Free-space Optical Communication Links Research Articles

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.

BER performance of an FSOC system over atmospheric turbulence channels based on computational temporal ghost imaging.

In a free space optical communication (FSOC) system, atmospheric turbulence will increase the bit error ratio (BER) and impair FSOC link reliability. Since computational temporal ghost imaging (CTGI) has anti-interference, we present an FSOC system over atmospheric turbulence based on CTGI. The simulation results show that the BER performance of CTGI is better than on-off keying under different atmospheric turbulence regimes. To improve the performance of the CTGI scheme, the influence of the number of transmission samples and code length is analyzed. It is shown that BER performance improves with the increment of the number of samples, while code length has no impact. This scheme provides an idea for reliable communication over atmospheric turbulence and an important reference for improving wireless optical communication in an extreme environment.

Machine learning based performance estimation of terrestrial Ro-FSO Link

In this work, a radio over free-space optical communication (Ro-FSO) link has been examined considering quadrature amplitude modulation (64-QAM) based orthogonal frequency division multiplexing (OFDM) technique for a turbulence channel. The performance of the system has been investigated considering log normal and gamma-gamma atmospheric scintillation models under clear air, rain and fog weather conditions. Artificial neural network (ANN), k-nearest neighbour (KNN), and decision tree (DT) machine learning (ML) techniques have been applied for estimation of quality of received signal in terms of bit error rate BER. ANN model exhibits the highest value of R-squared (R2) of 0.9967 and lowest value of root mean square error (RMSE) of 0.0134 as compared to other ML techniques resulting in the best fit model.

Image transmission performance analysis through free space optical communication link using coherent QPSK modulation under various environmental conditions

AbstractFree space optics (FSO) technology has been used recently in many applications due to its various advantages such as high bandwidth, easy deployment, high security, free installation license and immunity to electromagnetic interference. However, this type of communication is vulnerable to meteorological conditions such as rain, haze and fog which significantly degrade the system's efficiency. To enhance the effectiveness of the FSO systems, Quadrature Phase Shift Keying (QPSK) modulation based on coherent detection constitutes a promising technique for achieving high spectral efficiency and long‐distance FSO links with the characteristics of low‐cost implementation and simple design. This work aims to investigate the performance of FSO communication links on image transmission under the presence of atmospheric attenuations. A comparative study has been made using QPSK modulation scheme and on–off‐keying (OOK) modulation scheme to estimate the maximum range that was used to recover an RGB transmitted image over an FSO link under different meteorological weathers. The obtained results clearly show that the quality of the received image has degraded significantly during transmission through free space yielding to limit the propagation range. Moreover, the proposed FSO communication system performs well when using QPSK modulation rather than OOK modulation in different environmental conditions. Thus, to improve the quality of the received image under adverse fog weather condition, high power and small beam divergence angle have been suggested to increase system's performance. The proposed solution increases the communication range of the FSO link by 230 m and 100 m for moderate and heavy fog, respectively.

Multi-Level Logarithmic Amplification-Based Fixed Threshold Circular Polarized On-Off Keying Detection for Free-Space Optical Communications

This study investigates a multi-level logarithmic amplification (MLA)-based fixed threshold circular polarized on-off keying (CP-OOK) detection for free-space optical (FSO) communication links. OOK signal is polarized into a single circular polarization state by a linear polarizer (LP) and a quarter-wave plate (QWP). In the receiver terminal, firstly, circular polarization is transformed into linear polarization utilizing QWP without polarization coordinates alignment between transmitter and receiver. Then, the background noises are decreased by polarization filtering using LP. Then, CP-OOK signal intensity variation is eliminated by nonlinear gains from MLAs in the low gain nonlinearity condition. Finally, fixed threshold decision (FTD) is realized by optimizing cascaded LAs to reduce the extinction ratio distortion of the CP-OOK signal. The proposed CP-OOK transmission is analyzed under various strengths of turbulence channel and different configurations of MLAs. Simulation results demonstrated that the proposed CP-OOK signal was effectively detected by FTD with optimized MLAs.

