Performance Characteristics of Perfect Vortex Beam Operating as Free-space Optical Communication Links

Atmospheric turbulence is known to significantly degrade the efficiency and reliability of free space optical communication link. Use of coded-orthogonal frequency division multiplexing (OFDM) technique to mitigate the effect of adverse atmospheric conditions on free space optical (FSO) communication link has been proposed here. With Gamma–Gamma distribution for channel modeling, the error performance of the proposed RS8 (Reed Solomon) coded, 128 sub-carrier OFDM link has been investigated using coherent BPSK and QPSK modulation scheme. The results obtained from this analysis have also been compared with intensity modulated/direction detection (IM/DD) based OOK-OFDM FSO link. In case of strong turbulence and for target BER of 10−4, it was observed that BPSK and QPSK modulated OFDM FSO link achieve a descent coding gain of 18.2 dB and 12.6 dB respectively over non coded OOK-OFDM FSO link. Also, it was observed that as the link conditions worsened from weak to strong turbulence, the effect of atmospheric impairments on FSO link becomes significantly pronounced. Additionally, in terms of BER performance, the BPSK modulated link out-performed QPSK and OOK under all the considered channel conditions.

Intensity scintillation and beam wander caused by atmospheric turbulence are two significant phenomena that affect free space optical (FSO) communication links. We have constructed an imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links. A He-Ne laser beam propagates over a range of 863 meters in atmospheric turbulence conditions that vary diurnally and seasonally from weak to strong. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. The captured image frames are analyzed in Labview to evaluate the turbulence index parameter, temporal and spatial intensity variances, and aperture averaging. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. This paper will present the most accurate empirical data to date for the weak and intermediate turbulence regime. Such results can help build upon existing empirical data and lead to the development of new theories. Aperture averaging of the received irradiance is also shown to be independent of the shape of the receiver aperture, and depends only on its area. This finding allows the use of refractive or catadioptric receivers, whichever is convenient, and the same amount of aperture averaging will be achieved for equal unobscured aperture areas. This can make the telescope design for an FSO receiver more compact.

Even after several decades of study, inconsistencies remain in the application of atmospheric turbulence theories to experimental systems, and the demonstration of acceptable correlations with experimental results. We have developed a flexible empirical approach for improving link performance through image analysis of intensity scintillation patterns coupled with frame aperture averaging on a free space optical (FSO) communication link. Aperture averaging calculations are invaluable in receiver design. A receiver must be large enough to collect sufficient power and reduce scintillation effects at a given range, but must also be of practical size. We have constructed an imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links. A He-Ne laser beam propagates over a range of 863 meters in atmospheric turbulence conditions that vary diurnally and seasonally from weak to strong. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. The captured image frames are analyzed in Labview to evaluate the turbulence parameter C n 2 , temporal and spatial intensity variances, and aperture averaging. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. The reduction in scintillation with aperture size guides the selection of optimum receiver aperture. Spatial and temporal variance analyses within single frames and between frames are compared and show good agreement.

Free Space Optical (FSO) Communication is the transmission of data using optical beam through free space. It requires the line of sight condition to be maintained between the transmitter and receiver in order to transmit data. High data rates, license-free spectrum and low deployment cost are some of the advantages of FSO link. But the attenuation due to atmospheric conditions is a major factor responsible for the degradation of performance of FSO link. Depending upon the weather conditions of a particular place, the performance of FSO system varies accordingly. So in this paper, we have analysed the attenuation coefficient for different weather conditions. The effects of rain, snow and fog on the performance of FSO link has been analyzed in terms of attenuation coefficient along with the study of turbulent nature of free space optical communication link employing Gamma-Gamma Turbulence Model.

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.

This study proposes a threshold decision technology to compensate for the turbulence effect in free-space optical (FSO) communication links, which integrates deep learning (DL) with a low-pass filter (LPF) to enhance system performance. Firstly, we introduce DL model of fully connected neural network (FCNN) for the sake of adaptive threshold decision (ATD) capability improvement. Then, in the cascaded LPF and FCNN approach, in order to improve the accuracy of channel state information (CSI) signal acquired from LPF, FCNN model is deployed behind LPF with a fixed cut-off frequency set for different turbulence channel degrees and data rates. For the adaptive cut-off frequency scheme of FCNN-based LPF technology, we utilize the FCNN model to determine the cut-off frequency value of LPF according to the estimated turbulence channel characteristics, enabling flexible variation of cut-off frequency values across diverse turbulence channel degrees and data rates. Finally, we conducted simulations to evaluate this technology. Simulation results demonstrate that FCNN-based adaptive cut-off frequency LPF technology outperforms LPF-based ATD with a fixed cut-off frequency, the FCNN-based ATD, and our proposed cascaded LPF and FCNN approach. Furthermore, its performance is approximate to theoretical ATD with comprehensive CSI knowledge. Therefore, the proposed method is a promising solution to compensate turbulence effect in FSO links. Received: 27 November 2024 | Revised: 18 March 2025 | Accepted: 11 June 2025 Conflicts of Interest The authors declare that they have no conflicts of interest to this work. Data Availability Statement Data are available from the corresponding author upon reasonable request. Author Contribution Statement Yan Gao: Writing – original draft. Yan-Qing Hong: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Visualization, Supervision, Project administration, Funding acquisition. Chao-Yue Zhai: Writing – original draft, Writing – review & editing. Xiao-Xue Ren: Writing – review & editing.

