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Photonics
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Advances in Free Space Optical Communications and Optical Turbulence Modeling
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Advances in Free Space Optical Communications and Optical Turbulence Modeling

A special issue of Photonics (ISSN 2304-6732). This special issue belongs to the section "Optical Communication and Network".

Deadline for manuscript submissions: closed (15 December 2022) | Viewed by 12365

Special Issue Editors

Dr. Italo Toselli

E-Mail Website
Guest Editor
Fraunhofer Institute of Optronics, System Technologies and Image Exploitation IOSB, Gutleuthausstraße 1, 76275 Ettlingen, Germany
Interests: turbulence; turbulence modeling; modeling and simulation; optics; adaptive optics; atmospheric optics; lidar; optics and photonics; laser; light
Dr. Rubén Boluda-Ruiz

E-Mail Website
Guest Editor
Wireless Optical Communications Lab., Institute of Telecommunication Research (TELMA), University of Málaga, E-29071 Málaga, Spain
Interests: optical wireless communication systems; multiple-input systems; multiple-output (MIMO) systems; channel modeling; cooperative communications; physical layer security aspects
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue aims to discuss the latest advances on free-space optical communications and optical turbulence modeling. Although interest in FSO systems has recently been increasing and many systems have been implemented, most of them cannot yet successfully operate in strong fog and turbulence conditions. Such challenging scenarios require the use of new techniques including modulation formats, laser beams that are less affected by turbulence, wavefront sensors that are more resilient to scintillation, adaptive optics and so on. In the atmosphere, propagation though deep turbulence can be present at low elevation angles for satellite communications or already after a few kilometers of horizontal paths for terrestrial links. Even more challenging conditions are present in the ocean, where propagation is limited to a hundred meters or less and scattering is dominant. This Special Issue encourages discussions on new methods and techniques to increase the effectiveness of FSO systems operating in the atmosphere and/or underwater.

Topics include, but are not limited to:

  1. New modulation techniques;
  2. Correction codes (FEC) capable of performing better in turbulent channels;
  3. Laser beams with different shapes and coherence properties (OAM, “exotic” beams);
  4. New wavefront sensors (sensor-less, oleographic, improved Shack–Hartmann);
  5. Adaptive optics techniques;
  6. New power spectrums for turbulence modeling in the atmosphere or ocean;
  7. Investigations on laser beam propagation through the atmosphere, ocean or both (airplane to submarine case), including sea-surface (waves) refractive effects;
  8. Optical system performance (BER, SNR, fading, outage probability);
  9. New probability density function (Pdf) models to describe the statistics at the focal plane of beam intensity and phase, coupling efficiency;
  10. Budget link analysis of real FSO scenarios.

Dr. Italo Toselli
Dr. Rubén Boluda-Ruiz
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Photonics is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • New modulation techniques
  • Correction codes (FEC) capable of performing better in turbulent channels
  • Laser beams with different shapes and coherence properties (OAM, “exotic” beams)
  • New wavefront sensors (sensor-less, oleographic, improved Shack–Hartmann)
  • Adaptive optics techniques
  • New power spectrums for turbulence modeling in the atmosphere or ocean
  • Investigations on laser beam propagation through the atmosphere, ocean or both (airplane to submarine case), including sea-surface (waves) refractive effects
  • Optical system performance (BER, SNR, fading, outage probability)
  • New probability density function (Pdf) models to describe the statistics at the focal plane of beam intensity and phase, coupling efficiency
  • Budget link analysis of real FSO scenarios

Published Papers (7 papers)

