Free-space Optical Communication Links Research Articles

All-in-One BPSK/QPSK Switchable Transmission and Reception for Adaptive Free-Space Optical Communication Links

Adaptive free-space optical (FSO) communication links have been extensively studied in order to adapt to variable atmospheric channel environments due to factors such as atmospheric turbulence. As a supporting technology, an all-in-one BPSK/QPSK switchable transmission and reception method is proposed and experimentally demonstrated for adaptive modulation format switching in FSO links. The transmission and reception of both modulation formats are realized based on the same IQ modulator and single-photodetector coherent receiver. Simulation and experimental results show that the QPSK signal has a power penalty of about 3–4 dB compared to the BPSK signal with a BER of about 1 × 10−3. The basis for format switching is given according to the various atmospheric channel conditions. The proposed method provides a flexible and efficient solution for variable FSO communication environments to improve their performance.

Maritime free space optical communications field test and link budget statistics

During sub-optimal weather, a free-space optical (FSO) link range degrades depending on attenuation (atmospheric extinction) and turbulence effects. The ability to predict the system level performance can be exceedingly challenging as the atmospheric variability in a maritime link can be large and difficult to model. Link budget estimation for FSO systems often takes a nominal view of atmospheric conditions; here, we use statistical atmospheric predictions specific to a geographic area of interest to enable performance trades to be evaluated through link budget analysis. We compare these models to field-collected data to show the utility of the statistical atmospheric analysis in predicting FSO link performance for specific parts of the world. We have performed shore-to-ship FSO communications field tests at 10 Gb/s with links reaching out to a horizon limit over 40 km away in times of moderate extinction to clear weather. We provide further analysis by describing the expected performance of the link using statistical probabilities via cumulative distribution functions of both extinction and turbulence. The atmospheric variability can be determined for nearly any region of interest through the implementation of numerical weather prediction data to calculate the atmospheric performance drivers. These conditions are specifically evaluated for the 2017 Trident Warrior field test off the coast of San Diego, California, USA.

Entanglement-Based CV-QKD with Information Reconciliation over Entanglement-Assisted Link.

An entanglement-based continuous variable (CV) QKD scheme is proposed, performing information reconciliation over an entanglement-assisted link. The same entanglement generation source is used in both raw key transmission and information reconciliation. The entanglement generation source employs only low-cost devices operated in the C-band. The proposed CV-QKD scheme with information reconciliation over an entanglement-assisted link significantly outperforms the corresponding CV-QKD scheme with information reconciliation over an authenticated public channel. It also outperforms the CV-QKD scheme in which a classical free-space optical communication link is used to perform information reconciliation. An experimental demonstration over the free-space optical testbed established at the University of Arizona campus indicates that the proposed CV-QKD can operate in strong turbulence regimes. To improve the secret key rate performance further, adaptive optics is used.

Coding techniques for diversity enhancement of dense wavelength division multiplexing MIMO‐FSO fault protection protocols systems over atmospheric turbulence channels

AbstractAn enhanced transmission is presented in a multiple‐input‐multiple‐output (MIMO) dense‐wavelength division multiplexed (DWDM) free‐space‐optical (FSO) communication link using diversity coding techniques under the effect of turbulent weather phenomenon. The findings show good performance with an (8 channels × 2.5 Gbps data rate/channel) 20 Gbps 1500 m transmission distance. The bit‐error‐rate (BER), outage probability (OP), and signal‐to‐noise ratio (SNR) of the diversity combining techniques using maximum‐ratio combining (MRC), selection combining (SC), and equal‐gain combining (EGC) technique are evaluated in this work. The obtained results illustrate that Alamouti, space‐time coding (STC), space‐time block coding (STBC), space‐time trellis code (STTC), orthogonal STBC (O‐STBC), and quasi‐orthogonal STBC (QO‐STBC) on the minimum mean‐square‐error, and MRC are worth implementing on the DWDM‐FSO wireless communication systems. The mitigation of atmospheric turbulence is achieved using MIMO diversity combining techniques coding. The simulation results for diversity coding techniques using QO‐STBC/STTC and SC/MRC in the MIMO‐DWDM FSO communication system can improve BER performance, OP, and SNR. The MRC exhibits the lowest OP and BER when compared with the SC and EGC. The numerical results demonstrate that the FSO communication link using DWDM QO‐STBC/STTC improves the power penalty at both BER values under varying atmospheric turbulence conditions for ST, MT, and WT, in comparison to FSO systems without DWDM QO‐STBC/STTC diversity coding techniques.

