Free-space Optical Links Research Articles

Average symbol error rate analysis of reconfigurable intelligent surfaces-assisted free-space optical link over log-normal turbulence channels

<span>Optical wireless communication (OWC) has attracted significant interest recently in academia and industry. Free-space optical (FSO) communication systems are where free space acts as a communication channel between transceivers that are line of sight (LOS) for the successful transmission of optical signals. The FSO transmissions through the atmosphere, nevertheless, bring significant challenges, besides the uncertainty of atmospheric channels, especially the signal fading due to the atmospheric turbulence, attenuation and pointing errors caused by the random beam misalignments between transceivers, signal obstruction due to buildings or trees can pre-vent the transmitted message to reach the destination. This study theoretically investigates the average symbol error rate (ASER) of reconfigurable intelligent surfaces (RIS) assisted FSO link over log-normal turbulence channels. The RIS effect is examined by considering the influence of link distance, transmitted optical power, and quadrature amplitude modulation (QAM) scheme on the ASER.</span>

English

The spatial diversity techniques are broken down and explained systematically along with its combination which includes Maximum Ratio Combining (MRC), Equal Gain Combining (EGC) and Simple Selection Combining (SC) under the influence of atmospheric-induced turbulence fading in Free Space Optical (FSO) communication. Diversity is a technique used to prevent or reduce multipath. Multipath is a phenomenon describing propagation that results in the signals reaching the antenna by two or more paths. There is an extreme difficulty for a transmitted signal to be detected by a receiver in a case of increased turbulence. This research provides a forum for the receiver with some forms of diversity. The validation of the recommended simulation is through Additive White Gaussian Channel without combiner of the same process. The results display that Maximum Ratio Combining (MRC) has the greatest mitigation level of fading in comparison with others. Consequently, where fading is more dominant like on a terrestrial FSO link, Maximum Ratio Combining (MRC) can be used. Overall, the findings suggest the capability of the model in mitigating atmospheric turbulence especially for the 5G wireless networks in the terrestrial link. Key words: Free Space Optical Communication (FSO), Atmospheric Turbulence, Additive White Gaussian Channel (AWGC), Maximum Ratio Combining (MRC), Equal Gain Combining (EGC), Selection Combining (SC) and Additive White Gaussian Noise (AWGN).

Enriching Capacity and Transmission of Hybrid WDM-FSO Link for 5G Mobility

A dramatic increase in user and capacity demands has been noted after the COVID-19 pandemic. These challenges have damaged the 5G communication system mobility. Therefore, developing mobility and enhancing capacity transmission of 5G advanced services are the focused research gaps in the current era. In this paper, the free space optics (FSO) link is modeled with wavelength division multiplexing (WDM) technology based optical fiber system, purposing to enhance the 5G capabilities in multi-channel, high distance, and bidirectional transmissions. In addition, the presented hybrid FSO-WDM supported optical fiber network is analyzed for 4, 8, and 16 × 10 Gbps downlink and uplink transmission. The paper also includes the mathematical discussion of merged fiber length (SMF = 30 km) and FSO (600 m) with improved mobility management. In another contribution, the tolerance against Rayleigh backscattering (RB) noises is developed through various wavelengths of downlink and uplink channels. Finally, we perform the simulation analysis and reliability of the proposed structure for the 5G advanced communication system.

Backhaul-Aware User Association and Throughput Maximization in UAV-Aided Hybrid FSO/RF Network