Secrecy analysis of a free-space laser communication system with a coherent main channel

Physical layer security of a free-space optical (FSO) communication link is investigated for certain eavesdropping scenarios in which the eavesdropper exploits the divergence of the transmitted laser beam, which is subject to atmospheric turbulence, to compromise the security of the transmission link. A solution for physical layer security in which the legitimate receiver is positioned to receive only the coherent component of the transmitted laser beam is proposed. The irradiance fluctuations are modeled with separate distributions for the coherent and non-coherent components of the transmitted beam, corresponding to the main channel and the eavesdropper’s channel, respectively. Complete secrecy analysis of this setup is presented, and closed form expressions of the probability of strictly positive secrecy capacity, average secrecy capacity, and secrecy outage probability are derived. The analytical results are verified by Monte-Carlo simulations. Secrecy performance for worst-case scenarios corresponding to an adversary with powerful resources to capture all of the leaked information is explored. Results are analyzed for different turbulence regimes, and it was found that secure communication is possible even in the region of strong turbulence.

Demonstration of Turbulence-Resilient Self-Homodyne 12-Gbit/s 16-QAM Free-Space Optical Communications Using a Transmitted Pilot Tone

Free-space optical (FSO) communication links can benefit from recovering both amplitude- and phase-encoded data (e.g., quadrature-amplitude-modulation, QAM) by mixing a local oscillator (LO) with a received data beam in a coherent detector. However, atmospheric turbulence can cause power coupling from a fundamental Gaussian mode of the data beam to higher-order modes, resulting in a significantly degraded mixing efficiency between the data and a fundamental Gaussian LO. Previously, a turbulence-resilient pilot-assisted self-coherent FSO QAM link has been demonstrated by transmitting a frequency-offset Gaussian pilot beam along with the Gaussian data beam. The pilot and data beams experienced similar modal coupling and thus can efficiently mix in a self-heterodyne detector. However, a frequency guard band was used to avoid signal-signal beating interference, thereby reducing the utilization efficiency of the detector's bandwidth. To increase the detector's bandwidth utilization, we experimentally demonstrate in this paper a turbulence-resilient 12-Gbit/s 16-QAM FSO link using pilot-assisted self-homodyne detection. The pilot beam is located at the same center optical frequency as the data channel but on an orthogonal polarization. At the receiver, we utilize self-homodyne detection to achieve the efficient mixing of the data and pilot. Compared to self-heterodyne, our approach has ∼2X the utilization efficiency of the detector's bandwidth. Compared to conventional LO-based coherent (homodyne or intradyne) detection, the pilot-assisted self-homodyne link shows up to ∼20-dB improvement of optical-to-electrical mixing efficiency under a turbulence strength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2{{\bm{w}}}_0/{\bm{\ }}{{\bm{r}}}_0 = \ \sim 7.5$</tex-math></inline-formula> . The turbulence resilience of the link is demonstrated with a bit-error rate below the 7% forward error correction limit for 400 different turbulence realizations.

Deep learning-based approach for detection of turbulence-induced distortions in free-space optical communication links

This study aims to develop a deep-learning-based approach that is able to detect turbulence-induced mode distortion in orbital angular momentum-based free-space optical communication links. The proposed and tuned deep learning-based models have been trained with a dataset that is created based on the intensity beam profiles that propagate 5 Km in four different levels of turbulent atmosphere. The random search algorithm has been adopted for conducting a hyperparameter tuning process to select the best structures for two groups of deep learning models each of which contains three different deep learning models. The proposed approach is able to not only distinguish the distorted beams but also to recognize the level of distortion. The obtained results indicate that the utilized tuned models gave 100% classification accuracy in terms of detecting distorted beams. Besides that, the proposed and tuned models obtained a very high classification accuracy reaching 97, 94.99, and 97.78% in terms of assessing the amount of distortion exposed by the transmitted beams. We believe that the obtained results will be a milestone in free-space optical communication systems that utilize orbital angular momentum.

Synchronization and clock recovery in a ferroelectric liquid crystal spatial light modulator based free-space optical communication link.

Synchronization of the transmitter and receiver is crucial in a free-space optical communication system for the proper transfer and retrieval of user information. In this work, we propose a method for the synchronization and recovery of the clock signal at the receiver from the optical signal modulated by a ferroelectric liquid crystal spatial light modulator (FLCSLM) in the transmitter. We have demonstrated our scheme by building an experimental arrangement that comprises an FLCSLM based computer generated holography assembly for modulating the laser beam in the transmitter and a photodiode cum micro-controller circuit in the receiver to generate the synchronized clock. We present the experimental results to demonstrate the accuracy of the recovered clock and the successful retrieval of the transmitted user information. The scheme can work for amplitude modulated, phase modulated, or complex amplitude modulated information transfer based on the FLCSLM.

