Free-space Optical Communication Systems Research Articles

Adaptive-optics-based turbulence mitigation in a 400 Gbit/s free-space optical link by multiplexing Laguerre–Gaussian modes varying both radial and azimuthal spatial indices

In general, atmospheric turbulence can degrade the performance of free-space optical (FSO) communication systems by coupling light from one spatial mode to other modes. In this Letter, we experimentally demonstrate a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) FSO mode-division-multiplexing (MDM) coherent communication link through emulated turbulence using four Laguerre Gaussian (LG) modes with different radial and azimuthal indices (LG10, LG11, LG−10, and LG−11). To mitigate turbulence-induced channel cross talk and power loss, we implement an adaptive optics (AO) system at the receiver end. A Gaussian beam at a slightly different wavelength is co-propagated with the data beams as the probe beam. We use a wavefront sensor (WFS) to measure the wavefront distortion of this probe beam, and this information is used to tune a spatial light modulator (SLM) to adaptively correct the four distorted data-beam wavefronts. Using this adaptive-optics approach, the power loss and cross talk are reduced by ∼10 and ∼18 dB, respectively. © 2023 Optical Society of America

Intelligent-reflecting-surfaces-assisted hybrid FSO/RF communication with diversity combining: a performance analysis

Free space optics (FSO) and radio frequency (RF) communication systems exhibit complementary characteristics, with FSO being susceptible to fog, turbulence, and pointing errors, whereas RF is susceptible to rain and small scale fading. These inherent complementary characteristics between FSO and RF communication systems enable the hybrid configuration to effectively alleviate the impact of environmental impediments. This paper presents the performance analysis of an intelligent reflecting surfaces (IRSs)-assisted hybrid FSO/RF system under atmospheric turbulence (AT), pointing errors (PE), small scale fading effects, and attenuation due to climatic conditions such as fog, rain, etc. To characterize the AT, gamma–gamma distribution is used, and the multipath fading in the RF link is modeled using Nakagami-m distribution. At the receiver, signals from both the FSO and RF links are combined using either the selection combining (SC) or maximal-ratio combining (MRC) technique. We derive the exact closed-form expressions for outage probability and average symbol error rate (ASER) for both diversity combining schemes. The proposed system is compared with the direct link (DL) FSO system, IRSs-assisted FSO system, and hybrid FSO/RF system with diversity combining schemes. Further, the performance comparison between the SC and MRC schemes is also reported. Finally, the accuracy of the analytical expressions is verified by utilizing Monte Carlo simulations.

Improving the performance of free space optical systems: a space-time orthogonal frequency division modulation approach

Free space optical (FSO) communication systems are known for high capacity and information security. The overall system performances of FSO systems are however significantly affected by atmospheric turbulence induced fading. This paper, therefore, proposes a technique to mitigate this effect through the introduction of an additional degree of error correction capacity by exploiting the spectral dimension in the coding space. A space-time trellis coded orthogonal frequency division modulation (OFDM) scheme was developed, simulated and evaluated for optical communication through a Gamma-Gamma channel. The evaluation of the coding gain obtained from the simulation results, the mathematical analysis and the truncation error analysis shows that the proposed technique is a promising and viable technique for improving the error correction performance of space-time codes for free space optical communication links.

Signal quality enhanced and multicast transmission via cylindrical vector beams with adaptive optics compensation in free space optical communications

Signal quality is critical in free space optical (FSO) communications. Many technologies and methods are applied to enhance the signal quality of an optical communication system, which might require one of the methods employing structured light. We introduce structured light of cylindrical vector beams with adaptive optics compensation to improve signal quality in FSO communications. The multiplexing/demultiplexing of coaxial cylindrical vector beams is used to increase the capacity of the communication channel. Adaptive optics compensation and cylindrical vector beams are applied to mitigate influence of the turbulent atmospheric disturbances and improve the signal quality. An optical vortex grating is employed to demultiplex signal beams and then send the signals to a receiver plane to measure the bit-error-rate. Consequently, the signal quality of the cylindrical vector beams multiplexing/demultiplexing in FSO communication systems improves.