Average symbol error rate analysis of reconfigurable intelligent surfaces based free-space optical link over Weibull distribution channels

Optical wireless communication (OWC) enables wireless connectivity using ultraviolet bands, infrared or visible. With its advantages features as high bandwidth, low cost, and operation in an unregulated spectrum. Free-space optical (FSO) communication systems are near terrestrial as a communication link between transceivers, the link is line-of-sight and successfully transmitted optical signals. Nevertheless, the optical signals transmissions over the FSO channels bring challenges to the system. To overcome the challenges posed by the FSO channels, the most common technique is to use relay stations, the most recent is the reconfigurable intelligent surfaces (RISs) technique. This study introduces a Weibull distribution model for a free-space optical communication link with RISs assisted, the parameter used to evaluate the performance of the system is the average symbol error rate (ASER). The RISs effect is examined by considering the influence of the transmitter beam waist radius, shape parameter, aperture radius, scale parameter, and signal-to-noise ratio on the ASER.

Influence of different codes on the performance of SAC‐OCDMA over FSO link

SummaryThis paper evaluates the performance of a spectral‐amplitude‐coding optical code division multiple access (SAC‐OCDMA) system over a free space optical (FSO) communication link under various weather conditions such as fog, snow, and rain. The system uses different code families, including modified quadrature congruence (MQC), random diagonal (RD), and diagonal eigenvalue unity (DEU), to address user codes and reduce the effects of multiple access interference (MAI). The system's bit error rate (BER) performance has been analyzed taking into account receiver noises and MAI, and calculates the required optical power for a BER value of 10−9 under different weather conditions. The results show that fog has the most significant impact on the system's performance, while rainy conditions require the lowest optical power. The performance of the system also declines with increasing transmission length and concurrent user count. Finally, the paper compares the performance of different code families under unfavorable weather conditions and concludes that DEU performs better than MQC and RD in bad weather.

A deep-learning approach for turbulence correction in free space optical communication with Laguerre–Gaussian modes

Free space optical communication has become increasingly popular in the past decade due to its terahertz bandwidth, unlicensed spectrum, and enhanced security features. This technology has been attracting significant interest from the network community due to its broad applications in future military and civilian protocols. Recently, the use of orthogonal spatial states of the light field, including those with nonzero orbital angular momentum, has increased communication system capacities and bit-transfer rates per photon. However, atmospheric turbulence disrupts the orthogonal spatial modes over propagation. We present a novel deep learning architecture, cGULnet, to mitigate the effects of atmospheric turbulence on spatial multiplexing of Laguerre–Gaussian structured light modes. The model predicts the phase correction patterns required for an adaptive optical system. It is a U-shaped architecture trained adversarially to a patch discriminator similar to conditional generative adversarial neural-networks. The network is trained for exact phase recovery systems without approximating Zernike polynomials, enabling its use in highly phase-sensitive but high-speed spatial multiplexed communication systems. The proposed architecture can predict the successive three frames in a single shot using the previous ten frames. The method was evaluated for a wide range of beam waist sizes in a simulated Laguerre–Gaussian spatial mode multiplexed 1 km free space optical communication link. We have considered 20 dB optical signal-to-noise ratio, with turbulence conditions parameterized by the refractive index structure parameter of Cn2≤5×10−15m−23. It was found to reduce the bit error rate to the International Telecommunication Union recommended forward error correction limit of 3.8×10−3 under high turbulence conditions.

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.

