Free-space Optical Communication Network Research Articles

Mitigating attenuation effects in free-space optics using WDM under variable atmospheric conditions

Abstract This research delves into the optimization of wavelength division multiplexing (WDM) within free space optical (FSO) communication systems, aiming to enhance the system’s resilience against atmospheric disruptions. FSO communication, known for its high bandwidth capabilities in unlicensed frequency bands, often encounters reliability issues due to weather-related signal degradation. By integrating a novel WDM-FSO approach, this study seeks to extend the operational range by using switching technique, thereby ensuring consistent and reliable communication. The introduction of a sophisticated switching and fork mechanism is pivotal in this context, facilitating dynamic signal routing and the strategic placement of optical amplifiers to counteract the impact of weather fluctuations and geographical challenges on signal integrity. This system, engineered to support a data transmission rate of 25 Gbps, caters to the needs of applications requiring high bandwidth. Through rigorous performance evaluation based on the Q-factor and BER under various atmospheric conditions, the effectiveness of the WDM-FSO system is demonstrated. The findings are detailed through graphs and tables, providing a comprehensive understanding of the system’s performance. This study makes a significant contribution to the field of wireless optical communication by presenting an optimized WDM-FSO system capable of overcoming weather-related obstacles especially at places where weather conditions are changed very frequently, marking a step forward in establishing more reliable FSO communication networks.

Leveraging InGaN solar cells for visible light communication reception

AbstractSolar cells are increasingly being utilised for both energy harvesting and reception in free‐space optical (FSO) communication networks. The authors focus on the implementation of a mid‐band p‐In0.01Ga0.99 N/p‐In0.5Ga0.5 N/n‐In0.5Ga0.5 N (PPN) solar cell, boasting an impressive 26.36% conversion efficiency (under 1.5AM conditions) as a receiver within an indoor FSO communication network. Employing a solar cell with dimensions of 1 mm in length and width, the FSO system underwent simulation using Optisystm software, while the solar cell's behaviour was simulated using SCAPS‐1D. The received power from the solar cell was then compared to that of four commercially available avalanche photodiode (APD) receivers. Exploring incident wavelengths spanning 400–700 nm within the visible spectrum, across transmission distances of 5, 10, 15, and 20 m, the study presented current‐voltage (IV) and power‐voltage curves. Notably, the InGaN solar cell exhibited superior electrical power output compared to all commercial APDs. In conclusion, the findings underscore that augmenting received power has the potential to enhance FSO network quality and support extended transmission distances.

An optoelectronic system for controlling a direct current motor

The software for an optoelectronic system for controlling a direct current (DC) motor is presented in Part 2 of the article. The structure of the designed control system was described in Part 1. The developed system processes data received from an infrared transmitter. The project was upgraded in successive stages of development, and it ultimately evolved into a small computer with a motor controller. The designed system automatically adjusts the motor’s rotation and speed. The user is tasked only with conveying operational commands. The entire system is based on a single microcontroller. The designed optoelectronic system receives user commands (the program can be modified to support free-space optical communication networks conforming to all communication standards). The system activates the motor, counts the number of rotations and adjusts the motor’s position. The designed system operates on the following principle: the user sends commands to the motor via a remote control with an infrared diode. The keys on the remote control have been programmed with different commands. The transmitted data are processed by the system which activates the motor and sets the desired motor speed. The task is completed, and the system is ready to process the next command. If the number of rotations differs from the preset value, the motor’s position is adjusted. If the physical position of the rotor axis is altered, the system corrects the offset to the last programmed position. The designed system can be easily adapted to various types of motors and IR controllers.

An optoelectronic system for controlling a direct current motor

An optoelectronic system for controlling a direct current (DC) motor is presented in Part 1 of the article. The software for the designed motor is described in Part 2. A system for processing data from an infrared transmitter was built. The project was upgraded in successive stages of development, and it ultimately evolved into a small computer with a motor controller. The designed system automatically adjusts the motor’s rotation and speed. The user is tasked only with conveying operational commands. The entire system is based on a single microcontroller. The designed optoelectronic system receives user commands (the program can be modified to support free-space optical communication networks conforming to all communication standards). The system activates the motor, counts the number of rotations and adjusts the motor’s position. The designed system operates on the following principle: the user sends commands to the motor via a remote control with an infrared diode. The keys on the remote control have been programmed with different commands. The transmitted data are processed by the system which activates the motor and sets the desired motor speed. The task is completed, and the system is ready to process the next command. If the number of rotations differs from the preset value, the motor’s position is adjusted. If the physical position of the rotor axis is altered, the system corrects the offset to the last programmed position. The designed system can be easily adapted to various types of motors and IR controllers.