Alignment and performance analysis of free-space optical communications for broadband video transmission.

We describe a free-space optical communications system operating with an externally modulated infrared carrier signal at 1550nm. The purpose of the system is to explore an alternative to radio-frequency wireless communications, which are subject to spectral congestion and bandwidth limitations. We provide details of the optical alignment procedure and a means for extrapolating the obtained results to larger transmission distances. To illustrate the versatility of the system, a relatively wideband signal was chosen for transmission: an NTSC analog video signal, whose instantaneous bandwidth was 6MHz. We describe the performance of the system by examining its output image quality and signal-to-noise ratio.

High-sensitivity few-mode heterodyne receiver with a few-mode optical fiber amplifier for turbulence resistance in free space optical communication

In the field of free-space optical communication (FSOC), the sensitivity of the receiver can be affected by atmospheric turbulence, leading to light-intensity scintillation or beam drift. This paper offers a solution to this challenge by proposing a few-mode heterodyne receiver with a few-mode optical fiber amplifier (FM-OFA). The FM-OFA is used in the few-mode heterodyne receiver to enhance the output power of different modes of signal light, and the amplified signal light is detected by the few-mode heterodyne detection based on Kramers–Kronig (KK) relations, which effectively suppresses the atmospheric turbulence effect and improves the sensitivity of the receiver. In addition, we build an FSOC system and verify the sensitivity performance of this receiver under different turbulence conditions. The experimental results show that the sensitivities of the few-mode heterodyne receiver with an FM-OFA are −42.6dBm, −41.3dBm, and −40.7dBm, respectively, which are 8.8 dB, 8.5 dB, and 9.3 dB higher than those of the few-mode heterodyne receiver without an FM-OFA at average BER = 10−3 under low, moderate, and high turbulence intensities.

Performance Analysis of Multi-Hop FSOC over Gamma-Gamma Turbulence and Random Fog with Generalized Pointing Errors

The multi-hop amplify-and-forward free-space optical communication (FSOC) system is studied in random fog using the I-function, considering Gamma-Gamma atmospheric turbulence and Beckmann pointing error. Outage probability, average bit error rate and average ergodic channel capacity are obtained. Channel-state-information assisted relay performs better than fixed-gain relay under high transmitted power. Increasing the hop number significantly improves the performance. More hops are needed in medium fog than in light fog to achieve the same performance. In addition, on a single-hop link, the influence of fog channel on system performance is dominant, while atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have little effect on the system performance. However, on a multi-hop link, atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have serious effects on system performance. Compared with correcting the normalized boresight error, compensating the normalized jitter standard deviation greatly improves the multi-hop FSOC system performance. Furthermore, optimizing beam width can further improves the performance. To ensure good communication, the system should select a low-order modulation scheme.

UAV-assisted FSO communication system with amplify-and-forward protocol under AOA fluctuations: A performance analysis

Free space optical (FSO) communication has recently aroused great interest in academia due to its unique features, such as large transmission band, high data rates, and strong anti-electromagnetic interference. With the aim of evaluating the performance of an FSO communication system and extending the line-of-sight transmission distance, we propose an unmanned aerial vehicle (UAV)-assisted dual-hop FSO communication system equipped with amplify-and-forward protocol at the relay node. Specifically, we consider impairments of atmospheric absorption, pointing errors, atmospheric turbulence, and link interruptions due to angle-of-arrival fluctuations in the relay system. The Gamma-Gamma and Malaga distributions are used to model the influence of atmospheric turbulence on the source-to-UAV and UAV-to-destination links, respectively. We derive closed-form expressions of the probability density function (PDF) and cumulative distribution function (CDF) for the proposed communication system, in terms of the Meijer-G function. Based on the precise PDF and CDF, analytical expressions for the outage probability, average bit error rate, and ergodic capacity are proposed with the aid of the extended generalized bivariate Fox's H function. Finally, we show that there is a match between the analytical results and numerical results, and we analyze the influence of the system and channel parameters on the performance.