Free-space optical (FSO) communication is expected to play an indispensable role with high data rates and low system complexity in beyond fifth-generation (B5G) networks. However, infrequent adverse weather conditions can incapacitate its performance. The combination of FSO and radio frequency (RF) has emerged as an effective alternative for meeting the growing need for high data rates in wireless communication networks. Unmanned aerial vehicles (UAVs) are also anticipated to play an instrumental role in B5G networks due to their flexible movement and deployment. In this paper, a UAV-aided hybrid FSO/RF backhauling system using a matching game theory (GT) and reinforcement learning (RL) framework is investigated. We deploy a UAV to provide a user offloading service to an already existing ground base station (GBS), which is facing a reduced backhaul capacity due to weather attenuation (e.g., fog). It is considered that the GBS has a pre-installed FSO backhaul connection to a macro-base station (MBS). However, during adverse weather conditions, the FSO backhaul is severely affected, compromising the reliability of the FSO link. With the reduced FSO backhaul capacity, the GBS needs an additional backhaul link to support its backhaul data transmission to the destination MBS. As a result, instead of building an expensive permanent parallel RF link for the rare foggy situation, a UAV can be hired to serve a portion of the users, thereby reducing the GBS load. The users perform a matching game-based procedure to select the base station (BS) of their choice to maximize their utility. The UAV is deployed at an optimal altitude, and the bandwidth partition between the GBS and the UAV is optimized to maximize the system throughput using RL. Real weather data from the cities of Edinburgh and London in the U.K. are used to evaluate the performance of the system. The numerical results show the superiority and effectiveness of the proposed scheme compared to conventional methods.

Daylight entanglement-based quantum key distribution with a quantum dot source

Entanglement-based quantum key distribution can enable secure communication in trusted node-free networks and over long distances. Although implementations exist both in fiber and in free space, the latter approach is often considered challenging due to environmental factors. Here, we implement a quantum communication protocol during daytime for the first time using a quantum dot source. This technology presents advantages in terms of narrower spectral bandwidth—beneficial for filtering out sunlight—and negligible multiphoton emission at peak brightness. We demonstrate continuous operation over the course of three days, across an urban 270 m-long free-space optical link, under different light and weather conditions.

High-Speed Spiral-Phase Donut-Modes-Based Hybrid FSO-MMF Communication System by Incorporating OCDMA Scheme

Hybrid free-space optics (FSO) and optical fiber have been viewed as vital transmission techniques to satisfy high bandwidth and extended transmission range requirements under adverse environment conditions in the future last-mile obstruction problem. In this investigation, 80 Gbps data is transmitted on a hybrid FSO and multimode fiber (MMF)-based network using mode division multiplexing of two donut modes, Donut mode 0 and 1, and optical code-division multiplexing (OCDMA) schemes. For the OCDMA schemes, modified new zero-cross-correlation (MNZCC) codes are used, whereas, to add the phases into donut modes, a spiral phase diffuser is used. The purpose of the investigation is to provide an economical, high-speed and advanced last-mile network with adequate resource utilization for hybrid wired/wireless-based systems. The results obtained show achievement of an acceptable BER up to a fixed 100 m FSO link, with the combination of a 385 m MMF link under clear weather conditions. In another case, when the MMF link was fixed at 100 m, an acceptable bit error rate (BER) is achieved at 2.07 km FSO link. Furthermore, the results were obtained in the presence of strong and weak turbulences. A comparison of log-normal and gamma-gamma modeling for scintillations is presented.

Secured End-to-End FSO-VLC-Based IoT Network With Randomly Positioned VLC: Known and Unknown CSI

In this article, we determine the secrecy performance of a dual-hop hybrid free space optics (FSO)-visible light communication (VLC) system with a randomly located legitimate Internet of Things (IoT)-based receiver in the presence of two distinct eavesdroppers (EDs). We consider one FSO-based ED located near the relay, wiretapping the FSO link. However, another ED wiretaps the information from the multiple VLC sources embedded on the ceiling of the room. We consider the three different scenarios for VLC-based ED: 1) the effect of uniformly distributed ED in the absence of any secured zone in the VLC coverage area; 2) the impact of uniformly distributed ED in the presence of VLC secured area; and 3) the effect of random waypoint (RWP) distributed ED on the system’s security. The secrecy performance of the system is evaluated based on known and unknown instantaneous channel state information (CSI) of EDs. Furthermore, the closed form expression for secrecy outage probability (SOP) in terms of Meijer’s-G and Gauss hypergeometric functions is derived. In order to have a better insight into the system’s performance, analytical results are expressed asymptotically in the high signal-to-noise ratio (SNR) regime. Following that, Monte Carlo simulations are done to validate the theoretical results. Additionally, a secrecy threshold is obtained in order to comprehend the system’s security and reliability.

