Optical Wireless Communication System Research Articles

Adaptive Optics for Orbital Angular Momentum-Based Internet of Underwater Things Applications

Orbital angular momentum (OAM) has the potential to dramatically enhance the amount of information in the Internet of Underwater Things (IoUT) system. Nevertheless, underwater-turbulence-induced scintillation will destroy the orthogonality of OAM modes, hence degrading the performance of the system. In this article, a random-amplitude-mask-based adaptive optics (AOs) technique is proposed for the sake of mitigating the turbulence effects in the OAM-based underwater wireless optical communication (UWOC) system. Combined with phase retrieval algorithms, the magnitudes of linear measurements obtained from the distorted OAM beams modulated with a series of random amplitude masks and focused by a lens are employed for the phase estimation. Furthermore, we present a comprehensive performance comparison against state-of-the-art phaseless wave-front sensing techniques. Moreover, the mixture exponential-generalized gamma (EGG) distribution is applied for characterizing the probability density function (PDF) of reference-channel irradiance of OAM beams coupled into a single-mode fiber (SMF). In the end, the performance metrics, such as the outage probability, the average bit-error-rate (BER), and the ergodic capacity are analyzed with the aid of PDF for both single-input-single-output (SISO) and multiinput-multioutput (MIMO) systems. In a nutshell, this article provides new insights for the applications of AO in the OAM-based UWOC system, which can serve as a candidate for supporting IoUT devices.

A high-speed multi input multi output Ro-IsOWC system incorporating Manchester coding and E-LP, O-LP modes

Abstract Aiming at the link length enhancement in intersatellite optical wireless communication (IsOWC) system, considering the hybrid radiofrequency (RF)-enabled multi input multi output (MIMO) system, we accentuate the realization of medium earth orbit (MEO)-based IsOWC system by incorporating different modulation formats such as return to zero (RZ), non-RZ (NRZ), and Manchester coding. A 40 Gbps and 16 × 8 MIMO system enabling 40 GHz RF signal is proposed by using even linear polarized (E-LP) mode, and odd linear polarized (O-LP) mode over a 22,000 km IsOWC channel. Further, a performance comparison of NRZ, RZ, and Manchester coding is performed at different IsOWC distances in terms of the Q factor. Results revealed that the proposed system with Manchester coding and O-LP mode can successfully cover the 22,000 km IsOWC link length by exhibiting a 6.35 Q factor value.

Capacity enhancement of WDM visible light communication system employing 3-SOPs/channel/LD color

Abstract Nowadays, polarization division multiplexing (PDM) employing different states of polarizations (SOP) is a prominent candidate to offer high capacity and improved performance. However, the incorporation of PDM in indoor wireless optical communication (WOC) systems is limited to horizontal and vertical SOPs only. Therefore, in this research article, a cost-effective and high capacity 90 Gbps non-return to zero (NRZ) line coding based 18 channels VLC system is presented by incorporating red–green–blue laser diodes (RGB LDs). Performance investigations of different SOPs such as SOP 0°, SOP 45°, and SOP 90° are carried out at diverse VLC link distances in terms of log BER and Q factor. Results revealed that the proposed VLC system can successfully cover the 6 m distance and red LD together with SOP0 is found to be the highest performing.

Demonstration of Spatial Modulation Using a Novel Active Transmitter Detection Scheme with Signal Space Diversity in Optical Wireless Communications.

Line-of-sight (LOS) indoor optical wireless communications (OWC) enable a high data rate transmission while potentially suffering from optical channel obstructions. Additional LOS links using diversity techniques can tackle the received signal performance degradation, where channel gains often differ in multiple LOS channels. In this paper, a novel active transmitter detection scheme in spatial modulation (SM) is proposed to be incorporated with signal space diversity (SSD) technique to enable an increased OWC system throughput with an improved bit-error-rate (BER). This transmitter detection scheme is composed of a signal pre-distortion technique at the transmitter and a power-based statistical detection method at the receiver, which can address the problem of power-based transmitter detection in SM using carrierless amplitude and phase modulation waveforms with numerous signal levels. Experimental results show that, with the proposed transmitter detection scheme, SSD can be effectively provided with ~0.61 dB signal-to-noise-ratio (SNR) improvement. Additionally, an improved data rate ~7.5 Gbit/s is expected due to effective transmitter detection in SM. The SSD performances at different constellation rotation angles and under different channel gain distributions are also investigated, respectively. The proposed scheme provides a practical solution to implement power-based SM and thus aids the SSD realization for improving system performance.

