DC-biased Optical Orthogonal Frequency Division Research Articles

Clipping Noise in Visible Light Communication Systems with OFDM and PAPR Reduction

This paper presents an analytical study of signal clipping that leads to the noise/distortion in the waveform of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM)-based visible light communication (VLC) systems. The pilot-assisted (PA) technique is used to reduce the high peak-to-average power ratio (PAPR) of the time-domain waveform of the DCO-OFDM system. The bit error rate (BER) performance of the PA DCO-OFDM system is investigated analytically at three different clipping levels as well as without any clipping. The analytical BER performance is verified through simulation and then compared to that of the conventional DCO-OFDM without PAPR reduction at the selected clipping levels. The PA DCO-OFDM system shows improved BER performance at all three clipping levels.

Vector Coding Optical Wireless Links

The quasi-static nature of the optical wireless channel means that the channel state information (CSI) can be readily available at the transmitter and receiver prior to data transmission. This implies that electrically band-limited optical wireless communication (OWC) systems can make use of optimal channel partitioning or vector coding based multi-channel modulation (MCM) to achieve high throughput by mitigating the non-linearities arising from the optical and electrical channel. This paper proposes a pulse amplitude modulation (PAM) based DC-biased optical vector coding (DCO-VC) MCM scheme for OWC. The throughput performance of DCO-VC is evaluated and compared to the well known DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) over hybrid (line-of-sight and diffuse) and diffuse (non line-of-sight only) visible light communication (VLC) channels with additive white Gaussian noise. For the completeness of the VLC physical layer, the performance comparison is based on an uncoded and a forward error correction transmission mode using well-known convolutional codes with Viterbi decoder. The results show that the coded DCO-VC outperforms DCO-OFDM system by achieving up to 2 and 3 dB signal to noise ratio gains over hybrid and diffuse VLC channels, respectively.

LiNEV: Visible Light Networking for Connected Vehicles

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been introduced to visible light networking framework for connected vehicles (LiNEV) systems as a modulation and multiplexing scheme. This is to overcome the light-emitting diode (LED) bandwidth limitation, as well as to reduce the inter-symbol interference caused by the multipath road fading. Due to the implementation of the inverse fast Fourier transform, DC-OFDM suffers from its large peak-to-average power ratio (PAPR), which degrades the performance in LiNEV systems, as the LEDs used in the vehicles’ headlights have a limited optical power-current linear range. To tackle this issue, discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) has been proposed as an alternative modulation scheme for LiNEV systems instead of DCO-OFDM. In this paper, we investigate the system performance of both schemes considering the light-emitting diode linear dynamic range and LED 3 dB modulation bandwidth limitations. The simulation results indicate that DCO-OFDM has a 9 dB higher PAPR value compared with DFTS-OPAM. Additionally, it is demonstrated that DCO-OFDM requires an LED with a linear range that is twice the one required by DFTS-OPAM for the same high quadrature amplitude modulation (QAM) order. Furthermore, the findings illustrate that when the signal bandwidth of both schemes significantly exceeds the LED modulation bandwidth, DCO-OFDM outperforms DFTS-OPAM, as it requires a lower signal-to-noise ratio at a high QAM order.

Reducing peak to average power ratio (PAPR) for visible light communications (VLC) system using Walsh–Hadamard precoded discrete Hartley transforms

Abstract In visible light communication (VLC), intensity modulation with direct detection (IM/DD) based on DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) is used to transfer data at a high rate. Because OFDM signals are modulated using IM/DD, the peak of the transmitted signals is scaled up, resulting in a high peak-to-average power ratio (PAPR). In this article, the Walsh–Hadamard transform (WHT) is used as a precoder for discrete Hartley transform (DHT) to decrease the PAPR and the DC bias and achieve a good bit error rate (BER) performance in such a system. PAPR reduction results from the decrease of superposition of the transmitted signals owing to the use of WHT as a precoder. The proposed scheme and conventional DCO-OFDM indoor VLC system based on discrete Hartley transform (DHT) are compared to assure the new system’s benefits and efficiency.