A comprehensive survey of free-space optical communication – modulation schemes, advantages, challenges and mitigations

Abstract The demand for large bandwidth and high data rates in communication systems has become the main cause of the upgrade of traditional networks into free space optical (FSO) technology. FSO technology has gained significant popularity due to its easy deployment, high data rates, abundant bandwidth, enhanced security, and license-free spectrum utilization. However, the performance of FSO communication systems can be affected by certain limiting factors, such as changes in weather conditions during data transmission. To overcome these challenges and improve FSO performance, various modulation techniques are employed. This article presents a concise overview of the FSO communication system, highlighting different modulation techniques used to enhance its performance, as well as discussing its advantages, applications, and existing challenges. Some advanced modulation formats which are recently introduced in the field of FSO communication such as QPSK, DP-QPSK, QAM, and OFDM are also made part of this paper.

Li-Fi technology based long range free-space communication data transmit system evaluation

AbstractThe most flexible and reliable technological system is Wi-Fi, which is made possible by a wireless connection that transmits data using radio frequencies. Wi-Fi networks, however, encounter numerous issues related to power supply, availability, efficiency, and security as a result of the various access points. While relational waves describe the medical device, Wi-Fi radios produce radio waves that are very dangerous for patients. This document offers line-of-sight communication between the transmitter and receiver using LED technology. Li-Fi technology is a method that transmits audio data using LED light, which is faster and more efficient than Wi-Fi. Since it is practically ubiquitous, light can be used for communication as well. A cutting-edge technology called optical communication includes a subset called light fidelity. By sending out visible light, the Li-Fi device enables wireless intranet communication. This paper is an in-depth study and analysis of Light Fidelity (Li-Fi), a novel technology that transmits data at high speeds over a wide spectrum by using light as a medium of transmission. The research fields that are pertinent to Li-Fi networks are thoroughly analyzed and categorized in this paper: high speed data transmission, receiving, sharing, broadcasting through light in free space optical communication system by Li-Fi technology. In this paper, we followed some methods and developed a unique method to develop this study: VLC, OOK, a Lambertian discharge mechanism, LOS, NLOS, or a CMOS optical receiver. The proposed model tested transmits and receives audio, video, and other data, which is very high-rated and near the 2 GB/s range.

Study of Nonlinearities in FSO Communication System: A Detailed Review

In the world of today where growing demand for a fast, reliable and secured communication system for the growing population of the world is in dire need, the Free Space Optics (FSO) provides a very good alternative to the conventional RF communication. When compared in terms of free space communication it is better than RF communications in some aspects. Also in regard to optical mode of communication, it is superior to fiber optics when it comes to surpassing the geographical installation related issues and being the “last mile” solution to the impeding problem of providing access of communication link to remote areas across the world. FSO also offers a good access for local operation usage in civil and military areas. Considering such wide usage and promising future applications of FSO technology, this paper brings out various challenges presently faced in the usage of FSO communication system. In the present paper study of non linearities faced and a brief discussion regarding possible mitigation techniques has been carried out.

On the error performance and channel capacity of a uniquely decodable coded FSO system over Malaga turbulence with pointing errors.

Our previous work has proved that the uniquely decodable code (UDC) has the ability of enhancing the throughput of a free space optical communication (FSO) system. This paper quantitatively analyzes the error performance and channel capacity of the UDC-FSO system under Malaga turbulence and pointing errors. We first propose the minimum distance of the superimposed patterns (MDSP) approximation to reveal the universal symbol error rate (SER) for UDC-FSO systems. A closed form expression of SER is further deduced for a special case of 2 TXs. Based on the deduced SER, the upper and lower bounds of bit error rate (BER) can be obtained. Additionally, the discrete channel capacity of the UDC-FSO system is defined and deduced according to different superposition patterns, as well as the channel capacity gain. Both simulation and experiment verify the accuracy of the MDSP and SER's expressions. It's also discovered that the channel capacity of the UDC-FSO system is superior to the conventional end-to-end (E2E) link, where maximal channel capacity is limited by the UDC codebooks.