LSTM Attention Neural-Network-Based Signal Detection for Hybrid Modulated Faster-Than-Nyquist Optical Wireless Communications.

In order to improve the accuracy of signal recovery after transmitting over atmospheric turbulence channel, a deep-learning-based signal detection method is proposed for a faster-than-Nyquist (FTN) hybrid modulated optical wireless communication (OWC) system. It takes advantage of the long short-term memory (LSTM) network in the recurrent neural network (RNN) to alleviate the interdependence problem of adjacent symbols. Moreover, an LSTM attention decoder is constructed by employing the attention mechanism, which can alleviate the shortcomings in conventional LSTM. The simulation results show that the bit error rate (BER) performance of the proposed LSTM attention neural network is 1 dB better than that of the back propagation (BP) neural network and outperforms by 2.5 dB when compared with the maximum likelihood sequence estimation (MLSE) detection method.

Performance analysis of 2λ × 160 Gbps optical single sideband (OSSB) generation in polarization division multiplexed (PDM) QPSK based satellite wireless optical systems (SWOS)

Abstract Capacity enhancement in satellite wireless optical systems (SWOS) systems is an ultimate task to accomplish due to ever-increasing internet services and it can be achieved by incorporating polarization division multiplexed (PDM) quadrature phase shift keying (PDM-QPSK). Herein, 2λ × 160 Gbps intersatellite optical wireless communication (IsOWC) system is proposed using PDM-QPSK. Spectral efficiency is prime requirement to enhance the performance of system and therefore, a suppression of optical single sideband (OSSB) in PDM-QPSK carrier spectrum is performed by employing fiber Bragg grating with 0.99 reflectivity. Proposed system is investigated in terms of log BER and error vector magnitude percentage (EVMP) at varied SWOS distances. Further, effects of the digital signal processing (DSP) in the receiver are analyzed by considering and neglecting the module. Results revealed that suppression of sidebands and employment of DSP enhance the system performance and competent to cover 52,000 km SWOS distance.

Optical wireless communication system performance in natural water turbulence of any strength.

The recently introduced power spectrum model for natural water turbulence, i.e., that at any average temperature, average salinity, and stratification [J. Opt. Soc. Am. A37, 1614 (2020)JOAOD61084-752910.1364/JOSAA.399150], is extended from weak to moderate-to-strong regimes with the help of the spatial filtering approach. Based on the extended spectrum, the expressions for the scintillation index (SI) are obtained, and based on its signal-to-noise ratio and bit error rate of the underwater wireless optical communication (UWOC) system with the on-off-keying modulation and gamma-gamma irradiance distribution model, the analysis is performed. The obtained results are compared with those derived from the widely used Nikishov and Nikishov spectrum. It is shown that the natural water turbulence results in the SI for plane (spherical) waves attaining higher maxima values at shorter propagation distances, about 20 m (40 m) with respect to 30 m (50 m) of Nikishovs turbulence. Therefore, it predicts a stronger degradation of the UWOC system performance in weak and moderate turbulence regimes.

A Review on Feasible and Reliable Underwater Wireless Optical Communication System for achieving High Data Rate and Longer Transmission Distance

A Review on Feasible and Reliable Underwater Wireless Optical Communication System for achieving High Data Rate and Longer Transmission Distance

Adaptive symbol-by-symbol decision feedback threshold detection receiver to suppress oceanic turbulence scintillation effect

In this paper, an adaptive symbol-by-symbol decision feedback (DFB) threshold detection receiver is proposed that can suppress the scintillation effect of oceanic turbulence in underwater wireless optical communication (UWOC) system. The DFB receiver has a simple structure and can estimate the instantaneous channel state information (CSI) through the previously detected signal after adding pilot-symbols to the transmission data header. It thereby adaptively adjusts the decision threshold of the UWOC system receiver. The DFB receiver is independent of perfect CSI (PCSI). The channel model and transmitted signal power information are not essential for it. Moreover, the channel bandwidth can accommodate the inserted pilot-symbols. Using the gamma–gamma oceanic turbulence model, the performance of the DFB receiver under intensity fluctuation scintillation index σI2=0.15 and σI2=1.60 with different signal-to-noise ratios (SNRs) γ and different pilot-symbol lengths L was simulated. The results showed that the performance of the proposed DFB receiver was highly similar to the performance of the ideal receiver with PCSI known in advance. When σI2=1.60, γ=33dB, L=10, the bit error rate performance of the decision threshold TDFB of the DFB receiver was the same as that of the decision threshold TPCSI of the PCSI receiver, and the SNR gain loss was 0 dB. Furthermore, a wavelet denoising method was designed to improve the DFB receiver performance. When the sampling frequency of the DFB receiver was 25 MHz, σI2=1.60, γ=24dB, L=10, the proposed wavelet denoising preprocessing could obtain a SNR gain of 2.1 dB. These findings suggest that the proposed DFB receiver can promote the development of real-time applications of UWOC system.

