Optical Wireless Communication System Research Articles

Blue-LED based electrical and optical PD-NOMA systems: Characterization in underwater environment

This paper presents the characterization of blue LED-based two different power domain non-orthogonal multiple access(PD-NOMA)-assisted underwater optical wireless communication (UOWC) systems, which use on-off keying (OOK) signal, in an emulated seawater channel. We first characterize the electrical PD-NOMA (EPD-NOMA) system having different power allocation ratios (PAR). In the next study, we characterize two configurations of optical PD-NOMA (OPD-NOMA) systems. The first configuration of OPD-NOMA uses LEDs positioned over a 2-D plane at the transmitter. Conventional OPD-NOMAis found to maintain the desired PAR only at a particular location at the receiver. We next propose, design, and characterize a new architecture of OPD-NOMA transmitter, in which LED lights are combined to produce a collinear OPD-NOMA beam. For collinear beam OPD-NOMA transmitter, the PAR value is maintained within 0.1 of its targeted value over viewing angle of ±4 degrees. Both EPD-NOMA and Collinear-beam OPD-NOMA systems with 10Mbps data rate are characterized using two different users at distances of 2.5 m (far user (FU)) and 1 m (near user (NU)) in tap water and in an emulated seawater channel. The received optical power of approximately −30.2 dBm(−30 dBm)is found to produce a pre-FEC BER of 10−3 in EPD-NOMA (collinear beam OPD-NOMA)for both the users and in tap water channels at a data rate of 10 Mbps with a PAR value of 0.5. For the saline water channel, corresponding values are −29.85 dBm (−29.7 dBm) for the EPD-NOMA (collinear beam OPD-NOMA) system at a 10 Mbps data rate with PAR =0.5.

WIRELESS TRANSMITTER FOR OPTICAL COMMUNICATION WITH FREQUENCY-MODULATED LASER CARRIER

Interest in the use of unguided optical systems is growing, due to the development of wireless mobile and indoor communication systems as well as the applications of lasers in space technology. The proposed laser transmitter, with frequency-modulated carrier, represents the optical transmitter of a wireless optical communication system, for transmitting audio signals on laser carriers, which can be used both inside buildings and outside, for distances estimated according to system parameters. In the paper are presented elements of design of the electrical diagram of principle, as well as analysis by SPICE simulation of the designed circuit. The results obtained by SPICE simulation synthesize the operation of the laser transmitter, allowing an optimization of the parameters of the component circuits.

Adaptive-memory-length look-up table (LUT)-based digital pre-distortion for IM/DD underwater wireless optical communications.

A digital pre-distortion (DPD) scheme based on an adaptive-memory-length look-up table (AML-LUT) is proposed and experimentally demonstrated in a four-level pulse amplitude modulation (4-PAM) underwater optical wireless communication (UOWC) system. By implementing adaptive memory length for each pattern in the AML-LUT-based DPD, the size of the AML-LUT can be significantly reduced without sacrificing performance compared to both the full-size LUT and the multi-symbol simplified look-up table (MSS-LUT)-based DPDs. The performance of the proposed AML-LUT-based DPD is experimentally evaluated for a 625 Mbit/s 4-PAM UOWC over 1 m transmission length. Experimental results show that compared with the full-size LUT with a memory length of 7 (LUT-7)-based DPD, the proposed AML-LUT-based DPD (i) incurs a marginal power penalty of 0.5 dB at both the 7% hard-decision forward error correction (HD-FEC) and KP4-FEC threshold limits, while simultaneously reducing the implementation complexity (i.e., the LUT size) by 93%; (ii) achieves comparable transmission performance compared to the MSS-LUT-based DPD, while reducing the implementation complexity by 89%; and (iii) shows great potential for high-speed, low-complexity and memory-efficient intensity modulation and direct detection (IM/DD) UOWC and short-reach optical interconnects.

Self-powered graphene/4H-SiC nanowire array-based ultraviolet photodetectors with fast response time and low dark current for promising wireless ultraviolet communication

Self-powered ultraviolet photodetectors (UV PDs) capable of detecting UV light signals without using an external power supply have become particularly attractive because of their high sensitivity, consistent optical response, ultra-small size, low energy consumption, and long-term robustness for the future. Here we reported a self-powered UV PDs coupling of two-dimensional (2D) single-layer graphene and one-dimensional (1D) 4H-SiC nanowire arrays. The fabricated PDs can feature an ultra-high on-off ratio of 6301 at 0 V and the 2D-1D architectural exhibits a low dark current of ∼4 pA, an ultra-fast response of 4.89 ms. The high performance was associated with the formed heterojunction between the 2D graphene and 1D 4H-SiC nanowire array and the high carrier mobility of the graphene. Moreover, the 2D-1D PDs with a large switching ratio and ultra-fast response time can be beneficial to achieve efficient, real-time, and accurate information transmission under 275 nm UV light, shedding some light on the development of high-performance wireless optical communication systems.

