Performance Of Free Space Optical Link Research Articles

Exploring FSO link performance in varied atmospheric conditions to optimize 5G communication with a polarized quasi-diffuse transmitter

Abstract Free space optical (FSO) communication is an innovative technology that holds immense promise for numerous applications thanks to its high data transmission rates, rapid scalability, cost-effectiveness, superior security, and comprehensive capacity access techniques for Gbps data transmission. FSO communication relies on the Earth’s atmosphere as the medium, which introduces atmospheric disturbances that have driven the development of spatial diversity techniques aimed at enhancing the technology’s performance. Our proposed system employs a polarized quasi-diffused system with a fork as a spatial diversity scheme, which eliminates the need for multiple transmitters. We have developed a model that integrates both simulation designs to achieve optimal results. The proposed model has demonstrated excellent performance under various atmospheric turbulences, including rain, fog, and haze, exhibiting a very high maximum quality factor, improved received power, and better bit error rate (BER). Finally, we have obtained, analyzed, and extensively discussed the simulation results to provide a comprehensive understanding of the proposed model’s potential benefits. Our simulations show a maximum quality factor of 23 compared to existing models in much better, a 34 % increase in received power, and a 31 % decrease in BER compared to existing models. These results highlight the potential benefits of the proposed model for FSO communication systems.

Improving FSO Link Performance using PolSK Modulation

Free Space Optical (FSO) communication systems are based on the use of infrared light to transmit information at high speed through the atmospheric channel. Their performance is therefore affected by the characteristics of the atmospheric channel. One of the most widely used methods of compensating for the effects of atmospheric factors is modulation.The main aim of this research is to demonstrate the improvement in FSO link performance using PolSK modulation in a subtropical environment such as Cˆote d’Ivoire. The performance indicators considered are bit error rate, channel capacity, outage probability and energy efficiency. The mathematical expression of the evolution of each of these indicators as a function of the atmospheric parameters of our environment was determined using the gamma-gamma model. A numerical evaluation of the results obtained was carried out using Matlab software. The results show that PolSK modulation offers the best results in terms of BER reduction, with a BER of the order of 10-11 for an SNR of 50dB, compared with BERs of 10-4 and 10-2 for BPSK and OOK modulations respectively. We obtain a probability of interruption of the order of 10-3 for PolSK modulation compared with 10-2 for BPSK modulation and 10-1 for OOK modulation. As for the channel capacity, we obtain the same value (4:1458bit/s) for PolSK and BPSK modulation compared with 3:8181bit/s for OOK modulation. BPSK modulation, on the other hand, stands out for its excellent energy efficiency.

Maritime free space optical communications field test and link budget statistics

During sub-optimal weather, a free-space optical (FSO) link range degrades depending on attenuation (atmospheric extinction) and turbulence effects. The ability to predict the system level performance can be exceedingly challenging as the atmospheric variability in a maritime link can be large and difficult to model. Link budget estimation for FSO systems often takes a nominal view of atmospheric conditions; here, we use statistical atmospheric predictions specific to a geographic area of interest to enable performance trades to be evaluated through link budget analysis. We compare these models to field-collected data to show the utility of the statistical atmospheric analysis in predicting FSO link performance for specific parts of the world. We have performed shore-to-ship FSO communications field tests at 10 Gb/s with links reaching out to a horizon limit over 40 km away in times of moderate extinction to clear weather. We provide further analysis by describing the expected performance of the link using statistical probabilities via cumulative distribution functions of both extinction and turbulence. The atmospheric variability can be determined for nearly any region of interest through the implementation of numerical weather prediction data to calculate the atmospheric performance drivers. These conditions are specifically evaluated for the 2017 Trident Warrior field test off the coast of San Diego, California, USA.