High CRI RGB Laser Lighting With 11-Gb/s WDM Link Using Off-the-Shelf Phosphor Plate

In this letter, a practical structure enabling both high-speed visible light communication (VLC) and high-quality white-light is presented. The proposed structure directly mixes red-green-blue colors from semiconductor laser diodes (LD) without dichroic mirrors. The three colors are mixed by a thermally stable off-the-shelf phosphor plate. The broad yellow photoluminescence spectrum excited by a portion of blue LD light fills the vacant spectrum, leading to a high color rendering index (CRI). It is experimentally verified that the phosphor plate works as an excellent diffuser even for the green and red channels. Consequently, an incoherent Lambertian source suitable for indoor lighting is created. It is also found that the impact of the broad yellow spectrum on the green and red communication channels is insignificant. A proof-of-concept demonstration shows a 90 CRI (3503 K) white-light delivering 11 Gb/s of wavelength division multiplexing VLC link, using channel adaptive DC-biased optical orthogonal frequency division multiplexing.

DFT Spread-Optical Pulse Amplitude Modulation for Visible Light Communication Systems

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been proposed in visible light communication (VLC) to overcome the limited modulation bandwidth of light emitting diode (LED). Due to the implementation of the inverse fast Fourier transform at the DCO-OFDM transmitter, DCO-OFDM suffers from its high peak-to-average power ratio (PAPR), which restricts its use in some VLC applications, especially where the optical power efficiency is a crucial requirement. That is because the LEDs used in VLC systems have a limited optical power-current linear range. To this end, a novel discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) signal scheme based on the single carrier-interleaved frequency division multiple access (SC-IFDMA) signal is introduced in this paper to address the high PAPR issue of OFDM. DFTS-OPAM is achieved by considering a PAM as an SC-IFDMA data symbol and duplicate the output vector of the fast Fourier transform at the SC-IFDMA transmitter side. Simulation results show that the PAPR of the proposed scheme is 7 dB lower than that of DCO-OFDM. Furthermore, this significant PAPR improvement is experimentally investigated where the practical results show that the proposed scheme can provide more 2.5 dB reduction in the average transmitted power requirement compared to DCO-OFDM and can subsequently increase the maximum achieved distance between the transmitter and the receiver up to 44%.

Artificial Noise Design in Time Domain for Indoor SISO DCO-OFDM VLC Wiretap Systems

In this paper, we investigate physical-layer security for the indoor single-input single-output (SISO) DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) visible light communication (VLC) wiretap systems. To increase system security, we propose a precoding scheme using time-domain artificial noise (AN) to exploit the degrees of freedom (DOFs) provided by the cyclic prefix (CP) of OFDM. We further propose a convex optimization method to restrict the increase of peak to average power ratio (PAPR) introduced by AN precoding and then maximize the secrecy rates. Numerical results suggest that the proposed scheme can significantly boost the secrecy performance and alleviate the impact of PAPR increment.

Effect of Sunlight on Photovoltaics as Optical Wireless Communication Receivers

This paper explores the effects of sunlight on using a low-cost off-the-shelf silicon solar panel as an optical wireless communication (OWC) receiver. A receiver circuit structure has been proposed to maximize simultaneous energy harvesting and data communication performance. An equivalent circuit model of the solar panel for simultaneous energy harvesting and wireless optical communication is discussed. By using the solar panel model, the effects of varying sunlight conditions on the performance of the model as an OWC receiver are estimated. Furthermore, an experimental setup is developed to study the effects and verify the simulated estimations. The experimental setup consists of a 3.5 m wireless optical link with a full Transmission Control Protocol and Internet Protocol (TCP/IP) network stack. The system uses DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) to use the available communication bandwidth efficiently. A 940 nm low-cost off-the-shelf laser device along with an off-the-shelf off-axis-parabolic mirror is used as the transmitter. The maximum user throughput achieved over the air is 28.3 Mb/s while simultaneously harvesting energy using the maximum power point tracking (MPPT) technique. The peak power harvested with simultaneous communication is 4.5 W. The harvested energy is stored in a 38 Wh lithium-ion (Li-ion) battery.