Transmitter design and AE–AR region characterization for NOMA-SLIPT UWOC systems with uniformly distributed message and Imp-SIC

This paper investigates simultaneous lightwave and information transfer (SLIPT) based non-orthogonal multiple access (NOMA) in underwater optical wireless communication (UWOC) systems. At the user signal separation (SS) based SLIPT scheme is employed. Imperfect successive interference cancellation (SIC) at the near user is considered. Firstly, we derive the maximum allowable electrical power for message symbols at the transmitter, ensuring input to the laser diode lies within the linear region and. A novel linearity-constrained transmitter design is proposed for the uniformly distributed message, followed by calculations of instantaneous data rate and instantaneous harvested energy for both users. Then, closed-form expressions for average data rates (AR) and average energy harvested (AE) under weak turbulence-induced fading channels using Holtzman’s approximation are derived. The tightness of the approximation is confirmed by Monte-Carlo simulations. AE–AR boundaries for both users show an initial rise and subsequent decline in data rates with increased harvested energy, owing to higher bias values restricting transmitter power. The study also examines trade-offs between AE and AR across various system parameters including beam divergence angles, imperfection of SIC, power allocation coefficients, and water types, is conducted. We observe shifting and shrinking/expansion of the AE–AR region depending upon the variation of the various system parameters. We also show that the discussed SS scheme is general and encompasses time-switching (TS), and power-splitting (PS) schemes as special cases. We also briefly discuss as to how the developed framework could be extended to more than two users. In summary, the overall effective energy consumption deduces owing to the adoption of SLIPT at the users, making the considered setup more energy-efficient.

Free space optics communication system design using iterative optimization

Abstract Free Space Optics (FSO) communication provides attractive bandwidth enhancement with unlicensed bands worldwide spectrum. However, the link capacity and availability are the major concern in the different atmospheric conditions. The reliability of the link is highly dependent on weather conditions that attenuate the signal strength. Hence, this study focuses to mitigate the weather and geographic effects using iterative optimization on FSO communication. The optimization maximizes the visibility distance while guaranteeing the reliability by minimizing the Bit Error Rate (BER). The wireless optical communication system is designed for the data rate of 10 Gbps. The performance of the proposed wireless optical communication is compared against the literature in terms of visibility distance, quality factor, BER, and Eye diagram at different atmospheric conditions. The simulation results have shown that the proposed work has achieved better performance.

Experimental study of laser spot tracking for underwater optical wireless communication.

In this paper, a novel laser spot tracking algorithm that incorporates the Kalman filter with the continuously adaptive Meanshift algorithm (Cam-Kalm) is proposed and employed in an underwater optical wireless communication (UOWC) system. Since the Kalman filter has the advantage of predicting the state information of the target spot based on its spatial motion features, the proposed algorithm can improve the accuracy and stability of the moving laser spot tracking. A 2 m optical wireless communication experimental system with auto-tracking based on a green laser diode (LD) is built to evaluate the tracking performance of different algorithms. Experimental results verify that the proposed algorithm outperforms conventional tracking algorithms in aspects of tracking accuracy, interference resistance, and response time. With the proposed Cam-Kalm algorithm, the experimental system can establish an effective communication link, while the maximum tracking speed is 20 mm/s given the forward-error-correction (FEC) threshold.