Analysis on the performance of reconfigurable intelligent surface-aided free-space optical link under atmospheric turbulence and pointing errors

<span lang="EN-US">Free-space optical (FSO) communication can provide the cost-efficient, secure, high data-rate communication links required for applications. For example, it provides broadband internet access and backhauling for the fifth-generation (5G) and the sixth-generation (6G) communication networks. However, previous solutions to deal with signal loss caused by obstructions and atmospheric turbulence. In these solutions, reconfigurable intelligent surfaces (RISs) are considered hardware technology to improve the performance of optical wireless communication systems. This study investigates the pointing error effects for RIS-aided FSO links under atmospheric turbulence channels. We analyze the performance of RIS-aided FSO links influenced by pointing errors, atmospheric attenuation, and turbulence for the subcarrier quadrature amplitude modulation (SC-QAM) technique. Atmospheric turbulence is modeled using log-normal distribution for weak atmospheric turbulence. Several numerical outcomes obtained for different transmitter beam waist radius and pointing error displacement standard deviation are shown to quantitatively illustrate the average symbol error rate (ASER).</span>

Modeling and investigation on the performance enhancement of hovering UAV-based FSO relay optical wireless communication systems under pointing errors and atmospheric turbulence effects

This paper investigates and enhances unmanned aerial vehicle (UAV) relay-assisted free-space optics (FSO) optical wireless communication (OWC) systems under the effects of pointing errors (PEs) and atmospheric turbulences (ATs). The incorporation of UAVs as buffer-aided moving relays in the conventional FSO (CFSO) relay-assisted systems is proposed for enhancing the performance of PEs through AT. Using M-PSK (phase shift keying) and M-QAM (quadrature amplitude modulation), the impact of PEs on transmission quality is evaluated in this work. We evaluate and optimize the symbol error rate, outage probability (OP), and signal-to-noise ratio (SNR) for the UAV-to-ground station-based FSO communications systems. The spatial diversity-based relay-assisted CFSO systems can enhance the performance of the UAV-UAV FSO links. In this paper, a new FSO (NFSO) channel model for the hovering UAV-FSO OWC fluctuations under the PEs, AT effects, jitter, deviation, receiving an error, and wind resistance effects are established. To improve the performance of hovering UAV-based FSO relay OWC systems. We reduce the influence of UAV-FSO OWC fluctuations under PEs and AT effects. By receiving incoherent signals at various locations, the spatial diversity-based relay-assisted NFSO systems can significantly increase the system's redundancy and enhance connection stability. Numerical results show that to achieve a bit-error-rate (BER) of le 10^{ - 5}, the required SNR is ≥ 23 dB when the wind variance of the UAVs sigma_{alpha }^{2} increases from 0 to 7 mrad with FSO link distance L = 2000 m. The required SNR is ≥ 25 dB when the wind variance sigma_{alpha }^{2} is 1 mrad at an OP of 10^{ - 6}. To obtain an average BER of 10^{ - 6}, the SNR should be 16.23 dB, 17.64 dB, and 21.45 dB when sigma_{alpha }^{2} is 0 mrad, 1 mrad, and 2 mrad, respectively. Using 8-PSK modulation without PEs requires 23.5 dB at BER of 10^{ - 8} while 16-QAM without PEs requires 26.5 dB to maintain the same BER of 10^{ - 8}. Compared with 16-QAM without PEs, the SNR gain of 8-PSK without PEs is 3 dB. The results show the relay-assisted UAV-FSO system with five stationary relays can achieve BER 10^{ - 8} at 25 dB SNR in the ideal case and 10^{ - 5} at 27 dB SNR with AT and PE at FSO length 1000 m. The results show the relay UAV-FSO system outperforms the CFSO at the BER and SNR performance. The effects of UAV’FSO s fluctuation increase when the UAV-FSO link length, {text{L}}_{{{text{fso}}}} increases. The results of the weak turbulence achieve better SER compared with MT and ST. The obtained results show that decreasing {text{L}}_{{{text{fso}}}} can compensate for the effects of UAV-FSO link fluctuation on the proposed system. Finally, we investigated the CFSO relay-assisted UAV-FSO system with aided NFSO-UAVs spatial diversity-based relay-based on NFSO OWC and revealed the benefits of the resulting hybrid architecture.