Long-Wavelength Semipolar (11–22) InGaN/GaN LEDs with Multi-Gb/s Data Transmission Rates for VLC

Monolithic integration of red, green, and blue (RGB) LEDs is crucial for white light visible light communications (VLC), display technology, and general illumination but is intrinsically limited by the fundamental differences between the InGaN and AlGaInP epitaxial material systems typically used for visible wavelength emission. Increasing the indium content in traditional c-plane InGaN/GaN to achieve longer wavelength LEDs enhances the piezoelectric-induced polarization resulting in a low quantum efficiency and long radiative lifetimes. These issues can be resolved by growing along a nonpolar or semipolar orientation. This paper demonstrates the feasibility of the (11–22) semipolar LEDs for long-wavelength VLC with an additional capability of monolithic integration to produce a single white light RGB chip. Based on our high-performance semipolar InGaN/GaN LEDs, we report record data transmission rates of 4.22 Gb/s, 3.72 Gb/s, and 336 Mb/s under the forward error correction (FEC) threshold standard for reliable communication for our green (515 nm), yellow (550 nm), and amber (600 nm) semipolar LEDs using adaptively bit-loaded DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM).

Reflection based coupling efficiency enhancement in a fluorescent planar concentrator for an optical wireless receiver.

Fluorescent planar concentrators have been proposed as optical concentrators that can have both a wide field of view and a high optical gain stemming from a large collection area for optical wireless communications. However, the fluorescent concentrators with such a large collection area often lead to a low light coupling efficiency due to the edge coupling mechanism leading to a considerable optical power loss. In this work, an analysis of the light coupling efficiency enhancement in the electrical power gain is presented. In particular, a practical method to improve the coupling efficiency by introducing edge and back reflection using Lambertian-, specular-, and retro-reflectors is presented. It is demonstrated that by choosing the optimal reflectors, the received signal strength can be improved by more than a factor of two. Also demonstrated with the proposed method is a data rate more than 1.12 Gbps with bit error rate less than 3.8 × 10-3 using a DC-biased optical orthogonal frequency division multiplexing. This is, to the best of our knowledge, the first Gbps class demonstration using a commercial fluorescent planar concentrator.

BER enhanced receiver for PHO-OFDM-based optical wireless communications

In this paper, a soft-equalization-based single iterative receiver is proposed for the recently proposed PAM-DMT-based hybrid optical orthogonal frequency division multiplexing (PHO-OFDM) system to enhance its bit error rate (BER) performance. The interlayer interference and clipping noise of the pulse amplitude modulated discrete multitone (PAM-DMT) branch and DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) branch are first reduced by a soft-equalization method in the frequency domain. Then, the pairwise clipping (PC) and pairwise averaging (PA) are further introduced to correct the regenerated time-domain branch signals according to the corresponding branch symmetry properties. Numerical simulations demonstrate that the proposed receiver can realize improved BER performance over the conventional counterpart under both the linear and nonlinear transmission model.

Orthogonal Time-Frequency Multiplexing With 2D Hermitian Symmetry for Optical-Wireless Communications

Orthogonal time-frequency-space (OTFS) modulation is a promising two-dimensional (2D) signal technique to tackle high mobility time-varying wireless channels. However, its good performance on time-frequency plane multiplexing for optical-wireless communications (OWC) has never been fully investigated. In this article, we take advantage of OTFS and propose a new orthogonal time-frequency multiplexing (OTFM) scheme for DC-biased OWC. We first design a 2D Hermitian symmetry in the DC-biased optical OTFM (DCO-OTFM) system to generate the real-valued signal. The bit error rate (BER) of a DCO-OTFM system is studied on the OWC channel. Numerical results demonstrate that the proposed scheme outperforms the conventional DC-biased optical orthogonal frequency division multiplexing scheme.