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Research

14 pages, 4011 KiB  
Article
Polar-Coded Transmission over 7.8-km Terrestrial Free-Space Optical Links
by Shingo Fujita, Eiji Okamoto, Hideki Takenaka, Hiroyuki Endo, Mikio Fujiwara, Mitsuo Kitamura, Ryosuke Shimizu, Masahide Sasaki and Morio Toyoshima
Photonics 2023, 10(4), 462; https://doi.org/10.3390/photonics10040462 - 17 Apr 2023
Viewed by 1312
Abstract
Free-space optical (FSO) communications can offer high-capacity transmission owing to the properties of the laser beams. However, performance degradation caused by atmospheric turbulence is an urgent issue. Recently, the application of polar codes, which can provide capacity-achieving error-correcting performance with low computational cost [...] Read more.
Free-space optical (FSO) communications can offer high-capacity transmission owing to the properties of the laser beams. However, performance degradation caused by atmospheric turbulence is an urgent issue. Recently, the application of polar codes, which can provide capacity-achieving error-correcting performance with low computational cost for decoding, to FSO communications has been studied. However, long-distance and real-field experiments have not been conducted in these studies. To the best of our knowledge, this study is the first to present the experimental results of polar-coded transmission over 7.8-km FSO links. Using experimental data, we investigated the performance of polar codes over atmospheric channels, including their superiority to regular low-density parity-check codes. We expect that our results will offer a path toward the application of polar codes in high-speed optical communication networks including satellites. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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8 pages, 1989 KiB  
Communication
Performance Analysis of LDPC-Coded OFDM in Underwater Wireless Optical Communications
by Jianzhong Guo, Jinpeng Xiao, Jing Chen, Xin Shan, Dejin Kong, Yan Wu and Yong Ai
Photonics 2023, 10(3), 330; https://doi.org/10.3390/photonics10030330 - 20 Mar 2023
Cited by 2 | Viewed by 1503
Abstract
The performance of Low-Density Parity-Check (LDPC)-coded Orthogonal Frequency Division Multiplexing (OFDM) is investigated over turbulence channels in underwater wireless optical communications (UWOC). The relation between the bit error ratio (BER) and parameters such as the scintillation coefficient, signal-to-noise ratio (SNR), length of LDPC [...] Read more.
The performance of Low-Density Parity-Check (LDPC)-coded Orthogonal Frequency Division Multiplexing (OFDM) is investigated over turbulence channels in underwater wireless optical communications (UWOC). The relation between the bit error ratio (BER) and parameters such as the scintillation coefficient, signal-to-noise ratio (SNR), length of LDPC code, and order of OFDM is quantified by simulation. Results show that while the OFDM with subcarrier quadrature amplitude modulation (QAM-OFDM) has slightly better anti-turbulence performance than the OFDM with subcarrier phase shift keying modulation (PSK-OFDM), the LDPC-coded QAM-OFDM has a much better performance than the QAM-OFDM and the LDPC-coded PSK-OFDM, and, at SNR = 12, it decreases the BER by four orders of magnitude compared to the 16QAM-OFDM system when the scintillation coefficient σξ2 = 0.05. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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12 pages, 4775 KiB  
Communication
CoolMomentum-SPGD Algorithm for Wavefront Sensor-Less Adaptive Optics Systems
by Zhiguang Zhang, Yuxiang Luo, Huizhen Yang, Hang Su and Jinlong Liu
Photonics 2023, 10(2), 102; https://doi.org/10.3390/photonics10020102 - 18 Jan 2023
Cited by 3 | Viewed by 1855
Abstract
Instead of acquiring the previous aberrations of an optical wavefront with a sensor, wavefront sensor-less (WFSless) adaptive optics (AO) systems compensate for wavefront distortion by optimizing the performance metric directly. The stochastic parallel gradient descent (SPGD) algorithm is pervasively adopted to achieve performance [...] Read more.
Instead of acquiring the previous aberrations of an optical wavefront with a sensor, wavefront sensor-less (WFSless) adaptive optics (AO) systems compensate for wavefront distortion by optimizing the performance metric directly. The stochastic parallel gradient descent (SPGD) algorithm is pervasively adopted to achieve performance metric optimization. In this work, we incorporate CoolMomentum, a method for stochastic optimization by Langevin dynamics with simulated annealing, into SPGD. Numerical simulations reveal that, compared with the state-of-the-art SPGD variant, the proposed CoolMomentum-SPGD algorithm achieves better convergence speed under various atmospheric turbulence conditions while requiring only two tunable parameters. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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19 pages, 742 KiB  
