Optical Wireless Communication System Research Articles

Artificial Neural Network-Based Transmission Power Control for Underwater Wireless Optical Communication System

Several scientists have proposed using an underwater wireless optical communication system (UWOC) to deliver high-speed data services using the abundant optical spectrum. However, wireless optical signal propagation faces an antagonistic environment when using undersea channels due to various factors like scattering, absorption, turbulence, and optical link misalignment between the transmitter and the receiver. These factors will attenuate the optical signal and lead to degrading system performance. To reduce these factors impact on the communication system's performance, transmitted optical power (OTP) should be increased. Since the UWOC system is battery-powered, increasing OTP will consume more electrical power. Therefore, it is necessary to adjust OTP to a value commensurate with the underwater channel changes. So, an ANN model is proposed in this article for link adaptation, which can adjust the OTP level in tandem with the underwater channel conditions. Data for training, testing, and validation of the proposed system reliability was collected experimentally, and tap water was used as a transmission medium. Evaluation of the proposed model outcome demonstrates that reliable performance is achieved in predicting OTP needed in multiple scenarios. The MSE of the predicted OTP is(9.5×10-3,1.5×10-2, and 1.7×10-2) dBm in the training, testing, and validation stages, respectively. The regression values of the training, testing and validation sets are (0.9997,0.9990, and 0.9996). The results achieved by the proposed model prove it is reliable to be applied in UWOC systems.

Unified statistical thermocline channel model for underwater wireless optical communication.

Understanding the effect of ocean turbulence on optical beam propagation is critical to the design and performance evaluation of underwater wireless optical communication systems. In this Letter, we propose a unified Weibull-generalized gamma distribution to characterize the laser beam irradiance fluctuations of turbulent underwater thermocline wireless optical channels. The proposed model shows an excellent agreement with the measured data under various experimental emulated channel conditions that cover turbulences induced by temperature, salinity, and air bubbles. To the best of our knowledge, this is the first model that comprehensively describes the statistics of the laser beam irradiance fluctuations in underwater wireless optical channels due to both thermohaline gradient and air bubbles.

Optimization of LED Array Spatial Coverage Characteristics in Underwater Wireless Optical Communication

To achieve uniform spatial coverage characteristics in optical signals in an underwater wireless optical communication (UWOC) system, and therefore reduce the requirement of the alignment between the receiver and the transmitter, we propose an optimized scheme of optical signal coverage based on a light-emitting diode (LED) array in this paper. For high-efficiency coverage of the optical signals, the pitch angle of the LED light source is first optimized on the basis of the light beam geometry. Then, the layout of the LED array and the horizontal deflection angle of the light source are jointly optimized by an improved particle swarm optimization (PSO) algorithm. Taking a 16-LED array as an example, the performances of the spatial coverage characteristics with three different LED array layouts are analyzed in detail under four typical seawater environments. The results show that the LED array with the PSO-optimized layout can achieve better uniformity in the power distribution for the received optical signals, and enhance the robustness of the UWOC system in complex seawater environments.

Non-Data-Aided SNR Estimation for Bandlimited Optical Intensity Channels

Powerful and reliable estimation of transmission parameters is an indispensable task in each receiver unit-not only for radio frequency, but also for optical wireless communication systems. In this context, the signal-to-noise ratio (SNR) plays an eminent role, especially for adaptive scenarios. Assuming a bandlimited optical intensity channel, which requires a unipolar waveform design, an algorithm for SNR estimation is developed in this paper, which requires no knowledge of the transmitted data. This non-data-aided approach benefits to a great extent from the fact that very long observation windows of payload symbols might be used for the estimation process to increase the accuracy of the result; this is in striking contrast to a data-aided approach based on pilot symbols reducing the spectral efficiency of a communication link. Since maximum likelihood, moment-based or decision-directed algorithms are not considered for complexity and performance reasons, an expectation-maximization solution is introduced whose error performance is close to the Cramer-Rao lower bound as the theoretical limit, which has been derived as well.