Generation and free-space transmission characterization of bottle vortex beam

Vortex beams carrying orbital angular momentum (OAM) with the doughnut-shaped intensity distribution can be employed in free-space optical (FSO) communication links to circumvent obstructions. However, the size of the receiver aperture is proportional to the size of obstructions, which seriously constrains the application of OAM beams in this scenario. In this paper, we propose a method to generate bottle vortex beams (BVBs) with a parabolic trajectory by manipulating the radial phase distribution of conventional OAM beams. Meanwhile, the trajectory of BVBs generated are highly compatible with the predesigned trajectory by using this method. Moreover, we evaluate the free-space transmission performance of BVBs under atmospheric turbulence and limited receiving aperture. The results show that BVBs have better OAM FSO communication link performance compared with conventional OAM beams and Bessel beams. In addition, the performance of the BVBs circumventing obstructions is further investigated. The simulation results show that when setting the atmospheric turbulence strength D/r0 = 2 and the obstruction size of 40 mm, the average received optical power of the BVBs captured by a limited receiving aperture diameter (d = 40 mm) is improved about 7 dB and 3 dB compared to conventional OAM beams and Bessel beams, respectively.

Optical Intelligent Reflecting Surface for Cascaded FSO-VLC Communication System

For optical wireless communication, laser-based free-space optical communication (FSO) has high energy utilization efficiency and speed, but is easily blocked and cannot support multiple users. Visible light communication (VLC) based on light-emitting diode (LED) can support multiple mobile users, but has low energy efficiency in long-distance communication. Aiming at the above problems, this paper proposes an OIRS-assisted cascaded FSO-VLC system. In this system, we adopt a FSO link with OIRSs at the front end. Multiple OIRSs are utilized to screen the optimal channel, thereby improving the flexibility and robustness of the system. The VLC link is used in the back end to support multi-user mobile communication. By jointly considering atmospheric turbulence, pointing error, OIRS's physical model and VLC geometric model, the closed-form solutions of the system's precise outage probability, asymptotic outage probability and BER have been derived. It is shown through numerical results that the derivation results are accurate. We can observe that both the increase of VLC source's coverage radius in VLC link and the decrease of pointing error in FSO link can effectively improve the system performance. Based on theoretical analysis and simulation results, the effect of each parameter and system design have been discussed.

Performance Analysis of Adopting FSO Technology for Wireless Data Center Network

Free Space Optical Communication (FSO) is a promising technology to address wired Data Center Network (DCN) challenges like power consumption, low scalability and flexibility, congestion and cabling. Scholars have developed indirect line-of-sight (LoS) FSO schemes by reflecting the FSO beams via switchable mirrors. These schemes have introduced extra overhead delay to establish indirect LoS links, defined herein as the rack-to-rack FSO link setup process. The purpose of this work is to study and model this setup process with the consideration of the DC workloads. We found that the process involves a sequence of i.i.d random variables that contribute differently to its delay. Also, the process shows a statistical characteristic close to M/M/K. However, the number of FSO links, K, is random with time, which necessitates careful modeling. Finally, the PDF of the process total response time is close to the hypoexponential distribution, and it maintains its main characteristics even with different distributions for the service time.

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.

Estimation of intelligent-based quality of service in terrestrial FSO links: A case of South Africa propagation paths

Free Space Optical (FSO) communication is one of the popular technologies’ dominants in the optical communication domain. Despite all the advantages, FSO links are grossly affected by severe atmospheric conditions such as fog, rain, snow, smoke, and dust or aerosol particles suspended in the air. Fog is one of the major challenges for free-space optics to achieve carrier-class availability and causes extreme attenuation. A 10-year (2010––2019) visibility data were obtained from the South African Weather Services (SAWS) with a special focus on Cape Town (–33.925° S, 18.424° E). This extracted and processed Cape Town data was used to estimate the needed fog-induced attenuation. The intelligent algorithm is rooted in the theory of Artificial Neural Networks (ANN) systems which intelligently control the sub-tropical climate conditions of the chosen region. The adopted network-based technique is capable to interact with ever-changing weather by generating appropriate improvements to the free space optical communication link quality of service for efficient services to the customers. In this study, an Intelligent Algorithm (IA) based on an Artificial Neural Network (ANN) has been utilized to improve the reliability of QoS that can satisfy customer SLAs and improve received signal over FSO communications. The results obtained by implementing the IA system indicates that the power transmitted range from 8 to 100 dB and the adjusted Signal to Noise Ratio (SNR) between 390–––420 dB at 650 nm optical wavelength, whereas the same trend could be observed when the optical wavelength of 1550 nm was applied. Subsequently, the model effectively estimated the automatic transmit power control for enhancement in SNR and resulting in better QoS at the receiving end, regardless of the related weather changes along with the transmission link.

