Free Space Optical Communication Communication Research Articles

Atmospheric turbulence recognition with deep learning models for sinusoidal hyperbolic hollow Gaussian beams-based free-space optical communication links

The integration of artificial intelligence technology to improve the performance of free-space optical communication (FSO) systems has received increasing interest. This study aims to propose a novel approach based on deep learning techniques for detecting turbulence-induced distortion levels in FSO communication links. The deep learning-based models improved and fine-tuned in this work are trained using a dataset containing the intensity profiles of Sinusoidal hyperbolic hollow Gaussian beams (ShHGBs). The intensity profiles included in the dataset are the ones of ShHGBs propagating for 6 km under the influence of six different atmospheric turbulence strengths. This study presents deep learning-based Resnet-50, EfficientNet, MobileNetV2, DenseNet121 and Improved+MobileNetV2 approaches for turbulence-induced disturbance detection and experimental evaluation results. In order to compare the experimental results, an evaluation is made by considering the accuracy, precision, recall, and f1-score criteria. As a result of the experimental evaluation, the average values for accuracy, precision, recall and F-score with the best performance of the improved method are given; average accuracy 0.8919, average precision 0.8933, average recall 0.8955 and average F-score 0.8944. The obtained results have immense potential to address the challenges associated with the turbulence effects on the performance of FSO systems.

Capacity Enhancement Analysis of an OAM-OFDM-SMM Multiplexed Free Space Communication System in Atmospheric Turbulence

To overcome atmospheric turbulence (AT) distortion during signal propagation through the optical link, orbital angular momentum (OAM) mode states employing multiple inputs and multiple outputs (MIMO) techniques have recently gained prominence in free space optical communication (FSO). As the various OAM modes propagate through the free space optical link, signal attenuation and crosstalk may occur, reducing system capacity and increasing the likelihood of bit errors. In this work, our objective is to propose a spectrally efficient, high-speed and channel capacity efficient crosstalk FSO communication system by combining the features of orthogonal frequency division multiplexing (OFDM), spatial mode multiplexing (SMM), and a mode diversity scheme into an existing OAM-FSO communication system. The incorporation of the OFDM-MIMO concept and spatial mode diversity into the existing OAM-MIMO-FSO system is extremely beneficial in enhancing the transmission capacity, mitigating multipath fading and atmospheric turbulence distortions. The Gamma–Gamma (GG) model is used to assess the performance of the proposed system under various atmospheric turbulence conditions in terms of the performance metrics such as BER vs. number of OAM states for different refractive index structure and Rytov constants, link distance, and an optical signal to noise ratio (OSNR). A FEC limit of 3.8 × 10−3 and a maximum link distance of 2 km are set to evaluate these performance parameters. Finally, the transmission capacity of the proposed system is compared to that of the existing MIMO and OAM-SMM-MIMO systems for different OSNR values under atmospheric turbulence conditions for the OAM state of l = +1, yielding an overall improvement of 3.3 bits/s/Hz compared to conventional MIMO systems and 1.6 bits/s/Hz for the OAM-SMM-MIMO system.

Four-access, 80 mm aperture all phase-controlled liquid crystal laser antenna

High speed, wide range, and high precise beam steering technology are fundamental to free-space optical communication (FSO). Liquid crystal optical phased array (LC-OPA) is capable of achieving non-mechanical beam steering. This paper demonstrates a parallel, four-communication access beam deflection antenna based on LC-OPA, providing a large aperture, wide steering range, and high pointing precision. This antenna is composed of a coarse tracking subsystem using cascaded liquid crystal polarized grating (LCPG) and a fine tracking subsystem based on LC-OPA. The optical aperture is 80 mm × 80 mm, arranged in 2 × 2 subapertures, each serving a single wavelength beam. Besides, a cascaded control platform based on field programmable gate array is proposed to realize low-latency response. This optical antenna achieves a steering resolution of 20 μrad and covers a range of ±12 deg in two dimensions, laying the foundation for simultaneous access of multiple terminals in FSO communication network.

Research on spatial multiplexing using BGSM beam in FSO communication

We propose a spatial division multiplexing scheme based on the Bessel–Gaussian​ Schell-mode beam. Sucha communication scheme can enhance the capacity of free-space optical communication (FSO) in turbulence. This multiplexing utilizes the relationship between the characteristics of coherence degree distribution and the distant facula, realizing the separation of several BGSM beams with different Bessel parameter from each other, which makes the multiplexing technique deploying coherence modulation possible. The advantage of this scheme is its resistance to the atmospheric influence which exists widely in the optical wireless communication’s application environment. According to the simulation, for the transmission length of about 1300 m, under the weak turbulence environment (Cn2=10−16 m2/3), the BGSM-SDM scheme can support about 5 times more capacity compared with the traditional optical wireless communication scheme with Gaussian beam, and while the turbulence becomes strong (Cn2=10−13 m2/3), the SDM scheme degenerates to single channel because of the beam expansion, but it can still support more than 2 times more capacity. The scheme is proved work stable for each channel, and applicable in strong turbulent environment by measuring the signal and noise in the experiment. I believe it provides a promising choice for the future 6G ultrahigh-speed wireless communication.