Designing of high-speed inter-satellite optical wireless communication (IsOWC) system incorporating multilevel 3-D orthogonal modulation scheme

Designing of high-speed inter-satellite optical wireless communication (IsOWC) system incorporating multilevel 3-D orthogonal modulation scheme

Mitigation of PAM4 rolling shuttered pattern grayscale ambiguity in demodulation utilizing long short term memory neural network (LSTM-NN) in optical wireless communication systems

One realization of optical wireless communication (OWC) is optical camera communication (OCC), which utilizes mobile-phone and surveillance cameras for communication. One main limitation of OCC system is the low frame rate of typical cameras. To enhance the OCC data rate with limited frame rate cameras, rolling shuttered camera mode can be utilized. However, decoding the rolling shuttered pattern is very challenging. In addition, utilizing multi-level modulation formats, such as four-level pulse-amplitude-modulation (PAM4) can increase the OCC bit-per-symbol; however, it would be a great challenge to identify the multiple intensity levels owing to the uneven aggregated light exposure of each pixel-row. In this work, we put forward and demonstrate utilizing the Long-Short-Term Memory neural-network (LSTM-NN) to alleviate the decoding of high inter-symbol-interference (ISI) of PAM4 rolling shuttered pattern. The experiment is performed in a practical room with 3-m free-space transmission distance. Results show that the proposed LSTM-NN method can significantly enhance the decoding of multiple intensity level rolling shuttered pattern. The proposed scheme also allows a large tilting angle of ±70° between the transmitter (Tx) and receiver (Rx).

RIS-Assisted Visible Light Communication Systems: A Tutorial

Recent intensive and extensive development of the fifth-generation (5G) of cellular networks has led to their deployment throughout much of the world. As part of this implementation, one of the challenges that must be addressed is the skip-zone problem, which occurs when objects such as trees, people, animals, and vehicles obstruct the transmission of signals. In free-space optical (FSO) and radio frequency (RF) systems, dead zones are most often caused by buildings and trees, while in visible light communications (VLC), obstructions are caused by individuals moving around a room or objects placed in the room. A signal obstruction can significantly reduce the signal-to-noise ratio in RF and indoor VLC systems, whereas in FSO systems, where the transmitted signals are directional, the obstruction can completely disrupt data transmission. Therefore, the skip-zone dilemma must be resolved to ensure the smooth and efficient operation of 5G and beyond networks. By placing a relay between a transmitter and a receiver, the effects of obstacles can be mitigated. As a result, the signal from the transmitter will reach the receiver. In recent years, reconfigurable intelligent surfaces (RISs) that are more efficient than relays have become widely accepted as a method of mitigating skip-zones and providing reconfigurable radio environments. However, there have been limited studies of RISs for optical wireless communication (OWC) systems. Through the RIS technology, OWC and RF communication channels can be reconfigured. This paper aims to provide a comprehensive tutorial on indoor VLC systems utilizing RIS technology. The article discusses the basics of VLC and RISs and reintroduces RISs for OWC systems, focusing on RIS-assisted indoor VLC systems. We also provide a comprehensive overview of optical RISs and examine the differences between optical RISs, RF-RISs, and optical relays. Furthermore, we discuss in detail how RISs can be used to overcome line-of-sight blockages and the device orientation issue in VLC systems while revealing key challenges such as RIS element orientation design, RIS elements to access point/user assignment design, and RIS array positioning design problems that need to be studied. Moreover, we discuss and propose several research problems on integrating optical RISs with other emerging technologies, including non-orthogonal multiple access, multiple-input multiple-output systems, physical layer security, and simultaneous lightwave and power transfer in VLC systems. Finally, we highlight other important research directions that can further improve the performance of RIS-assisted VLC systems.

Demonstrating Intra-Spacecraft Optical Wireless Links

In a Spacecraft (SC), a huge amount of cabled electrical connections is needed by many different units to exchange digital data. This increases the overall mass, the space occupancy, and the routing design time. Here, we propose and demonstrate an Optical Wireless Communication (OWC) system that can perfectly replace the MIL-STD-1553B cables inside the SC, providing wireless communication among the satellite units, with full backward compatibility with pre-existing units and protocols. We present the experimental demonstration in the challenging environment of a populated mockup of a real SC, where we connect four different units and exploit the Non Line-of-Sight (NLOS) configuration, i.e. take advantage of the scattering from the walls to overcome the shadowing effects. This OWC technique can effectively reduce the overall weight (more than 50%) and cost of the SC data network. The proposed transceivers are based on a transparent, fully analog solution and are made of Commercial Off-the-Shelf (COTS) components, with features compatible with the space environment. The on-field experiment shows that this is a practical solution for the data networks in future SCs.