Shaping the transmission trajectory of vortex beam by controlling its radial phase.

Vortex beam carrying orbital angular momentum (OAM), which features a helical phase front, has shown its potential applications in diverse areas, especially in free-space optical (FSO) communications. However, when generating vortex beams, the radial phase distribution is usually disregarded in previous reports. In this paper, by controlling the radial phase distribution, we propose a method for the generation of vortex beams with arbitrary convex trajectories. By using this method, we successfully generate vortex beams with different predesigned trajectories with high accuracy. Moreover, we also demonstrate the transmission of the radial phase-controlled vortex beams in FSO links for different scenarios in simulation. Firstly, we generate vortex beams with different OAM states (l=+1, + 3, and +6), which have the same ring diameter at the receiver side. Secondly, we generate three vortex beams (l=+3) with the same ring diameter at different transmission distances (z = 100 m, 200 m, and 300 m). Finally, by carefully controlling the radial phase of the vortex beam, we generate vortex beams that can almost keep the same ring diameter for a long distance. The proposed method for shaping the transmission trajectory of vortex beams may pave the way for more applications in OAM-based FSO communications.

Non-mechanical self-alignment system for free-space optical communications based on a cascaded liquid crystal optical antenna.

Free-space optical (FSO) communication has attracted extensive attention in recent years. To maintain a reliable FSO link, two main issues need to be addressed: beam drift and vibration. In this paper, we demonstrate a non-mechanical self-alignment system based on a cascaded liquid crystal optical antenna, in which a frequency decoupled hybrid integration Kalman filter (FDHI-KF) method is proposed to achieve predictive beam drift tracking and vibration mitigation. By leveraging the integrated control on our lab-made liquid crystal phase modulation devices, and implementing the adaptive algorithm on a heterogeneous field programmable gate array (FPGA), this system is capable of realizing precise self-alignment without any moving parts. Experiments are conducted to verify its performance in practical applications. We envision it to set a benchmark for future liquid crystal non-mechanical beam-steering systems in FSO communications.

Modeling of optical wave propagation through turbulent atmosphere using fractional approach for FSO wireless communication

Free space optical (FSO) wireless communication has emerged as a viable alternative to the existing fiber optics and radio frequency (RF) communications due to its ability to operate in unlicensed spectrum, offering huge data handling capacities and being cost effective with easy deployment. The underlying communication mechanism in the FSO link is based upon the laser beam propagation model. The propagating free space optical beam undergoes signal degradation due to refraction and diffraction caused by atmospheric turbulence measured by refractive index structure parameter (C n 2). In this work, a novel, robust analytical model for propagating Gaussian-beam through atmospheric turbulence is presented, by solving the space-fractional paraxial wave equation. We report analytical expressions for the intensity and long-term beam spreading of a Gaussian beam in terms of space-fractional parameter D, for the range 2 < D ≤ 3. This range of parameter D, defines the effective number of euclidean space corresponding to atmospheric turbulence levels faced by the propagating Gaussian beam in the FSO link. The results of the proposed fractional model and the existing models agreed well, therefore the D-dimensional parameter can be used to effectively express the value of refractive index structure parameter (C n 2). The classical values of C n 2 ranges form 10−13 to 10−16 for strong to weak fluctuations respectively. However, in fractional-dimension scale, these fluctuations can be express as D = 2.668 (strong fluctuations) to D = 2.999 (weak fluctuations). The ideal case of free space beam propagation can be expressed as D = 3, with no turbulence. Moreover, we have studied the fractional-dimension model performance for varying wavelengths. Further, based upon a practical example, we proposed a self-sustainable FSO link architecture to efficiently minimize the effect of weak and strong fluctuations on the propagating optical beam, based upon the D-dimension fractional parameter, hence ensuring reliability of FSO link.

