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Free Space Optical (FSO) communication is extensively utilized in the telecommunication industry for both ground and space wireless links, as well as last-mile applications, as a result of its lesser Bit Error Rate (BER), free spectrum, and easy relocation. However, atmospheric turbulence, also known as Wavefront Aberration (WA), is considered a serious issue because it causes higher BER and affects coupling efficiency. In order to address this issue, a Sensor-Less Adaptive Optics (SLAO) system is developed for FSO to enhance performance. In this research, the compensation of WA in SLAO is obtained by proposing the Brownian motion and Directional mutation scheme-based Coati Optimization Algorithm, BDCOA. Here, the BDCOA is developed to search for an optimum control signal value of actuators in Deformable Mirror (DM). The incorporated Brownian motion and directional mutation are used to avoid the local optimum issue and enhance search space efficiency while searching for the control signal. Therefore, the dynamic control signal optimization for DM using BDCOA helps to enhance the coupling efficiency. Thus, the WAs are compensated for and optical signal concentration is enhanced in FSO. The metrics used for analyzing the BDCOA are Root Mean Square (RMS), BER, coupling efficiency, and Strehl Ratio (SR). The existing methods, such as Simulated Annealing (SA) and Stochastic Parallel Gradient Descent (SPGD), Advanced Multi-Feedback SPGD (AMFSPGD), and Oppositional-Breeding Artificial Fish Swarm (OBAFS), are used for evaluating the performance of BDCOA. The RMS of BDCOA for iterations 500 is 0.12, which is less than that of the SA-SPGD and OBAFS. [ABSTRACT FROM AUTHOR]

Summary: The present research work aims to investigate the reliability of the mixed free space optical (FSO)/radio frequency (RF) decode‐and‐forward (DF) relaying system where the satellite intends to communicate with the ground station through the unmanned aerial vehicles (UAV) as the relay node. Moreover, it has been considered that a second UAV interferes with the intended UAV. The operation of the UAVs has been represented considering the small scale fading, path loss, 3‐D location, and probability of maintaining line‐of‐sight (LoS) and non line of sight (NLoS) links, while FSO link undergoes Malaga distributed turbulence. Using the aforementioned model, the closed form expressions for the outage probability and bit error rate (BER) have been derived. The exact expressions have been extended to obtain the high signal‐to‐noise ratio (SNR) results for the outage probability and BER. The analytical expressions have been numerically evaluated and the results obtained through these expressions have been verified using the Monte Carlo simulations. More importantly, long short‐term memory (LSTM)‐based deep learning model has been trained for prediction of outage probability. The model is trained offline and then utilized to predict the values of the outage probability in online mode with mean square error (MSE) and root mean square error (RMSE) of MSE = −45.02 and RMSE = −18.34 dB. [ABSTRACT FROM AUTHOR]

An analysis of the prospects for the use of atmospheric optical communication lines at industrial nuclear energy installations, including thermonuclear (fusion) reactors and energy facilities with on-site fuel reprocessing plants, was carried out. It is shown that modern atmospheric communication terminals make it possible to implement high-speed data exchange within the perimeter of energy complexes, as well as to provide an external backup communication channel protected by use of quantum key distribution technology. The absence of the need to lay special cables through limited-access areas to organize high-speed data transmission provides a significant advantage of atmospheric communication systems over any wired communication systems. A methodology is presented for assessing the feasibility of using atmospheric optical communications at nuclear facilities, and based on long-term meteorological observations in the area where ITER (International Thermonuclear Experimental Reactor) is located, graphs of the availability of atmospheric communications are constructed. The high prospects of using atmospheric laser communication at nuclear and fusion facilities are shown. [ABSTRACT FROM AUTHOR]

In this study, we propose a novel wavelength shift keying (WSK) technique that is combined with the conventional intensity modulation scheme for the transmission of point-to-point bidirectional data at the rate of 10 Gbps in each direction. We observe that WSK technique has not been investigated for implementation in point-to-point free space optical (FSO) links. Therefore, to the best of our knowledge, our study is the first one to perform this investigation. Our proposed link uses WSK in the downlink direction while we re-use the optical carriers for the transmission of uplink data. The use of WSK in the downlink direction enables us to perform balanced detection at the receiver, resulting in 3 dB improvement in receiver sensitivity compared to simple direct detection. We present bit error rate (BER) results for the signals transmitted in both the directions under different turbulence conditions and FSO link lengths. It was observed that the downlink signals generally perform better compared to the uplink due to the use of balanced detection and higher intensity fluctuations induced over the re-used optical carrier transmitted in the uplink direction.Please check and confirm that all the author names (given name, family name) and initials are correctly identified. The author names and initials are CORRECT. [ABSTRACT FROM AUTHOR]