Free-space mid-IR communications using wavelength and mode division multiplexing

There has been growing interest in free-space optical (FSO) communications in the mid-infrared (mid-IR) region. In this paper, we review FSO communications in the mid-IR using wavelength division multiplexing (WDM) and orbital angular momentum (OAM)-based mode division multiplexing (MDM). We introduce the effects of mid-IR wavelengths in FSO communication links. The methods available to modulate and detect signals on mid-IR wavelengths are discussed. Subsequently, we introduce WDM and OAM-based MDM techniques in the mid-IR. Finally, we focus on the performance of a mid-IR FSO communication system combining both multiplexing techniques.

Turbulence-resistant high-capacity free-space optical communications using OAM mode group multiplexing.

Twisted light carrying orbital angular momentum (OAM), which features a helical phase front, has shown its potential applications in diverse areas, especially in free-space optical (FSO) communications. Multiple orthogonal OAM beams can be utilized to enable high-capacity FSO communication systems. However, for practical OAM-based FSO communication links, atmospheric turbulence will cause serious power fluctuations and inter-model crosstalk between the multiplexed OAM channels, impairing link performance. In this paper, we propose and experimentally demonstrate a novel OAM mode-group multiplexing (OAM-MGM) scheme with transmitter mode diversity to increase system reliability under turbulence. Without adding extra system complexity, an FSO system transmitting two OAM groups with a total of 144 Gbit/s discrete multi-tone (DMT) signal is demonstrated under turbulence strength D/r0 of 1, 2, and 4. In our experiments, the proposed OAM-MGM scheme helps to achieve bit-error-rate (BER) mostly less than 3.8 × 10-3 under turbulence strength D/r0 of 1 and 2 with a total transmitted power of 10 dBm. Compared with the conventional OAM mode multiplexed system, the system interruption probability decreases from 28% to 4% under moderate turbulence strength D/r0 of 2.

Teletraffic analysis and simulation on FSO systems: the traffic generators approach

Free Space Optical Communications (FSO) has become an interesting topic to researchers in recent years since the amount of data generated by devices is growing, and it is necessary to use data links that support high bandwidth to transmit them from one device to another. This technology is used to establish not only terrestrial links but also space links.A CubeSat satellite is generally deployed in the Low Earth Orbit (LEO) region. To maintain the altitude, the satellite must have a high orbital speed. For that reason, the time to transmit data between a particular ground station and the CubeSat satellite is limited. On the other hand, the volume, mass, and energy storage capacity are restricted in a CubeSat. The greater the bandwidth capacity of radio frequency links, the greater the demand for volume, mass, and energy they require. For that reason, to transmit a significant amount of data, traditional radio frequency links are not suitable and are becoming replaced by FSO as technology improves.Despite research in physical layer technologies on FSO (modulation schemes, error mitigation techniques, pointing and tracking systems), there is very little research in the literature about data link layer protocols for FSO. Secondly, there is little research to measure the data traffic demand on CubeSat satellites so that a certain data link layer protocol can be selected or adapted to be implemented in an FSO system.This paper presents research to address the issue of teletraffic through the use of traffic generators. The result is the design and development of a traffic generator for a discrete event simulator that will later be used to observe the behavior and to measure the performance of the Selective Repeat ARQ protocol in a simulated satellite FSO link, in order to propose improvements to adapt the protocol to this scenario.

An Improved Wavelength Diversity Based Free Space Optical Link: Effects of Fog and Atmospheric Turbulence

Free-space optical (FSO) communication systems have received a lot of attention from academia and industries due to their low cost, high security and data rates for various wireless communication applications. However, factors such as fog and atmospheric turbulence degrade the performance of the FSO communication system. Therefore, this study presents an improved wavelength diversity based free space optical link using a single channel approach. The authors employed both Kim and Al-Naboulsi’s models to model the attenuation-induced FSO link. Furthermore, the Gamma-Gamma turbulent model was utilized to explain the turbulent nature of the FSO communication link. Experimental results showed improved performance in terms of Bit Error Rate (BER) and Quality factor of the FSO link up to a distance of 7km compared to the multiple channel approach that gives a signal output for distances less than 3km only. Hence, the impact of attenuation on the FSO link was reduced and the performance of the system was enhanced under fog and atmospheric turbulence conditions.