DC-biased Optical Orthogonal Frequency Research Articles

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.

On BER analysis of nonlinear VLC systems under ambient light and imperfect/outdated CSI

Visible light communication (VLC) has emerged as a promising supplement to the existing radio frequency (RF) technology, which facilitates high speed communication for next generation communication systems. Although promising, the performance in a practical VLC system is severely impaired by ambient light interference. Furthermore, nonlinearity of light emitting diodes (LEDs) and imperfect/oudated channel state information (CSI) are other factors that degrade the achievable bit error rate (BER) performance. In this work, an analytical framework for analyzing the expression for BER of a wide class of modulation techniques is derived for an impaired VLC system considering: (a) ambient light interference, (b) LED nonlinearity, and (c) imperfect and outdated CSI. The derived analytical expressions for BER are validated through extensive computer simulations performed over typical VLC channels. Lastly, the utility of the derived analytical framework is demonstrated in the context of high data-rate VLC systems, wherein the performance of DC biased optical orthogonal frequency division multiplexing (DCO-OFDM) is quantified for VLC links impaired by the aforementioned degradations.

A Blind Timing Synchronization Algorithm for DCO-OFDM VLC Systems

Visible Light Communication (VLC) systems are more vulnerable to timing synchronization problems due to the employed incoherent modulation scheme. In this work, a blind timing synchronization algorithm is introduced to DC biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) systems, based on cyclostationarity of the received signal that relies on second-order statistics only. It does not require any channel impulse response, as it is a blind estimator. The performance of the proposed estimator is evaluated using computer simulations in the Lab-view software and also validated through experimental results obtained using the VLC testbed using Universal Software defined Radio Peripheral (USRP) hardware.

Performance studies of MIMO based DCO-OFDM in underwater wireless optical communication systems

The research in underwater communication system is focused towards wireless networks in view of recent developments in wireless technology and increasing need for deep sea data mining. The underwater wireless optical communication (UWOC) have more advantages for short-range wireless links due to very high bandwidth and data rate than acoustic communication. Although UWOC offers high capacity links at low latencies, they suffer from limited communication range due to various distinctive characteristics of underwater channel. A major degrading effect associated with the underwater channel is turbulence-induced fading. To mitigate this fading and to extend the viable communication range, spatial multipath diversity techniques can be adopted over UWOC links. The proposed work evaluates the performance of a multicarrier modulation scheme, i.e., MIMO based DC biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) for underwater wireless optical channel. The simulation of MIMO-DC biased optical OFDM technique employing spatial diversity in underwater optical communication is a novel approach towards the determination of an optimal modulation technique for underwater optical wireless channels. The performance is evaluated based on the various parameters such as Signal to Interference plus Noise Ratio (SINR), Throughput and Bit Error Rate (BER). A simple experiment is conducted to demonstrate the BER performance assessment for different link distance of 0.45 m and 1.5 m with different transmitter and receiver configurations (SIMO and MIMO) for both clear water and coastal water using MIMO DCO OFDM technique.

Pre-Distorted Enhanced ADO-OFDM for Hybrid VLC Networks: A Mutual-Interference-Free Approach

Mutual interference is a crucial issue for the visible light communication (VLC) systems to be addressed, including both co-channel interference (CCI) and inter-carrier interference (ICI). In this paper, to solve the CCI problem, a novel optical attocell network is introduced, which can be regarded as a hybrid network based on both single frequency network and multi-frequency network. Meanwhile, to eliminate the ICI, a pre-distorted enhanced asymmetrically clipped DC biased optical orthogonal frequency division multiplexing (PEADO-OFDM) approach is designed. In addition, the corresponding properties of PEADO-OFDM are analyzed, and different modulation schemes are employed for downlink and uplink in the same ADO-OFDM modulation framework to reduce the complexity of user processing. Simulation results indicate that the proposed PEADO-OFDM scheme outperforms the conventional ADO-OFDM approach in terms of bit error rate, while the processing latency and computational complexity are greatly reduced at the receiver.

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.

Advanced ADO-OFDM With Adaptive Subcarrier Assignment and Optimized Power Allocation

Visible light communication (VLC) is emerging as a promising technology thanks to the superior advantages of light-emitting diodes. In this letter, an advanced asymmetrically clipped dc biased optical orthogonal frequency division multiplexing (AADO-OFDM) scheme with adaptive subcarrier assignment is proposed to accommodate the demands of multi-users. Additionally, the optimization of optical power allocation for AADO-OFDM under the constrained total optical power is investigated and formulated to maximize the tight lower bound of the total channel capacity. Simulation results show that compared with the schemes without optimal power allocation, the proposed scheme can maximize the lower bound of channel capacity.

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.