry (HS) constraint is imposed in frequency-domain (FD) to obtain a real-valued time-domain (TD) signal for intensity modulation-direct detection (IM-DD) implementation of VLC. Howbeit, HS results in an increase of PAPR for O-SCFDMA. In this regard, we propose HS free (HSF) O-SCFDMA (HSFO-SCFDMA). We compare HSFO-SCFDMA with several approaches in key parameters, such as, bit error rate (BER), optical power penalty, PAPR, quantization, electrical power efficiency and system complexity. BER performance and optical power penalty is evaluated considering multipath VLC channel and taking into account the bandwidth limitation of LED in combination with its optimized driver. It is illustrated that HSFO-SCFDMA outperforms other alternatives.

Performance of MIMO Modulation Schemes With Imaging Receivers in Visible Light Communication

In multiple-input multiple-output (MIMO) indoor visible light communication (VLC) systems, the elements of the channel matrix can be highly correlated which is a cause for degraded bit error performance. Imaging receiver techniques can resolve the spatial correlation in VLC channels because of the increased spatial resolution possible in imaging receivers. In this paper, we investigate the performance of MIMO modulation schemes using imaging receivers that employ an imaging lens above the receiving plane to focus the energy from the transmitting LEDs onto the receiving plane. Hemispherical lens and fish-eye lens for imaging are considered in the study. The MIMO modulation schemes considered include spatial multiplexing, generalized spatial modulation, quad-LED complex modulation, and dual-LED complex modulation. Analytical and simulation results show that imaging receivers can provide significant performance gains compared to nonimaging receivers. We analytically capture the high SNR performance of the modulation schemes with nonimaging and imaging receivers by computing the normalized minimum distance of the received signal set. Simulation results validate the analytical results. Using this distance measure, we study the spatial distribution of the best performing modulation scheme with nonimaging and imaging receivers.

An anti-noise modem for visible light communication systems using the improved M-ary position phase shift keying

In this paper, we propose an anti-noise modem based on the improved M-ary position phase shift keying (MPPSK) for visible light communication (VLC) systems. The line spectral interferences in the original MPPSK are theoretically analyzed and approximately mitigated by adopting a set of improved waveform samples. Then, within one symbol duration, the peak envelopes of the received signals are captured at each of synchronization bit with a quantity of sampling offsets to improve the proportion of the reliable data tones. In addition, a novel demodulation scheme using an amplitude-position detector based on soft decision is proposed to recover the MPPSK symbols, which can minimize the contamination influence of channel noise on symbol decision. With this scheme, the false alarm error and wrong slot error caused by the multipath fading in VLC channel can be effectively corrected and the bit error rate (BER) performance of the system is significantly improved. Simulation results demonstrate that the proposed scheme reduce the required signal-to-noise ratio (SNR) at least by 2 dB for BER = 10−4, which show the feasibility and validity of this anti-noise modem.

Achievable information rate enhancement of visible light communication using probabilistically shaped OFDM modulation.

We present the first experimental demonstration of orthogonal frequency division multiplexed (OFDM) modulation using the probabilistic shaping (PS) technique in visible light communication (VLC) systems, in order to increase the achievable information rate (AIR) according to the pre-estimated signal to noise ratio (SNR) of VLC channel. We numerically investigate the performance of PS technique and make a fair comparison with bit-loading technique under different scenarios. By using a phosphor-LED based VLC system with available bandwidth of ~45-MHz, OFDM with PS technique can experimentally realize an AIR of 204.1-Mb/s over 1-m free space transmission, leading to a 26.8% increment in comparison with OFDM using bit-loading technique at the expense of 16% overall forward error correction (FEC) overhead (OH).

Channel estimation for wideband underwater visible light communication: a compressive sensing perspective.

With the rapid development of light emitting diode (LED), visible light communication (VLC) becomes an important technique for information transmission including underwater applications. However, accurate channel estimation for underwater VLC is still challenging due to the complex environment of the underwater VLC channel. In this paper, by utilizing a proper approximation, where the channel attenuation is linear with the frequency, a new compressive sensing (CS) based channel estimation approach is proposed. Utilizing the sparse property of the reflection path length for the underwater VLC channel, the CS framework is modeled to estimate the reflection path length, which can further recover the underwater VLC channel. Moreover, a Bayesian CS recovery algorithm is investigated to overcome the problem of high coherence for the sensing matrix which outperforms the conventional greedy algorithm such as orthogonal matching pursuit (OMP). Simulation results illustrate that our proposed channel estimation for underwater VLC systems has a superior performance which can significantly reduce the pilot overhead, improve the spectral efficiency, and enhance the estimation accuracy.

Multi-Element VLC Networks: LED Assignment, Power Control, and Optimum Combining

Visible light communications (VLCs) are a promising technology to address the spectrum crunch problem in radio frequency networks. A major advantage of VLC networks is that they can use the existing lighting infrastructure in indoor environments, which may have large number of LEDs for illumination. While LEDs used for lighting typically have limited bandwidth, presence of many LEDs can be exploited for indoor VLC networks, to serve each user by multiple LEDs for improving link quality and throughput. In this paper, LEDs are grouped and assigned to the users based on received signal strength from each LED, for which different solutions are proposed to achieve maximum throughput, proportional fairness, and quality of service. Additionally, power optimization of LEDs for a given assignment is investigated, and the Jacobian and Hessian matrices of the corresponding optimization problem are derived. Moreover, for multi-element receivers with LED grouping at the transmitter, an improved optimal combining method is proposed. This method suppresses interference caused by simultaneous data transfer of LEDs and improves the overall signal-to-interference-plus-noise-ratio by 2–5 dB. Lastly, an efficient calculation of channel response is presented to simulate multipath VLC channel with low computational complexity.

