Free-space Optical Networks Research Articles

Fundamentals and recent developments of free-space optical neural networks

Machine learning with artificial neural networks has recently transformed many scientific fields by introducing new data analysis and information processing techniques. Despite these advancements, efficient implementation of machine learning on conventional computers remains challenging due to speed and power constraints. Optical computing schemes have quickly emerged as the leading candidate for replacing their electronic counterparts as the backbone for artificial neural networks. Some early integrated photonic neural network (IPNN) techniques have already been fast-tracked to industrial technologies. This review article focuses on the next generation of optical neural networks (ONNs), which can perform machine learning algorithms directly in free space. We have aptly named this class of neural network model the free space optical neural network (FSONN). We systematically compare FSONNs, IPNNs, and the traditional machine learning models with regard to their fundamental principles, forward propagation model, and training process. We survey several broad classes of FSONNs and categorize them based on the technology used in their hidden layers. These technologies include 3D printed layers, dielectric and plasmonic metasurface layers, and spatial light modulators. Finally, we summarize the current state of FSONN research and provide a roadmap for its future development.

An alternative computing technique for machine learning

Free-space optical neural networks (FSONNs) offer speed, efficiency, and versatility in machine learning applications, overcoming the limitations of electronic integrated circuits.

Leveraging InGaN solar cells for visible light communication reception

AbstractSolar cells are increasingly being utilised for both energy harvesting and reception in free‐space optical (FSO) communication networks. The authors focus on the implementation of a mid‐band p‐In0.01Ga0.99 N/p‐In0.5Ga0.5 N/n‐In0.5Ga0.5 N (PPN) solar cell, boasting an impressive 26.36% conversion efficiency (under 1.5AM conditions) as a receiver within an indoor FSO communication network. Employing a solar cell with dimensions of 1 mm in length and width, the FSO system underwent simulation using Optisystm software, while the solar cell's behaviour was simulated using SCAPS‐1D. The received power from the solar cell was then compared to that of four commercially available avalanche photodiode (APD) receivers. Exploring incident wavelengths spanning 400–700 nm within the visible spectrum, across transmission distances of 5, 10, 15, and 20 m, the study presented current‐voltage (IV) and power‐voltage curves. Notably, the InGaN solar cell exhibited superior electrical power output compared to all commercial APDs. In conclusion, the findings underscore that augmenting received power has the potential to enhance FSO network quality and support extended transmission distances.

Optimal Power Allocation in Optical GEO Satellite Downlinks Using Model-Free Deep Learning Algorithms

Geostationary (GEO) satellites are employed in optical frequencies for a variety of satellite services providing wide coverage and connectivity. Multi-beam GEO high-throughput satellites offer Gbps broadband rates and, jointly with low-Earth-orbit mega-constellations, are anticipated to enable a large-scale free-space optical (FSO) network. In this paper, a power allocation methodology based on deep reinforcement learning (DRL) is proposed for optical satellite systems disregarding any channel statistics knowledge requirements. An all-FSO, multi-aperture GEO-to-ground system is considered and an ergodic capacity optimization problem for the downlink is formulated with transmitted power constraints. A power allocation algorithm was developed, aided by a deep neural network (DNN) which is fed channel state information (CSI) observations and trained in a parameterized on-policy manner through a stochastic policy gradient approach. The proposed method does not require the channels’ transition models or fading distributions. To validate and test the proposed allocation scheme, experimental measurements from the European Space Agency’s ARTEMIS optical satellite campaign were utilized. It is demonstrated that the predicted average capacity greatly exceeds other baseline heuristic algorithms while strongly converging to the supervised, unparameterized approach. The predicted average channel powers differ only by 0.1 W from the reference ones, while the baselines differ significantly more, about 0.1–0.5 W.

A CDMA Method for Multibeacon Codomain Detection by a Quadrant Detector in Free-Space Optical Networking

A CDMA Method for Multibeacon Codomain Detection by a Quadrant Detector in Free-Space Optical Networking

Free-Space Optical (FSO) Satellite Networks Performance Analysis: Transmission Power, Latency, and Outage Probability

Free-Space Optical (FSO) Satellite Networks Performance Analysis: Transmission Power, Latency, and Outage Probability

Optical satellite network architecture [Invited Tutorial

This paper addresses the architecture of optical satellite networks from the physical to the transport and application layers. New frontiers exist for much more research before an efficient architecture can be constructed. Optical wireless networks have the potential to serve space–space, space–terrestrial/aircraft, and aircraft–aircraft data centers and metropolitan area networks. Though the technology in these applications is similar in nature, the architecture constructs and protocol tuning can be very different. Free-space optical networks have two attributes that are not encountered in fiber networks and those are their ability to connect without predeployment of infrastructures and the ability to reconfigure their connection topology by beam steering in time scales of milliseconds to seconds to adapt to traffic loads (as high as Tbps per connection), satellite and mobile platform movements, switching node states, and atmospheric conditions. This paper emphasizes a multi-layer approach to optical wireless networks and how the network architecture can be tuned to specific applications.