Performance analysis of underwater vertical wireless optical communication system in the presence of weak turbulence, pointing errors and attenuation losses

Performance analysis of underwater vertical wireless optical communication system in the presence of weak turbulence, pointing errors and attenuation losses

Physical-Layer Security for UAV-Assisted Air-to-Underwater Communication Systems with Fixed-Gain Amplify-and-Forward Relaying

We analyze a secure unmanned aerial vehicle-assisted two-hop mixed radio frequency (RF) and underwater wireless optical communication (UWOC) system using a fixed-gain amplify-and-forward (AF) relay. The UWOC channel was modeled using a mixture exponential-generalized Gamma distribution to consider the combined effects of air bubbles and temperature gradients on transmission characteristics. Both legitimate and eavesdropping RF channels were modeled using flexible α-μ distributions. Specifically, we first derived both the probability density function (PDF) and cumulative distribution function (CDF) of the received signal-to-noise ratio of the system. Based on the PDF and CDF expressions, we derived the closed-form expressions for the tight lower bound of the secrecy outage probability (SOP) and the probability of non-zero secrecy capacity (PNZ), which are both expressed in terms bivariate Fox’s H-function. To utilize these analytical expressions, we derived asymptotic expressions of SOP and PNZ using only well-known functions. We also used asymptotic expressions to determine the suboptimal transmitting power to maximize energy efficiency. Furthermore, we investigated the effect of levels of air bubbles and temperature gradients in the UWOC channel, and studied the nonlinear characteristics of the transmission medium and the number of multipath clusters of the RF channel on the secrecy performance. Finally, all analyses were validated using a simulation.

High-Speed Trains Access Connectivity Through RIS-Assisted FSO Communications

Free-space optic (FSO) is a promising solution to provide broadband Internet access for high-speed trains (HSTs). Besides, reconfigurable intelligent surfaces (RIS) are considered as hardware technology to improve performance of optical wireless communication systems. In this paper, we propose a RIS-assisted FSO system to provide access connectivity for HTSs, as an upgrade for the existing direct and relay-assisted FSO access setups. Our motivation is mainly based on well-proven results indicating that a RIS-assisted optical wireless system, with a large enough number of RIS elements, outperforms a relay-assisted one thanks to its programmable structure. We firstly compute the statistical expressions of the considered RIS-assisted FSO channels under weak and moderate-to-strong fading conditions. Then, the network's average signal-to-noise ratio and outage probability are formulated based on the assumed fading conditions, and for two fixed- and dynamic-oriented RIS coverage scenarios. Our results reveal that the proposed access network offers up to around 44% higher data rates and 240% wider coverage area for each FSO base station (FSO-BS) compared to those of the relay-assisted one. The increase of coverage area, on average, reduces 67% the number of required FSO-BSs for a given distance, which results in fewer handover processes compared to the alternative setups. Finally, the results are verified through Monte-Carlo simulations.

Average Capacity Evaluation Performance for Transdermal Optical Wireless Communications under the Effect of Pointing Error

In this paper, we investigate the performance of average capacity of transdermal wireless optical communication system under the effect of pointing error and skin attenuation. The channel statistical model is assumed to be the product of a stochastic process due to pointing error, and a constant channel gain caused by the skin attenuation. Furthermore, the transdermal optical wireless system employs intensity modulation direct detection with on-off signaling (IMDD/OOK), and the pointing error stochastic process only considers the zero boresight case. We derive novel closed form expression for the average capacity which takes into account the effect of geometric spreading due to pointing error, and the transdermal channel pathloss. Numerical results for the average capacity are provided as a function of the received signal-to-noise ratio, and the results are shown for different pointing error severity levels, and transdermal pathloss due to skin attenuation