Performance of Spatial Diversity DCO-OFDM in a Weak Turbulence Underwater Visible Light Communication Channel

The performance of underwater visible light communication (UVLC) system is severely affected by absorption, scattering and turbulence. In this article, we study the performance of spectral efficient DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in combination with the transceiver spatial diversity in turbulence channel. Based on the approximation of the weighted sum of lognormal random variables (RVs), we derived a theoretical exact bit error rate (BER) for DCO-OFDM systems with spatial diversity. The simulation results are compared with the analytical prediction, confirming the validity of the analysis. It is shown that spatial diversity can effectively reduce the turbulence-induced channel fading. The obtained results can be useful for designing, predicting, and evaluating the DCO-OFDM UVLC system in a weak oceanic turbulence condition.

A New Selective Mapping Scheme for Visible Light Systems

The huge bandwidth and immunity to electromagnetic interference make Visible Light Communication (VLC) systems as preferred technique for designing the physical layer of 5G applications. Unfortunately, the superimposition of multiple subcarriers in DC-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) system leads to a high peak to average power ratio (PAPR). The well-accepted nondistorting PAPR reducing schemes like selective mapping (SLM) require an obligatory transmission of side information (SI) reducing the bandwidth efficiency of the system. To address these issues, a simple SI cancellation algorithm using pilots associated with channel estimation is proposed in this work. In short, the DCO-OFDM system is presented with a modified cluster architecture and each cluster is assigned with at least one pilot. Every cluster is restricted to have same phase and this idea provides the pilots to perform joint channel estimation and SI cancellation. The experimental results obtained by using Universal Software Radio Peripheral (USRP) as hardware and LabVIEW as software indicate a significant reduction in PAPR without effecting the real-time channel response.

Optical single carrier-interleaved frequency division multiplexing for visible light communication systems

In this paper, a novel optical single carrier-interleaved frequency division multiplexing (OSC-IFDM) signaling scheme for an intensity modulation and direct detection (IM/DD) based visible light communications (VLC) system is presented. We show that, OSC-IFDM significantly improves the peak-to-average-power ratio (PAPR) of the DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM). The reduction in the PAPR value is achieved by exploiting the symmetrical characteristics of the single carrier-interleaved frequency division multiple access (SC-IFDMA) time domain symbol, whereby the imaginary and real parts of the SC-IFDMA samples are separated and transmitted through two sub-symbols. Simulation results show that, the PAPR of OSC-IFDM is 10 dB lower than that of the DCO-OFDM scheme. We also show that the proposed scheme offers higher spectral efficiency compared to our previous work.

Non‐linear companding transform for DCO‐OFDM‐based VLC systems

In this study, a novel non-linear companding transform (NCT) is proposed for the DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) based visible light communications (VLC). The authors lower the peak-to-average power ratio (PAPR) of the original DCO-OFDM signals to mitigate the transmission performance degradation induced by the non-linearity of the light-emitting diodes (LEDs) transmitter. Firstly, the positive and negative amplitudes of the original DCO-OFDM signals are separated into two groups. Then, the group with the maximum peak is first processed with a given compress coefficient u 1 and power compensation coefficient c. After that, another coefficient u 2 is calculated by the emission power conservation criteria to further compress the rest group. Finally, these two groups of amplitude samples are combined together before adding a suitable DC bias for driving LEDs. As the mean power is not changed, a PAPR reduction can be obviously realised by the proposed NCT scheme.