Dust storms are considered a new influence on free space optical communication (FSOC) link. It is referred to the suspended particle in the atmosphere at the desert environment. A few information about dust storms impose on us to explore the dust effect on the performance of (FSOC). This paper investigated the realization of FSO link under dust situations and take into account the effect of dust concentration. Some of the metrics used for the assessment of system including received optical power, channel capacity, signal to noise ratio (SNR), bit error rate (BER). It has been found that dust is a severe deterioration that give rise to the link decay under high concentration of dust. Moreover, we noted that the system suffers from working for the long optical link. The increase in the concentration of dust makes for low visibility, this reason leads to an increase in errors of transmitter data. Furthermore, the system can be worked with efficient channel capacity. Therefore, dust can be deemed as the decadence optical link in FSO.

Free space optics is a line-of-sight technology, which uses LASERS and Photo detectors to provide optical connections between two points—without the fiber. Increased needs for bandwidth require technology that goes beyond traditional copper lines. The technology advancement in internet scales faster than Moore's law which makes it infeasible to dig installed lines in metropolitan areas; the industry now realized the benefits of free space optical communication. Free-space optical (FSO) communication links are attractive because they provide a means of low cost, high bandwidth, reduced time-to-market, secure and are a reliable means of communications. This paper describes the benefits of FSO along with operation of such links working outdoors in an open atmosphere under various effecting field parameters such as atmospheric attenuation, beam divergence and scintillation where the performance of FSO link is evaluated in terms of transmitted and received power.

Free-space optical communication with perfect optical vortex beams multiplexing

The Log Normal model for weak turbulences and Gamma-Gamma model for moderate to strong turbulences are being used for calculating the path losses for engineering Free Space Optical (FSO) communication link. Since FSO has been an attractive substitute to overcome the issues of RF spectrum license, high infrastructure cost and difficult terrains to ensure the availability of a network. Their application in mountainous and high density built up areas needs to be exploited. Atmospheric turbulence degrades the FSO communication performance arising due to variations in C n 2 {{C}}_{{n}}^2{\ } (Refractive index structure parameter). Both these models have the limitation of NOT considering Rainfall parameter while calculating C n 2 C_n^2 . In this present paper we have included effect of Rainfall and presented the modified analytical model by re modeling C n 2 C_n^2 . To validate the impact, comparison of existing model and the modified path loss model has been presented using Meteorological (MET) data for a designed FSO communication link between geographically known mountainous locations of North eastern region of Arunachal Pradesh. The results emphasize that, the modified model is essential to ensure reliable and fail safe FSO link.

Free Space Optical (FSO) communication links are although extremely vulnerable to atmospheric adversities, multi-hop relay transmission can however, significantly improve the link performance and reliability. This paper proposes 120 Gbps DP-16 QAM modulated multi-hop serial FSO link with coherent reception for delivering medical consultation services in remote and isolated locations. Considering the current situation wherein pandemic of highly infectious nature, COVID-19 has affected millions of people globally; doctors can use the proposed high-speed architecture for “contact-less” supervision of quarantined patients and suspects through video conferencing. Each relay terminal uses an all-optical amplify-and-forward technique with Erbium-doped fiber amplifier (EDFA) and gain optimization as its core elements. As a possible last-mile application for delivering medical/health care services, the proposed link has been evaluated for reliability by exposing the link to varied atmospheric conditions. Our results reveal that at target BER of $10^{-5}$ , the useful communication link range of proposed multi-hop link increments by 1.8 kms as compared to direct link that operates under similar channel conditions. Furthermore, compared to results from recent literature on high-speed FSO [30], the proposed link shows enhancement in link range by approximately 0.7 km. The analysis also reveals that as the number of relay nodes increases, the error performance of the link for different atmospheric conditions approaches a state of convergence.

