Optical Wireless Communication System Research Articles

Orthogonal Frequency Division Diversity and Multiplexing for 6G OWC: Principle and Underwater Use Case

In this paper, we, for the first time, propose and demonstrate an orthogonal frequency division diversity and multiplexing (OFDDM) scheme for the sixth-generation (6G) underwater optical wireless communication (UOWC) systems. In OFDDM, the subcarriers are grouped into subblocks; the subcarriers within each subblock transmit the same constellation symbol through diversity transmission, while different subblocks transmit different constellation symbols via multiplexing transmission. As a result, OFDDM can support hybrid diversity and multiplexing transmission simultaneously. Moreover, the combination of subblock interleaving and low-complexity diversity is further proposed to efficiently mitigate the adverse low-pass effect and substantially reduce the computational complexity, respectively. The feasibility of OFDDM adapting to the various transmission conditions in UOWC systems has been verified via both simulations and experiments. Experimental results demonstrate that a striking 106.1% effective bandwidth extension can be obtained using OFDDM in comparison to conventional orthogonal frequency division multiplexing (OFDM) for a fixed spectral efficiency of 1 bit/s/Hz. Furthermore, OFDDM with adaptive bit loading can also gain a remarkable 13.3% capacity improvement compared with conventional OFDM with adaptive bit loading.

OFDM system based on parametric DFT transform

This study introduces a specific type of orthogonal frequency division multiplexing (OFDM) system that relies on the parametric discrete Fourier transform (DFT-alpha), where alpha is a parameter randomly selected within the range [-2pi, 0]. We investigate the performance of the proposed system across various channel types and modulation formats, including binary phase shift keying (BPSK), 16 quadratic amplitude modulation (QAM), analyzing aspects such as bit-error rate (BER), peak-to-average power ratio (PAPR), and computational complexity. This work considers the most interesting special case of the one-parameter DFT, which is the DFT-π/6 . The simulated results confirm the suggested OFDM system’s ability to minimize the computational complexity of the conventional OFDM system, standing as the primary contribution of the current study, while maintaining the performance identical. Therefore, the parametric DFT-OFDM system would be a good alternative to the classical DFT-OFDM. The findings suggest that parametric DFT based OFDM holds significant potential for high-speed optical wireless communication systems.

Water-Triboelectrification-Complemented Moisture Electric Generator.

Energy harvesting from ubiquitous natural water vapor based on moisture electric generator (MEG) technology holds great potential to power portable electronics, the Internet of Things, and wireless transmission. However, most devices still encounter challenges of low output, and a single MEG complemented with another form of energy harvesting for achieving high power has seldom been demonstrated. Herein, we report a flexible and efficient hybrid generator capable of harvesting moisture and tribo energies simultaneously, both from the source of water droplets. The moisture electric and triboelectric layers are based on a water-absorbent citric acid (CA)-mediated polyglutamic acid (PGA) hydrogel and porous electret expanded polytetrafluoroethylene (E-PTFE), respectively. Such a waterproof E-PTFE film not only enables efficient triboelectrification with water droplets' contact but also facilitates water vapor to be transferred into the hydrogel layer for moisture electricity generation. A single hybrid generator under water droplets' impact delivers a DC voltage of 0.55 V and a peak current density of 120 μA cm-2 from the MEG, together with a simultaneous AC output voltage of 300 V and a current of 400 μA from the complementary water-based triboelectric generator (TEG) side. Such a hybrid generator can work even under harsh wild environments with 5 °C cold and saltwater impacts. Significantly, an optical alarm and wireless communication system for wild, complex, and emergency scenarios is demonstrated with power from the hybrid generators. This work expands the applications of water-based electricity generation technologies and provides insight into harvesting multiple potential energies in the natural environment with high output.

A Hybrid Network Integrating MHSA and 1D CNN–Bi-LSTM for Interference Mitigation in Faster-than-Nyquist MIMO Optical Wireless Communications

To mitigate inter-symbol interference (ISI) caused by Faster-than-Nyquist (FTN) technology in a multiple input multiple output (MIMO) optical wireless communication (OWC) system, we propose an ISI cancellation algorithm that combines multi-head self-attention (MHSA), a one-dimensional convolutional neural network (1D CNN), and bi-directional long short-term memory (Bi-LSTM). This hybrid network extracts data features using 1D CNN and captures sequential information with Bi-LSTM, while incorporating MHSA to comprehensively reduce ISI. We analyze the impact of antenna numbers, acceleration factors, wavelength, and turbulence intensity on the system’s bit error rate (BER) performance. Additionally, we compare the waveform graphs and amplitude–frequency characteristics of FTN signals before and after processing, specifically comparing sampled values of four-pulse-amplitude modulation (4PAM) signals with those obtained after ISI cancellation. The simulation results demonstrate that within the Mazo limit for selecting acceleration factors, our proposal achieves a 7 dB improvement in BER compared to the conventional systems without deep learning (DL)-based ISI cancellation algorithms. Furthermore, compared to systems employing a point-by-point elimination adaptive pre-equalization algorithm, our proposal exhibits comparable BER performance to orthogonal transmission systems while reducing computational complexity by 31.15%.