Reliability analysis of optical IRS-enabled FSO system with multiple detectors and PCB under composite turbulence fading channels considering the impact of GML

Free space optical (FSO) communication with optical intelligent reflecting surface (IRS) assistance is one promising and evolving technology that can relax the line-of-sight requirement and enable more efficient and reliable link performance in obstacles areas. In this work, an optical IRS-enabled FSO system is proposed with partially coherent beam (PCB) and multiple detectors. Specifically, for an end-to-end (e2e) IRS-assisted FSO link, the statistical model of optical signal fluctuations due to atmospheric turbulence is assumed to follow Fisher–Snedecor F-distribution, and the random movements of IRS, transmitter (Tx), and receiver (Rx) due to building sway is modeled by geometric and misalignment losses. Considering the Gaussian–Schell model and intensity modulation/direct detection, the analytical average bit error rate (ABER) expressions of the studied system are obtained for a single detector and multiple detectors, respectively, with selected combining (SC), equal gain combining (EGC) and maximum ratio combining (MRC) aperture diversity schemes over independent identically distributed composite fading channels. And the correctness of the analytical results is verified through the Monte-Carlo based statistical simulation method. Results show that the ABER and outage probability performance of IRS-assisted FSO link degrades with the increase in the strength of atmospheric turbulence and building sway. And even the position fluctuations variances of Tx, IRS, and Rx are the same, their impacts on the e2e IRS-assisted FSO link are not the same. Interestingly, aperture diversity techniques can significantly mitigate these adverse effects, especially in the case of MRC scheme. Besides, the studied system with PCB can achieve better system performance compared to that using coherent beam, and the performance gap would become more obvious as the spatial coherence radius decreases. This work could serve as a guide for the design and research of IRS-enabled FSO communication systems.

Ergodic Capacity Analysis of Satellite Communication Systems With SAG-FSO/SH-FSO/RF Transmission

Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles.

Performance Analysis of FSO Systems Over a Lognormal-Rician Turbulence Channel With Generalized Pointing Errors

In this paper, an approximate closed-form probability density function expression for the Lognormal-Rician atmospheric turbulence with generalized pointing errors model is developed by using a series representation. The Kolmogorov-Smirnov (KS) goodness-of-fit statistical tool is employed to verify the accuracy of the proposed approximation. We demonstrate that a more efficient approximation can be achieved for the smaller jitter variances and nonzero boresight displacements. To reveal the importance of the proposed approximation, the approximate analytic expressions for the ergodic capacity and outage performance of single-input single-output free-space optical (FSO) systems are derived. It is observed that the pointing errors significantly degrade the performance of FSO links. By using the obtained asymptotic expressions, the impact of pointing errors on the ergodic capacity is considerably mitigated by selecting the optimum beam width at the transmitter side. We find that the optimum beam width is independent of atmospheric turbulence and the Pearson correlation coefficient in high signal-to-noise ratio conditions. All of the derived analytical results are confirmed by Monte Carlo simulation results.

Adaptive subcarrier intensity modulation for free space optical communication

Increased capacity demands and radio frequency (RF) congestion impacts on current communication networks have brought greater attention to free-space optical (FSO) communication as a viable augmentation technology for terrestrial, aerial, and space-based communication infrastructure. As a complementary alternative to RF communication systems, FSO can support high link bandwidths and provide high data security without RF spectral constraints. The performance of FSO links, however, can be significantly impacted by receive power variation caused by propagation and scattering losses along with losses due to atmospheric turbulence. Depending on the FSO application, these loss mechanisms can dynamically change, impacting link performance at different time scales. We investigate subcarrier phase-shift keying (PSK) and quadrature amplitude modulation (QAM) intensity modulation that can be adapted to dynamically changing link conditions to optimize bandwidth utilization. Using custom subcarrier intensity modulation (SIM) modems, the performance of binary PSK (BPSK), QPSK, 8PSK, 16APSK, and 16-QAM waveforms is reported. The impact of adaptive equalization is also characterized, and the initial performance of a subcarrier multiplexed system is presented. This work represents the first experimental evaluation of SIM waveforms using a laboratory scintillation playback system based on scintillation recorded over real-world propagation paths.

