For Free Space Optical Communication Research Articles

Design and Experimental Demonstration of an Atmospheric Turbulence Simulation System for Free-Space Optical Communication

In this paper, the design of an atmospheric turbulence simulation system for free-space optical (FSO) communication is proposed. The system can accurately simulate repeatable atmospheric turbulent fading channels. It has a dynamic execution range of 30 dB with an execution rate of 1 MHz. The execution accuracy of the system is higher than 0.1 dB. In addition, the reliable TCP/IP protocol is used for communication, which ensures the convenience and versatility of the system. Experiments are carried out to investigate the performance of the system, and the results prove its superiority. Hence, the atmospheric turbulence simulation system can effectively simulate the influence of atmospheric turbulence channels on FSO communication systems.

Performance Analysis of Passive Retro-Reflector Based Tracking in Free-Space Optical Communications With Pointing Errors

In this correspondence, we propose a diversity-achieving retroreflector-based fine tracking system for free-space optical (FSO) communications. We show that multiple retroreflectors deployed around the communication telescope at the aerial vehicle save the payload capacity and enhance the outage performance of the fine tracking system. Through the analysis of the joint-pointing loss of the multiple retroreflectors, we derive the ordered moments of the received power. Our analysis can be further utilized for studies on multiple input multiple output (MIMO)-FSO. After the moment-based estimation of the received power distribution, we numerically analyze the outage performance. The greatest challenge of retroreflector-based FSO communication is a significant decrease in power. Still, our selected numerical results show that, from an outage perspective, the proposed method can surpass conventional methods.

Orbital angular momentum shift-keying communication through atmospheric turbulence and high scattering channel using a deep learning decoder

To increase the data rate and the channel capacity, orbital angular momentum shift keying (OAM-SK) has been studied extensively for free space optical (FSO) communication. However, the beams carrying OAM are disturbed into speckle patterns via the atmospheric turbulence (AT) and high scattering (HS) channel, which causes difficulty in decoding. Herein, we propose an AT and HS channel model to simulate the propagation of OAM beams, showing that the formed speckle size depends on the topological charge l of the OAM mode, which means the OAM mode can be recognized from the receiving speckle patterns. Proof-of-concept​ experiments of an OAM-SK system using a deep learning decoder are implemented. The results show that under dynamic AT with the refractive index structure constant (the AT strength, Cn2=1×10−16K2m−2/3) and static HS (a ground glass), we achieve a hybrid eight-mode OAM-SK, where the decoding accuracy of the encoding set l=1,2,3,4,5,6,7,8 reaches >0.95 within a 500 m transmission distance. Moreover, increasing the encoding mode interval can improve the decoding accuracy; when the AT strength increases to Cn2=1×10−14K2m−2/3 and the scattering media becomes dynamic, the accuracy of the encoding set l=1,4,7,10,13,16,19,22 with a mode interval of 3 can still achieve >0.94 at 100 m. Furthermore, error-free image transmission can be achieved in the OAM-SK system under AT strengths of Cn2=1×10−16K2m−2/3 and 1×10−15K2m−2/3 and HS at 100 m. This work provides a potential approach for FSO communication based on OAM modes under harsh AT and HS channels.

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.

Transmission characteristics of vortex frozen waves in different obstacle channels.

The obstacle in a channel is a typical scenario for free-space optical (FSO) communications, however, it will destroy the information in channels, especially for the orbital angular momentum (OAM) multiplexing systems and cause performance degradation. Motivated by the feature of predefining intensity profile, here we propose to use frozen wave (FW) carrying OAM for the FSO communications to mitigate the influence of obstacles on the beam propagation. The key idea is to design the longitudinal intensity profile of FW to distribute the beam energy of the location where the obstacle exists over a large region and focus again on the central region after the obstacle for propagation. By analyzing the cases under different sizes, positions, and shapes of the obstacles with on-axis and off-axis scenarios, it has been demonstrated that the detection probability of OAM mode carried by FW can be improved by 0.35 and 0.15 in short-distance and long-distance transmission scenarios, respectively, when compared to that carried by Bessel-Gaussian beam. It demonstrates the FWs have great potential in the OAM-based FSO communications, especially for the obstacle channels.

Secure turbulence-resistant coherent free-space optical communications via chaotic region-optimized probabilistic constellation shaping.

We propose a chaotic region-optimized probabilistic constellation shaping (CRPCS) scheme to enhance the security and the resistance to turbulence for free-space optical (FSO) communications. For this approach, a four-dimensional hyperchaotic system generates a pseudorandom sequence to rotate and encrypt the constellation. Constellation distribution of short pseudorandom sequences behaves as the law of a non-uniform character. Grouping long pseudorandom sequences and counting the characteristics of constellation distribution can realize probabilistic constellation shaping with low and fixed redundant information. We demonstrate a 56 Gbyte/s coherent FSO communication system based on log-normal and Gamma-Gamma turbulence models with a key space of 1075. The results show that the optical receiver sensitivity is improved by 0.3-1.1 dB, and the transmission distance is also improved by 3.2%-7.0% in different shaping cases.