GAN Based Channel Estimation and Interference Cancellation for Mixed RF/FSO Cooperative Relaying Systems

Free space optical (FSO) communication systems have been considered as a valid option to be integrated with the existing hardware of radio frequency (RF) systems in cooperative relaying technique called the dual hop RF/FSO asymmetric relay. However, the advantages of cooperative relaying is strongly hampered due to the presence of interference in practical systems. Moreover, the modelling of FSO systems need proper attention due to complex nature of the atmospheric turbulence, pointing errors and beam wander which involves tedious mathematical computations. The present research work focuses on the capability of deep learning models for developing a framework for enhancing the performance of the mixed RF/FSO relaying systems using interference cancellation. Specifically, generative adversarial networks (GANs) have been trained to represent the channel conditions of the co-channel interferer (CCI) and FSO links. The GAN model consists of two competing deep learning models — the generator and the discriminator. Two separate deep learning GAN models namely: CCI-GAN and FSO-GAN have been trained for estimating the channel state information (CSI) for the interferer and FSO links, respectively. The CCI-GAN model has been trained offline and tested with mean square error (MSE) of −53.01 dB and root mean square error (RMSE) of −11.003 dB. On the other hand, the FSO-GAN model has been trained with error metrics of MSE of -80.14 dB and RMSE of −11.12 dB. The trained models have been utilized to summarize the performance of the decode-and-forward (DF) mixed RF/FSO relaying network by analysing the outage probability, bit error rate (BER) and the ergodic capacity of the system. The obtained results demonstrate that the FSO-GAN generates the samples from the probability density function (PDF) representing the FSO link. More importantly, the interference cancellation mechanism adopted using the CCI-GAN enhances the system performance.

Bidirectional Atmospheric Channel Reciprocity-Based Adaptive Power Transmission

In atmosphere free-space optical communication (FSO) systems, the scintillation effect produced by turbulence effects increases the bit error rate (BER) of the communication system and reduces the system’s performance. However, a high correlation of turbulent noise occurs in the two transmission channels when a signal transmitted in the bidirectional atmospheric channel with channel reciprocity. The performance of the FSO system can be increased by extracting channel state information (CSI) in forward transmission and using adaptive power technology to reduce turbulence in inverse transmission. In this research, we propose a bidirectional atmospheric channel reciprocity-based adaptive power transmission (CR-APT) technique that lowers the bit error rate of the transmitted signal by using the CSI of the relevant channel. To verify the effectiveness of the technique, a bidirectional atmospheric channel with various turbulence intensities is built in the simulation program, along with various background sounds to vary the channel reciprocity, and the impact of reciprocity on signal transmission is examined. The simulation findings demonstrate that adaptive power transmission with high reciprocity is excellent under the weak turbulence condition, and its future development is promising.

Integrated spatial light receivers based on inverse design.

Photonic integrated spatial light receivers play a crucial role in free space optical (FSO) communication systems. In this paper, we propose a 4-channel and 6-channel spatial light receiver based on a silicon-on-insulator (SOI) using an inverse design method, respectively. The 4-channel receiver has a square receiving area of 4.4 µm × 4.4 µm, which enables receiving four Hermite-Gaussian modes (HG00, HG01, HG10, and HG02) and converting them into fundamental transverse electric (TE00) modes with insertion losses (ILs) within 1.6∼2.1 dB and mean cross talks (MCTs) less than -16 dB, at a wavelength of 1550 nm. The 3 dB bandwidths of the four HG modes range from 28 nm to 46 nm. Moreover, we explore the impact of fabrication errors, including under/over etching and oxide thickness errors, on the performance of the designed device. Simulation results show that the 4-channel receiver is robust against fabrication errors. The designed 6-channel receiver, featuring a regular hexagon receiving area, is capable of receiving six modes (HG00, HG01, HG10, HG02, HG20, and HG11) with ILs within 2.3∼4.1 dB and MCTs less than -15 dB, at a wavelength of 1550 nm. Additionally, the receiver offers a minimum optical bandwidth of 26 nm.

A Free-Space Optical Communication System Based on Bipolar Complementary Pulse Width Modulation.