I.BACKGROUND & OBJECTIVES Owing to its high-bandwidth, robustness to EMI, and operation in unregulated spectrum, free-space optical communication (FSO) is uniquely qualified as a promising alternative or complementary technology to fiber optic and wireless radio-frequency (RF) links. Despite the vibrant advantages of FSO technology and the variety of its applications, its widespread adoption has been hampered by rather disappointing link reliability for long-range links due to atmospheric turbulence-induced fading and sensitivity to detrimental climate conditions. A major challenge of such hybrid systems is to provide a strong backup system with soft-switching capabilities when the FSO link becomes down. The specific objective of this work is to study for the first time in Qatar and the GCC the link capacity, link availability, and link outage of an FSO system with RF back up (i.e. hybrid FSO/RF) under harsh environment. II.METHODS In this work, a practical demonstration of hybrid FSO/RF link system is shown. The system has a capacity of 1 Gbps and 100 Mbps for FSO and RF, respectively. It is installed in Qatar University at two different buildings 600 m away and 20 feet high. This system is basically a point-to-point optical link that uses Infrared laser lights to wirelessly transmit data. Moreover, the proposed system has capability to make parallel transmission between links. In order to analyze the two transport media, we used the tool IPERF. This Java based GUI (jperf) application can either act as a server or client, and is available on a variety of platforms. We have tested end-to-end throughput by running IPERF tool in server mode on one Laptop and in client mode on another. III.RESULTS Figure1 shows a block diagram of the system used. Initial results were obtained for the two links under same climatic and environmental conditions, where the average ambient temperature reached 50°C and RH above 80% (July-August 2014). Both FSO and RF links allowed transfer rates of around 80% of their full capacity. During all experiments while running both links simultaneously, there was no FSO link failure. In case of an FSO failure, the RF is expected to back up within 2 seconds (hard switching), which might cause a loss of data. Detailed results on FSO-to-RF switching and induced packet loss will be reported in the full manuscript and during the presentation. IV.CONCLUSION Tests on FSO/RF link have been carried for the first time in Qatar. Initial results showed that both FSO and RF links operated close to their capacity. During summer, Qatari weather did not induce FSO link outage. The team is focusing on developing a seamless FSO-RF soft switching using NetFPGA boards and raptor coding.

Atmospheric turbulence causes severe degradation in the performance of free space optical (FSO) communication links. Among the various techniques used to mitigate the effect of turbulence, aperture averaging is one of the simplest. We experimentally investigate the effect of aperture averaging on the performance of FSO communication links for different intensity modulation (IM) schemes namely on-off keying (OOK), pulse position modulation (64-PPM) and differential pulse position modulation (64-DPPM). Atmospheric turbulence is artificially generated within an optical turbulence generator (OTG) chamber and for the above mentioned IM schemes beam variance measurements are carried out for different temperature induced turbulence conditions. The value of beam variance is used to determine the refractive index structure parameter and hence the Rytov variance. Finally, the bit error rate (BER) in presence of aperture averaging for the above modulation schemes is calculated. It is observed that the PPM scheme is a power efficient modulation scheme with lower error rate in comparison to DPPM and OOK schemes with its highly peaked pulses encountering the effect of turbulence effectively. Also, the link performance improves with aperture averaging for all the modulation schemes with the improvement more pronounced in the case PPM scheme. Thus PPM becomes the preferred modulation scheme in designing a FSO communication link.

In this paper, we consider and examine fog measurement data, coming from several locations in Europe and USA, and attempt to derive a unified model for fog attenuation in free space optics (FSO) communication links. We evaluate and compare the performance of our proposed model to that of many well-known alternative models. We found that our proposed model, achieves an average RMSE that outperforms them by more than 9 dB. Furthermore, we have studied the performance of the FSO system using different performance metrics such as signal-to-noise (SNR) ratio, bit error rate (BER), and channel capacity. Our results show that FSO is a short range technology. Therefore, FSO is expected to find its place in future networks that will have small cell size, i.e., <1 km diameter. Moreover, our investigation shows that under dense fog, it is difficult to maintain a communications link because of the high signal attenuation, which requires switching the communications to RF backup. Our results show that increasing the transmitted power will improve the system performance under light fog. However, under heavy fog, the effect is minor. To enhance the system performance under low visibility range, multi-hop link is used which can enhance the power budget by using short segments links. Using 22 dBm transmitted power, we obtained BER=10−3 over 1 km link length with 600 m visibility range which corresponds to light fog. However, under lower visibility range equals 40 m that corresponds to dense fog, we obtained the same BER but over 200 m link length.

Free Space Optical (FSO) communication link under clear weather conditions is affected by irradiance fluctuations caused due to random atmospheric temperature variations. In this work, bit error rate (BER) performance of free space optical (FSO) communication link is analysed. A 60 GHz millimeter wave is generated using frequency quadrupling technique and transmitted via free space optical (FSO) communication link. Different atmospheric turbulence conditions such as weak, moderate and strong are observed for FSO channel and corresponding simulation results have been obtained. Channel models such as Gamma-gamma model for strong atmospheric turbulence conditions and Log normal model for weak to moderate turbulence are considered to evaluate system bit error rate (BER). Further, BER performance is analysed for certain parameters such as transmitter antenna aperture diameter, receiving antenna aperture diameter, beam divergence and optical wavelength used.