Article
Systematic Performance Analysis of Hybrid FSO/RF System over Generalized Fading Channels with Pointing Errors
by Yan Wu, Mengwan Jiang, Gang Li and Dejin Kong
Photonics 2022, 9(11), 873; https://doi.org/10.3390/photonics9110873 - 18 Nov 2022
Cited by 1 | Viewed by 1604
Abstract
Hybrid free space optical (FSO)/radio frequency (RF) system has attracted extensive attention because of its advantages of both the FSO and RF links. From the viewpoint of overall system performance, this paper presents a systematic analysis method of communication performance and security performance [...] Read more.
Hybrid free space optical (FSO)/radio frequency (RF) system has attracted extensive attention because of its advantages of both the FSO and RF links. From the viewpoint of overall system performance, this paper presents a systematic analysis method of communication performance and security performance of the hybrid FSO/RF system with the Málaga turbulence channel and the α−μ fading channel. The hybrid FSO/RF system adopts the diversity method of maximum ratio combining (MRC) to receive signals. The new expressions of communication performance parameters (i.e., the bit error rate, the outage probability, the ergodic channel capacity) of the only FSO system and the hybrid system are obtained. Then, the new expressions of the security performance parameters (i.e., the security outage probability and the strictly positive secrecy capacity) of the hybrid system with the FSO or RF links eavesdropping are derived, respectively. Our derived analytical expressions present an efficient tool to investigate the impact of system parameters on the overall performance of the hybrid system, namely modulation scheme, turbulence intensity, pointing errors, target rate, and eavesdropper output signal-to-noise ratio. The simulation results show that compared with the only FSO system, the hybrid system can significantly improve the communication performance of the system; the communication performance of the hybrid system using coherent binary phase shift keying (CBPSK) modulation is obviously better than the other two modulation technologies; with the deterioration of atmospheric environment (increasing turbulence intensity and pointing errors), the communication performance and security performance of the hybrid system will decline; both RF link eavesdropping and FSO link eavesdropping have a greater impact on the security performance of the hybrid systems; whether it is FSO link eavesdropping or RF link eavesdropping, the reduction of target rate and output signal-to-noise ratio of the eavesdropper can improve the security performance of the hybrid system. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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15 pages, 805 KiB  
Article
Performance Analysis of a Multi-Hop Parallel Hybrid FSO/RF System over a Gamma–Gamma Turbulence Channel with Pointing Errors and a Nakagami-m Fading Channel
by Yan Wu, Jing Chen, Jianzhong Guo, Gang Li and Dejin Kong
Photonics 2022, 9(9), 631; https://doi.org/10.3390/photonics9090631 - 02 Sep 2022
Cited by 7 | Viewed by 1270
Abstract
Due to the influence of the atmospheric environment and pointing errors, the performance of free space optical communication is greatly limited. In this paper, we propose a parallel multi-hop hybrid free space optical (FSO)/radio frequency (RF) system to improve the system performance. The [...] Read more.
Due to the influence of the atmospheric environment and pointing errors, the performance of free space optical communication is greatly limited. In this paper, we propose a parallel multi-hop hybrid free space optical (FSO)/radio frequency (RF) system to improve the system performance. The FSO sub-link and RF sub-link are modeled by Gamma–Gamma turbulence with pointing errors and Nakagami-m distributions, respectively. Based on the selective combination scheme, the probability density function (PDF) and cumulative distribution function (CDF) of the output signal-to-noise ratio (SNR) of the hybrid FSO/RF one-hop or direct link are obtained. Then, the PDF and CDF of the output SNR of the parallel multi-hop hybrid system are derived with the decoded forward (DF) protocol considered. Finally, the expressions of the average bit error rate (ABER) and outage probability are derived for the parallel multi-hop hybrid system, the hybrid FSO/RF direct link, and the FSO-only direct link. The results show that the parallel multi-hop hybrid system can effectively mitigate the negative impact of atmospheric turbulence and pointing errors and can significantly improve the system performance. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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13 pages, 3127 KiB  