Π-ML: a dimensional analysis-based machine learning parameterization of optical turbulence in the atmospheric surface layer.

Turbulent fluctuations of the atmospheric refraction index, so-called optical turbulence, can significantly distort propagating laser beams. Therefore, modeling the strength of these fluctuations (C n2) is highly relevant for the successful development and deployment of future free-space optical communication links. In this Letter, we propose a physics-informed machine learning (ML) methodology, Π-ML, based on dimensional analysis and gradient boosting to estimate C n2. Through a systematic feature importance analysis, we identify the normalized variance of potential temperature as the dominating feature for predicting C n2. For statistical robustness, we train an ensemble of models which yields high performance on the out-of-sample data of R2 = 0.958 ± 0.001.

Bootstrapped low complex iterative LDPC decoding algorithms for free-space optical communication links

In recent years, much research has been devoted to free-space optical communication (FSO). The unregulated spectrum, low implementation costs, and robust security of FSO systems all are of great importance which lead to a wide range of applications for FSO links, from terrestrial communications to satellite communications. However, the fundamental limitation with FSO links is atmospheric turbulence (AT) caused by fading, significantly reducing link performance. Random phenomena are the best characteristic of atmospheric turbulence caused by changes in the air’s refractive index over time. Numerous probability density functions of the AT models were presented to model the randomness in AT channels. The Log-Normal (LN) channel model is for weak atmospheric turbulence, while the Gamma–Gamma (G–G) channel is selected for moderate and strong atmospheric turbulence. The impacts of geometric losses, attenuation due to weather, and errors due to misalignment are addressed using LN and G–G channels. Channel coding is one of the possible solutions for mitigating such FSO channel impairments as the low-density parity check (LDPC) codes. In this article, the Weighted Bit Flipping (Algorithm (1)), Implementation Efficient Reliability Ratio Weighted Bit Flipping (Algorithm (2)), and Min-Sum (Algorithm (3)) algorithms are compared and evaluated against FSO atmospheric turbulence channels. In addition, two novel algorithms are proposed to enhance the complexity or Bit Error Rate performance of LDPC decoding over FSO channels. The results showed an impressive improvement of the coded FSO system by employing the proposed algorithms compared to the existing LDPC decoders for FSO communications from the point of all comparison parameters.

Performance Analysis of Free Space Optical Communication under Scintillation Effect

Free-Space Optical communication (FSO) offers unregulated bandwidth of up to 200 THz, security, higher speed, unlimited data rate and shortest installation time. However, weather attenuation, especially scintillation, has a massive impact on the performance of the FSO transmission channel. This paper evaluates link availability and effect of scintillation on FSO performance in terms of eye diagrams, Bit Error Rate (BER), Q-factor, and signal-to noise ratio. Our work used data collected from January 2018 to December 2021 by Tanzania Meteorological Authority (TMA) in Mwanza and Arusha regions for link availability and performance analysis. Simulation was performed to determine the FSO link availability, and we analyzed the scintillation effect on FSO performance using Submarine Laser Communication II and Hufnagel Valley (HV) day prediction models. It is observed that the HV day model performs better in predicting scintillation intensity compared to the former. In both regions, for the same transmission power of 20 dBm, the signal quality is better for a distance of 6 km where the BER value is found to be 10-8. These results suggest that, to achieve high-speed data transmissions, the maximum separating distance between FSO transceivers should not exceed 6 km.