Cross-layer modeling and analysis of TCP communications over turbulent FSO links

This paper addresses the accurate characterization of the performance of transmission control protocol (TCP) for end-to-end transport services over free-space optical (FSO) links. Unlike previous works on this topic, we introduce a second-order Markovian assumption for the variation of the TCP congestion window to capture the memory from turbulence channels and further propose a Markov chain model that maps two consecutive congestion windows along time into the state space to interpret TCP operations. The analytical expression of steady-state probability distribution vector of the proposed model in general cases where relay routers exist behind the TCP connection is derived, based on which the throughput efficiency of TCP is formulated. The high accuracy of the proposed model is verified by Monte-Carlo simulations and experiments. We further analyze the impact of different link/channel parameters on the TCP performance, and discuss briefly the effectiveness of bundle protocol-based schemes for improving the performance of FSO networks from a transport layer perspective.

Effects of Eavesdropper on the Performance of Mixed $\eta$–$\mu$ and DGG Cooperative Relaying System

Free-space optical (FSO) channels offer line-of-sight wireless communication with high data rates utilizing unlicensed optical spectrum. The hybrid radio frequency (RF)–FSO system is currently receiving a great deal of research interest because atmospheric turbulence prevents enhanced secrecy performance. Except for the double generalized gamma (DGG) model, traditional FSO models typically are unable to show secrecy performance across all turbulence severity ranges. Due to this, we propose a dual-hop <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\eta$</tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> and unified DGG mixed RF–FSO network while taking into account the following three different eavesdropping scenarios: 1) the eavesdropper is at the RF link; 2) the eavesdropper is at the FSO link; and 3) both eavesdroppers remains active simultaneously. These proposed scenarios' security is examined in terms of the probability, of strictly positive secrecy capacity, secure outage probability and effective secrecy throughput, and further investigation is done into the implications of various system settings on the secrecy performance. The analysis shows that, when compared to the other two scenarios, the third scenario exhibits the worst secrecy condition. Intensity modulation/direct detection (IM/DD) and heterodyne detection (HD) approaches are compared, with HD outperforming IM/DD in terms of secrecy performance. Finally, Monte Carlo simulations are used to confirm all analytical findings.

FSO-5G Networks with Enhanced Throughput, Reliability and Low Latency

Due to the necessity for high-speed and reliable communication, the combination of Free Space Optics (FSO) and 5G wireless networks has received substantial attention in recent years. However, atmospheric turbulence can significantly damage the quality of the FSO link, leading to high error rates and poor reliability. In this paper, a concatenated dynamic coding approach has been proposed and presented to limit the effects of atmospheric turbulence, hence increasing the overall reliability and security of FSO-5G networks. The proposed strategy combines various coding algorithms in FSO-5G networks to improve error correction, minimise latency, and increase throughput. The proposed approach's performance was examined using simulation and compared to other traditional coding systems.

Mitigating Jamming in FSO Cooperative Networks: A Buffer-Aided Relaying Approach

In this paper, we propose a buffer-aided relaying-based approach to counter the effects of jamming in free space optical (FSO) communication networks. An FSO cooperative system with a source node, a relay node, and a destination node is considered. The relay node consists of a finite-size buffer, and both relay and destination nodes are assumed to be affected by jamming. We use a Markov chain (MC) based approach to obtain the state transition matrix of the system, which is later used to evaluate the steady state probability of the proposed setup. A mixture of jamming and Gaussian noise is considered to evaluate the system’s performance. Novel expressions of the outage probability and average bit error rate of the considered FSO system are obtained. The system’s performance is then analyzed for different jamming powers and jamming probabilities. The proposed system is then compared with a non-buffer-aided (NBA) cooperative FSO system affected by a jammer; it is observed that the proposed buffer-aided FSO system significantly outperforms the NBA counterparts.