Visible Light Communication Channel Modeling for Underwater Environments With Blocking and Shadowing

In this paper, we present a comprehensive channel modeling and characterization study for underwater visible light communications. Our study is based on the advanced ray tracing, which allows for an accurate description of the interaction of rays emitted from the lighting source within an underwater environment. Contrary to existing works, which are mainly limited to simplified underwater scenarios, i.e., empty sea, we take into account the presence of human and man-made objects to investigate the effects of shadowing and blockage. The reflection characteristics of the sea surface and sea bottom as well as the water characteristics, i.e., extinction coefficient and scattering phase function of particles, are precisely considered. As case studies, we consider various underwater scenarios with different transmitter/receiver specifications (i.e., viewing angle, aperture size) and different depths from the sea surface. For each environment, we obtain channel impulse responses and present a characterization study where channel parameters, such as channel DC gain, path loss, and delay spread, are obtained.

Subcarrier intensity modulation for MIMO visible light communications

In this paper, subcarrier intensity modulation (SIM) is investigated for multiple-input multiple-output (MIMO) visible light communication (VLC) systems. A new modulation scheme called DC-aid SIM (DCA-SIM) is proposed for the spatial modulation (SM) transmission plan. Then, DCA-SIM is extended for multiple subcarrier case which is called DC-aid Multiple Subcarrier Modulation (DCA-MSM). Bit error rate (BER) performances of the considered system are analyzed for different MIMO schemes. The power efficiencies of DCA-SIM and DCA-MSM are shown in correlated MIMO VLC channels. The upper bound BER performances of the proposed models are obtained analytically for PSK and QAM modulation types in order to validate the simulation results. Additionally, the effect of power imbalance method on the performance of SIM is studied and remarkable power gains are obtained compared to the non-power imbalanced cases. In this work, Pulse amplitude modulation (PAM) and MSM-Index are used as benchmarks for single carrier and multiple carrier cases, respectively. And the results show that the proposed schemes outperform PAM and MSM-Index for considered single carrier and multiple carrier communication scenarios.

Physical-Layer Security Against Known/Chosen Plaintext Attacks for OFDM-Based VLC System

This letter introduces a chaos-based physical-layer security approach for an orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) system with the capability against known-plaintext attacks (KPAs) and chosen-plaintext attacks (CPAs). This approach generates dynamic cyphertexts by exploiting the random characteristics of chaotic keys and input data. It performs chaotic permutation of subcarrier allocation in time/frequency domains and chaotic polarity reversal of subcarriers. For secret-key generation, we present chaotic key generation protocol with the help of the position-sensitive and real-valued channel state information of the VLC channel. The results show that our approach supports superior confidentiality for OFDM-based VLC systems against KPAs and CPAs.

Study on VLC Channel Modeling Under Random Shadowing

Recently, visible light communication (VLC) based on LEDs has attracted much attention. In order to realize multiusers indoor VLC systems, a reasonable channel model considering random shadowing, which is first proposed to the best of our knowledge, is significant. Taking the nonquantitative obstructions with random movement into account, by introducing the Poisson process and the probability density function, we propose a weighting function and set up a channel model. The expressions of the received optical power, the horizontal illuminance and the impulse responses are given. The verification of the model's feasibility is presented through simulation. Moreover, based on the proposed channel model, the simulation analysis of the effect of random shadowing on communication performance is conducted, and the corresponding simulation optimization is also made.

Channel estimation in ACO-OFDM employing different transforms for VLC

The pronounced eminency of assuring simultaneous illumination and communication has driven Visible Light Communication (VLC) to gain significant ubiquity in recent times. This paper proposes comb type pilot arrangement based channel estimation for Asymmetrically Clipped Optical OFDM (ACO-OFDM) and different multicarrier transmission systems like Discrete Hartley Transform (DHT)-based ACO-OFDM and Fast-Walsh Hadamard transform (FWHT)-based Hadamard Coded Modulation (HCM) over dispersive VLC channel. Various channel estimation algorithms like Least Square (LS), Minimum Mean Square Error (MMSE), and Interpolation techniques namely Linear, Spline, and Low-Pass are evaluated and compared for the aforesaid systems employing different orders of constellation. Here, an elaborate mathematical analysis is accomplished and Cramer Rao Lower Bound (CRLB) is derived for the channel estimation error. Simulated results emphasize that, ACO-OFDM and DHT-based ACO-OFDM have improved Bit Error Rate (BER) performance than HCM at lower Signal to Noise Ratio (SNR), while at higher SNRs HCM dominates the former. Furthermore, the simulated results evidences that, in all multicarrier systems MMSE algorithm has reducible probability of error than LS because, at higher SNRs LS is more susceptible to noise and Spline Interpolation outperforms both LS and MMSE. The simulated results are validated analytically demonstrating good agreement.