Link-Layer Retransmission-Based Error-Control Protocols in FSO Communications: A Survey

Free space optical (FSO) communication has established a reputation for itself capable of delivering high-speed data services over long distances without exhausting radio frequency (RF) resources. FSO communication can be considered in different network scenarios, including inter-satellite/deep-space links, ground-station/vehicles, satellite/aerial links, and terrestrial links. It is expected to be one of the key enabling technologies for the next generation of 6G wireless networks. Nevertheless, despite the great potential of FSO communications, its performance suffers from various limitations and challenges: atmospheric turbulence, clouds, weather conditions, and pointing misalignment. The error-control solutions, including physical layer (PHY) and link-layer methods, aim to mitigate the transmission errors caused by such adverse issues. While the existing surveys on error-control solutions in FSO systems primarily focussed on the PHY methods, we instead provide a review of link-layer solutions. In particular, we conduct an extensive literature survey of state-of-the-art retransmission protocols, both automatic repeat request (ARQ) and hybrid ARQ (HARQ), for various FSO communication scenarios, including point-to-point terrestrial, cooperative, multi-hop relaying, hybrid FSO/RF, satellite/aerial, and deep-space systems. Furthermore, we provide a survey of recent literature and insightful discussion on the cross-layer design frameworks related to link-layer retransmission protocols in FSO communication networks. Finally, the lessons learned, design guidelines, related open issues, and future research directions are exposed.

Emerging Military Applications of Free Space Optical Communication Technology: A Detailed Review

Free Space Optical Communication (FSOC) technology, designed to operate through free space as medium is rapidly emerging as reliable, fast & secure alternative for broadband communication. It is being researched & investigated for applications and technical utilities in both civil & military domain systems owing to its immense benefits including high security, better data rates & fast installations, no requirement of licensed spectrum, best costs & simplicity of design as compared to contemporary Radio Frequency (RF) systems. FSOC networks once established are difficult to detect & intercept in view of small divergence of the laser beams. This paper reviews the FSOC technology and presents features based merits as well as unmatched advantages & associated major applications in various fields collating them into a single reference point for future research. Efforts have also been invested to present a review of FSOC’s limitations & innovative emerging mitigation techniques which can prove to be a one stop feeder & a launch pad for future research in FSOC domain. A literature survey has been undertaken of available FSOC related military applications to review & gather relevant inputs to throw light on emerging trends in military applications including recent experiments & researched areas pertaining to laser systems & weapons, Unmanned Aerial Vehicles (UAVs), under sea usages, terrestrial applications, aerial, naval ships/shore based applications & RF/hybrid systems. It has been endeavored to shed light on findings & developments in these classified military domains to generate inputs for future work in this domain. Finally, a future technical roadmap and a way ahead & suggestions have been coined up pointing towards required impetus & focus areas in FSOC research domain.

Multi-Tier Heterogeneous Beam Management for Future Indoor FSO Networks

To meet the growing demand for wireless capacity, communications in the Terahertz (THz) and optical bands are being broadly explored. Communications within these bands provide massive bandwidth potential along with highly directional beam steering capabilities. While the available bandwidth offers incredible link capacity, the directionality of these technologies offers an even more significant potential for spatial capacity or area spectral efficiency. However, this directionality also implies a challenge related to the network’s ability to quickly establish a connection. In this paper, we introduce a multi-tier heterogeneous (MTH) beamform management strategy that utilizes various wireless technologies in order to quickly acquire a highly directional indoor free space optical communication (FSO) link. The multi-tier design offers the high resolution of indoor FSO while the millimeter-wave (mmWave) system narrows the FSO search space. By narrowing the search space, the system relaxes the requirements of the FSO network in order to assure a practical search time. This paper introduces the necessary components of the proposed beam management strategy and provides a foundational analysis framework to demonstrate the relative impact of coverage, resolution, and steering velocity across tiers. Furthermore, an optimization analysis is used to define the top tier resolution that minimizes worst-case search time as a function of lower tier resolution and top tier range.