Availability, outage, and capacity of spatially correlated, Australasian free-space optical networks

Network capacity and reliability for free space optical communication (FSOC) is strongly driven by ground station availability, which is dominated by local cloud cover causing an outage. Here, we combine remote sensing data and novel methods to provide a generalized framework for assessing and optimizing optical ground station networks. This work is guided by an example network of eight Australian and New Zealand optical communication ground stations that span approximately 60° in longitude and 20° in latitude. Utilizing time-dependent cloud cover data from five satellites, we present a detailed analysis that determines the network availability and diversity, which showed that the Australasian region is well-suited for an optical network with a 69% average site availability and low spatial cloud cover correlations. Employing methods from computational neuroscience, we provide a Monte Carlo method for sampling the joint probability distribution of site availabilities for an arbitrarily sized and point-wise correlated network of ground stations. Furthermore, we develop a general heuristic for site selection under availability and correlation optimizations and combine it with orbital propagation simulations to compare the data capacity between optimized networks and the example network. We show that the example network may be capable of providing tens of terabits per day to a low Earth orbit satellite and up to 99.97% reliability to geostationary satellites. We therefore used the Australasian region to demonstrate, to the best of our knowledge, novel, generalized tools for assessing and optimizing FSOC ground station networks, as well as the suitability of the region for hosting such a network.

Artificial Bee Colony Based Iterative Heuristic Signal Detection Technique for Optical Wireless Communications

In this paper, we consider a problem of optical signal detection and present a novel artificial bee colony (ABC) based heuristic approach for optical spectrum sensing in a free-space optical wireless network that overcomes channel impairment due to noise uncertainty. In particular, we propose a generic ABC-based optimization framework in conjunction with eigenvalue-based detection (EVD). The uniqueness of the proposed algorithm is that it is executed iteratively by comparing various local solutions until an optimum solution is obtained. The channel is characterized as correlated multivariate fading and the received signal is quantified as a continuous waveform wideband detector. We derive the joint statistic of the correlated multivariate channel fading in the multiple-input multiple-output (MIMO) optical network. Subsequently, the marginal probability density function of the channel coefficients is obtained. For comparison purposes with existing optical spectrum sensing techniques, noise uncertainty is considered. It is demonstrated from simulations that the proposed ABC-EVD spectrum sensing yields performance improvement and the analytical results derived are verified. From the comparative study, it is also found that the proposed scheme outperforms existing optical spectrum sensing techniques, particularly in a regime of low signal-to-noise ratio. When compared to EVD, it is shown that, at a received signal-to-noise ratio of -15 dB, the performance gain achieved in terms of the probability of detection is 15% higher over the entire range of target probability of false alarm. When compared with the ED technique, the proposed technique outperforms it with approximately 50% higher probability of detection at the noise uncertainty of 1 dB and received SNR of -10 dB. The proposed algorithm offers fast convergence, strong robustness, and high flexibility.

Integration of Optical and Free Space Optics Network Architecture for High-Speed Communication in Adverse Weather using suitable Optical Bands

Free Space Optics (FSO) is a highly viable solution for high-speed wireless communication and is widely preferred over radio frequency communication systems because of its faster data transmission, no regulatory requirements and highly secure long-range operations. However, the capacity and availability of FSO optical bands are a significant concern in varying atmospheric conditions. Our objective is to enhance network flexibility and expand wireless network coverage in adverse weather conditions by combining optical and FSO links using optical bands C, S, and O. The study analyzed the performance of a hybrid 4 channels FSO-WDM system with a 100GHz or 0.8 nm channel spacing under different conditions, including adverse weather and varying data rates. An attenuation of 0.25 dB/km was fixed, and the system's performance was analyzed up to 3 km. The results showed that as the data rate increased, the system's performance declined, and the O band was the best performer up to 25 Gbit/s. BER values were analyzed at different weather conditions using the Kim model, and the O band consistently outperformed the S and C bands. Eye diagrams were used to evaluate the signal quality, and the O band was shown to perform better than the other two bands, even in adverse weather conditions. Overall, the study suggests that FSO is a viable solution for high-speed wireless communication, particularly when using the O band.

Compact lensless convolution processor for an optoelectronic convolutional neural network

To our knowledge, optical 4f systems have been widely used as a convolutional layer to perform convolutional computation in free-space optical neural networks (ONNs), which makes ONNs too bulky to be easily applied to miniaturized smart systems. Hence, we propose a compact lensless optoelectronic convolutional neural network (LOE-CNN) architecture in which a single optimized diffractive phase mask acts as an analog convolution processor to perform convolutional operation without a Fourier lens or lenslet array. We demonstrate that this LOE-CNN can be functionally comparable to existing electronic counterparts in classification performance, achieving a classification accuracy of 98.07% and 95% over the Modified National Institute of Standards and Technology dataset in simulation and experiment, respectively, which not only opens new application prospects for free-space ONNs based on a compact single-chip convolution processor, but also facilitates the development of ONN-based smart devices.