Enhanced underwater optical wireless communication using optical code division multiple access with sigma shift matrix code

A high-speed 40 Gbps Underwater Optical Wireless Communication (UOWC) system exploiting code division multiple access is proposed in this paper. Optical Code Division Multiple Access (OCDMA) systems play a crucial role in enhancing transmission capacity by allowing multiple channels to transmit data simultaneously. In this proposed work, four channels each assigned a distinct Sigma Shift Matrix (SSM) code and modulated with a data at 10 Gbps. Laser Diode (LD) sources operating in the green spectrum are used in UOWC systems due to their low attenuation. To ensure the feasibility of implementing the proposed system in real-world environments, the actual scattering and absorption coefficients from five different types of water are considered: Pure Sea (PS), Clear Ocean (CL), Coastal Sea (CS), Harbor I (HI), and Harbor II (HII). The system's performance is evaluated via the Quality Factor (Q-factor), Bit Error Rate (BER), and eye diagrams opening. The simulation results indicate that in PS water, which exhibits the lowest attenuation, the four channels achieve the longest underwater transmission distance of 48 m, with a Q-factor ≥ 4.1, log(BER) ≤ −4.7, and wide eye openings. Since CL and CS have higher attenuation than PS, they enable shorter transmission distances of 27.5 m and 16 m, respectively, while maintaining nearly the same Q-factor, log(BER), and eye diagrams. Conversely, the highest attenuation observed in HI and HII waterbodies results in the shortest underwater spans of 8.2 m and 5.05 m, respectively.

Adaptive TLS-OQSM technology of a wireless optical MIMO communication system over an oceanic vertical turbulence channel

Adaptive TLS-OQSM technology of a wireless optical MIMO communication system over an oceanic vertical turbulence channel

Compound parabolic concentrator for LED-based underwater optical communication transmitter

Laser sources have been used at the transmitter unit of underwater wireless optical communication (UWOC) systems since their invention. In recent years, light-emitting diodes (LEDs) have begun to be used as UWOC transmitters due to their low cost, long lifespan, and high energy efficiency. However, data transmission distances in UWOC systems using bare LEDs and LED arrays with collimating lens are limited due to LED’s large divergence angles and insufficient gains provided by the low cost lens designs. In this paper, we propose to use a compound parabolic concentrator (CPC), whose transmission efficiency curve is very close to the ideal concentrator, as a collimator that narrows the divergence angle of LEDs to design low-cost UWOC transmitter with long communication distance. Using the Bouguer–Beer–Lambert channel model, we derived the received optical power expression at the receiver side for the LED-based UWOC system with CPC at the transmitter. Furthermore, unlike existing studies in the literature, the full spectrum of the LED has been taken into account when deriving the power expression. The results show that using the CPC collimator instead of a typical collimation lens in an LED-based UWOC system can narrow the divergence angle by approximately 10 times, resulting in a 70 dB increase in signal-to-noise ratio (SNR) at the receiver and up to a 20 times increase in the communication distance.

Investigating performance of optical communication system with FSO/OWC channel

Abstract In recent years, the evolution of optical wireless communication (OWC) system has emerged as a viable alternative to radio frequency communication. These technologies provide an effective solution for addressing the need for point-to-point communication, offering benefits such as higher bandwidth, faster data rates, no licensing requirements, low power usage, quick and simple installation, enhanced security, and resistance to electromagnetic interference. In this article, we analyze two wireless optical communication systems: one using an FSO channel and the other using an OWC channel. The analysis focuses on range and quality factor as performance metrics. We examine the performance of one-to-many Tx/Rx FSO/OWC channel under three different atmospheric conditions: clear weather, haze, and fog, using eye diagrams. The system analysis includes mathematical models for the received optical power and the pointing error. Additionally, we investigate the impact of spatial diversity on the performance of FSO/OWC channel with configurations of 1 × 1, 2 × 2, 4 × 4, and 8 × 8. Our findings indicate that the 8 × 8 FSO/OWC configurations yield better results compared to other configurations and the OWC channel performs well over long distances up to 110 km, while the FSO channel is more suitable for short range communication up to 37 km.

