Free-space Optical Communication Links Research Articles

Multi-element full-duplex FSO transceiver design using optimum tiling

With ever-growing demand of high-speed mobile data and in vision of smart cities, optical wireless, a.k.a. free-space optical (FSO), communication deem to be a critical technology due to its significantly faster data transfer rate, higher security, lower costs, and reduced power usage. These beneficial effects have led to interest in exploring FSO technologies for mobile platforms such as drone-based cell tower to provide Internet services to remote areas, or even for wireless communications on the ground with jammed radio channels as in battlefields. Direct line-of-sight (LOS) is required to establish secure directional FSO communication (FSOC) links which are highly susceptible to random and erratic movements of the mobile nodes as well as the turbulence in the free-space medium. The performance of FSOC links can be improved by designing multi-element tiling of the laser-based transceivers which is capable of in-band full-duplex (IBFD) communication. In this work, we propose a genetic algorithm framework to explore optimized multi-element FSO transceiver tiling patterns to ensure maximal signal-to-interference and noise ratio (SINR) and minimize the effects of vibration of the mobile platform and atmospheric turbulence.

Link budget analysis of free space optical communication link for atmospheric conditions of India

The performance of FSO links gets degraded significantly due to fog weather conditions. This paper aims to perform the link budget analysis for different locations of India by analyzing the impact of foggy weather on the operation of FSO links. The meteorological data of daily visibilities of potential locations of India, have been analyzed carefully to calculate the probability of exceeding visibility, ranging from 1 to 5 km, for each of the locations. It has been observed that Ludhiana has recorded maximum fog events having visibility less than 1 km and Chennai has recorded maximum events having visibility more than 5 km. In addition to this, the average attenuation coefficients due to fog have been calculated for each of the locations, based on the meteorological data. Further, the maximum feasible link distances for various locations of India have been computed for three different practical configurations of FSO links by using the link margin equation. The coastal area of Chennai has delivered maximum feasible link ranges as compared to other considered locations of India.

Modeling and Spatial Diversity-Based Receiving Improvement of In-Flight UAV FSO Communication Links

An in-flight unmanned aerial vehicle (UAV) free-space optical (FSO) communication channel model is proposed by considering the beam deviation of the UAV under different motion states and the phase distortion caused by atmospheric turbulences. The influence of the different motion states and turbulences on the communication quality is evaluated through phase screen and Monte Carlo methods. When the average bit error rate (BER) is 10−5, the signal-to-noise ratio (SNR) should be increased from 13 dB to 20 dB when the tilt angle of the UAV increases from 0 to 5 mrad. An SNR of up to 20 dB is required when the variance of the wind σα2 is 2 mrad. The performance of the in-flight UAV FSO link can be effectively improved through spatial diversity receiving technology. The BER of lower than 10−5 can be obtained just with an SNR of 13 dB if the spatial diversity array with four receivers is used.

Performance Analysis of Dual-Hop Mixed Power Line Communication/Free-Space Optical Cooperative Systems

In this paper, we study a mixed cooperative communication system consisting of power line communication (PLC) and free-space optical communication (FSO) links, where the PLC link suffers from log-normal fading and is affected by both impulsive and background noises. Meanwhile, the FSO link undergoes Gamma-Gamma fading with both atmospheric turbulence and pointing errors. More specifically, we present closed-form expressions for the probability density function and the cumulative distribution function of the end-to-end signal-to-noise ratio of the proposed model. Consequently, the outage probability and the bit error rate (BER) performance are derived in terms of univariate Fox-H and bivariate Fox-H functions. Finally, the analytical results are verified using Monte Carlo simulations, providing useful insights into the capabilities of the proposed system.

Adaptive optics model characterizing turbulence mitigation for free space optical communications link budgets.