Performance Analysis of Hybrid Radio Frequency and Free Space Optical Communication Networks with Cooperative Spectrum Sharing

This paper investigates the impact of cooperative spectrum sharing policy on the performance of hybrid radio frequency and free space optical wireless communication networks, where primary users and secondary users develop a band of the same spectrum resource. The radio frequency links obey Nakagami-m distribution with arbitrary fading parameter m, while the free space optical link follows gamma-gamma distributed atmospheric turbulence with nonzero pointing error. Because the secondary users access the spectrum band without payment, their behavior needs to be restricted. Specifically, the power of the secondary users is dominated by the tolerable threshold of the primary users. Considering both heterodyne and intensity modulation/direct detection strategies in optical receiver, the performance of optical relaying networks is completely different from that of traditional networks. With the help of bivariable Fox’s H function, new expressions for cumulative distribution function of equivalent signal to noise ratio at destination, probability density function, outage probability, ergodic capacity and symbol error probability are built in closed forms.

Research on dynamic handover decision algorithm based on fuzzy logic control in mobile FSO networks

Because of multiple advantages such as high bandwidth, high security, and flexible networking, the free space optical communication (FSO) has attracted more and more attention. However, the narrow beam of laser and the constrained access degree of optical node limit the development of mobile FSO networks, especially in the aspect of handover. In this paper, we propose a fuzzy logic control-based handover decision (FLC-HD) algorithm to ensure real-time and reliable communication. First, according to the predictive position and the motion information of mobile nodes obtained by the error correction extend Kalman filter algorithm, we obtain the received signal strength and the relative motion angles at next transmission. Second, we put forward the concept of threshold to ensure the completion of handover operations before the current link breaking. Third, the fuzzy logic control method is designed to select the optimal access point (AP) with three critical parameters, i.e., the received signal strength, the relative motion angles, and the access degrees of nodes. Simulation results show that the FLC-HD algorithm can improve the handover efficiency and relieve the ping-pong effect in mobile FSO networks.

Urban Free-Space Optical Network Optimization

This paper presents a set of graph optimization problems related to free-space optical communication networks. Such laser-based wireless networks require a line of sight to enable communication, thus a visibility graph model is used herein. The main objective is to provide connectivity from a communication source point to terminal points through the use of some subset of available intermediate points. To this end, we define a handful of problems that differ mainly in the costs applied to the nodes and/or edges of the graph. These problems should be optimized with respect to cost and performance. The problems at hand are shown to be NP-hard. A generic heuristic based on a genetic algorithm is proposed, followed by a set of simulation experiments that demonstrate the performance of the suggested heuristic method on real-life scenarios. The suggested genetic algorithm is compared with the Euclidean Steiner tree method. Our simulations show that in many settings, especially in dense graphs, the genetic algorithm finds lower-cost solutions than its competitor, while it falls behind in some settings. However, the run-time performance of the genetic algorithm is considerably better in graphs with 1000 nodes or more, being more than twice faster in some settings. We conclude that the suggested heuristic improves run-time performance on large-scale graphs and can handle a wider range of related optimization problems. The simulation results suggest that the 5G urban backbone may benefit significantly from using free-space optical networks.

Double-layer graphene optical modulators based on Fano resonance in all-dielectric metasurfaces

In this paper, two double-layer graphene optical modulators based on all-dielectric metasurfaces are proposed. The double-layer graphene modulators remove the requirement of doped silicon back gates which would cause excessive loss and limit the modulation speed. The first structure is based on the electromagnetically induced transparency phenomenon, and the second one is a polarization-independent modulator that is based on the Fano resonance. The structures are simulated and analyzed using the finite element method. According to the simulation results, large modulation depth of about 95% at the wavelength of 1.55 μm can be obtained. The required voltages of 4.95 V are obtained for both of the modulators. By assuming the device of 50 × 50 unit cells, the 3 dB bandwidth of the first (second) structure is calculated as 630 MHz (482 MHz). It is estimated that by improving the quality of graphene, the 3 dB bandwidth of more than 2 GHz could be attained. The achieved modulation performances are much better than the previously reported free-space modulators with the same device area. The proposed high-performance optical modulators are promising for free-space optics technology especially free-space optical communication networks.