Effects of Atmospheric Turbulence on Optical Wireless Communication in NEOM Smart City

The foundation of any smart city requires an innovative and robust communication infrastructure. Many research communities envision free-space optical communication (FSO) as a promising backbone technology for the services and applications provided by such cities. However, the channel through which the FSO signal travels is the atmosphere. Therefore, the FSO performance is limited by the local weather conditions. The variation in meteorological variables leads to variations of the refractive index along the transmission path. These index inhomogeneities (i.e., atmospheric turbulence) can significantly degrade the performance of FSO systems. Thus, a practical implementation of the FSO link must carefully consider the atmospheric turbulence effect. This paper aims to investigate the feasibility of FSO communication for NEOM, a promising smart city in Saudi Arabia. We study the effect of weather conditions on FSO links using the micrometeorology model, taking into account actual weather data. The FSO performance in winter and summer was compared in terms of the bit error rate, signal-to-noise ratio (SNR), link availability, and transmission distance. The study shows that the atmospheric turbulence strength is moderate and strong in winter and summer, respectively. The temperature has the biggest impact on the FSO system when compared to the other meteorological elements included in this study. Furthermore, at transmission distances less than 300 m, atmospheric turbulence does not significantly affect the FSO for the operating wavelength of 1550 nm. Furthermore, it has been shown that at transmission distances greater than 300 m, the SNR in summer is more than 18% higher than in winter. The findings of this research enable understanding of the effect of turbulence caused by NEOM weather on the FSO link, thus assisting engineers in establishing a reliable FSO backbone link by adjusting the relevant parameters.

Improved Performance in the Detection of ACO-OFDM Modulated Signals Using Deep Learning Modules

Free space optical communication (FSO) is widely deployed to transmit high data rates for rapid communication traffic increase. Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation is a very efficient FSO communication technique in terms of transmitted optical power. However, its performance is limited by atmospheric turbulence. When the channel includes strong turbulence or is non-deterministic, the bit error rate (BER) increases. To reach optimal performance, the ACO-OFDM decoder needs to know accurate channel state information (CSI). We propose novel detection using different deep learning (DL) algorithms. Our DL models are compared with minimum mean square error (MMSE) detection methods in different turbulent channels and improve performance especially for non-stationary and non-deterministic channels. Our models yield performance very close to that of the MMSE estimator when the channel is characterized by weak or strong turbulence and is stationary. However, when the channel is non-stationary and variable, our DL model succeeds in improving the performance of the system and decreasing the signal to noise ratio (SNR) by more than 8 dB compared to that of the MMSE estimator, and it succeeds in recovering the received data without needing to know accurate CSI. Our DL decoders also show notable speed and energy efficiency improvement.

Estimating the Performance of Free Space Optical Communication in Rain Weather Conditions Using Various Models and Modified Duobinary Return to Zero Technique

Free space optical communication (FSO) system has remarkably gained importance of late due to the various advantages associated with it. However, before deployment of FSO communication system, an understanding of the laser beam scintillation due to the atmospheric as well as other weather-induced losses need to be understood. This paper mainly focuses on simulative performance analysis of an FSO system under rain conditions using Optisystems commercial software and rainfall data. Thus, the available models which are the Suriza, Carbonnea, Japan and Samir models were used to determine the rainfall-induced attenuation for a given rainfall rate. Unlike in the previous articles that only used on off keying as the modulation scheme at the transmitter, we have used modified duobinary return to zero (MDRZ) modulation. Further, to improve the MDRZ-based FSO performance in rain conditions, we have used the single-input multiple-output (SIMO) concept with equal gain combining (EGC) at the receiver and again proved that the improvement in BER, Q factor and received optical power suggests that MDRZ-based SIMO FSO communication results in a remarkable improvement.

Investigation on Hollow Beam Propagation through Turbulence Conditions in Free Space Optical Communication

In Free Space Optical Communication (FSO), the optical signal from the laser source severely affects while travelling through free space atmospheric channel due to scattering, absorption and other effects of atmospheric turbulence conditions. This degrades the performance of FSO communication. In this article, we have generated the hollow beam from the laser output by the inverse axicon lens called inverted axicon beam. An artificial controlled turbulence chamber is created and the test signal has been transmitted through the turbulence conditions by employing various modulation schemes such as PAM, PWM, PPM, ASK, BPSK and QPSK separately. In all cases, we measured parameters phase jitter and Time jitter experimentally and the results are compared. Two types of wavelength laser sources such as red and green lasers are used for the analysis. In which, ASK provides a better withstand ability to jitter than other modulation techniques with atmospheric turbulence.