Free-space optical communications (FSOC) is becoming an important option for both atmospheric and space-based high data rate networks. Long-range, mobile FSOC links in the former environments must mitigate the effects of turbulence if they are to provide reliable, high link availability under cloud-free atmospheric conditions. Adaptive optics (AO) has been proposed as one means of reducing link degradation in turbulence, but field validated AO performance models are few and not definitive. These models are important if credible FSOC high performance links are expected to be deployed using AO systems. This paper reviews the Strehl ratio for the Andrews AO model and provides comparisons between predicted AO-based FSOC link budgets and measured link performance derived from several field trials. These results suggest that AO systems perform well under weak turbulence or short range under 10 km, but only offer limited tip/tilt gain in moderate turbulence and no gain under high/saturation regime turbulence at longer ranges. Long range links close to the ground in high moderate turbulence may degrade AO performance further.

Using Machine Learning Algorithms for Accurate Received Optical Power Prediction of an FSO Link over a Maritime Environment

The performance prediction of an optical communications link over maritime environments has been extensively researched over the last two decades. The various atmospheric phenomena and turbulence effects have been thoroughly explored, and long-term measurements have allowed for the construction of simple empirical models. The aim of this work is to demonstrate the prediction accuracy of various machine learning (ML) algorithms for a free-space optical communication (FSO) link performance, with respect to real time, non-linear atmospheric conditions. A large data set of received signal strength indicators (RSSI) for a laser communications link has been collected and analyzed against seven local atmospheric parameters (i.e., wind speed, pressure, temperature, humidity, dew point, solar flux and air-sea temperature difference). The k-nearest-neighbors (KNN), tree-based methods-decision trees, random forest and gradient boosting- and artificial neural networks (ANN) have been employed and compared among each other using the root mean square error (RMSE) and the coefficient of determination (R2) of each model as the primary performance indices. The regression analysis revealed an excellent fit for all ML models, indicative of their ability to offer a significant improvement in FSO performance modeling as compared to traditional regression models. The best-performing R2 model found to be the ANN approach (0.94867), while random forests achieved the most optimal RMSE result (7.37).

Analyzing the Impact of Fog and Atmospheric Turbulence on the Deployment of Free-Space Optical Communication Links in India

Analyzing the Impact of Fog and Atmospheric Turbulence on the Deployment of Free-Space Optical Communication Links in India

Experimental Study and Analysis of Meteorological and Wavefront Profile for Terrestrial Free Space Optical Communication Link at Lat.10.66° and Long.79.05°

Experimental Study and Analysis of Meteorological and Wavefront Profile for Terrestrial Free Space Optical Communication Link at Lat.10.66° and Long.79.05°

Performance of a free space optical link employing DCO-OFDM modulated Gaussian-beam

Abstract The paper studies the performance of a free space optical communication link, which consists of a Gaussian Laser beam employing direct-current-biased optical frequency division multiplexing (DCO-OFDM) modulation, to carry information. While optical multi-carrier modulation is capable of alleviating multi-path fading, higher transmit power by virtue of dc-bias, helps the optical beam in partially off-setting signal intensity loss due to beam divergence in free space channel. In order to reduce the overall transmitted power apart from avoiding nonlinearities due to high peak-to-average-power-ratio (PAPR), we utilize a PAPR-dependent dc-bias, to generate a DCO OFDM light beam from a pre-clipped electrical baseband OFDM signal. The radiated optical beam encounters turbulence-induced impairments like beam-spreading, beam-wander, etc., which affect the signal-to-noise ratio (SNR) of the received data symbols. Further signal pre-clipping and optical source linearity generate clipping noise components resulting in compromised receiver performance or reduction of span length, to achieve target bit rate error (BER). In this paper, we derive an analytical model for evaluating the performance of the free-space optical (FSO) link, by modeling various impairments as noise variances and characterizing atmospheric turbulence by refractive index structure parameter under weak- and strong-turbulent conditions. Numerical results are obtained by varying system design parameters in the model, which graphically provide useful insight on the power penalties and various trade-offs involved in operating the link on a longer span, at a higher data rate, and with reduced transmitted power through pre-clipping while ensuring desired signal BER.