Simultaneous Wavelength and Format Conversions Based on the Polarization-Insensitive FWM in Free-Space Optical Communication Network

Free-space optical (FSO) network is an effective way to enhance the interoperability and improve the resource utilization of the FSO communication systems. Since the structure is relatively simple and the nodes are not so many, wavelength conversion-based optical circuit switching (WC-OCS) is an appropriate candidate for the FSO network to solve the blocking problem. Meanwhile, format conversion to lower the data rate is also necessary in case the atmospheric channel deteriorates. In this paper, we construct a 3-node FSO network and experimentally investigate the feasibility of simultaneous WC-OCS and format conversion based on straight-lined single-pump four-wave mixing (FWM) process. The biterror-ratio (BER) results of the on-off key, binary phase-shift keying (BPSK), and quadrature phase-shift keying (QPSK) signals show that comparing to the corresponding back-to-back (B2B) transmission at the reference BER level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> , power penalties caused by WC-OCS are less than 1-dB for all the three formats. In the format conversion experiment from QPSK to BPSK, the power penalties, comparing to the original BPSK signal after B2B and WCOCS transmission, are also less than 1 dB at the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> reference BER level. Consequently the feasibility and practicability of the FWM-based optical signal processing at FSO network nodes are experimentally validated.

空间激光通信网络中的全光相位再生技术

空间激光通信网络中的全光相位再生技术

Time synchronization over a free-space optical communication channel

Future high-precision free-space optical (FSO) clock networks will require laser links to transmit time-frequency between sites and, with this information, to synchronize the times across widely separated clocks. FSO communication networks already use laser links between remote sites to transmit high-speed data. Here we repurpose a FSO digital communication system and use it directly for two-way time-frequency transfer. We demonstrate synchronization of the time between two sites separated by a turbulent air path of 4 km using binary-phase-modulated continuous-wave laser light. Under synchronization, the two sites exhibit a fractional frequency deviation below 10−15 at 1500 s averaging time and a time deviation below 1 ps at averaging times of seconds to hours. Over an 8 h period, the peak-to-peak wander is 16 ps. This method should be applicable to future ground-to-space and intra-satellite links and could lead to an improved global navigation and satellite system.

WiFO: A hybrid communication network based on integrated free-space optical and WiFi femtocells

Developments in smart home technology and the Internet of Things have significantly increased the demand for high-speed indoor wireless links. Although the majority of the research conducted in this area is still focused on the efficient usage of radio frequency (RF) spectrum, free-space optical (FSO) networks have also been explored as an alternative due to their large bandwidth potentials and low interference. In this paper we present a hybrid FSO and WiFi system (WiFO) that seamlessly integrates optical femtocell architecture with high mobility WiFi networks. Each FSO femtocell in this indoor communication system is capable of transmitting and receiving data at a rate of 50 Mbps over a distance of up to three meters with a field-of-view of ±15°, while still achieving a low bit error rate between 10−6, and 10−4. Reed–Solomon forward error correction codes were also applied to the data stream to further reduce the bit error rate to below 10−7. Different than many other free-space optical communication network using static transceivers, mobility in our WiFO system is achieved through the integration of a WiFi channel in the network protocol. The WiFi channel provides a feedback mechanism and allows for seamless handoffs between FSO femtocells. Additionally, we have experimentally demonstrated the advantage of this WiFO architecture by comparing the throughput of our system with a standard WiFi link in a realistic scenario. Our investigation has shown that the WiFO system presented in this work offers a cost-effective and easy-to-implement approach to significantly increase the capacity of current WiFi networks.

Technologies and applications of free-space optical communication and space optical information network

Technologies and applications of free-space optical communication and space optical information network

Multiple User Pair Scheduling in TWR Assisted FSO Systems

In this paper, various multiuser scheduling schemes are applied to a two-way relay (TWR) assisted multiple user pair free-space optical (FSO) communication network, when the optical beam is subjected to path loss, misalignment errors, and atmospheric turbulence. The FSO network consists of a single, half-duplex, decode-and-forward, two-way optical relay that serves multiple user pairs by employing a scheduler. Three types of schedulers based on absolute signal-to-noise ratio (SNR), normalized SNR, and selective multiuser diversity are considered. Further, the atmospheric turbulence is modeled by a recently proposed statistical model, namely the Malaga M-distribution. For the presented system and channel models, closed-form and asymptotic expressions for outage probability and bit error rate are derived. The mathematical analysis is accompanied by Monte Carlo simulations and several numerical examples to illustrate the effect of the key system parameters.