An ADO‐OFDM with integrated precoding, companding and optimized power allocation methods for high‐speed data transmission in visible light communication

AbstractVisible light communications (VLC) has received a lot of attention in recent studies because of its benefits over radio‐frequency (RF) communications. It is a short‐range optical wireless communication system that uses light‐emitting diodes (LEDs) as transmitters. Optical orthogonal frequency division multiplexing (OFDM) is an auspicious technology for VLC high‐speed data transfer. The use of OFDM in a VLC system raises the system's peak‐to‐average power ratio (PAPR). The intrinsic non‐linearity of LED is a key concern in an asymmetrically clipped DC‐biased optical OFDM (ADO‐OFDM) system due to its high PAPR. Also, conventional ADO‐OFDM modulation scheme cannot be used for accommodating different demands of services in downlink multiple access due to the restriction of direct current biased optical OFDM (DCO‐OFDM) and asymmetrically clipped optical (ACO‐OFDM) transmission on odd and even subcarriers. To tackle this issue, a modified ADO‐OFDM (MADO‐OFDM) is proposed that adjusts the numbers of subcarriers required for the transmission of ACO‐OFDM and DCO‐OFDM adaptively based on the requests of services in downlink multiple access. Also, the proposed MADO‐OFDM is integrated with a discrete Hartley Matrix transform (DisHMT) precoder and Generalized Piecewise Linear Compander (GPLD) to provide high‐speed data transmission with less PAPR. In addition, the power is allocated to the proposed MADO‐OFDM system optimally by maximizing the channel capacity based on the Aquila optimizer algorithm. The simulation results reveal that the suggested system's PAPR is reduced by 2.4 dB and 0.8 dB, respectively, compared to conventional ADO and hybrid ADO‐OFDM. It also confirms that the suggested generalized PLC can greatly increase BER without affecting the PAPR performance.

Underwater optical wireless communication system based on dual polarization states with optical code division multiple access: performance evaluation

Underwater optical wireless communication system based on dual polarization states with optical code division multiple access: performance evaluation

Performance Analysis of Subcarrier Index Modulation OFDM in a DWDM-FSO link

Free Space Optics (FSO) is a wireless optical communication system using light to establish high-capacity links between points without the need for physical cables. However, its susceptibility to atmospheric conditions such as fog, rain, and turbulence presents challenges to signal quality maintenance. To address this, Dense Wavelength Division Multiplexing (DWDM), traditionally employed in fiber optics, is leveraged to optimize bandwidth utilization and scalability in high-capacity communications. The integration of DWDM into FSO systems offers redundancy and resilience against signal degradation induced by atmospheric conditions. To the best of our knowledge, this is the first time a paper explores the performance of the Subcarrier Index Modulation Orthogonal Frequency Division Multiplexing (SIM-OFDM) technique within a DWDM-FSO system. The objective is to reinforce FSO link robustness and improve transmission rates. Through a comparative analysis with conventional OFDM in an Additive White Gaussian Noise (AWGN) channel, the study reveals a notable Signal-to-Noise Ratio (SNR) gain of at least 3dB with enhanced SIM-OFDM (ESIM-OFDM). Simulations, conducted with realistic parameters within the DWDM-FSO architecture, demonstrate that ESIM-OFDM modulation enables a transmission range of 2 km with an aggregated rate of 10.1 Gb/s. These results underscore the potential of SIM-OFDM in enhancing FSO system performance through the integration of DWDM technology. The proposed ESIM-OFDM DWDM-FSO scenario could represent a highly interesting solution for establishing 5G Backhaul links.

Investigation of non-line-of-sight underwater optical wireless communications with wavy surface.

Underwater optical wireless communication (UOWC) systems have been widely researched to achieve high-speed and secure wireless communications. The non-line-of-sight (NLOS) UOWC system that uses the water surface to reflect signal light is widely studied to overcome the line-of-sight (LOS) channel limitation, particularly the channel blockage issue by marine biology or complex underwater topography. However, most previous NLOS UOWC studies have assumed a flat water surface or a general sine or cosine surface wave model for simplicity, leading to inaccurate performance estimations. In this paper, we build a theoretical NLOS UOWC framework with the Pierson wave model which considers both spatial correlation and time relativity information of wave incorporating wind speed, and investigate the signal-noise-ratio (SNR) and bit-error-rate (BER) performance. Results show that compared with the previous flat surface, the wavy surface can reduce the probability of achieving a satisfying signal level by up to 70%, affecting the performance of NLOS UOWC systems. Furthermore, we investigate the multiple-input-multiple-output (MIMO)-based NLOS UOWC under wavy surfaces. Results show that the MIMO principle can reduce the impact of the wavy surface, where the probability of achieving a satisfying signal level can be increased by up to 50% using the 2 × 4 MIMO configuration. However, results also show that further increasing the number of receivers may not further improve the system performance. The proposed model enables more accurate design and analysis of NLOS UOWC systems by accounting for the overlooked impact of wavy surfaces.