50 m/187.5 Mbit/s real-time underwater wireless optical communication based on optical superimposition

In this paper, an optical pulse amplitude modulation with 4 levels (PAM-4) using a fiber combiner is proposed to enhance the data rate of a field-programmable gate-array-based long-distance real-time underwater wireless optical communication system. Two on–off keying signals with different amplitudes are used to modulate two pigtailed laser diodes, respectively, and the generated optical signals are superimposed into optical PAM-4 signals by a fiber combiner. The optical PAM-4 scheme can effectively alleviate the nonlinearity, although it reduces the peak-to-peak value of the emitting optical power by 25%. A real-time data rate of 187.5 Mbit/s is achieved by using the optical PAM-4 with a transmission distance of 50 m. The data rate is increased by about 25% compared with the conventional electrical PAM-4 in the same condition.

Analysis and Simulation of BER Performance of Chaotic Underwater Wireless Optical Communication Systems

It is shown that modulation methods based on the phenomenon of dynamic chaos and correlation processing can improve the noise immunity of underwater wireless communication systems. This provides the possibility of signal recovery at negative values of the signal-to-noise

Solution for Self-Interference of NOMA-Based Wireless Optical Communication System in Underwater Turbulence Environment

Non-orthogonal multiple access (NOMA) is a multiple access technology of great potential and it has superior performance in underwater wireless optical communications (UWOC). In this paper, we investigate the self-interference problem caused by the superimposed transmission of NOMA-based wireless optical communication system in the underwater turbulent environment, and then propose an alternating composite phase shift (ACPS) coding scheme based on constellation optimization, which can effectively eliminate the decoding error caused by the self-interference within the NOMA paired group. To enhance the optimization efficiency of ACPS in the medium- and strong-turbulence environments, we design a golden-section-approach-based power allocation (GSPA) algorithm that minimizes the bit error rate (BER). The simulation results show that the BER of the low-power user within the NOMA paired group can be reduced by up to 3 orders of magnitude in the ACPS-based UWOC-NOMA system compared to the conventional UWOC-NOMA system. In addition, the proposed GSPA algorithm fully takes into account the light-intensity scintillation caused by the ocean turbulence, which not only significantly reduces the average BER of the NOMA system, but also accelerates the convergence rate.

Performance analysis of the hybrid MIMO-LD/LED link for the UWOC system in sea water

Abstract Theoretical analysis of underwater wireless optical communications (UWOC) systems based on a hybrid LD/LED link in sea water was reported in this research. Multiple input-multiple output (MIMO) and single input-single output (SISO) approaches are used to implement the modulation schemes NRZ-OOK, RZ-OOK, 4-QAM, and 64-PPM. As carriers, waves with the wavelengths 715, 650, 532, and 412 nm are employed. The receiver unit includes the photodiodes PIN and APD to recognize the received signal. The findings indicate that the hybrid LD/LED link’s performance using the MIMO technique is superior to SISO in terms of BER, Q factor, and data rate. Additionally, 64-PPM is a reliable modulation technique for achieving a low BER and a high Q factor.

Hybrid opto-electronic deep neural network based orbital angular momentum mode recognition scheme in oceanic turbulence

Orbital angular momentum (OAM) has been widely used in underwater wireless optical communication (UWOC) systems due to the mutual orthogonality between modes. However, wavefront distortion caused by oceanic turbulence (OT) on the OAM mode seriously affects its mode recognition and communication quality. In this work, we propose a hybrid opto-electronic deep neural network (HOEDNN) based OAM mode recognition scheme. The HOEDNN model consists of a diffractive DNN (DDNN) and convolutional neural network (CNN), where the DDNN is trained to obtain the mapping between intensity patterns of a distorted OAM mode and intensity distributions without OT interference, and the CNN is used to recognize the output of the DDNN. The diffractive layers of the trained DDNN model are solidified, fabricated, and loaded into a spatial light modulator, and the results recorded by a charge-coupled device camera are processed and fed into the trained CNN model. The results show that the proposed scheme can overcome the interference of OT to OAM modes and recognize accurately azimuthal and radial indices. The OAM mode recognition scheme based on HOEDNN has potential application value in UWOC systems.