Terrestrial Free Space Optical Communication Systems Availability based on Meteorological Visibility Data for South Africa

In spite of the numerous advantages of employing free space optical (FSO) communication systems as viable complementary platforms for next-generation networks, the presence of atmospheric disturbances such as fog and scintillations are major sources of signal impairment which degrade system performance. Consequently, it becomes imperative to investigate and contextualize the unique climatic conditions in those locations where FSO links are to be deployed. Statistical evaluation of meteorological visibility data collected for various cities in South Africa is thus hereby employed in estimating the availability performance of FSO links transmitting at both 850 nm and 1550 nm. It is determined that the cities of Mbombela and Cape Town have the lowest performance due to the high occurrence of fog events as compared to other regions in South Africa. During foggy periods, FSO links in Mbombela and Cape Town would have availabilities of ∼99.6% for link distances of 500 and 600 metres, respectively. The bit error rate (BER) estimations of intensity modulation and direct detection (IM/DD) FSO links in the presence of weak atmospheric turbulence were also investigated for the identified locations during foggy weather; with the cities of Mafikeng and Kimberley showing the lowest BER performances because of their high wind velocities, altitudes and refractive index values. In order to obtain a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> , receive signal-to-noise ratio (SNR) values ranging from ~46 to ~51 dB are required for FSO links deployed for data transmission in the various cities investigated in this work.

High-speed modulating retro-reflectors with optical phase conjugation compensation

A modulating retro-reflector (MRR) with optical phase conjugation (OPC) compensation is presented and demonstrated to mitigate the turbulence-induced channel distortion for high-speed coherent free-space optical (FSO) communications. The impact of the turbulence on the beam spot, the phase fluctuation and the bit error rate (BER) is analyzed for the MRR FSO link carrying quadrature phase-shift keying (QPSK) signals with spatial or single-mode fiber (SMF)-based OPC. The mitigation of the atmospheric turbulence is verified by comparing the transmission performance of the MRR FSO links with and without OPC. The compensation using SMF-based OPC is experimentally demonstrated in a turbulence-tunable atmospheric chamber. The BER is improved by two orders of magnitude in the turbulent channel with the temperature difference of 250 °C, and the power penalty is reduced by about 2.0 dB, which agree well with the simulation results. A larger receiving aperture size will improve the compensation effect

The BER performance of a FSO system with polar codes under weak turbulence

The key challenge in free space optical (FSO) communications is combating turbulence-induced fading. As the channel fading in FSO is quasi-static, the transmission parameters such as the code rates, transmit power and modulation schemes can be modified with respect to the channel state information transmitted via the feedback path. As a result, adaptive channel coding is considered as one of the practical approaches to improve the FSO link performance. In this study, the FSO system with polar codes is investigated and its performance is analysed by determining the optimum code-rate required to achieve a bit error rate of 10−9 under weak turbulence. It is shown that, using Monte-Carlo simulations for the scintillation indices of 0.12 and 0.2, the successive cancelation list (SCL) decoder offers coding gains of 2.5 and 0.3 dB, respectively, as compared with SC decoder, and for the scintillation index of 0.31, the SC decoder offers a coding gain of 2.5 dB compared to that of the SCL decoder for the code rate.

Spatial Jitter Influence on the Average BLER Performance of SIMO FSO Links over Atmospheric Turbulence Channels

In the recent years, Free Space Optics (FSO) technology has attracted significant research and commercial interest mostly because of its many advantages in comparison with other radio systems used for point-to-point connections. However, the reliable operation of these systems significantly depends on the conditions of the atmosphere in the area in which the optical beam propagates. The most important of these conditions are atmospheric turbulence and the misalignment between the optical beam and the receiver, which is also known as the pointing errors effect. In this work, in order to obviate the performance mitigation caused by these phenomena, we examined the most widely accepted and one of the most effective techniques, i.e., the implementation of receivers’ diversity. Various metrics have been investigated to evaluate the performance of such systems, but most of them do not take into account that the ultra-fast modern optical communication systems use blocks of bits for the transmission and codes for the detection and/or correction of erroneous bits. Thus, by taking these aspects into account, in this work, we investigated the combined impact of spatial jitter and atmospheric turbulence on the total average block error rate of an optical wireless system with receivers’ diversity. Novel closed-form analytical formulas were derived.

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.

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.