Experimental evaluation of the impact of physical beam misalignment on the performance of an underwater wireless optical communication network utilizing machine learning

Underwater wireless optical communication systems have garnered significant interest due to advantages in speed and security over radio-frequency and acoustic underwater transmission. One example utilizes Laguerre–Gaussian beams carrying orbital angular momentum to increase bit transfer rate by spatially multiplexing information into an alphabet of symbols, sized 2N, where N is the number of bits encoded in each symbol. This system uses a convolutional neural network (CNN) to demultiplex the information, which has been shown to be capable of high levels of accuracy even in turbid environments or in the presence of significant optical turbulence. However, it has been observed that CNNs struggle when shown images affected by physical environmental effects (different levels of turbulence, optical magnification, beam misalignment) that were not present in the training set. In this paper we further investigate physical beam misalignment under two experimental conditions, quiescent and strong optical turbulence. Physical beam misalignment is defined as a translation of the beam at the receiver. Our optically turbulent environment is characterized using the scintillation of a Gaussian beam, and is estimated to have a refractive index structure constant value of ∼10−11m−2/3. We use two alphabet sizes, of 16 and 256 symbols, and demonstrate classification of received images in a combination of scenarios with adequate success (>90% accuracy) when training and testing under the same conditions, and using a pre-trained CNN, AlexNet. However, when training and testing under physically misaligned conditions, we demonstrate an issue with impractical rates of classification, at 5%–50%. Our investigation provides CNN classification at significantly higher levels of complexity than previously seen, both in terms of experimental turbulence strength and number of classifiable symbols (alphabet size), and clearly demonstrates the importance of carefully designed experiments to aid the machine learning training process for communication systems.

Signal recovery in optical wireless communication using photonic convolutional processor.

Deep neural networks (DNNs) have been applied to recover signals in optical communication systems and have shown competence of mitigating linear and nonlinear distortions. However, as the data throughput increases, the heavy computational cost of DNNs impedes them from rapid and power-efficient processing. In this paper, we propose an optical communication signal recovery technology based on a photonic convolutional processor, which is realized by dispersion delay unit and wavelength division multiplexing. Based on the photonic convolutional processor, we implement an optoelectronic convolutional neural network (OECNN) for signal post-equalization and experimentally demonstrate on 16QAM and 32QAM of an optical wireless communication system. With system parameters optimization, we verify that the OECNN can achieve accurate signal recovery where the bit error ratio (BER) is below the 7% forward error correction threshold of 3.8×10-3 at 2Gbps. With adding the OECNN-based nonlinear compensation, compared with only linear compensation, we improve the quality (Q) factor by 3.35 dB at 16QAM and 3.30 dB at 32QAM, which is comparable to that of an electronic neural network. This work proves that the photonic implementation of DNN is promising to provide a fast and power-efficient solution for optical communication signal processing.

A Novel Combined Double Binary Turbo Coding and Color Shift Keying Technique for Flicker Mitigation in Multi Carrier Visible Light Communications

Although radio frequency (RF) systems have been developed at a level that can operate almost within theoretical limits in terms of spectral efficiency due to increasing demands, the RF spectrum allocated for wireless mobile communication has started to fill up rapidly and become insufficient. Visible light communication (VLC), one of the optical wireless communication systems, stands out as a promising technology that can be an alternative to RF technology in some application areas and can help meet the demands by playing a complementary role in some application areas. However, VLC systems face the flickering problem. In this study, it is proposed to combine the double binary Turbo coding (DBTC) technique with color shift keying (CSK) modulation, which does not have a flickering problem, as a solution to the flicker problem of VLC systems. The proposed combined DBTC-CSK method is tested on optical OFDM systems from multi-carrier wireless communication systems. Monte-Carlo simulations are performed to verify the performance of the proposed DBTC-CSK-OFDM system in terms of bit-error-rate (BER) over AWGN channel, single-path optical LOS channel and multi-path optical channel. According to the simulation results it is observed that, DBTC-CSK-OFDM system has better BER performance than that of RS–CC coded CSK-OFDM and un-coded CSK-OFDM systems, providing 5.5 dB and 14 dB SNR gain at multi-path optical channel environments, respectively.

Scintillation index for the optical wave in the vertical oceanic link with anisotropic tilt angle.