Combined Photobleaching and Temperature Effects on 1550 nm Radiation-Induced Attenuation of Germanosilicate Optical Fiber

Combined temperature and photobleaching (PB) effects on the 1550 nm radiation-induced attenuation (RIA) levels and kinetics of a commercial Ge-doped single-mode optical fiber have been investigated. The main goal of this study was to evaluate if synergetic effects could impact the performances of free space optical communication links in space. For this, we performed accelerated (0.5 Gy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s) irradiation runs with 100 kV X rays up to the total ionizing dose of 150 kGy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), varying the irradiation temperature between -80 °C and 80 °C. First, we evaluate the irradiation temperature effects: infrared RIA largely increases at negative temperatures. Second, the effects of injecting different optical powers at the different temperatures have been characterized: if at temperatures exceeding room temperature (RT), PB effects are almost negligible, a PB effect is clearly observable at lower temperatures where the germanosilicate optical fiber present large RIA levels due to the contribution of metastable defects absorbing at 1550 nm. Increasing the injected powers allows reducing the concentration of these radiation-induced unstable defects, but not as much as thermal treatment consecutive to the irradiation. These results highlight the complex physics of radiation induced defects absorbing at 1550 nm in Ge-doped optical fiber, but also the vulnerability of this class of fiber for space telecommunication links. Regarding the unstable nature of point defects causing the RIA at lower temperature, it seems possible to mitigate the loss excess by thermal cycles at RT or higher temperatures. We also could expect that our accelerated tests represent a worst-case scenario in terms of RIA levels at the maximum dose.

Analysis on the performance of pointing error effects for RIS-aided FSO link over gamma-gamma channels

In this study, we analysis on the performance of pointing error effects for reconfigurable intelligent surfeces (RISs) aided free-space optical (FSO) communication link over moderate to strong atmospheric turbulence channels. Among these solutions, RISs are considered as hardware technology to improve performance of optical wireless communication systems. Performance evaluation of systems is affected by atmospheric attenuation, pointing errors and moderate to strong atmospheric turbulence channels for quadrature amplitude modulation (QAM) technique. Atmospheric turbulence channel from moderate to strong is modeled using gamma-gamma distribution. Several numerical results obtained the average symbol error rate (ASER) performance versus beam waist radius and signal to noise ratio for three value of pointing error displacement standard deviation. From the results, it is show that with RIS assisted can effectively improve the performance and link distance of the wireless communication systems.

Securing Non-Terrestrial FSO Link with Public Key Encryption Against Flying Object Attacks

Free Space Optical (FSO) communication has potential terrestrial and non-terrestrial applications. It allows large bandwidth for higher data transfer capacity. Due to its high directivity, it has a potential security advantage over traditional radio frequency (RF) communications. However, eavesdropping attacks are still possible in long non-terrestrial transmission FSO links, where the geometry of the link allows foreign flying objects such as Unmanned Aerial vehicles (UAVs) and drones to interrupt the links. This exposes non-terrestrial FSO links to adversary security attacks. Hence, data security techniques implementation is required to achieve immune FSO communication links. Unlike the commonly proposed physical layer security techniques, this paper presents a lab-based demonstration of a secured FSO communication link based on data cryptography using the GNU Radio platform and software-defined radio (SDR) hardware. The utilized encryption algorithm (Xsalsa20) in this paper requires high-time complexity to be broken by power-limited flying objects that interrupt the FSO beam. The results show that implementing cryptographic encryption techniques into FSO systems provided resilience against eavesdropping attacks and preserved data security. The experiment results show that, at a distance of 250 mm and laser output power of 10 mW, the system achieves a packet delivery rate of 92% and transmission rate of 10 Mbit/s. This is because the SDR used in this experiment requires a minimum received electrical amplitude of 27.5 mV to process the received signal. Long distance and higher data rates can be achieved using less sensitive SDR hardware.

Effect of MIMO scheme on MWIR FSO link &amp; comparison with SWIR wavelength in the presence of different visibility and strong turbulence circumstances

Abstract In this article, the authors analyse the performance of the “multiple input multiple output (MIMO)” technique on a free space optical (FSO) communication link using the mid wavelength infrared (MWIR) and commonly used short wavelength infrared (SWIR) in the presence of various visibility conditions like clear weather, light haze, dense haze, fog, etc., and strong turbulence regime. A 5 km link range has been considered with different diversity MIMO schemes. The performance analysis has been made for both wavelengths in terms of bit error rate (BER) and link outage probability. As the MIMO diversity is increased, the performance has been improved compared to the single input single output (SISO) scheme. In the presence of foggy conditions and higher turbulence strength, the MWIR wavelength works properly, whereas the SWIR wavelength has not worked for the same signal to noise (SNR) ratio and the same link distance.