Structured light signal transmission through clouds

Dynamic media such as atmospheric clouds and fog form a formidable barrier to light propagation for free-space optical communication (FSO). To overcome such an obstacle, we propose to make use of the acoustic properties of a laser filament coupled together with a donut-shaped signal beam. A filament generated by an ultrafast laser is accompanied by an acoustic wave that clears a cylindrical chamber around the filament’s plasma column that can mimic a transmission channel. We present a method to couple a Laguerre–Gauss beam through the obstacle-free channel. We image and measure the transmitted signal carried by the structured beam to demonstrate an efficient method for FSO through cloudy conditions, which requires low energy, is resilient to noise, and is unaffected by the filament.

Non-orthogonal PS-OOK transmission for FSO communications

This paper proposes a non-orthogonal polarization shift on-off keying (NOPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, NOPS-OOK signal is modulated with the non-orthogonal polarizations of bits ‘1’ and ‘0’ characteristics employing a single Mach-Zehnder modulator (MZM). At the receiver end, NOPS-OOK signal is detected using the cascaded polarization-insensitive and gain saturated semiconductor optical amplifier (SOA) and sloped-linear polarizer (SLP) with the polarization perpendicular to bit ‘0.’ The proposed technique is verified in experiments. The experimental results showed that the bit-error-rate (BER) performance of the proposed technique was close to the orthogonal polarization shift OOK (OPS-OOK) signal transmission.

Multi-modulation compatible miniaturization system for FSO communication assisted by chirp-managed laser.

In recent years, the thriving satellite laser communication industry has been severely hindered by the limitations of incompatible modulation formats and restricted Size Weight and Power (SWaP). A multi-modulation compatible method serving for free-space optical (FSO) communication has been proposed assisted by chirp-managed laser (CML). The corresponding demonstration system has been established for realizing free-switching between intensity (OOK) and phase modulation (RZ-DPSK). The feasibility and performance of system have been evaluated sufficiently when loading with 2.5 and 5 Gbps data streams, respectively. Additionally, a control-group system has been operated utilizing Mach-Zehnder modulator (MZM) for comparison between CML-based and MZM-based compatibility solutions. The OOK receiving sensitivities of CML-based system are -47.02 dBm@2.5 Gbps and -46.12 dBm@5 Gbps at BER of 1×10-3 which are 0.62 dB and 1.11 dB higher than that of MZM; the receiving sensitivities of RZ-DPSK are -50.12 dBm@2.5 Gbps and -47.03 dBm@5 Gbps which are 0.79 dB and 0.47 dB higher than that of MZM respectively. Meanwhile, CML-based transmitter abandoned the traditional modulator and its complicated supporting devices which can effectively contribute to the reduction of SWaP. The CML-based system has been proven to have the compatibility between intensity and phase modulation while also possesses a miniaturized design. It may provide fresh thinking to achieve a practical miniaturization system for satisfying the requirements of space optical network in future.

Performance of improved mode diversity reception for free-space optical communication under atmospheric turbulence

This paper first proposes and verifies two simplified mode diversity receivers (MDRs) for free-space optical (FSO) communication systems under moderate-to-strong turbulence. The MDRs are designed and implemented for coherent detection of the FSO communication system based on few-mode-fiber (FMF) coupling. The first MDR replaces the coherent receiver with a Kramers–Kronig (KK) receiver, which reduces the number of photodetectors (PDs) and high-speed analog-to-digital converters by 3/4. The second MDR realizes few-mode KK detection by utilizing a few-mode local oscillator and only one PD. This method simplifies the system of a MDR to the extreme. Experimental results show that the average received power of a FSO communication system based on six-mode fiber has approximately 6 dB gain compared with the one based on single-mode fiber (SMF) under moderate-to-strong turbulence. The two proposed MDRs have approximate performance, improving by 5 dB to 6 dB compared to SMF-based heterodyne detection under moderate-to-strong turbulence. In addition, the simplified schemes reduce the volume and the cost of the optical receiver, and it has some practical significance for FSO communication.