Performance Analysis of Smart Grid Wide Area Network With RIS Assisted Three Hop System

In this paper, we investigate the performance of a wide area network (WAN) with three hops over a mixed radio frequency (RF), reconfigurable intelligent surface (RIS) assisted RF and Free space optics (FSO) channel. Here RIS and decode-and-forward (DF) relays are used to improve the coverage and system performance. For general applicability, the RF and FSO links are modelled with Saleh-Valenzuela (S-V) and Gamma-Gamma distribution, respectively. In the smart grid (SG) communication network, the smart meter (SM) communicates the information from the devices to the data aggregator unit (DAU) through the RIS-assisted RF link. As all the devices are not switched on all the time, the Markov chain is used to model the number of active devices. The DAU retransmits the data to the meter data management system (MDMS) through the FSO link, where the atmospheric turbulence and pointing error impairments exist. In this context, closed-form expressions for average bit error rate (ABER) and outage probability (OP) are derived using the H-fox function and extended generalized Bi-variant H-fox function. Here the numerical results demonstrate the effect of different FSO parameters, the number of reflectors in RIS, traffic intensity, and the number of total devices in the SG network.

Variable antennas positions solution to reduce pointing errors due to wind speed and temperature coupled effects during free space optical link using matrix Rician pointing error model

Pointing errors (PE) during free space optical (FSO) transmission can be caused by laser beam wander due to thermal and wind dynamic instability. The aim of this work is to study the coupled effects of temperature and wind speed on PE using matrix Rician pointing error (MRPE) model; then show how variable antennas height can reduce PE due to wind speed and temperature coupled effects. To achieve this purposes, average PE expression was established using MRPE model. Then considering a Gaussian beam wave and Monin–Obukhov similarity functions for the structure parameters of temperature, explicit relationship was established between average PE, temperature and wind speed. It comes out of this study that under dynamic turbulence, one can appropriately modify temperature to reduce PE due to dynamic instability and reciprocally. Depending on turbulence large cells or frozen turbulence eddies distribution, PE can be reduced by appropriately modified antennas height or the distance between transmitter and receiver. That is why this work suggests to install variable or dynamic antennas (rather than fixed ones) which could intelligently modify its positions according to laser beam wander created by atmospheric turbulence.

Hybrid SAG-FSO/SH-FSO/RF Transmission for Next-Generation Satellite Communication Systems

Recent advances demonstrate satellite communication (SatCom) as a potent enabler for future Terabit/s global connectivity. Existing SatCom systems, however, rely mostly on radio frequency (RF) transmission, whose limited available bandwidth is the main bottleneck for further data rate increases. Free-space optical (FSO) communication links, with huge license-free bandwidth, have emerged as an attractive alternative. Despite their ability to deliver high-throughput transmissions, FSO communications are weather-dependent and susceptible to atmospheric turbulence. Hybrid FSO/RF and space-air-ground (SAG) FSO transmissions are possible solutions to increase the reliability of FSO-based transmission systems. A strategically deployed unmanned-aerial vehicle, acting as a relay, can successfully mitigate the adverse effect of atmospheric turbulence, while the hybrid FSO/RF transmission can address weather-dependent effects. These solutions can also be integrated to create a system with significantly improved performance and reliability. To evaluate the performance of the resulting integrated transmission system, we analyze the outage probability and average symbol error probability in this work. Asymptotic expressions are also derived to get further insight into the system behavior and calculate the overall diversity gain. Furthermore, we consider the optimal design of switching thresholds. The numerical results show that the integrated transmission system achieves about 10 dB performance gain over existing solutions for both downlink and uplink scenarios.

The electro-optic spatial light modulator of lithium niobate metasurface based on plasmonic quasi-bound states in the continuum.