Performance analysis of FSO communications over a generalized turbulence fading channel with pointing error

SummaryQuantitatively assessing the performance of free space optical (FSO) communication systems requires an accurate description of turbulence induced fading. However, it is well known that common irradiance distributions inaccurately model intensity tail regions. Recently, a general fading model as the product of arbitrary number of Gamma with inverse Gamma was proposed and shown to accurately predict measurements, besides containing many well‐known models as special cases. This paper is concerned with analyzing the effect of discrepancies among different fading distributions on optical system performance in terms of bit error rates and capacity. Based on the general fading model a unified analysis of the channel structure effects on FSO communications in the presence of turbulence and pointing error is presented. The outage probability is investigated for both homodyne and direct detection schemes. Closed form expressions for the average probability of bit error for intensity modulation, phase shift keying and polarization shift keying, single‐input‐single‐output systems are given and verified by Monte‐Carlo simulations. Spatial diversity is also studied through single‐input‐multiple‐output systems. Also, the ergodic capacity is derived. Comparisons between the proposed distributions and other widely used distributions are carried out and show significant differences among existing fading models. This suggests possible improvements in the spectral efficiency and error performance design of optical wireless communication systems.

Energy efficiency optimization in a parallel relay-assisted UWOC system with simultaneous lightwave information and power transfer.

In this paper, we investigate the energy efficiency optimization for a parallel relay-assisted underwater wireless optical communication (UWOC) system with simultaneous lightwave information and power transfer (SLIPT) over an aggregate channel. In this system, relay nodes are equipped with energy harvesting devices, getting energy from the direct current component of the received signal transmitted by the source node. These nodes utilize the harvested energy to transmit the signal to the destination node with the decoding and forwarding strategy. The harvested energy for each relay node is derived by the Gauss-Laguerre quadrature formula and the outage probability is deduced by the Meijer-G function. Then, the system's energy efficiency can be calculated and an energy efficiency maximization problem is built up with respect to the bias current. We propose a three-level-iteration algorithm to solve this problem. In the first level, the Dinkelbach method is used to represent energy efficiency in a parametric subtractive form. In the second level, we use the penalty function method to convert the object function and constraint. In the third level, the objective function is transformed into a quadratic function by using a successive convex approximate method, thereby solving for the bias current. The effects of system parameters on energy efficiency are also analyzed. Theoretical results and Monte Carlo simulations suggest that employing the solved bias current can significantly improve the system's energy efficiency.

Performance analysis of OSSK-UWOC systems considering pointing errors and channel estimation errors.

In this paper, we present the bit error rate (BER) performance of the underwater wireless optical communication (UWOC) systems using the optical space shift keying (OSSK) on the gamma-gamma turbulent fading channel, which also considers pointing errors and channel estimation errors. Firstly, we develop the new expressions for the probability density function (PDF) based on the Gamma-Gamma distribution with error factors. Subsequently, we analyze the statistical characteristic of the difference in attenuation coefficients between two channels in the OSSK system, by which we provide analytical results for evaluating the average BER performance. The results show that the effective improvement of spectral efficiency (SE) and BER performance is achieved by rationally allocating the number of lasers and detectors in the system. The OSSK-UWOC system performs better when a narrow beam waist is used. Furthermore, the presence of channel estimation error brings the BER performance advantage to the system, and the system with a high channel estimation error (ρ = 0.7) shows a 4 dB improvement in signal-to-noise ratio (SNR) gain compared to the system with a low channel estimation error (ρ = 0.95). The findings in this paper can be used for the UWOC system design.

ОРГАНИЗАЦИЯ ПОДВОДНОЙ БЕСПРОВОДНОЙ СИСТЕМЫ СВЯЗИ

The article discusses underwater wireless communication systems. Research in the field of underwater optical wireless communication system will allow developing science, industry, to find solutions to defense tasks and emergencies, to carry out remote monitoring of environmental pollution of the underwater world and control underwater objects and equipment of oilrigs, to conduct underwater research and much more. The purpose of the work is to develop a classification of the principles of construction and organization of underwater wireless communication, taking into account modern achievements and development of technologies in the field of underwater wireless communication systems, to assess the strengths and weaknesses of underwater acoustic, radio frequency and optical communication systems. It is shown that the underwater optical wireless communication system has a great potential to enhance the traditional underwater acoustic communication system and underwater radio frequency communication system due to the high speed of information transmission, low latency, lower power consumption and compact size. To select the equipment and organize the communication channel, a generalized classification of an underwater optical wireless communication system has been developed, depending on the routing protocols used, on the configuration of the communication channel, on the optical properties of water, on the type of water in which the transmission channel is organized, on the mobility of underwater vehicles and the coverage area, on the type of modulation used, the method of underwater communication and from the factors affecting the organization of the communication channel.