Partial Dual-Relay Selection Protocols in Two-Way Relayed FSO Networks

In this paper, we present relay selection protocols for parallel relayed free-space optical (FSO) communication network, which consists of two non-line-of-sight terminal nodes and multiple two-way relays (TWR). The TWRs use the two time-phase decode-and-forward relaying protocol to establish a bidirectional link between the terminals. For the considered system, we propose a partial dual-relay selection (P-DRS) scheme, in which two TWRs are selected to participate in the communication process. By using two TWRs, the advantages of cooperative diversity are achieved. For the proposed relay selection scheme and assuming Gamma–Gamma fading channels with pointing errors and path loss, we derive an exact closed form expression for the system outage. Further, we derive an asymptotic expression for average error probability. The diversity order of the system is also presented. It is shown by numerical analysis that by using the proposed P-DRS scheme, the system performance can be significantly improved.

RoFSO: A universal platform for convergence of fiber and free-space optical communication networks

The ever-increasing demand for capacity and quality in wireless communication links has continued to inspire researchers to innovate new design methodologies and concepts over wireless systems and networks with the ultimate aim of achieving a next-generation network. Among the emerging technologies is the radio on free-space optics system described in this article. With this technology it is possible to simultaneously transmit multiple RF signals comprising heterogeneous wireless services over FSO links using wavelength-division multiplexing technology. Moreover, the RoFSO technology can be applied as a universal platform for enabling seamless convergence of fiber and free-space optical communication networks, thus extending broadband connectivity to underserved areas. We present the design concept and highlight some experimental results obtained from performance evaluation of the RoFSO system we have developed. Using the quality metric parameters defined for the different RF service signals transmitted over the experimental RoFSO system, the results demonstrate satisfactory performance in terms of reliability and stability. Considering the potential of RoFSO technology, as demonstrated in our studies, we propose a study for standardization work in the ITU, which can lead to its rapid adoption.

Optical networks, last mile access and applications

Free Space Optical (FSO) links can be used to setup FSO communication networks or to supplement radio and optical fiber networks. Hence, it is the broadband wireless solution for closing the “last mile” connectivity gap throughout metropolitan networks. Optical wireless fits well into dense urban areas and is ideally suited for urban applications. This paper gives an overview of free-space laser communications. Different network architectures will be described and investigated regarding reliability. The usage of “Optical Repeaters”, Point-to-Point and Point-to-Multipoint solutions will be explained for setting up different network architectures. After having explained the different networking topologies and technologies, FSO applications will be discussed in section 2, including terrestrial applications for short and long ranges, and space applications. Terrestrial applications for short ranges cover the links between buildings on campus or different buildings of a company, which can be established with low-cost technology. For using FSO for long-range applications, more sophisticated systems have to be used. Hence, different techniques regarding emitted optical power, beam divergence, number of beams and tracking will be examined. Space applications have to be divided into FSO links through the troposphere, for example up- and downlinks between the Earth and satellites, and FSO links above the troposphere (e.g., optical inter-satellite links). The difference is that links through the troposphere are mainly influenced by weather conditions similar but not equal to terrestrial FSO links. Satellite orbits are above the atmosphere and therefore, optical inter-satellite links are not influenced by weather conditions. In section 3 the use of optical wireless for the last mile will be investigated and described in more detail. Therefore important design criteria for connecting the user to the “backbone” by FSO techniques will be covered, e.g., line of sight, network topology, reliability and availability. The advantages and disadvantages of different FSO technologies, as well as the backbone technology are discussed in this respect. Furthermore, the last mile access using FSO will be investigated for different environment areas (e.g., urban, rural, mountain) and climate zones. The availability of the FSO link is mainly determined by the local atmospheric conditions and distance and will be examined for the last mile. Results of various studies will complete these investigations. Finally, an example for realizing a FSO network for the last mile will be shown. In this example FSO transmitters with light emitting diodes (LED) instead of laser diodes will be described. By using LEDs, problems with laser-and eye safety are minimized. Some multimedia applications (like video-conferences, live TV-transmissions, etc.) will illustrate the range of applications for FSO last mile networks.