Enhancing Performance of Coherent Optical OFDM FSO Communication Link Using Cascaded EDFA

Coherent communication aims to support high-speed dynamic networks by making their installation easier along with their monitoring and maintenance. In addition, Orthogonal Frequency Division Multiplexing (OFDM) which has shown gradual emergence in radio frequency (RF) domain during the past years, has now encroached into the optical domain. Combination of these two provides vigorous dispersion transmission as well as enhanced spectral efficiency. Free Space Optical (FSO) communication link performance deteriorates with scintillations, atmospheric turbulences and weather induced attenuations. The optical amplifiers, their arrangement used in the FSO link design has a drastic influence on the performance of the link. This work involves evaluating the proposed Coherent Optical OFDM FSO communication link at 30 Gbps data rate using Cascaded Erbium Doped Fiber Amplifier. The system has been evaluated in clear weather, rainfall and fog (Moderate to Dense) in terms of Q-Factor, BER, SNR and constellation diagram. The system has been designed considering moderate turbulent environment and geometric losses. The FSO communication link shows optimal performance up to 26 km for clear weather, 3.73 km under rainfall, 1.44 km under moderate foggy and 0.66 km under dense foggy environment.

Transmitter Diversity With Phase-Division Applied to Optical GEO Feeder Links

The high data rates enabled by free-space optical communication links and additional factors, such as the unregulated spectrum, make them an attractive solution in future geostationary feeder link applications with capacities beyond one Terabit per second. However, optical links through the atmosphere are heavily affected by atmospheric turbulence, leading to significant scintillation. Transmitter diversity is an effective way to improve the signal quality of optical links. Transmitter diversity with Phase-Division in Bit-Time is a novel scheme for mitgating interference and thus enabling the use of equal wavelengths for multiple transmitters. This paper will evalute the Phase-Division in Bit-Time transmitter diversity scheme and present measurement results from a lab testbed.

Multi-Tier Heterogeneous Beam Management for Future Indoor FSO Networks

To meet the growing demand for wireless capacity, communications in the Terahertz (THz) and optical bands are being broadly explored. Communications within these bands provide massive bandwidth potential along with highly directional beam steering capabilities. While the available bandwidth offers incredible link capacity, the directionality of these technologies offers an even more significant potential for spatial capacity or area spectral efficiency. However, this directionality also implies a challenge related to the network’s ability to quickly establish a connection. In this paper, we introduce a multi-tier heterogeneous (MTH) beamform management strategy that utilizes various wireless technologies in order to quickly acquire a highly directional indoor free space optical communication (FSO) link. The multi-tier design offers the high resolution of indoor FSO while the millimeter-wave (mmWave) system narrows the FSO search space. By narrowing the search space, the system relaxes the requirements of the FSO network in order to assure a practical search time. This paper introduces the necessary components of the proposed beam management strategy and provides a foundational analysis framework to demonstrate the relative impact of coverage, resolution, and steering velocity across tiers. Furthermore, an optimization analysis is used to define the top tier resolution that minimizes worst-case search time as a function of lower tier resolution and top tier range.

Experimental Study of the Free Space Optics Communication System Operating in the 8–12 µm Spectral Range

(1) Background: Free space optics communication (FSO) has improved wireless communication and data transfer thanks to high bandwidth, low power consumption, energy efficiency, a high transfer capacity, and a wide applicability field. The FSO systems also have their limitations, including weather conditions and obstacles in the way of transmission. (2) Methods: This research assesses the atmospheric conditions’ influence on the intensity of received radiation, both experimentally and theoretically. The construction of a laboratory test stand of the FSO system, which is operating in the third-atmosphere transmission window (8–12 µm), is proposed. Next, considering different atmospheric conditions, the experimental validation was conducted, both in a laboratory and real conditions. (3) Results: The measurements were carried out for two optical links working with wavelengths of 1.5 µm and 10 µm. It was found that optical radiation with a wavelength of about 10 µm is characterized by better transmission properties in the case of limited visibility (e.g., light rain and fogs) than in the case of near-infrared waves. The same conclusion was found in analytical investigations. (4) Conclusions: The results obtained show that optical radiation with a wavelength of about 10 µm in limited visibility is characterized by better transmission properties than near-infrared waves. This demonstrates the validity of designing FSO links operating in the range 8–12 µm band, e.g., based on quantum cascade lasers and HgCdTe photodiodes.