Performance investigation of UOWC system based on OAM beams for various Jerlov water types

This paper introduces a novel high-speed underwater optical wireless communication (UOWC) system employing three distinct orbital angular momentum (OAM) beams. A single green laser source operating at 532 nm generates these beams: LG0,0,LG0,20,andLG0,50, each transmitting data at 10 Gbps. The paper provides a comprehensive analysis of absorption and scattering coefficients for five Jerlov water types: I (JI), IA (JIA), IB (JIB), II (JII), and III (JIII). Simulation results demonstrate the system ability to transmit multiple data streams simultaneously using distinct OAM modes, achieving an overall capacity of 30 Gbps. The longest underwater (UW) transmission distance of 22 m is achieved in JI water as it exhibits the lowest attenuation. This range decreases by 9.09 %, 31.82 %, 59.09 %, and 80.91 % in JIA, JIB, JII, and JIII, respectively, due to increased attenuation in these water types. These results are obtained with a log (BER) below −5 and a Q-factor above 4, indicating successful data reception. The findings highlight the potential of OAM multiplexing for enhancing data capacity in challenging underwater environments.

Joint optimization of spatial correlation for space-constrained MIMO underwater optical wireless communication system in weak turbulence

For the space-constrained underwater installations, spatial correlation is an essential factor affecting the reliability of the underwater optical wireless communication (UOWC) system with multiple-input multiple-output (MIMO) technology. In this paper, a joint optimization method for the layout and transmit power allocation of the light-emitting diode (LED)-based UOWC system is proposed and demonstrated under the weak turbulent channel. To analysis the transmission performance of the MIMO UOWC system, a composite channel model incorporating absorption, scattering and weak turbulence effects is established, and the channel spatial correlation characteristics with various system parameters is analyzed based on this model. Among these parameters, we take a 9 × 9 MIMO UOWC system as an example to simulate the communication performance of the optimization method under different turbulence intensities. The simulation results show the optimized system exhibits a channel capacity improvement of 13bps/Hz compared to the original system at the signal-to-noise ratio (SNR) of 20 dB and the same turbulence intensity. Moreover, the SNR of jointly optimized system can achieve a gain of 5 dB at the same bit error rate (BER). By extending the joint optimization algorithm to a 16 × 16 MIMO UOWC system, the optimized SNR can also obtain a gain of 7.5 dB which effectively demonstrates the universality of the proposed joint optimization algorithm.

End-to-end performance of mixed RF/UWOC system with AF protocol based on Mellin transform

This paper proposes an approach to analyze the performance of radio frequency/underwater wireless optical communication (RF/UWOC) systems using Mellin inverse transform. This analysis combines heterodyne detection (HD) and direct detection /intensity modulation (IM/DD). The RF link from land ground stations to relay stations follows the Nakagami-m distributed channel. The UWOC link from the relay station to the underwater vehicle is modeled using the double generalized gamma (GG) distribution with pointing errors. By employing the Fixed Gain Amplification and Forwarding (AF) protocol, the probability density function (PDF) of the RF/UWOC link is derived using the Mellin inverse transform, expressed in the form of the Meijer-G function. In addition, expressions for the outage probability and bit error rate of the mixed system were derived. Monte Carlo simulations validated the analysis results, which demonstrated the superior performance of HD technology.

Microwave-assisted in-situ synthesis of low-dimensional perovskites within metal-organic frameworks for optoelectronic applications

Halide perovskites are renowned for their remarkable optoelectronic properties, yet their application is hindered by stability challenges. Metal-organic frameworks (MOFs) have emerged as promising matrices for enhancing perovskite durability. In this study, we achieved the in-situ synthesis of a series of perovskites within the MOF-5 matrix, spanning three-dimensional, quasi two-dimensional, and two-dimensional structures, via microwave-assisted reaction techniques. This microwave synthesis method has proven to be a rapid and efficient approach for the high-yield production of perovskite materials. These MOF-5⊃perovskites exhibited superior optical properties, positioning them as promising candidates for various optoelectronic applications, including white light-emitting diodes and optical wireless communication (OWC) systems. Significantly, our perovskites demonstrated high and consistent communication rates, making them suitable for both space and underwater OWC deployments. Overall, our study underscores the potential of microwave-assisted synthesis in advancing high-performance perovskite materials, offering valuable insights for the development of future optoelectronic devices.