Article
The Polar Code Construction Method in Free Space Optical Communication
by Yang Cao, Wenqing Li, Jing Zhang, Xiaofeng Peng and Yue Li
Photonics 2022, 9(9), 599; https://doi.org/10.3390/photonics9090599 - 24 Aug 2022
Cited by 1 | Viewed by 1447
Abstract
In order to solve the problem of the high complexity of polarization code construction under free-space optical communication, this paper proposes a segmented turbulent partial order construction method, which effectively reduces the complexity of polarization code construction while obtaining the highest possible quality [...] Read more.
In order to solve the problem of the high complexity of polarization code construction under free-space optical communication, this paper proposes a segmented turbulent partial order construction method, which effectively reduces the complexity of polarization code construction while obtaining the highest possible quality of communication performance. The method introduces the generalized partial order technique into the atmospheric strong turbulent channel, determines the reliability relationship between each sub-channel in the strong turbulence using Monte Carlo simulation, and combines it with the polarization weight formula to find the optimal parameter values in different signal-to-noise ratio ranges to construct the polarization code. The simulation results showed that the method had a similar communication performance compared with the conventional Monte Carlo construction method for different turbulence intensities, code rates, and code lengths, and its construction complexity was negligible due to its offline operation nature. For different code rate code lengths and BER of 10-4, this method generated about 0.03–0.17 dB loss compared with the Monte Carlo method; for different turbulence strengths and BER of 10-4, only a loss of about 0.05–0.07 dB was generated. It provides a solution for the efficient application of polarization codes under free-space optical communication. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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30 pages, 19937 KiB  
Article
Analysis of Scintillation Effects on Free Space Optical Communication Links in South Africa
by Olabamidele O. Kolawole, Thomas J. O. Afullo and Modisa Mosalaosi
Photonics 2022, 9(7), 446; https://doi.org/10.3390/photonics9070446 - 25 Jun 2022
Cited by 3 | Viewed by 2183
Abstract
The performance of free space optical communication (FSOC) systems is severely degraded by certain atmospheric conditions prevalent in places where they are deployed, in spite of their numerous advantages. In clear weather conditions, the random fluctuation in the atmosphere’s refractive index causes substantial [...] Read more.
The performance of free space optical communication (FSOC) systems is severely degraded by certain atmospheric conditions prevalent in places where they are deployed, in spite of their numerous advantages. In clear weather conditions, the random fluctuation in the atmosphere’s refractive index causes substantial scintillation losses to transmitted optical signals. It is therefore imperative to estimate the potential losses due to atmospheric turbulence in locations where FSOC links are to be deployed. This will provide the necessary fade margin for FSOC systems so that designed links withstand such atmospheric disturbances. In this paper, statistical analysis of wind speed data collected for various cities of South Africa is used for calculating the corresponding refractive index structure parameter (Cn2). These Cn2 values, as well as the zero inner scale and infinite outer scale model and finite inner and finite outer scale model, are used in computing the scintillation indices not exceeding 50%, 99%, 99.9%, and 99.99% of the time for the investigated locations. The Lognormal and Gamma–gamma distribution models are then employed for the computational analysis of the irradiance fluctuations and channel characteristics while considering the effect of pointing errors for weak and moderate to strong turbulence regimes. Finally, derived mathematical expressions for outage probabilities and bit error rate (BER) performances for FSOC links, employing various intensity modulation and direct detection (IM/DD) schemes, are presented. Full article
(This article belongs to the Special Issue Advances in Free Space Optical Communications and Optical Turbulence Modeling)
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