Performance evaluation of a UOWC system based on the FRS/OCDMA code for different types of Jerlov waters.

In this work, the fixed right shift (FRS) code is utilized for the optical code division multiple access (OCDMA) technique in an underwater optical wireless communication (UOWC) system. Additionally, in this system, a 532nm laser diode (LD) source is employed to generate optical signals. The investigation encompasses an analysis of five distinct Jerlov water types, each exhibiting diverse chlorophyll concentrations. The performance of the proposed system is evaluated when each channel that is assigned a unique FRS code sequence carries different data rates (2.5, 5, and 10Gbps). Underwater (UW) ranges, bit error rate (BER), eye diagrams, and quality factor (Q-factor) are the performance metrics used to evaluate the system performance. The proposed UOWC-FRS/OCDMA system is simulated, and the obtained results show that the eye diagram openings close, the BER increases, and the Q-factor decreases as the data rate per each channel increases from 2.5 to 10Gbps, and the attenuation of water becomes higher. Moreover, the lower attenuation values caused by the Jerlov type I (JI) waterbody allow each channel to carry 10Gbps of data to propagate longer UW for a range of 35m with a log(BER) ≤-6.33 and Q-factor greater than 4.9. On the other hand, at the same values of BER and Q-factor, the shortest ranges of 12 and 5.15m are obtained for JII and JIII waters, respectively, where their attenuation coefficient values are 0.5297 (JII) and 1.8998 m -1 (JIII). Furthermore, as our model uses three channels, the overall achieved capacity is 3×10G b p s=30G b p s.

Spectral and Energy Efficient Pilot-Assisted PAPR Reduction Technique for Underwater Wireless Optical Communication Systems

Spectral and Energy Efficient Pilot-Assisted PAPR Reduction Technique for Underwater Wireless Optical Communication Systems

Design and experimental demonstration of underwater wireless optical communication system based on semantic communication paradigm.

Underwater wireless optical communication (UWOC) has been widely studied as a key technology for ocean exploration and exploitation. However, current UWOC systems neglect semantic information of transmitted symbols, leading to unnecessary consumption of communication resources for transmitting non-essential data. In this paper, we propose and demonstrate a deep-learning-based underwater wireless optical semantic communication (UWOSC) system for image transmission. By utilizing a deep residual convolutional neural network, the semantic information can be extracted and mapped into the transmitted symbols. Moreover, we design a channel model based on long short-term memory network and employ a two-phase training strategy to ensure that the system matches the underwater channel. To evaluate the performance of the proposed UWOSC system, we conduct a series of experiments on an emulated UWOC experimental platform, in which the effects of different turbidity channel environments and bandwidth compression ratios are investigated. Experimental results show that the UWOSC system exhibits superior performance compared to the conventional communication schemes, particularly in challenging channel environments and low bandwidth compression ratios.

Dual-mode spatial index modulation for MIMO-OWC.

In this Letter, we propose and demonstrate a dual-mode spatial index modulation (DM-SIM) scheme for spectral efficiency enhancement of band-limited multiple-input multiple-output optical wireless communication (MIMO-OWC) systems. By performing dual-mode index modulation in the spatial domain, DM-SIM can transmit both spatial and constellation symbols. Since constellation design plays a vital role in the proposed DM-SIM scheme, we further propose three dual-mode constellation design approaches including phase rotation, amplitude scaling and joint phase rotation and amplitude scaling. Moreover, we also designed a differential log-likelihood ratio (LLR) detector for the proposed DM-SIM scheme. Experimental results show that the joint phase rotation and amplitude scaling approach can achieve a remarkable 3.2 dB signal-to-noise ratio (SNR) gain compared with the phase rotation approach in a 2×2 MIMO-OWC system applying DM-SIM.

Performance Assessment of Underwater-to-Air Optical Wireless Communication System With the Effect of Solar Noise and Sea Surface Conditions

Performance Assessment of Underwater-to-Air Optical Wireless Communication System With the Effect of Solar Noise and Sea Surface Conditions