OFDM-based 2D generalized optical quadrature spatial modulation for MIMO-OWC systems

An innovative scheme, named orthogonal frequency division multiplexing (OFDM)-based two-dimensional (2D) generalized optical quadrature spatial modulation (GOQSM), is proposed for bandlimited multiple-input multiple-output optical wireless communication (MIMO-OWC) systems. In this scheme, by way of combining two consecutive time slots into one time block, 2D GOQSM mapping can be executed in spatial domain and time domain. Therefore, more spatial bits can be transmitted, resulting in the increase of system spectral efficiency. Furthermore, discrete Fourier transform (DFT) spreading is further introduced in OFDM-based 2D GOQSM systems, so as to substantially reduce the peak-to-average power ratio (PAPR) of the transmitted signals. To evaluate and compare the performance of OFDM-based 2D GOQSM systems with other traditional systems, numerical simulations are conducted under various spectral efficiencies. The obtained simulation results prove that the proposed OFDM-based 2D GOQSM scheme with DFT spreading can achieve better bit error rate performance and lower PAPR than other benchmark schemes.

A reach enhanced hybrid OCDMA-MDM multiplexing based IsOWC system incorporating EDFA, SOA, and RFA

Abstract Reach enhancement in inter-satellite optical wireless communication systems (IsOWC) is an utmost requirement due to ever-increasing internet usage all over the globe. Optical code division multiplexing is prominently incorporated for security enhancement in IsOWC systems, however, limited in terms of total reach. Therefore, in this work, a 120 Gbps capacity-based hybrid OCDMA and mode division multiplexed (MDM) IsOWC is presented using different optical amplifiers such as semiconductor optical amplifier (SOA), erbium-doped fiber amplifier (EDFA), and Raman fiber amplifier (RFA). Moreover, the combined effects of Laguerre–Gaussian (LG) with EDFA/SOA/RFA and Hermite–Gaussian (HG) modes with EDFA/SOA/RFA amplifiers are investigated. Results revealed that the proposed system can cover 5000 km IsOWC link within acceptable BER limits and EDFA with LG modes is found to be the best technique.

Long‐Range Optical Wireless Communication System Based on a Large‐Area, Q‐Dots Fluorescent Antenna

AbstractFluorescent concentrators (FCs) have been recently proposed as optical condensers for visible light communications (VLC) and optical wireless communication (OWC) receivers, with advantages over conventional optical stages in terms of optical gain and field of view (FoV). However, the use of FC‐based receivers in real‐world scenarios is hampered by the need for large resilience of the involved chromophores to sunlight exposure, and availability of large‐area FC substrates with suitable optochemical properties. This paper presents an innovative OWC system, based on a high‐power blue LED and a large‐area FC‐based on CuInS2 quantum dots (Q‐Dots) as receiving optical stage. A thorough characterization of the FC material in terms of conversion efficiency, temporal response, and FoV is provided, along with a full set of outdoor measurements. The combination of the Q‐Dots fluorophores chemico‐physical properties with the strong DC rejection granted by the design receiver stage allows error‐free VLC link distances up to 60 m and baud rates up to 1 Mb/s. This work represents the first demonstration of long‐range VLC links under strong solar irradiance using large‐area FC antennas and paves the way to deployment of long‐range free‐space optical links with minimal susceptibility to misalignments and pointing instabilities between receiver and transmitter.