Adaptive learning architecture-based predistorter for nonlinear VLC system

A light-emitting diode (LED) acts as a transmitter in a visible-light communication (VLC) system. However, the nonlinear characteristics of LED limit the performance of the VLC system by degrading the quality of a transmitted signal. In recent years, several forms of predistorters have been proposed to mitigate the effects of LED nonlinearity; however, none of them have been able to approach the performance of a linear VLC system. In this paper, we propose an adaptive learning architecture (ALA)-based predistortion technique to estimate and compensate for LED nonlinearities in a VLC system. A DC-biased optical orthogonal frequency division multiplexing signal is considered. The performance with and without predistorter is analyzed assuming optical channel. It is shown that degradation due to LED nonlinearity can be compensated by using ALA-based predistortion, and the overall predistorter–LED system is able to approach near-linear performance. Further, the proposed predistorter architecture is also able to track the variations in LED nonlinearity and compensate them. Simulation results based on error vector magnitude, symbol error rate, amplitude distortion (AM/AM) curves and constellation plots validate the performance of our proposed technique.

Alleviation of LED nonlinearity impact in visible light communication using companding and predistortion

In order to suppress the nonlinearity impact of light-emitting diodes (LEDs) on the performance of visible light communication (VLC), the combined method including companding and predistortion was proposed in this work. The modified companding and predistortion based on the LED characteristic analysis were used corresponding to the output current -input voltage (I-V) and light output-injected current (L-I) nonlinearity, respectively. Using the combined method, the performances of the DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) VLC system were analysed. It is demonstrated that the non-linear distortion of OFDM transmitted signal is alleviated, thus the received signal quality is increased. As a result, the bit error rate (BER) performance of VLC system is improved obviously by using the combined method of companding and predistortion. Furthermore, the applicability of this combined method is also verified by analysing the nonlinearity suppression of the VLC system using different LED. The results reveal that both the impact from LED's I-V and L-I nonlinearity can be alleviated using the proposed method combining from companding and predistortion for DCO-OFDM VLC system.

Deployment Issues and Performance Study in a Relay-Assisted Indoor Visible Light Communication System

This paper investigates the deployment issues and analyzes the performance of a relay-assisted indoor visible light communications (VLC) system with dc-biased optical orthogonal frequency division modulation (DCO-OFDM). The study considers two light sources in an office environment. One light source works as the information source and is located at the ceiling while the other one works as the relay terminal, which usually is a desk light or a floor light. We analyze the general blockage in a room, discuss the path loss of the VLC channel, and compare received signal power from a line of sight (LOS) link and a non-LOS link. The constraints on the relay height in order to achieve LOS connection are investigated. Simplified optimal power allocation under illumination and blockage constraints is proposed. Bit error rate performance is analyzed and the required signal power is derived for the relay-assisted DCO-OFDM VLC system. The study shows that in a relay-assisted VLC system, the blockage of source to destination link has trivial influence on the overall system performance if relays are properly deployed.

Impact of Device Orientation on Error Performance of LiFi Systems

Most studies on optical wireless communications (OWC) have neglected the effect of random orientation in their performance analysis due to the lack of a proper model for the random orientation. Our recent empirical-based research illustrates that the random orientation follows a Laplace distribution for the static user equipment (UE). In this paper, we analyze the device orientation and assess its importance on system performance. The reliability of the OWC channel highly depends on the availability and alignment of line-of-sight (LOS) links. In this paper, the effect of receiver orientation, including both polar and azimuth angles on the LOS channel gain are analyzed. The probability of establishing the LOS link is investigated and the probability density function (PDF) of signal-to-noise ratio (SNR) for a randomly oriented device is derived. By means of the PDF of SNR, the bit-error ratio (BER) of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in additive white Gaussian noise (AWGN) channels is evaluated. A closed-form approximation for the BER of UE with random orientation is presented which shows a good match with Monte-Carlo simulation results. Furthermore, the impact of UE's random motion on the BER performance has been assessed. Finally, the effect of random orientation on the average signal-to-interference-plus-noise ratio (SINR) in a multiple access points (AP) scenario is investigated.