Performance of hybrid RF/FSO wireless communication system under low visibility

Abstract In the last ten years, there has been a significant increase in the importance of free-space optics (FSO). FSO can be a solution for the last mile, but its reliability and availability issues need to be looked into in depth before they can be fixed. Because FSO links depend so much on the weather, a number of weather-related issues reduce their availability. A hybrid network with an FSO link and a backup link in the GHz frequency band offers good availability and data speeds that are similar to those of a pure FSO network. Fog’s effects on hybrid RF/FSO systems are investigated. As a backup link, the GHz frequencies with the best complementing behavior should be used. The findings demonstrated that high temperatures had no adverse effects on the RF at 25 GHz in terms of impaired visibility. Despite the cold weather, the FSO link required excellent visibility.

Laser Intersatellite Link Range in Free-Space Optical Satellite Networks: Impact on Latency

Laser communication terminals for establishing laser intersatellite links (LISLs) are in high demand by companies, such as SpaceX and Telesat, who want to equip their satellites with LISLs to realize free-space optical satellite networks (FSOSNs). Such terminals offer LISLs with ranges spanning from 4500 to 45,000 km. How does LISL range affect network latency in FSOSNs? To investigate this effect, we employ the satellite constellation for Phase I of SpaceX's Starlink, and examine six different LISL ranges for satellites in this constellation in three different scenarios for long-distance intercontinental data communications. We observe that the satellite connectivity and thereby the network connectivity improves with an increase in the LISL range as more and farther satellites become available within the LISL range of a satellite. A higher LISL range leads to better shortest paths offering lower average network latency. Furthermore, improvement in average network latency with increase in LISL range is seen in all scenarios.

Space-Air-Ground FSO Networks for High-Throughput Satellite Communications

The next generation of satellite communication systems aims to achieve terabit-per-second throughput. Because of the limited spectrum available, traditional radio frequency (RF) communication links cannot provide such high throughput. Free-space optical (FSO) transmission is a possible alternative that has recently gained increased attention in the satellite community. However, FSO communications are vulnerable to the severe effects of atmospheric turbulence, such as beam-wandering-induced pointing errors and beam scintillation. To successfully remedy such effects, we propose a space-air-ground (SAG) FSO network with a strategically deployed high-altitude platform acting as a relay. We show that such a design can substantially mitigate the effects of atmospheric turbulence, especially when the satellite zenith angle is relatively high. Then we present a novel SAG satellite communication network that integrates the suggested SAG-FSO transmission and conventional hybrid single-hop FSO/RF transmission to improve overall system reliability and performance even further. The numerical results clearly show the potential of the proposed, highly innovative SAG-FSO network architecture.

Implementation of Shared Laser–LED Sources in a Free Space Optics (FSO) Network under Environmental Impact

This paper is devoted to evaluating the combined coherent and incoherent sources (CCIS) technique for different applications in the optical domain and future optical code division multiple access (OCDMA) networks. Spectral amplitude coding (SAC) has gained significant attention in optical processing systems due to its increased capabilities in dealing with multiple-access interference (MAI) efficiently. Fixed right shift (FRS) is adopted as a signature code in this study. Furthermore, performance analysis is studied in terms of bit error rate (BER) for the system using CCIS in both the free space optics (FSO) and sky mesh network using an aerial altitude platform system (AAPS). Simulation results confirmed that a CCIS design significantly improves system performance with moderate cost. An acceptable BER value of 10−9 at 1.25 Gbps data rate and 60 km, 38 km, and 6 km distances for the laser, CCIS, and LED sources, respectively, can be supported. In particular, at Q-factor ~4.5, the FSO ranges under low haze, moderate haze, and heavy haze are, respectively, 3.7 km, 2.5 km, and 1.5 km. The reason is that a CCIS design causes an increase in the effective transmitted power. It can be summarized that a CCIS design can provide reliable solutions and an affordable cost for future optical fiber and wireless network applications.

Reconfigurable multiple beams forming method based on liquid crystal on silicon

With the purpose of meeting the increasing requirement of free-space optical networking, reconfigurable multiple beams are required from a single terminal. In this paper, we demonstrate the role of co-modulation of amplitude and phase on incident laser beam using such a liquid crystal spatial light modulator. It enables arbitrary beam number, arbitrary pointing angle, and power occupation with multiple beams of excellent performance on a relative error and standard derivation when using typical groups of examples. Both numerical simulations and experimental results are presented demonstrating reconfigurable multiple beams forming via liquid crystal on silicon.