In this work, we propose a bipolar complementary pulse width modulation strategy based on the differential signaling system, and the modulation-demodulation methods are introduced in detail. The proposed modulation-demodulation strategy can effectively identify each symbol's start and end time so that the transmitter and receiver can maintain correct bit synchronization. The system with differential signaling has the advantages of not requiring channel state information and reducing background radiation. To further reduce the noise in the system, a multi-bandpass spectrum noise reduction method is proposed according to the spectrum characteristics of the received modulation signals. The proposed modulation method has an error bit rate of 10-5 at a signal-to-noise ratio of 7 dB. The fabricated optical communication system can stably transfer voice and text over a distance of 5.6 km.

Error performance analysis for chaotic pulse width position modulation in free space optical communication system

Abstract In this paper, a pulse time modulation method called chaotic pulse width position modulation (CPWPM) is studied for free space optical communication system. This is a digital modulation technique that combines pulse position modulation and pulse width modulation with chaos. In this scheme, the binary information is carried by both the position and width of the pulses, which are modulated by a chaotic signal. Hence, on a single pulse, two bits of information are encoded, which results in higher bit rates, better privacy, and larger effective bandwidth as compared to chaotic pulse position modulation (CPPM) and chaotic pulse width modulation (CPWM) techniques. Probability of error is investigated for different FSO scenarios, i.e., varying turbulence conditions and link lengths for CPWPM scheme. Finally, the performance of CPWPM-FSO system is compared with a commonly used differential chaos shift keying (DCSK) scheme. CPWPM-FSO system shows comparable results as DCSK-FSO system (for probability of error, P e = 10−4 average signal-to-noise ratio (SNR) required for DCSK and CPWPM scheme is 40 dB and 41 dB, respectively, for same length and system conditions), while data rate is doubled in latter, and this verifies the validity of using CPWPM in FSO communication system.

Hybrid FSO/RF system over proposed random dust attenuation model based on real-time data combined with G–G atmospheric turbulence

Free-space optical (FSO) communication links are thought to be most negatively impacted by fog and sandstorms. Several probabilistic models for signal attenuation in various types of fog have been proposed in the literature. To the best of our knowledge, there is no prior probabilistic signal attenuation model for dusty channels. As a result, in this study, we analyse the probabilistic behaviour of the FSO channel during sandstorms using real-time data from the Egyptian Meteorological Authority and propose the negative-exponential (NE) distribution as a novel statistical model for signal attenuation in dust. A probability distribution function (PDF) is then derived for the random dust channel state. Also, a closed-form channel state distributional formulation is derived for a stochastic FSO channel model that considers both dust effects and Gamma–Gamma (G–G) atmospheric turbulence-induced fading. Next, closed formulas are derived for both the average bit-error rate (BER) and outage probability for the FSO system operating in this channel environment. Using the forms derived, it can be shown that the dust storm has a significant impact on the system’s performance. Finally, to overcome this degradation, we suggest utilizing a hybrid FSO and radio frequency (RF) wireless communication system. The received signals are combined using the well-known diversity selection combining (SC) technique. Further, the hybrid system’s performance was compared to that of an FSO-only system.

Performance Analysis and Prediction for a Free-Space Optical Communication System under Foggy Absorption

Performance Analysis and Prediction for a Free-Space Optical Communication System under Foggy Absorption

Hybrid channel coding for OAM division multiplexing free space optical communication systems.

An orbital angular momentum division multiplexing free space optical communication (OAM-DM FSO) system with channel coding can compensate atmospheric channel fading and improve system performance. An OAM-DM FSO system based on hybrid channel coding is proposed in this paper. The coding gain is improved by taking into account mode dependent channel fading difference caused by atmospheric turbulence. Simulation results show that compared with single channel coding, the coding gain is increased by 1.85 dB under Cn2=1E-14, BER=1E-5 with non-uniform LDPC code (0.7 code rate) for an OAM1/OAM3 multiplexing system. In addition, for four OAM modes (+1,+3,+5,+7) multiplexing systems, the coding gain is increased by more than 3.8 dB under Cn2=1E-14 and BER=1E-5.