Performance comparison of various end-to-end learning technologies with a bandwidth-limited OWC system

Recently, end-to-end (E2E) learning methodologies have garnered significant attention as a compelling approach to attain global optimal communication within the domain of 6 G native intelligent systems. Nevertheless, a precise evaluation of the diverse E2E techniques is still lacking, leading to uncertainties regarding their applicable scenarios and effectiveness. In this paper, we present a comprehensive comparison applying three advanced E2E methods including the autoencoder-based geometric shaping (AEGS) model, comprehensive autoencoder (CAE) model, and wave-wise auto-equalization (WWAE) model in a real bandwidth-limited optical wireless communication (OWC) system. A novel attention-based comprehensive noise joint channel estimator (ACNJCE) is proposed to serve as a universal channel model adaptable to the existing E2E methods. Based on traditional carrier-less amplitude and phase modulation (CAP) modulation, AEGS, WWAE, and CAE are compared under the conditions of 2 GBaud and 3 GBaud respectively. The final results demonstrate that the CAE exhibits the capability to autonomously allocate bandwidth and achieves the highest dynamic adjustment range, which is increased by 69% compared with CAP based on neural network (NN) equalization. In contrast, AEGS has obvious advantages in terms of received optical power (ROP) gain. Based on bit-power loading discrete multi-tone modulation (DMT) modulation, WWAE can effectively compensate the signal spectrum after modulation order optimization and finally achieves the highest data rate under the condition that the −3 dB bandwidth of the channel is only close to 1 GHz. The BER of WWAE with DMT at this rate is 25.2% of that using the NN equalization. Furthermore, experimental results under turbulent conditions reveal that AEGS exhibits superior and more stable performance amidst the perturbations caused by turbulence due to its ability to achieve end-to-end autonomous optimization while integrating traditional modulation and bringing additional shaping gain. According to our knowledge, this marks the first comprehensive evaluation and comparison of existing major E2E algorithms and traditional communication algorithms in a real OWC system.

Capacity Optimization for RSMA-Based Multi-User System over Underwater Turbulence Channel

The underwater environment used for communication is harsh and complex, necessitating heightened standards for spectral efficiency and reliability in underwater wireless optical communication (UWOC) systems. The focus of this work is on the performance of multi-user UWOC systems operating in oblique channels of ocean turbulence downlink, where users are randomly distributed at a certain depth. A joint optimization scheme is proposed, which joints rate-splitting multiple access (RSMA) and power allocation so that the system’s ergodic sum capacity is optimized to improve the transmission bandwidth. Furthermore, the probability density function (PDF) and cumulative distribution function (CDF) models for the received signal-to-noise ratio (SNR) of a multi-user multiple-input multiple-output (MIMO) system operating in the turbulent underwater oblique channels are established, accounting for the avalanche photodiode (APD) shot noise and solar radiation noise. Theoretical derivations are presented to quantify the ergodic capacity and outage probability of the multi-user system utilizing the RSMA technology. Subsequently, a numerical analysis is conducted to investigate the influence of the power allocation coefficient, RSMA, and the joint optimization algorithm on the performance of a two-user MIMO system leveraging RSMA. The simulation results show that our optimization scheme effectively reduces the outage probability, thereby achieving the maximum system sum rate and validating the practical feasibility and efficacy of the proposed scheme.

Survey on Optical Wireless Communication with Intelligent Reflecting Surfaces

Optical Wireless Communication (OWC) technology has gained significant attention in recent years due to its potential for providing high-data-rate wireless connections through the large license-free bandwidth available. A key challenge in OWC systems, similar to high-frequency Radiofrequency (RF) systems, is the presence of dead zones caused by obstacles like buildings, trees, and moving individuals, which can degrade signal quality or disrupt data transmission. Traditionally, relays have been used to mitigate these issues. Intelligent Reflecting Surfaces (IRSs) have recently emerged as a promising solution, enhancing system performance and flexibility by providing reconfigurable communication channels. This paper presents an overview of the application of IRSs in OWC systems. Specifically, we categorize IRSs into two main types: mirror array-based IRSs and metasurface-based IRSs. Furthermore, we delve into modeling approaches of mirror array-based IRSs in OWC and analyze recent advances in IRS control, which are classified into system power or gain optimization-oriented, system link reliability optimization-oriented, system data rate optimization-oriented, system security optimization-oriented, and system energy optimization-oriented approaches. Moreover, we present the principles of metasurface-based IRSs from a physical mechanism perspective, highlighting their application in OWC systems through the distinct roles of light signal refraction and reflection. Finally, we discuss the key challenges and potential future directions for integrating IRS with OWC systems, providing insights for further research in this promising field.