Free-space optical (FSO) communication has become a focus of both study and commercial interest. FSO transfers data between sites in free space, typically the Earth's atmosphere or space, using laser-emitted light beams, in contrast to standard radio frequency (RF) wireless communication. The possibility for high data transmission speeds, the lack of licencing constraints, versatility, affordability, simplicity of deployment, and its environmentally friendly communication method which provides an alternative to crowded radio frequency-based communications are the driving forces behind this growing interest. To determine the feasibility of FSO communication in the cities of Alexandria and Aswan in Egypt, we utilized the micrometeorology model to analyze how weather conditions and atmospheric turbulence impact on FSO linkages based on real-time weather data. In both the winter and summer seasons, we evaluated the bit error rate, beam spreading, scintillation, and transmission range of FSO. Moreover, bit error rate performance was improved by the use of the multi-hop Decode and Forward relay. [ABSTRACT FROM AUTHOR]

The relative intensity noise (RIN) in a two‐section distributed Bragg reflector (DBR) tapered laser emitting around 1060 nm is being studied experimentally. By analyzing the RIN spectra, a resonance frequency and damping is obtained. Herein, the dependence of the resonance frequency and the maximum RIN at the resonance frequency is studied as functions of the ridge waveguide (RW) and tapered section currents independently. The low frequency RIN is approximately −162 dB Hz−1, and the RIN peak at currents providing high output power is as low as −154 dB Hz−1. The observed nonlinearity of the squared resonance frequency with the RW section current is attributed to carrier saturation. The measured high values of the resonance frequency, exceeding 2.5 GHz for RW currents higher than 100 mA at a tapered current of 3 A, indicate gain lever enhancement. [ABSTRACT FROM AUTHOR]

Smart cities like Neom require efficient and reliable transportation systems to support their vision of sustainable and interconnected urban environments. High-speed trains (HSTs) play a crucial role in connecting different areas of the city and facilitating seamless mobility. However, to ensure uninterrupted communication along the rail lines, advanced communication systems are essential to expand the coverage range of each base station (BS) while reducing the handover frequency. This paper presents the dual transceiver free space optical (FSO) communication system as a solution to achieve these objectives in the operational environment of HSTs in Neom city. Our channel model incorporates log-normal (LN) and gamma–gamma (GG) distributions to represent channel impairments and atmospheric turbulence in the city. Furthermore, we integrated the siding loop model, providing valuable insights into the system in real-world scenarios. To assess the system's performance, we formulated the received signal-to-noise ratio (SNR) of the network under assumed fading conditions. Additionally, we analyzed the system's bit error rate (BER) analytically and through Monte Carlo simulation. A comparative analysis with reconfigurable intelligent surfaces (RIS) and relay-assisted FSO communications shows the superior coverage area and efficiency of the dual transceiver model. A significant reduction of up to 76% and 99% in the number of required BSs compared to RIS and relay, respectively, is observed. This reduction leads to fewer handovers and lower capital expenditure (CAPEX) costs. [ABSTRACT FROM AUTHOR]

Free Space Optical (FSO) communication is extensively utilized in the telecommunication industry for both ground and space wireless links, as well as last-mile applications, as a result of its lesser Bit Error Rate (BER), free spectrum, and easy relocation. However, atmospheric turbulence, also known as Wavefront Aberration (WA), is considered a serious issue because it causes higher BER and affects coupling efficiency. In order to address this issue, a Sensor-Less Adaptive Optics (SLAO) system is developed for FSO to enhance performance. In this research, the compensation of WA in SLAO is obtained by proposing the Brownian motion and Directional mutation scheme-based Coati Optimization Algorithm, BDCOA. Here, the BDCOA is developed to search for an optimum control signal value of actuators in Deformable Mirror (DM). The incorporated Brownian motion and directional mutation are used to avoid the local optimum issue and enhance search space efficiency while searching for the control signal. Therefore, the dynamic control signal optimization for DM using BDCOA helps to enhance the coupling efficiency. Thus, the WAs are compensated for and optical signal concentration is enhanced in FSO. The metrics used for analyzing the BDCOA are Root Mean Square (RMS), BER, coupling efficiency, and Strehl Ratio (SR). The existing methods, such as Simulated Annealing (SA) and Stochastic Parallel Gradient Descent (SPGD), Advanced Multi-Feedback SPGD (AMFSPGD), and Oppositional-Breeding Artificial Fish Swarm (OBAFS), are used for evaluating the performance of BDCOA. The RMS of BDCOA for iterations 500 is 0.12, which is less than that of the SA-SPGD and OBAFS.