DeepGOMIMO: Deep Learning-Aided Generalized Optical MIMO with CSI-Free Detection

Generalized optical multiple-input multiple-output (GOMIMO) techniques have been recently shown to be promising for high-speed optical wireless communication (OWC) systems. In this paper, we propose a novel deep learning-aided GOMIMO (DeepGOMIMO) framework for GOMIMO systems, wherein channel state information (CSI)-free detection can be enabled by employing a specially designed deep neural network (DNN)-based MIMO detector. The CSI-free DNN detector mainly consists of two modules: one is the preprocessing module, which is designed to address both the path loss and channel crosstalk issues caused by MIMO transmission, and the other is the feedforward DNN module, which is used for joint detection of spatial and constellation information by learning the statistics of both the input signal and the additive noise. Our simulation results clearly verify that, in a typical indoor 4 × 4 MIMO-OWC system using both generalized optical spatial modulation (GOSM) and generalized optical spatial multiplexing (GOSMP) with unipolar nonzero 4-level pulse-amplitude modulation (4-PAM) modulation, the proposed CSI-free DNN detector achieves near the same bit error rate (BER) performance as the optimal joint maximum-likelihood (ML) detector, but with much-reduced computational complexity. Moreover, because the CSI-free DNN detector does not require instantaneous channel estimation to obtain accurate CSI, it enjoys the unique advantages of improved achievable data rate and reduced communication time delay in comparison to the CSI-based zero-forcing DNN (ZF-DNN) detector.

Adaptive Optics Compensation for Orbital Angular Momentum Optical Wireless Communications

Adaptive optics (AO) can efficiently compensate for turbulence-induced distortion in orbital angular momentum (OAM)-based optical wireless communication (OWC) systems. In this paper, we design a modified phase diversity algorithm (MPDA)-based wavefront sensor to enhance the reconstruction accuracy of distorted OAM wavefront information. Aiming to further strike a compelling trade-off between AO system complexity and compensation accuracy, we first construct a novel AO system that applies a quickly and electronically controlled focus-tunable lens (FTL). It decontaminates distorted OAM signaling beams while having a low systemic complexity and superior convergence performance. Furthermore, we propose the 3-modified phase diversity algorithm (3-MPDA) AO scheme relying upon a Fourier intensity and two defocused intensities as the prior information, which beneficially balances the compensation effect and the number of defocused intensities and exhibits good noise robustness against charge-coupled device (CCD) detectors. In summary, this paper provides new insight for designing AO schemes with high compensation performance in communication links.

Accurate Underwater Optical Wireless Communication Model With Both Line-of-Sight and Non-Line-of-Sight Channels

Underwater optical wireless communication (UOWC) systems have been widely studied to achieve high-speed wireless communications. To investigate and design UOWC systems, a mathematical model to characterize the accurate performance of UOWC systems is important. However, the previous mathematical models of UOWC systems have limitations, which mainly focused on the line-of-sight (LOS) link, and only considered the vertical incident background light or optical filters with constant transmittance for simplicity. To overcome these limitations, we establish an accurate UOWC system model, incorporating the previously overlooked impact of signal and sun lights incident angles and including both LOS and non-line-of-sight (NLOS) channels. Our proposed accurate UOWC system model is validated by Monte Carlo (MC) simulations. Results show that our proposed model can capture the changes of received powers of both signal and background lights with various transmitter and receiver alignments and orientations in both LOS and NLOS channels, which typically affect the signal-noise-ratio (SNR) and bit-error-rate (BER) performance of UOWC systems. Furthermore, we show that the incident angles of both signal and sun lights have significant impact on the system performance, and our proposed model can be used to optimize the PD rotation angle to improve the SNR performance. Therefore, the more accurate UOWC channel model established in this paper provides a fundamental framework for future UOWC system design and optimization.

On the Optimization of Integrated Terrestrial-Air-Underwater Architecture Using Optical Wireless Communication for Future 6G Network

Optical wireless communication (OWC) is one of the promising solution to enhance the capacity of the next generation cellular as well as fiber-wireless (FiWi) broadband access networks. The proposed work aims to complement an OWC system by utilizing unmanned aerial vehicles (UAV) to improve the performance of an integrated terrestrial-air-underwater (TAU) communication framework. Specifically, an integrated FiWi based UAV assisted hybrid free-space optical (FSO) and underwater wireless optical communication (UWOC) system is proposed to meet high data rate requirement between a terrestrial ground station and an autonomous underwater vehicle (AUV). The performance of the proposed scheme is compared against the conventional radio frequency (RF) based TAU communication framework. We also optimize the divergence angle of the optical beam of FSO link using a novel COgnition-based divergence angle tracking (CODAT) algorithm such that the outage probability of FSO links is minimized and the data rate requirement of AUV is also satisfied. The impact of the position of over-sea surface (OSS) relay and UAV altitude on the performance of the proposed system is also analyzed. Through the obtained results it has been shown that the proposed OWC based integrated TAU communication framework outperforms the conventional RF based TAU communication framework.