Reduction of the beam pointing error for improved free-space optical communication link performance

Free-space optical communication is emerging as a low-power, low-cost, and high data rate alternative to radio-frequency communication in short- to medium-range applications. However, it requires a close-to-line-of-sight link between the transmitter and the receiver. This paper proposes a robust H∞ control law for free-space optical (FSO) beam pointing error systems under controlled weak turbulence conditions. The objective is to maintain the transmitter–receiver line, which means the center of the optical beam as close as possible to the center of the receiving aperture within a prescribed disturbance attenuation level. First, we derive an augmented nonlinear discrete-time model for pointing error loss due to misalignment caused by weak atmospheric turbulence. We then investigate the H∞-norm optimization problem that guarantees the closed-loop pointing error is stable and ensures the prescribed weak disturbance attenuation. Furthermore, we evaluate the closed-loop outage probability error and bit error rate (BER) that quantify the free-space optical communication performance in fading channels. Finally, the paper concludes with a numerical simulation of the proposed approach to the FSO link’s error performance.

Performance analysis of free space optical communication system under rain weather conditions: a case study for inland and coastal locations of India

Free space optical communication (FSOC) has emerged as a potential candidate for providing huge bandwidth and solving the problem of spectrum congestion. But the atmospheric conditions limit the performance of FSOC link as the laser signal propagates through the atmospheric channel. This work aims to investigate the performance of FSOC link under average and heavy rainfall weather conditions of India. The meteorological data related to rainfall, over a duration of 4 years, was obtained from the Indian Meteorological Department for six different locations of India, representing the inland and coastal areas. The attenuation coefficient due to rain for all the locations has been calculated using mathematical models. The performance analysis of Wavelength Division Multiplexing based free space optical communication system has been analyzed by incorporating the attenuation due to rain for all considered locations. The inland location of Hyderabad has recorded the minimum attenuation coefficient of 1.91 dB/km due to rain and can support a link range of 5.43 km, corresponding to bit error rate of order of 10−9 under average rain weather conditions. On the other hand, the average rainfall is maximum for the coastal area of Mumbai, resulting in maximum attenuation coefficient of 4.08 dB/km. Moreover, the maximum link range for Mumbai is limited to 3.48 km, corresponding to bit error rate of order of 10–9 under average conditions of rain weather. Similarly, the inland locations of India have delivered a better link performance as compared to coastal areas under heavy rainfall weather conditions also.

Prediction of Attenuation using Visibility variations and other Meteorological Parameters in George, Western Cape, South Africa.

In this work, the impact of selected atmospheric parameters on attenuation of terrestrial and satellite signals over George, Western Cape in South Africa for the purpose of planning reliable and resilient free space optical communication links has been presented. The meteorological parameters data (visibility, temperature, rainfall rate, relative humidity, pressure, wind speed and wind direction) were obtained for ten years (2010 – 2019) from the South African Weather Service (SAWS). Total attenuation for the studied location was calculated from attenuation due to aerosol scattering, attenuation due to rain and attenuation due to scintillation. Three different wavelengths namely: 850 nm, 1200 nm and 1500 nm within the optical windows were used in calculating the wavelength dependent functions (attenuation due to aerosol scattering and attenuation due to scintillation). The result shows that attenuation due to rain was observed to account for 58% of the total attenuation and it is wavelength independent. Attenuation due to rain based on subtropics model was found to be about half of the calculated value based on temperate model. Further result shows total attenuation for worst visibility period (summer) throughout the year in the studied location is 7.7 dB/km (aerosol scattering: 0.53dB/km; rain: 4.5 dB/km and scintillation: 2.7 dB/km) at 850 nm. Overall results will be applicable in the area of the design and implementation of a reliable free space optical (FSO) links in the studied location.