The utilization of Free Space Optical (FSO) systems in upcoming 5G or beyond systems is appealing due to its secure transmission, unlicensed spectrum, and higher information rate. Despite the various advantages of the FSO system, long-range communication transmission is inadequate because the effectiveness of FSO systems is significantly impaired by environmental phenomena such as scintillation, fog, haze, and rain. This research intends to improve the FSO module's effectiveness in terms of channel capacity and long-range communication. The present research has employed a hybrid technique that integrates multiple-beam, Erbium-doped fiber amplifier amplification and Homodyne detection methodologies within a wavelength division multiplexing FSO system to achieve the desired outcome. The channel model employed in this study is the Gamma–Gamma model, and the framework is evaluated using various strategies including single beam (SB1 × 1), four beams (MB4 × 4), and eight beams (MB8 × 8). The findings indicate that optimal performance has been achieved through the utilization of MB8 × 8. A channel capacity of 320 Gb/s has been observed across a range of 1.6–12 km, with variations in transmission distance attributed to external environmental factors. The link effectiveness is evaluated through the calculation of the Bit Error Rate, Q factor and Eye Height analysis. [ABSTRACT FROM AUTHOR]

This paper provides a concise overview of hybrid Free Space Optical (FSO)/Radio Frequency (RF) systems operating under fog and smog channels. With the rapid growth of wireless communication and the emergence of the Internet of Things (IoT) have brought about unprecedented connectivity and transformed various aspects of our daily lives. However, certain environmental conditions pose significant challenges to wireless communication systems like FSO and RF technologies. Fog and smog with its dense moisture-laden atmosphere and presence of particulate matter cause absorption and scattering of light, leading to reduced link quality and limited range for FSO communication system. Whereas, RF systems are although less affected by fog/smog as compared to rain but still have inherent limitations in terms of bandwidth and capacity. To overcome these challenges and ensure reliable communication, researchers have turned their attention to hybrid FSO/RF systems which can combine the strengths of FSO and RF technologies and mitigate the limitations of each technology. This work summarizes and explores the hybrid system architecture and machine learning techniques used to enhance the performance and adaptability of wireless communication systems. Additionally, we present the existing experimental research on hybrid FSO/RF systems, performance evaluation metrics, and future research directions to pave the way for further advancement. Abbreviation FSO: Free Space Optical; RF: Radio Frequency; IoT: Internet-of-Things; OS: Operating Systems; OWC: Optical Wireless Communication; LOS: Line-of-Sight; LPF: Low Pass Filter; AM: Amplitude Modulation; FM: Frequency Modulation; PSK:Phase Shift key; QAM: Quadrature Amplitude Modulation; OFDM: OrthogonalFrequency Division Multiplexing; LF: Low frequency; M-L: Medium frequency; UHF:Ultra High Frequency; SHF: Super High Frequency; EHF: Extremely High frequency; PM: Phase Modulation; QPSK: Quadrature Phase Shift Key; DSSS: Direct SequenceSpread Spectrum; FHSS: Frequency Hopping Spread Spectrum; LAN: Local AreaAccess; D/A: Digital to Analog; LO: Local Oscillator; PA: Power amplifier; A/D:Analog to Digital; LNA: Low Noise Amplifier; FEC: Forward error correction; APC: Automatic Power Control; RSSI: Received Signal Strength Indicator; ML: Machine Learning; RL: Reinforcement Learning; DL: Deep Learning; LR:Logistic Regression; DT: Decision Tree; SVM: Support Vector Machine; KNN:K-Means Neighbour; PCA: Principal Component Analysis; QL: Q-Learning; DQN:Deep Q-NEtwork; R-F: Random Forest; GB: Gradient Boosting; NA: NaiveBayes; ANN: Artificial Neural Network; DTA: Decision Tree with AdaBoost; MIMO: Multiple-Input Multiple-Output; GEO: Geostationary Orbit; NIR: NearInfra-Red; VLC: Visible Light Communication; OP: Optical Communication; BER: Bit Error Rate; ECC: Ergodic Channel Capacity; SNR: Signal-to-NoiseRatio; AO: Adaptive Optics; ATP: Acquisition- Tracking and Pointing; LEO:Low-Altitude Earth Orbit; UAVs: Unmanned Aerial Vehicles; NTN: Non-TerrestrialNetwork; RSSI: Received Signal Strength Indicator; FSPL: Free Space Path Loss; ITU: International Telecommunication Union; GANs: Generative AdversarialNetworks [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communication is a wireless alternative to fiber-based communication as it may provide a backbone emergency communication link in disaster-hit areas. This paper proposes a full duplex millimeter wave (mm-wave) enabled FSO system to provide a high data rate communication link for search and rescue operations. The proposed system consists of a central unit which is connected to multiple unmanned aerial vehicles (UAVs) through an optical-amplify and forward relay over an FSO channel represented by Log-Normal channel model. An optical comb is generated and Laguerre-Gaussian modes of each wavelength of the optical comb are exploited to carry 10 Gbps differential phase shift keying modulated signals. 60 GHz mm-wave signals are generated at the remote UAVs by employing optical heterodyne detection for downlink transmission towards the user equipment. For the uplink transmission, a dedicated wavelength is used to carry 10 Gbps on-off keying baseband data for live streaming of the disaster-hit area. Furthermore, a single hop scheme is employed to counter the non-line of sight issue of the FSO link. The performance of the proposed model is evaluated under different weather conditions and refractive index structure parameter values. A maximum distance of 3.3 km is achieved for clear sky under low turbulence conditions. Moreover, the effect of misalignment due to hovering of the UAVs on the system performance is also investigated and tolerance in the x (horizontal) and y (vertical) directions is observed at the forward error correction BER limit of 10 - 9 . The simulation results reveal that the proposed system has the potential to provide reliable and quick emergency communication services in disaster-struck areas. [ABSTRACT FROM AUTHOR]