Performance Evaluation of System in Free Space Optic Utilizing Gaussian Optical Filter in Different Detection Scheme

Abstract In recent years, free space optical communication (FSO) has become a leader for its unique characteristics: large bandwidth, unlicensed spectrum, simple implementation, low power and high data rate. However, we use as a transmission medium for Spectral Amplitude Coding-Optical Code Division Multiple Access SAC OCDMA system. In this paper, we investigate the optimum received power of FSO communication system employing SAC OCDMA, by using different detection technique, such us Spectral Direct Detection SDD and Single Photodiode Detection (SPD) technique under optical Gaussian filter decoder schemes with Modified Double Weight code (MDW). In this work, the adverse effects of atmospheric channel limit the possibility of a large FSO communication, moderate turbulence and hazy weather conditions are considered. The results show that the performance of the proposed system with wavelength-division-multiplexing (WDM) multiplexer (MUX) based on Gaussian optical filter with SDP detection fares better than the system employing SDD technique.

Free-space optical channel characterization in a coastal environment

Recently, FSO (Free-Space Optical Communication) has received a lot of attention thanks to its high data-rate transmission via unbounded unlicensed bandwidth. However, some weather conditions lead to significant degradation of the FSO link performance. Based on this context and in order to have a better understanding of the capabilities of FSO communication in a coastal environment, the effects of temperature and humidity on an FSO system are investigated in this study. An experiment is conducted using an open source FSO system that achieves a transmission rate of 1 Gbit/s at a distance of 70 m. Two new mathematical models are proposed to represent the effects of temperature and humidity on our developed FSO system operating at a wavelength of 1 550 nm. The first model links the FSO attenuation coeffcient to the air temperature in coastal regions, while the second model links the FSO attenuation coeffcient to the humidity and the dew-point temperature. The key finding of this study is that FSO links can achieve maximum availability in a coastal city with normal variations in temperature and humidity.

Spatial-Mode Diversity and Multiplexing for FSO Communication With Direct Detection

This work investigates spatial-mode multiplexing (SMM) for practical free-space optical communication (FSO) systems using direct detection. Unlike several works in the literature where mutually incoherent channels are assumed, we consider mutually coherent channels that accurately describe SMM FSO systems employing a single laser source at the transmitter with a narrow linewidth. We develop an analytical model for such mutually coherent channels and derive expressions for aggregate achievable rate (AAR). Through numerical simulations, it was shown that there exist optimal transmit mode sets which result in the maximal asymptotic AAR at high transmitted power. Moreover, in order to resolve the reliability issues of such SMM FSO systems in the presence of turbulence, a so-called mode diversity scheme is proposed that can be easily implemented along with SMM FSO systems. It is demonstrated that mode diversity can significantly improve the outage probability and the outage achievable rate performance of the multiplexed channels in SMM FSO systems degraded by turbulence.

SER performance analysis of MPPM FSO system with three decision thresholds over exponentiated Weibull fading channels

In this work, the symbol error rate (SER) performance of the multiple pulse position modulation (MPPM) based free-space optical communication (FSO) system with three different decision thresholds, fixed decision threshold (FDT), optimized decision threshold (ODT) and dynamic decision threshold (DDT) over exponentiated Weibull (EW) fading channels has been investigated in detail. The effects of aperture averaging on each decision threshold under weak-to-strong turbulence conditions are further studied and compared. The closed-form SER expressions for three thresholds derived with the help of generalized Gauss–Laguerre quadrature rule are verified by the Monte Carlo simulations. This work is helpful for the design of receivers for FSO communication systems.

BER Analysis for BPSK Based SIM–FSO Communication System Over Strong Atmospheric Turbulence with Spatial Diversity and Pointing Errors

Free space optical communication (FSO) has much attention in recent years for the applications viz. inter-satellite, deep space communications, inter and intra chip communications. The performance of FSO systems sternly suffers from atmospheric turbulence due to the random nature of weather conditions. Spatial diversity is an emerging technique for improving the performance of the system over strong atmospheric turbulences. In this paper, the error rate performance of binary phase shift keying based subcarrier intensity modulated free space optical (SIM---FSO) communication system over gamma---gamma channel with pointing errors is investigated. Novel closed-form analytical expressions are derived for the average bit error rate of single-input multiple-output FSO (SIMO---FSO) system with various combining schemes. The error rate performance of SISO and SIMO---FSO systems are compared in terms of 2D and 3D plots.