Performance evaluation of free space optics communication system in the scenario of triple play service using probabilistic shaping scheme

An advanced triple-play-services (TPS) system based on probabilistic shaping (PS) scheme for free- space-optical (FSO) communication network is proposed and experimentally demonstrated for the first time. In this paper, the PS-32-quadrature amplitude modulation (QAM) 10 Gb/s data, PS-32-QAM 10 Gb/s voice and PS based 2.5 Gb/s HDTV signal are successfully transmitted over 600-m FSO link employing doublet lens scheme. The PS and polarization multiplexing technique both are helped to upgrade the spectral efficiency and improve the system performance. Normalized-generalized-mutual-information (NGMI) threshold is 0.92 at FEC code rate 9/10 for voice/data and 0.82 at FEC code rate 4/5 for video signal are selected to successfully execute the experiment. The spectral efficiency of our system is achieved by calculating the NGMI and shaping parameter (α), 13.6 bits/s/Hz (voice and data) and 10.41 bits/s/Hz (video) spectral efficiency are obtained at maximum α. Experimental results prove that PS based FSO communication system provides flexible data rate and greater spectrum efficiency and also be a powerful candidate to alleviate pressure on modern optical transport system.

Performance of FSO Communication Systems Employing Alamouti-Type Space-Time Encoding Over Málaga Channels With Pointing Errors

In this work, spatial diversity techniques have been proposed for free-space optical (FSO) communication systems to combat the deteriorating effects, such as atmospheric turbulence effects and pointing errors, and the performance of FSO communication systems with Alamouti encoding scheme over Málaga <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(\mathcal {M})$</tex-math></inline-formula> turbulence channel is investigated. We first derive the probability distribution function (PDF) of end-to-end channel gain under atmospheric turbulence and pointing error circumstances. Then, by capitalizing on this PDF, closed-form expressions of the average bit error rate (BER) and the outage probability (OP) for the proposed system are obtained. Additionally, to provide more insights, the asymptotic expressions for the average BER and the OP are also derived. In the analysis, intensity modulation/direct detection and heterodyne detection techniques are considered so that the obtained results can cover both cases. Furthermore, analytic results are successfully validated through Monte Carlo simulations. Our results highlight the gains in performance that can be achieved when Alamouti encoding scheme is employed in FSO communication systems.

Adaptive Probabilistic Shaping Using Polar Codes for FSO Communication

An adaptive polar coded probabilistic shaping scheme for free-space optical (FSO) communication is proposed and demonstrated to improve transmission performance. The input distribution can be adaptively adjusted by maximizing the achievable rate under different FSO turbulence channels. Furthermore, the transmission rate is less than the achievable rate to guarantee reliable transmission. In different FSO turbulent channels, the feasible regions of the optimization problem are explored with the aid of the exponential distribution, and the complexity of the single optimization is decreased to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$O( {\log ( {{\varepsilon ^{ - 1}}} )} )$</tex-math></inline-formula> . The systematic polar codes are adopted to combat the fading, and a sparse-dense encoder for polar codes is designed to maintain the target distribution. The achievable rate and block error rate (BLER) performance of this adaptive probabilistic shaping scheme are evaluated. The results indicate that the adaptive non-uniform distribution generated by the proposed scheme outperforms the uniform distribution under the same channel conditions. The code rate of 0.8 is more suitable than 0.9 in coping with various turbulence channels. The proposed scheme can obtain a range of 0.50∼0.90 dB shaping gains over the uniform distribution from weak to moderate turbulence channels for both the achievable rate and the BLER performance at 1.5 bits per channel use.

On the Design of FSO-Based Satellite Systems Using Incremental Redundancy Hybrid ARQ Protocols With Rate Adaptation

This paper presents a cross-layer design of error-control protocols with rate adaptation for free-space optical (FSO) burst transmission in satellite communication systems. While the prior works mostly focused on the design of error-control protocols for terrestrial-based FSO communications, using either hybrid automatic repeat request (HARQ) based stop-and-wait mechanism or pure sliding window ARQ, we instead propose a design of HARQ-based sliding window mechanism for the high data-rate and long-distance of FSO-based satellite systems. In particular, we model and analyze the performance of low-earth orbit (LEO) satellite-to-ground FSO systems over atmospheric turbulence channels when truncated incremental redundancy (IR) HARQ protocols, which combine the rate-compatible punctured convolutional (RCPC) codes and sliding window ARQ, are employed. For this purpose, the time-varying behavior of atmospheric turbulence channels is first captured by a finite-state Markov chain. Then, the channel model is used to develop the burst loss model to analytically derive the system performance metrics, including throughput, energy efficiency, and average frame delay. The results quantitatively demonstrate the impact of atmospheric turbulence and pointing error on the system performance and support the optimal selection of parameters. Additionally, the effectiveness of the proposed system is numerically confirmed. Monte Carlo simulations are also performed to validate the accuracy of theoretical derivations.