The tremendous speed and security that Free Space Optical Communication (FSOC) technology provides have led to its rapid expansion. This opens up a plethora of possibilities for terrestrial communication with small ranges of up to a few kilometers, such as multi-campus and building-to-building communication. However, the ever-varying nature of atmospheric channels poses a major challenge degrading the optical signal strength. Several constituents of atmospheric channels like fog, dust, smoke, rain, and wind turbulence will influence the performance of an FSOC channel. In this paper, we present a system model focusing on adverse atmospheric channels, primarily by replicating varying fog conditions. Additionally, we showcase the successful real-time implementation of diverse forward error correction (FEC) codes in adverse atmospheric conditions, specifically varying levels of fog, using a dedicated test bed. Our experiments demonstrate the capability to recover erroneous data up to 50% Bit Error Rates (BER). Furthermore, we delve into the selection of suitable FEC codes tailored to different fog conditions, aiming to optimize time efficiency with the encoded bitrate.

This paper investigates the technical aspects of the 1550 nm free space optical (FSO) link in extreme Arctic weather, specifically in falling snow, which is currently understudied. We measured the time variation of the received optical power in many snowfall events and derived attenuation statistics. Continuous measurements allowed us to estimate attenuation under varying temperatures and humidity to examine the after-snow channel impairments. Our results show several shortcomings in the International Telecommunication Union-Radio Communication (ITU-R) snow attenuation model and verify that a correlation exists between snow features, snowfall rate, temperature, and humidity in determining the total snow attenuation. We demonstrated the relationship between snow properties, temperature, humidity, and attenuation under both in-snow and post-snow environments. Furthermore, based on the experimental results, we recommend several mitigation techniques to realize FSO communication during challenging Arctic weather conditions. Lastly, we present a feasibility study of FSO in Svalbard at 78°N latitude.

This research paper presents the development and implementation of a real-time communication system that utilizes LEDs for transmitting text, image, and audio through Free Space Optical Communication (FSOC) and Underwater Wireless Optical Communication (UWOC) channels. The impact of atmospheric and oceanic attenuation and turbulences are considered for the performance analysis of the proposed system. The various atmospheric weather conditions like light fog, dense fog, and heavy rain are considered for FSOC while in the case of UWOC, effects like pure water, and seawater with turbulence and without turbulence are considered. The outcomes of the experiments and simulations, including quality factors, eye diagrams, and bit error rates, are plotted and discussed in the results. [ABSTRACT FROM AUTHOR]

Free Space Optical (FSO) communication systems are gaining popularity due to its tremendous speed, advanced capacity, cost effectiveness, secure and easy to deploy wireless networks. This technology has proven an effective choice for last mile applications and hard to reach areas where deployment of optical fibre links is not feasible. But FSO link is highly weather dependent and as signal passes through the atmospheric channel, the main impairments are the atmospheric turbulence, which induce fading and deteriorate the system performance. Delhi has a great potential for FSO communication because of its clear skies. Since there is no analysis for weather condition found in Delhi, this work provides analysis of typical Delhi weather condition ranging from heavy to light rain, fog and clear sky. The performance of FSO link is analysed in terms of attenuation and link length margin under different weather condition. The results are concluded to identify which atmospheric condition influences more on FSO link performance. [ABSTRACT FROM AUTHOR]

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis of the proposed hybrid system is then carried out, taking into account both the atmospheric and underwater channels and their respective degradation sources. Different networking scenarios and conditions are evaluated. To this end, the channel model of the terrestrial free-space optical (FSO) link is modeled using the Gamma–Gamma distribution, while the underwater optical link is modeled using the Weibull distribution. The former takes into account atmospheric and turbulence attenuation, geometric spread and pointing errors, while the latter takes into account underwater and turbulence attenuation and geometric spread. Accordingly, a new analytical closed-form expression for the bit error rate (BER), which depends on the cumulative distribution function of the holistic hybrid system, is derived. Analytical results show that pointing errors as well as atmospheric and oceanic turbulence seriously degrade the performance of the hybrid system. In addition, ocean turbulence leads to the occurrence of a BER floor in some scenarios. This is the first time that such a network is proposed and modeled under the assumption of critical channel impairments. [ABSTRACT FROM AUTHOR]