The influence of the ocean depth and anisotropic tilt angle on vertical underwater wireless optical communication (UWOC) systems is considered in this study. We propose a power spectrum model of oceanic turbulence with an anisotropic tilt angle for the first time. Thereafter, the expression of the scintillation index is derived for a spherical wave propagating over anisotropic oceanic turbulence in the vertical link. In addition, considering the temperature and salinity, relevant data of the Atlantic and Pacific oceans at different depths are selected to study further the effect of ocean depth on the scintillation index. The results indicate that the scintillation index strongly depends on the ocean depth and anisotropic tilt angle. Moreover, the scintillation index is also related to other parameters, such as temperature and salinity, kinematic viscosity, the anisotropic factor, optical wavelength, and propagation distance. The presented results can be beneficial in designing optical wireless communication systems in the ocean environment.

Theoretical analysis and experimental demonstration of gain switching for a PPM based UWOC system with picosecond pulses.

Shortening pulse width can improve the power efficiency and data rate of a pulse position modulation (PPM) based underwater wireless optical communication (UWOC) system at a fixed average optical power, which is more suitable for the energy-limited underwater environment. As a common method to generate short pulses, gain switching has the advantages of a tunable switching frequency and simple structure, facilitating the generation of high-order PPM signals. However, the output characteristics of electrical gain switching seriously affect the demodulation of PPM signals and limit the data rate. To study the performance of gain switching on a PPM communication system, simulation models of the semiconductor laser diode and the driving circuit are built to describe the generation of electrical and optical pulses. The pulse width, pulse peak value, and peak position of optical pulses are analyzed under different symbol durations and PPM orders. Furthermore, a 64-PPM/150-Mbps UWOC system with a 200-ps optical pulse width is demonstrated by using a gain-switched blue GaN-based laser diode in a water tank. The peak average power ratio (PAPR) is 19.5 dB. Via the statistical analysis of experiment results and the output characteristics of electrical gain switching, the main factor limiting the data rate attributes to the time delay fluctuation of gain switching. To the best of our knowledge, this is the first time that gain switching has been experimentally demonstrated and analyzed in a high-order PPM based UWOC system.

Analysis of aperture averaging effect and communication system performance of wireless optical channels with from weak to strong turbulence in natural turbid water

The aperture averaging technique can alleviate the effects of optical signal scintillation on the performance of underwater wireless optical communication (UWOC) systems. This paper derives a simplified formula that allows for an accurate and efficient analysis of the aperture averaging effect. Based on the OTOPS (oceanic turbulence optical power spectrum) model, the analytical formulas of the scintillation index from weak to strong turbulent channels for the optical signals of Gaussian, spherical and plane waves with the aperture averaging effect, are obtained. Then, the simple approximation expressions are deduced, which are appropriate to evaluate the performances of UOWC systems with the receiving aperture greater than 0.05 m and the link length not larger than 100 m, and simultaneously, they are proved by theoretical analyses and numerical processes.

Multihop Optical Wireless Communication Over ${\mathcal {F}}$-Turbulence Channels and Generalized Pointing Errors With Fog-Induced Fading

Multihop relaying is a potential technique to mitigate channel impairments in optical wireless communications (OWC). In this paper, multiple fixed-gain amplify-and-forward (AF) relays are employed to enhance the OWC performance under the combined effect of atmospheric turbulence, pointing errors, and fog. We consider a long-range OWC link by modeling the atmospheric turbulence by the Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathcal {F}}$</tex-math></inline-formula> distribution, pointing errors by the generalized non-zero boresight model, and random path loss due to fog. We also consider a short-range OWC system by ignoring the impact of atmospheric turbulence. We derive novel upper bounds on the probability density function (PDF) and cumulative distribution function (CDF) of the end-to-end signal-to-noise ratio (SNR) for both short and long-range multihop OWC systems by developing exact statistical results for a single-hop OWC system under the combined effect of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathcal {F}}$</tex-math></inline-formula> -turbulence channels, non-zero boresight pointing errors, and fog-induced fading. Based on these expressions, we present analytical expressions of outage probability (OP) and average bit-error-rate (ABER) performance for the considered OWC systems involving single-variate Fox's H and Meijer's G functions. Moreover, asymptotic expressions of the outage probability in high SNR region are developed using simpler Gamma functions to provide insights on the effect of channel and system parameters. The derived analytical expressions are validated through Monte-Carlo simulations, and the scaling of the OWC performance with the number of relay nodes is demonstrated with a comparison to the single-hop transmission.