Influence of Source Parameters and Non-Kolmogorov Turbulence on Evolution Properties of Radial Phased-Locked Partially Coherent Vortex Beam Array

Partially coherent optical vortices have been applicated widely to reduce the influence of atmospheric turbulence, especially for free-space optical (FSO) communication. Furthermore, the beam array is an effective way to increase the power of the light source, and can increase the propagation distance of the FSO communication system. Herein, we innovatively report evolution properties of the radial phased-locked partially coherent vortex (RPLPCV) beam array in non-Kolmogorov turbulence. The analytical expressions for the cross-spectral density and the average intensity of an RPLPCV beam array propagated through non-Kolmogorov turbulence are obtained. The numerical results reveal that the intensity distribution of the RPLPCV array propagated in the non-Kolmogorov turbulence is gradually converted to a standard Gaussian distribution. In addition, the larger the radial radius, radial number and waist radius are, the smaller the coherence length is. Moreover, the longer the wavelength is, the shorter the propagation distance required for the intensity distribution of the RPLPCV beam array to be converted into a Gaussian distribution in the non-Kolmogorov turbulence. The research in this paper provides a theoretical reference for the selection of light sources and the suppression of turbulence effects in wireless optical communication.

Design of 16 × 40 Gbps hybrid PDM-WDM FSO communication system and its performance comparison with the traditional model under diverse weather conditions of Bangladesh

Abstract A comprehensive design is proposed for free-space optical (FSO) communication system by hybridizing polarization division multiplexing (PDM) with wavelength division multiplexing (WDM) and its performance is investigated under diverse turbulent weather conditions of Bangladesh. Here we consider gamma–gamma (G–G) distribution for the turbulent FSO channel model. Moreover, a PDM-WDM technique not only maximizes the link capacity of FSO system but also enhances the spectral efficiency (SE) of the system. Besides, the performance of this hybrid PDM-WDM FSO system is compared with the traditional model and the proposed hybrid system exhibits excellent performance under diverse atmospheric conditions of Bangladesh. Performance analysis of the proposed model as well as the comparison with the traditional model is described in terms of optical power spectrum (OPS), optical signal to noise ratio (OSNR), bit error rate (BER), Q factor, constellation diagrams, and eye diagrams.

Impact of probability density function model choice in intensity-modulation direct-detection free-space optical communication analyses

Performance metrics such as mean-bit-error rate and probability of fade for free-space optical communication (FSOC) applications using intensity-modulation direct detection are theoretically calculated based on probability density functions (PDFs) describing irradiance fluctuations. Theoretical calculations using common PDF models can result in significant errors in prediction of performance metrics. In particular, these models do not consider the change in skewness of the distribution as the aperture size increases, and often positively skewed distributions (right tailed) are used to model scenarios where the true statistics are negatively skewed (left tailed). We evaluate the magnitude of errors in the prediction of bit-error rate and probability of fade based on simulation data for a collimated Gaussian beam in a realistic FSOC scenario for two strengths of turbulence and varying aperture sizes. The PDF models considered are lognormal, gamma–gamma, inverse-Gaussian-gamma, fractional exponential, exponentiated-Weibull, 3-parameter-Weibull, and normal distributions.

Pilot beam-assisted adaptive compensation for atmospheric turbulence in free-space optical transmission of beams carrying orbital angular momentum

One of the major challenges for free-space optical (FSO) communication through atmosphere is the turbulence caused by the random variations in temperature and convective motion of air. It induces fluctuation in intensity and phase of the propagating optical signal and impairs the transmission quality. The FSO communication using beams carrying orbital angular momentum would be vulnerable to such fluctuation since they have a unique spatial phase profile of the helical phase front. In order to overcome the issue, we propose a pilot beam-assisted adaptive compensation method for the turbulence-induced phase fluctuation by using spatial light modulators and a wavefront sensor. The proposed method is a potential candidate for real-time compensation due to its fast operation without algorithmic converging process. The propagation of Laguerre–Gaussian beam through atmospheric turbulence is numerically analyzed based on the modified von Karman model and split-step beam propagation method. The simulation results reveal that the compensation performances are considerably improved by using the pilot beam which has the same azimuthal index as that of the data beam.

Adaptive cooperation for free space optical communications

In this paper, we propose a new Adaptive Cooperation (AC) protocol for Free Space Optical (FSO) communications. Two versions of the protocol are presented. The first protocol uses the Instantaneous Signal to Noise Ratio (ISNR) to activate the relay only when the corresponding instantaneous throughput is better than that of the direct link. The second protocol uses the Average SNR (ASNR) to activate the relay when its average throughput is larger than that of the direct link. The proposed AC protocol has been extended to include Adaptive Modulation and Coding (AMC). Our results are valid for FSO communications in the presence of Gamma Gamma atmospheric turbulence.

Estimating refractive index structure constant and extinction coefficient under misalignment

This paper presents two methods, named the irradiance fitting method and the irradiance integration method, for the estimation of refractive index structure constant (Cn2) and extinction coefficient under misalignment. Validations of the methods are carried out based on experimentally obtained speckle images. These two methods are feasible and accurate regardless of whether the optical axis misalignment distance is calibrated, which provides an effective and convenient approach for free space optical communication (FSOC) atmospheric channel research.