Vortex beams carrying orbital angular momentum (OAM) have gained much interest in optical communications because they can be used to expand the number of multiplexing channels and greatly improve the transmission capacity. However, the number of states used for OAM-based communication is generally limited by the imperfect OAM generation, transmission, and demultiplexing methods. In this work, we proposed a dense space-division multiplexing (DSDM) scheme to further increase the transmission capacity and transmission capacity density of free space optical communications with a small range of OAM modes exploiting a multi-ring perfect vortex (MRPV). The proposed MRPV is generated using a pixel checkerboard complex amplitude modulation method that simultaneously encodes amplitude and phase information in a phase-only hologram. The four rings of the MRPV are mutually independent channels that transmit OAM beams under the condition of occupying only one spatial position, and the OAM mode transmitted in these spatial channels can be efficiently demodulated using a multilayer annular aperture. The effect of atmospheric turbulence on the MRPV was also analyzed, and the results showed that the four channels of the MRPV can be effectively separated under weak turbulence conditions. Under the condition of limited available space and OAM states, the proposed DSDM strategy exploiting MRPV might inspire wide optical communication applications exploiting the space dimension of light beams. [ABSTRACT FROM AUTHOR]

Free Space Optical (FSO) communication has evolved as a feasible technique for wireless implementations which offers higher bandwidth capacities over various wavelengths and refers to the transmission of modulated visible beams through atmosphere in order to communicate. Wavelength Division Multiplexing (WDM) is a technology that multiplexes numerous carrier signals onto single fiber using nonidentical wavelengths and enables the efficiency of bandwidth and expanded data rate. Multiple Input Multiple Output (MIMO) is implemented to improve the quality and performance of free space optical communication in various atmospheric conditions. In this paper, a WDM-based FSO communication system is being implemented that benefits from MIMO which receives multiple copies of the signal at receiver that are independent and analyzed for various streams of data in MIMO i.e. 2 × 2, 4 × 4, 8 × 8. Various factors like BER, Quality Factor are analyzed for the WDM-based FSO communication with MIMO using the OptiSystem for various data streams of MIMO under different atmospheric conditions. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid-subcarrier-intensity-modulation (hybrid-SIM) technique for the performance improvement of free-space-optical (FSO) communication system has been proposed. Subsequently, for further error performance improvement, avalanche photodiode (APD) based receiver is used in the proposed system. The system performance is analyzed at various atmospheric turbulence levels over weak and strong turbulence channels. The bit error rate (BER) is theoretically derived using Gauss–Hermite approximation and Meijer-G function and it is simulated in the MATLAB environment. The simulation result shows that the BER performance of hybrid-SIM is better than BPSK-SIM technique irrespective of the channel types and also the significant BER performance improvement is observed by APD receiver. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) channels between a low earth orbit (LEO) satellite and a ground station (GS) use a highly directional optical beam that necessitates a continuous line-of-sight (LOS) connection. In this paper, we propose a deep neural network (DNN)-based small target detection method that detects the position of a LEO satellite in an infrared image, which can be used to determine the receiver alignment for establishing the LOS link. For the infrared small target detection task without excessive down-sampling, we design a target detection model using a modified ResNest-based feature extraction network (FEN), a custom feature pyramid network (FPN), and a target determination network (TDN). ResNest utilizes the feature map attention mechanism and multi-path propagation necessary for robust feature extraction of small infrared targets. The custom FPN combines multi-scale feature maps generated from the modified ResNest to obtain robust semantics across all scales. Finally, the semantically strong multi-scale feature maps are fed into the TDN to detect small infrared targets and determine their location in infrared images. Experimental results using two widely used point spread functions (PSFs) demonstrate that the proposed algorithm outperforms the conventional schemes and detects small targets with a true detection rate of 99.4% and 94.0%. [ABSTRACT FROM AUTHOR]

Traditional optical spatial modulation techniques all rely on the premise that the system receiving end can obtain accurate channel state information.The optical differential spatial modulation (ODSM) system effectively avoids complex channel estimation, but related research only analyzes the system performance under a single turbulent state, and ignores the effects of factors such as pointing errors and path loss in actual free space optical communication systems.Therefore, the Málaga turbulence channel, which could characterize all atmospheric turbulence states, was used to derive the upper bound expression and diversity order of the bit error rate of the ODSM system under the atmospheric joint effect such as turbulence, pointing error, and path loss.The analysis results show that compared with other optical spatial modulation schemes, the ODSM system avoids complex channel estimation, making the system no longer affected by channel estimation errors, thus possessing higher anti-interference and stability.The bit error rate performance of the ODSM system decreases as the intensity of turbulence and pointing error increase.By optimizing parameters such as the number of optical antennas, the number of photodetectors, and the modulation signal order, the bit error rate performance of the ODSM system can be further improved.