Metasurface has attracted massive interest owing to its ability to control light arbitrarily in a wide range of applications, such as high-speed imaging, optical interconnection, and spectroscopy. Here we propose a free space light modulator combined with a gold grating metasurface based on lithium niobate (LiNbO3). The quasi-bound states in the continuum (quasi-BIC) are achieved in the metasurface. In addition, the plasmonic quasi-BIC and the guided-mode resonance (GMR) can be modulated by controlling the polarization of the incident light without any geometric adjustment. Thus, the working wavelength range from 1480 nm to 1620 nm was achieved, and the maximum resonance depth reached about 51% at the resonant wavelength. In addition, the insertion loss of the device was -2.8 dB at a wavelength of 1510 nm. Furthermore, the dynamic modulation speed reached up to 190 MHz and the highest signal-to-noise ratio (SNR) could reach about 49 dB at a frequency of 65 MHz. The data showed potential for the material for applications such as near-infrared imaging, beam steering, and free-space optical communication links.

MOEMS-based Lens-Assisted Beam Steering for Free-Space Optical Communications

Free-space optical communication (FSOC) systems require precise pointing, acquisition, and tracking to send and receive optical beams for effective operation. In a lens-assisted beam steering (LABS) system, light is steered by controlling the emission location in a focal plane. The emitted light is directed into the scene by a lens, like a camera but operating in reverse. In this paper, we demonstrate a novel free-space optical communication link using a micro-opto-electro-mechanical-system (MOEMS) based photonic integrated circuit (PIC) for LABS between multiple receive locations. The MOEMS PIC operates via selective electrical actuation of an array of small grating switches (30 µm x 30 µm footprint, 100 µm pitch). Data rates up to 10 Gbps and a 3 dB optical bandwidth covering the infrared C and L bands (1530 nm to 1625 nm) are measured over a 1 m free-space link distance. Eye diagrams indicate a quality communication link for data rates up to 10 Gbps and bit-error-rates < 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> are measured for on-off keying (OOK) modulation. A measured beam profile is propagated into the far-field via simulation, and used to calculate link budgets for example CubeSat crosslink and downlink scenarios. Link budget calculations indicate potential > 1 Gbps CubeSat FSOC crosslinks for link lengths > 1000 km and 1 W of input optical power, using identical 90 mm transmit and receive apertures and a commercially-available fiber-coupled InGaAs avalanche photodiode (APD) receive detector.

Physical Layer Security of Dual-Hop Hybrid FSO-mmWave Systems

Hybrid free-space optical (FSO) and millimeter wave (mmWave) systems have been proposed as an efficient solution to improve spectral efficiency and maintain link reliability. The comparable supported data rate and the complementary characteristics in different weather conditions of both links, i.e., FSO and mmWave) have attracted research efforts towards analyzing the different performance aspects of hybrid FSO-mmWave systems. Following this direction, in this article, the physical-layer security analysis of the dual-hop hybrid FSO-mmWave systems is addressed. In the considered system, both FSO and mmWave links are simultaneously used to carry data at each hop, while an eavesdropper is assumed to present to wiretap either one or both links. The analysis includes building a mathematical framework to derive different secrecy metrics, such as the average secrecy capacity, secrecy outage probability and effective secrecy throughput metrics, in mathematical expressions. Obtained analytical results are verified using Monte Carlo simulations where exact matching are achieved. Moreover, the presented results include investigation of the impact of the different operational parameters on the overall secrecy performance.

6G enabling FSO communication system employing integrated PDM-OAM-OCDMA transmission: impact of weather conditions inIndia.