A 4 × 20 Gbps inter-satellite optical wireless communication system based on orbital angular momentum multiplexing: performance evaluation

A 4 × 20 Gbps inter-satellite optical wireless communication system based on orbital angular momentum multiplexing: performance evaluation

BER and outage throughput analysis of DPIM/DHPIM coded QPSK-OFDM based outdoor optical wireless communications

With the growing demand for fast and reliable communication systems, particularly in outdoor environments, it is essential to investigate advanced encoding techniques. Digital Pulse Interval Modulation (DPIM) and Dual Header Pulse Interval Modulation (DHPIM) emerge as promising alternatives to traditional line coding methods, providing enhanced spectral efficiency and resistance to signal disruptions. This paper presents the performance of a Quadrature Phase-Shift Keying (QPSK) Orthogonal Frequency Division Multiplexing (OFDM)-based Optical Wireless Communication (OWC) system using these advanced encoding schemes. The analysis includes simulation results on QPSK-OFDM-based transmitter design, free space optical channel modeling based on Gaussian and log-normal distribution atmospheric turbulence, and recovery of input digital stream using the mentioned line coding techniques. The results demonstrate optimal bit error rate (BER) values for the QPSK-OFDM-based OWC system. The primary innovation of this research lies in the encoding schemes and their performance in outdoor optical wireless communication systems under turbulent conditions. Through extensive simulations and analysis, detailed insights into the bit error rate and outage throughput characteristics of DPIM/DHPIM coded QPSK-OFDM are provided, offering a unique perspective on the performance of these schemes in real-world scenarios. The findings not only highlight the optimal BER values achievable with the QPSK-OFDM-based OWC system but also offer insights into the system's ability to overcome transmission challenges under various atmospheric conditions.

Reconfigurable MIMO-based self-powered battery-less light communication system

Simultaneous lightwave information and power transfer (SLIPT), co-existing with optical wireless communication, holds an enormous potential to provide continuous charging to remote Internet of Things (IoT) devices while ensuring connectivity. Combining SLIPT with an omnidirectional receiver, we can leverage a higher power budget while maintaining a stable connection, a major challenge for optical wireless communication systems. Here, we design a multiplexed SLIPT-based system comprising an array of photodetectors (PDs) arranged in a 3 × 3 configuration. The system enables decoding information from multiple light beams while simultaneously harvesting energy. The PDs can swiftly switch between photoconductive and photovoltaic modes to maximize information transfer rates and provide on-demand energy harvesting. Additionally, we investigated the ability to decode information and harvest energy with a particular quadrant set of PDs from the array, allowing beam tracking and spatial diversity. The design was explored in a smaller version for higher data rates and a bigger one for higher power harvesting. We report a self-powering device that can achieve a gross data rate of 25.7 Mbps from a single-input single-output (SISO) and an 85.2 Mbps net data rate in a multiple-input multiple-output (MIMO) configuration. Under a standard AMT1.5 illumination, the device can harvest up to 87.33 mW, around twice the power needed to maintain the entire system. Our work paves the way for deploying autonomous IoT devices in harsh environments and their potential use in space applications.

Neural Network Equalisation for High-Speed Eye-Safe Optical Wireless Communication with 850 nm SM-VCSELs

In this paper, we experimentally illustrate the effectiveness of neural networks (NNs) as non-linear equalisers for multilevel pulse amplitude modulation (PAM-M) transmission over an optical wireless communication (OWC) link. In our study, we compare the bit-error-rate (BER) performances of two decision feedback equalisers (DFEs)—a multilayer-perceptron-based DFE (MLPDFE), which is the NN equaliser, and a transversal DFE (TRDFE)—under two degrees of non-linear distortion using an eye-safe 850 nm single-mode vertical-cavity surface-emitting laser (SM-VCSEL). Our results consistently show that the MLPDFE delivers superior performance in comparison to the TRDFE, particularly in scenarios involving high non-linear distortion and PAM constellations with eight or more levels. At a forward error correction (FEC) threshold BER of 0.0038, we achieve bit rates of ~28 Gbps, ~29 Gbps, ~22.5 Gbps, and ~5 Gbps using PAM schemes with 2, 4, 8, and 16 levels, respectively, with the MLPDFE. Comparably, the TRDFE yields bit rates of ~28 Gbps and ~29 Gbps with PAM-2 and PAM-4, respectively. Higher PAM levels with the TRDFE result in BERs greater than 0.0038 for bit rates above 2 Gbps. These results highlight the effectiveness of the MLPDFE in optimising the performance of SM-VCSEL-based OWC systems across different modulation schemes and non-linear distortion levels.

Modeling and field demonstration of water-to-ice wireless optical communication system based on highly-sensitive detectors

Modeling and field demonstration of water-to-ice wireless optical communication system based on highly-sensitive detectors