Smart cities like Neom require efficient and reliable transportation systems to support their vision of sustainable and interconnected urban environments. High-speed trains (HSTs) play a crucial role in connecting different areas of the city and facilitating seamless mobility. However, to ensure uninterrupted communication along the rail lines, advanced communication systems are essential to expand the coverage range of each base station (BS) while reducing the handover frequency. This paper presents the dual transceiver free space optical (FSO) communication system as a solution to achieve these objectives in the operational environment of HSTs in Neom city. Our channel model incorporates log-normal (LN) and gamma–gamma (GG) distributions to represent channel impairments and atmospheric turbulence in the city. Furthermore, we integrated the siding loop model, providing valuable insights into the system in real-world scenarios. To assess the system’s performance, we formulated the received signal-to-noise ratio (SNR) of the network under assumed fading conditions. Additionally, we analyzed the system’s bit error rate (BER) analytically and through Monte Carlo simulation. A comparative analysis with reconfigurable intelligent surfaces (RIS) and relay-assisted FSO communications shows the superior coverage area and efficiency of the dual transceiver model. A significant reduction of up to 76% and 99% in the number of required BSs compared to RIS and relay, respectively, is observed. This reduction leads to fewer handovers and lower capital expenditure (CAPEX) costs.

In this work, we have carried out a performance analysis of the Decode and Forward (DF), dual-hop hybrid Radio Frequency-Free Space Optical Communication (RF-(RF/FSO)) link. The Source to Relay (S → R) link is considered as an RF link, and Relay to Destination (R → D) link is a parallel RF and FSO link. The selection combining technique is employed at the destination. The RF links are modeled by Nakagami-m distribution, and the FSO link is modeled by and Malaga channel model with pointing errors. For the FSO link, both Intensity Modulation/Direct Detection (IM/DD) and Heterodyne detection schemes are considered. The closed form expressions of Probability Distribution Function (PDF), Cumulative Distribution Function (CDF), and Moment Generating Function (MGF) of end-to-end SNR are derived in form of Meijer's G function. Utilizing the above results, The expressions for outage probability, bit error rate (BER), and ergodic capacity have been obtained. The numerical results are compared for different strengths of fading of RF links, the strength of atmospheric turbulence, the severity of pointing errors, and detection techniques. The numerical results closely match with Monte-Carlo simulations which validate the analytical work. The RF-(RF/FSO) link's performance is found to be better than the equivalent RF-FSO link. The link (S → R or R → D) experiencing the stronger fading dominates the overall link performance. [ABSTRACT FROM AUTHOR]

Free Space Optical (FSO) communication has potential terrestrial and non-terrestrial applications. It allows large bandwidth for higher data transfer capacity. Due to its high directivity, it has a potential security advantage over traditional radio frequency (RF) communications. However, eavesdropping attacks are still possible in long non-terrestrial transmission FSO links, where the geometry of the link allows foreign flying objects such as Unmanned Aerial vehicles (UAVs) and drones to interrupt the links. This exposes non-terrestrial FSO links to adversary security attacks. Hence, data security techniques implementation is required to achieve immune FSO communication links. Unlike the commonly proposed physical layer security techniques, this paper presents a lab-based demonstration of a secured FSO communication link based on data cryptography using the GNU Radio platform and software-defined radio (SDR) hardware. The utilized encryption algorithm (Xsalsa20) in this paper requires high-time complexity to be broken by power-limited flying objects that interrupt the FSO beam. The results show that implementing cryptographic encryption techniques into FSO systems provided resilience against eavesdropping attacks and preserved data security. The experiment results show that, at a distance of 250 mm and laser output power of 10 mW, the system achieves a packet delivery rate of 92 % and transmission rate of 10 Mbit/s. This is because the SDR used in this experiment requires a minimum received electrical amplitude of 27.5 mV to process the received signal. Long distance and higher data rates can be achieved using less sensitive SDR hardware. [ABSTRACT FROM AUTHOR]

This paper explores the performance of a single input multiple output (SIMO) free space optical (FSO) communication system using differential chaos shift keying (DCSK) with diversity combining techniques. Specifically, the paper presents the use of maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) diversity techniques for the DCSK-FSO system. Adding chaos to FSO systems using diversity can significantly improve the reliability and the system performance, making them a promising option for various 5G/6G wireless applications. The inherent properties of chaos, such as security, wide band nature, and immunity to fading, make it an attractive option for FSO communication. The Gamma-Gamma turbulence model was used to characterize the FSO link, and the system’s performance was analyzed in terms of bit error probability across varying atmospheric turbulence levels, link lengths, and numbers of receivers. The results indicate that under the same system conditions, MRC/EGC performs 2–4 dB better in terms of error probability than the SC diversity technique. Furthermore, the SIMO-DCSK-FSO system presents a substantial improvement in diversity gain, with around 30 dB for moderate turbulence and 37 dB for strong turbulence.

Vortex beams' orbital angular momentum (OAM) has important application value in optical communication and other fields. Accurate measurement of their OAM topological charges is required for the application of vortex beams in the field of optical communication. As a type of convolutional neural network, DenseNet which combines the “Data Augmentation with Expansion” method, was introduced to measure the fractional topological charge of OAM images in the case of few samples. The OAM images used in this paper generally do not have high brightness, which simulates ultra-long-distance transmission situations in reality. The experiment shows that the classification accuracy of DenseNet network reaches 98.77%, with an average training time of about 1100 seconds, which is short. Our results showcase the potential of convolutional neural network approach to further study the OAM light in the application of free-space optical communication.