Currently, free space optics (FSO) transmission has gained attention due to its capability to deliver high data rates with a high level of security. In addition, using multiplexing techniques with FSO makes it capable of handling the excessive increase in traffic data and supporting the 5G and 6G wireless network requirements. Accordingly, in this paper, a novel, to the best of our knowledge, FSO system integrating three multiplexing techniques-orbital angular momentum (OAM) (four OAM beams are used) polarization-division multiplexing (two polarization states are used), and optical code division multiple access (four channels assigned with permutation vector codes are used)-is proposed. Additionally, the effects of rainy, foggy, dusty weather and the real weather for four different Indian cities that have different geographical locations are studied and investigated. The system performance is evaluated based on the bit error rate (BER), quality factor (Q-factor), maximum FSO range, and eye diagram opening. The simulation results show successful transmission of 320Gbps overall capacity with a maximum achievable FSO range of 7km under clear weather. On the other hand, the shortest FSO range of 0.105km is achieved when there are heavy dust storms. As for the Indian cities, Srinagar (hilly area of India) achieves the shortest FSO link, which is 4.2km while the largest range of 7km is observed for Chennai city (coastal area of India). All these ranges are evaluated for a log(BER) value <-7. Consequently, this proposed transmission model is suggested for use in 6G applications of FSO communication systems.

Enhanced Atmospheric Turbulence Resiliency With Successive Interference Cancellation DSP in Mode Division Multiplexing Free-Space Optical Links

We experimentally demonstrate the enhanced atmospheric turbulence resiliency in a 137.8 Gbit/s/mode mode-division multiplexing free-space optical communication link through the application of a successive interference cancellation digital signal processing algorithm. The turbulence resiliency is further enhanced through redundant receive channels in the mode-division multiplexing link. The proof of concept demonstration is performed using commercially available mode-selective photonic lanterns, a commercial transponder, and a spatial light modulator based turbulence emulator. In this link, 5 spatial modes with each mode carrying 34.46 GBaud dual-polarization quadrature phase shift keying signals are successfully transmitted with an average bit error rate lower than the hard-decision forward error correction limit. As a result, we achieved a record-high mode- and polarization-division multiplexing channel number of 10, a record-high line rate of 689.23 Gbit/s, and a record-high net spectral efficiency of 13.9 b/s/Hz in emulated turbulent links in a mode-division multiplexing free-space optical system.

Cooperative FSO and mmWave System for Reliable 200G Wireless Transmission

The use of free-space optics (FSO) for ultra-high-speed communications has been recently standing out as a promising alternative for fiber/wireless-based scenarios. However, FSO communications still suffer from variations in the received optical power due to atmospheric turbulence and weather instability, which limits its use in reliability-driven applications. To explore the full potential of FSO and maintain communications reliability, hybrid FSO and radio-frequency systems can be employed to leverage the strengths of the two technologies. In this letter, we demonstrate a 3m hybrid FSO and millimeter-wave system for 200G stable wireless transmission. Resorting to time-adaptive probabilistic constellation shaping and a cross-domain entropy loading scheme, we are able to compensate for up to 15% (30Gbps) of data-rate reduction in the FSO link. Comparing with FSO-only transmission, we demonstrate a cumulative capacity gain of over 150Gbps after 40 offline measurement iterations.

Atmospheric Turbulence and Fog Attenuation Effects in Controlled Environment FSO Communication Links

Free-space optical (FSO) communication can be seen as a promising technology for point-to-point and back-hauling links in the next generation wireless networks (5G and beyond) where cell size may shrink to a few hundred meters. In this work, we have experimentally investigated the laser beam propagation for FSO link under atmospheric turbulence and fog conditions. A controlled atmospheric environment chamber is designed to perform experiments under varying channel conditions. For fog attenuation, we have proposed an empirical model as a function of visibility against the measured average values, for visibility range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0\leq V \leq1000$ </tex-math></inline-formula> m. For atmospheric turbulence, we report the measured values of the refractive index structure parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{n}^{2}$ </tex-math></inline-formula> . The measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{n}^{2}$ </tex-math></inline-formula> is used to calculate the atmospheric coherence width along the propagation distance. This work would help in the design optimization of practical FSO links under adverse conditions like fog and atmospheric turbulence.