LDPC Decoding Techniques for Free-Space Optical Communications

There has been significant interest in free-space optical (FSO) communication by the research community in recent years. This is due to its high data rate, unlicensed spectrum, low cost, and immense security for FSO systems. Due to these advantages, FSO can have broader applications that extend from terrestrial to satellite communication. Atmospheric turbulence (AT) induced fading is a primary problem in the FSO link since it significantly impairs its performance. Atmospheric turbulence occurs due to the random variation of the air refractive index with time. Several statistical models are introduced to characterize the AT. The Log-normal (LN) model represents weak and moderate turbulence, and the Gamma-Gamma (G-G) model is employed for strong turbulence. These models are used with the effect of weather attenuation, geometric losses, and misalignment errors. One possible solution is channel coding, such as low-density parity-check (LDPC) codes. This paper proposed employing Weighted Bit Flipping (WBF) and Implementation Efficient Reliability Ratio Weighted Bit Flipping (IERRWBF) decoding techniques to improve FSO link performance. The results show a superior improvement in the bit error rate (BER) than the uncoded FSO system. In addition, the obtained results prove that the IERRWBF technique is more optimized than WBF from the point of the number of iterations, especially in weak and moderate turbulence FSO channels. In the calculation of decoders processing time, the WBF maintained lower decoding time than the IERRWBF technique, while in higher $E_{b}/N_{o}\text{s}$ , they have the same level. The same response for both techniques in the case of resultant throughput. Finally, both methods are evaluated from the point of convergence. IERRWBF technique achieved faster convergence than WBF in all FSO channels under study.

Simulation analysis of adaptive FSO/RF hybrid link under diverse weather conditions of Lahore, Pakistan

Free Space Optical (FSO) communication has higher bandwidth, data rate, quality factor, and secure transmission, making it ideal for future high capacity data links. However, fog, rain and dusty conditions are key hindrances in effective utilization of maximum performance of FSO link. This research work presents a simulation analysis of a typical FSO link of length 1 km (1550 nm) with Tx/Rx aperture diameters 10 cm and 15 cm, respectively, under weather conditions of the capital province Punjab, Lahore. Signal to noise ratio (SNR), Bit error rate (BER) and received power of link under consideration are calculated numerically in MATLAB ®. By proposing a switching technique, an year-round performance analysis was carried out using MATLAB ®. FSO is the primary link as the received power of the FSO link falls below a certain threshold; the communication link adapts the RF link as primary communication channel. Results show that an adaptive technique can successfully operate and enhance the performance of a FSO communication link.

Effect of fog and scintillation on performance of vertical free‐space optical link from Earth to low Earth orbit satellite

SummaryThis paper studies the effects of fog and scintillation attenuations on the performance of the vertical free‐space optical (FSO) link from the Earth to low Earth orbit (LEO) satellite. FSO link performance was investigated under different foggy conditions and turbulence by calculating the power received, achieved data rate, and link margin. Different parameters were studied such as the wavelength, the transmitted power, the divergence angle, and the diameter of receiving optical antenna. The results show that the FSO performance can be improved using 1,550‐nm wavelength, increasing the transmitted power to a range of 5 mW and reducing the divergence angle to a range of 1 μrad.

Performance analysis of NRZ and RZ variants for FSO communication system under different weather conditions

Abstract The evolving 5G technology is currently facing some huge challenges such as cost, bandwidth, network flexibility, deployment and capacity. Free space optics (FSO) is an emerging technology and viable approach to address the challenges of upcoming networks. This paper presents the performance comparison of advance variants of Return to zero (RZ) and Non-return to zero (NRZ) modulation formats in FSO communication. Detailed analysis of FSO link is carried out under various weather conditions by means of numerical simulations in which different data rates and distances are tested to evaluate the performance. The effects of different weather conditions over the performance, efficiency and availability of FSO link are analyzed on the basis of quality factor (Q-factor) of received signal. The work is performed on the licensed version of Optisystem v14.0.

Analysis of Free Space Optics Link Performance Considering the Effect of Different Weather Conditions and Modulation Formats for Terrestrial Communication

AbstractFree space optics (FSO) links provide an effectual and efficient way of transmission of data throughout free channels due to its higher bandwidth and inbuilt capability of security. In this research analysis, the FSO link is evaluated on the basis of Q value for different modulation formats and weather conditions. Gamma–Gamma and log-normal scintillation models have been analyzed for different index of refraction in terms of Q factor versus a range of a proposed FSO link. Attenuation increases during the occurrence of different fog conditions, and there might be complete link failure in the case of dense fog and for some of the cloud conditions.