The use of multiple transmitters in a turbulent free-space optical (FSO) communication link can enhance the systems performance because greater received power can improve receiver sensitivity. Nevertheless, the effect of having more power at the receiver can be severe. This paper investigates the impact of using multiple transmitters in preamplified FSO communication links limited by strong atmospheric turbulence (AT), amplified spontaneous emission noise, fixed path loss and pointing errors (PEs) using different preamplifier operating modes and different decision thresholding schemes. Results obtained show that regardless of the number of transmitters used, the best bit error rate (BER) performances are obtained with normalised decision threshold levels of about 0.2 and 0.5 when the decision thresholding scheme at the receiver is non-adaptive and adaptive, respectively. Also, in the strong AT regime, an additional transmitted power of about 7dB is required for the FSO communication systems under minimal PE effects to have the same performance as FSO communication systems without PE. The results also show that when the effects of PE are absent or minimal, a larger receiver would require about 15dB less transmitted power to record the same performance as a smaller receiver. However, with a non-adaptive decision threshold, smaller receivers perform better than larger receivers when the PE effects are severe. Additionally, it is shown in this paper that when the effects of PE is severe, the BER performances consistently get better with more transmitters regardless of the decision thresholding scheme employed at the receiver. However, in the absence of PEs, increasing the number of transmitters indefinitely will not always guarantee improved BER performances when the receiver decision threshold is non-adaptive. The use of multiple transmitters is particularly advantageous for applications where it is either necessary or unavoidable to use lower transmitted power.

Wireless optical coherent communication can be used as the backbone supporting technology of air-space-ground-sea-integrated network. In wireless optical coherent communication, the information can be demodulated from the carrier correctly only when the phase synchronization is accurate. Operating in different systems and diverse environments presents challenges for phase synchronization, especially lasers with different line-widths and atmospheric turbulence channels that cause scintillation and wave-front distortion. We propose a combination of adaptive fractionally-spaced blind equalization and adaptive kalman filter to realize phase synchronization under different line-widths and different scintillation and wave-front distortion. This combination which changed the limitation that the original settings of each part is not directly applicable to atmospheric channel combines spatial diversity in a natural way, jointly removes amplitude noise and phase noise more thoroughly with the optimal bandwidth, and can realize phase synchronization under time-varying conditions. The signal quality of the proposed scheme with constant parameter output can be improved 1-2dB in both mean square error (MSE) and symbol to error ratio (SER) compared to traditional equal gain combining (EGC) followed by Viterbi-Viterbi phase estimation (VVPE) and the proposed constant parameter scheme has more laser line-width options. The proposed scheme with noise parameter corrected by Autoregressive method outperforms the constant parameter scheme 1-2dB at both MSE and SER and has higher and wider line-width selections. With this design, phase synchronization can be more flexible to adapt to dynamic changes, which will be more suitable for future applications of wireless optical networks with dynamic,noisy, diverse environment.

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In this paper, an electroabsorption modulator-based relay is employed for the transmission of on-off keying (OOK) and differential phase shift keying (DPSK) signals over a free space optical (FSO) communication link. The FSO link is based upon the Gamma-Gamma (Γ - Γ ) channel model considering both medium and strong atmospheric turbulence conditions. 10 Gbps modulated OOK and DPSK signals are transmitted over the FSO link by employing all-optical amplify and forward (AOAF) and all-optical regenerate and forward (AORF) relay-assisted techniques. The bit-error-rate results are analyzed with respect to received optical power for the proposed model. It is observed that AORF relay-assisted OOK is performing better than AOAF at high data rates under medium atmospheric turbulence, whereas for DPSK-modulated signals, the performance is almost the same for both AORF and AOAF relay-based FSO links. The AORF-based relay for the transmission of OOK- and DPSK-modulated signals show significant improvement under strong atmospheric turbulence compared to the AOAF-based relay link. Furthermore, under multiple relay scenario for DPSK signal, AOAF results degraded while AORF remains almost the same under strong atmospheric turbulence. [ABSTRACT FROM AUTHOR]

Aiming at the problems of space resource waste and error bit performance limitation caused by fixed optical antenna combination set used in traditional optical generalized spatial modulation (OGSM), an improved scheme based on low complexity adaptive optical antenna combination set selection algorithm was proposed.First, the Euclidean distance matrix between optical antenna combinations was calculated using channel state information.Then, based on the principle of Euclidean distance equivalence and the idea of maximum norm antenna selection, the optical antenna combination with poor channel state was deleted, and the final reserved optical antenna combination was the optimal optical antenna combination set in the current state.The traversal of all optical antenna combinations was avoided and the complexity of the algorithm was reduced to a large extent.The simulation results show that the BER performance of the improved OGSM scheme outperforms the conventional OGSM under different conditions.