Statistical modeling of received signal strength for an FSO link over maritime environment

Free space optical communications (FSO) have the potential to substantially improve communications technology in terms of channel capacity and offer an alternative to their RF counterpart. Additional characteristics related to security, immunity, flexibility and low cost issues render FSO a reasonable candidate for military applications. FSO technology does not come without challenges. Its major issue is the local meteorological parameters that give rise to various atmospheric phenomena. The purpose of this work is to facilitate the performance prediction of an FSO communication link over a maritime environment by utilizing macroscopic meteorological parameters, i.e. air temperature, wind speed, relative humidity, air pressure, dew point, solar radiation and sea temperature, obtained from point measurements. The received signal strength indicator (RSSI) of the FSO receiver has been utilized as the performance metric of the channel and a closed form expression has been deduced. The model has then been validated against real meteorological data and the predicted RSSI values exhibited a reasonably strong correlation with the observed ones. Atmospheric turbulence has been taken into account using the Navy Surface Layer Model (NAVSLaM) to estimate the structure index parameter from the same meteorological data and thus allowed for a statistical correlation between the refractive index structure parameter and RSSI.

Performance evaluation of free space optical link by incorporating the device parameters of quantum cascade laser based transmitter

This article investigates the performance of a free space optical (FSO) communication link by incorporating the device parameters that influence the optical output power of the quantum cascade laser (QCL). The transmitter side of the FSO link consists of gain-switched QCL operating at 9 µm. Short optical pulses possessing minimum full width half maximum (FWHM) and peak power as a result of ON-OFF keying are transmitted into the channel. The pulses are attenuated by medium bound losses and reach the receiver which employs quantum well infrared photodetector operating in the same wavelength. Average signal-to-noise ratio (SNRavg), bit error rate (BER) and Channel Capacity are computed for the FSO link for all the variations in the device parameters namely optical confinement factor (Γ), spontaneous emission factor (β) and mirror reflectivity (R). The combination of device parameters that gives the best link performance is assessed. From the analysis it is observed that, under minimum FWHM condition, QCL with Γ = 0.32 provides the best average SNR of 4.04 dB, lowest BER of 13.02 × 10−2 and maximum capacity of 1.82 bps Hz−1. When peak power is the constraint, QCL with mirror reflectivity of Γ = 0.45 provides the best average SNR of 24.78 dB, lowest BER of 7.75 × 10−35 and a peak capacity of 8.24 bps Hz−1.

Parameter estimation of Gamma–Gamma fading channel in free space optical communication

Free-space optical communication links experience atmospheric turbulent light intensity fluctuation fading, which requires accurate channel parameter estimation to ensure stable communication. In this paper, the maximum likelihood (ML) method is proposed to estimate composite Gamma–Gamma fading channel parameters. The ML estimation expression based on the expectation maximization (EM) to compute unknown parameters is derived. Newton–Raphson (N–R) and generalized moment method (GMM) are used to compare EM algorithm with the estimation performance. We used the Cramer–Rao Bound (CRB) to evaluate these estimation algorithms. Each estimator’s performance is compared using simulation data as well as experimental measurements under different meteorological conditions. It is found that the proposed technique has better estimation performance for fading parameters of atmospheric turbulence channel.

Characterization of terrestrial FSO link performance for 850 and 1310 nm transmission wavelengths

Abstract Over past few years, optical wireless communication has gained increased interest due to high achievable data rates, unlicensed bandwidth, inherited secure links and ease of deployment. In this paper, we analyse the performance of terrestrial free space optical (FSO) communication link considering internal parameters of the system. The performance of the system has been investigated and optimized by examining quality factor of the received signal considering different input powers, type of detectors, link ranges, beam divergence, data rates and modulation schemes employing 850 and 1310 nm transmission wavelength. Better quality of received signal has been achieved in case of different modulation schemes at a wavelength of 1310 as compared to 850 nm.