The smart grid is playing a key role to enable smart cities and smart Internet of things. As an important part of the smart grid, the smart distribution network requires the intelligent terminal unit (ITU) to realize real-time electrical parameters acquisition and effective communication. The traditional ITU uses optical fiber or wireless mechanisms for data transmission among electrical equipment, other units, and master station, which is limited to deployment cost and ambient electromagnetic interference. Therefore, we propose a novel enhanced intelligent terminal unit (E-ITU) under the smart distribution network. First, we propose the E-ITU model that integrates free-space optical communication (FSO) and radio frequency (RF) for ITU to enhance the communication methods. Then, we propose the adaptive hybrid FSO/RF transmission mechanism for the E-ITU to improve the channel transmission performance. Finally, we evaluate our E-ITU performance, and the experiment results show that the E-ITU can effectively reduce system interrupts and improve the system transmission rate, which achieves high robust electrical parameters transmission under the smart distribution network. [ABSTRACT FROM AUTHOR]

We propose a novel all-optical technique for the transmission of multiple channels over a single optical carrier by employing pulse position modulation (PPM). The proposed technique partly solves the issue of low spectral efficiency associated with the PPM transmission scheme. Furthermore, our proposed technique also improves the security of the FSO link against eavesdroppers, especially if they are located close to the receiver and are able to detect some part of the wide beamwidth optical signal. We have employed the Gamma–Gamma channel model to observe the performance of the FSO link for different lengths. [ABSTRACT FROM AUTHOR]

This paper presents a fuzzy logic controlled based scheme that optimizes the throughput efficiency of the free space optical (FSO) communication system. The appropriate packet length required is computed using the particle swarm optimization method. The proposed scheme appropriately acquired the transmitted packet length and modulation order as per feedback of computed Channel State Information (CSI) at the receiver. The numerical results show a 0.4–1.8 dB gain improvement in throughput in comparison to the FSO communication system using fixed packet length. [ABSTRACT FROM AUTHOR]

In this paper, a high-speed short-reach free space optical (FSO) transmission system is proposed for data center interconnect (DCI). This system supports the transmission of 400 Gbps over a single wavelength by employing spatial vertical cavity surface emitting lasers. Space division multiplexing along with polarization division multiplexing of the Laguerre–Gaussian (LG) modes (LG 00 and LG 01 ) are exploited to achieve higher capacity over FSO channel. Intensity modulation direct detection scheme is used in the proposed system for cost-efficiency. Least mean square equalization is applied to the received data signals to mitigate the FSO channel's impairments. The performance of the proposed system is evaluated by observing the bit error ratio (BER) for weak, moderate and strong turbulences at different weather dynamics. The effects of polarization induced crosstalk and aperture misalignment induced crosstalk on BER degradation are also investigated. FSO channel lengths of 700 m, 500 m and 150 m are achieved for weak, moderate and strong turbulences, respectively. The receiver sensitivities at forward error correction limit of 3.8 × 10 - 3 for each FSO channel length are compared with back to back configuration. A penalty of 2.5 dB is observed for strong turbulence whereas 2 dB and 0.6 dB penalties are obtained for moderate and weak turbulences, respectively. The proposed FSO-DCI could be used to provide high aggregate transmission capacity of 800 Gbps and 1.6 Tbps for future DCI by adding more LG modes or other wavelengths. [ABSTRACT FROM AUTHOR]

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis of the proposed hybrid system is then carried out, taking into account both the atmospheric and underwater channels and their respective degradation sources. Different networking scenarios and conditions are evaluated. To this end, the channel model of the terrestrial free-space optical (FSO) link is modeled using the Gamma–Gamma distribution, while the underwater optical link is modeled using the Weibull distribution. The former takes into account atmospheric and turbulence attenuation, geometric spread and pointing errors, while the latter takes into account underwater and turbulence attenuation and geometric spread. Accordingly, a new analytical closed-form expression for the bit error rate (BER), which depends on the cumulative distribution function of the holistic hybrid system, is derived. Analytical results show that pointing errors as well as atmospheric and oceanic turbulence seriously degrade the performance of the hybrid system. In addition, ocean turbulence leads to the occurrence of a BER floor in some scenarios. This is the first time that such a network is proposed and modeled under the assumption of critical channel impairments.

Summary: The main aim of this study is to examine the performance of the free space optical communication (FSOC) system under the impact of rainfall in various regions of India. The meteorological data of rain intensities, during the years 2014–2017, were outsourced from the India Meteorological Department (IMD) for potential regions in India, comprising the coastal and inland locations for calculating the specific rain attenuation coefficient. Furthermore, we have proposed a 16‐channel FSOC system based on orthogonal frequency division multiplexing (OFDM), employing coherent detection and digital signal processing (DSP) for investigating the system performance for rainy weather conditions in India. For average rain intensities, the attenuation coefficient for the inland region of Hyderabad is 1.91 dB/km, which is the minimum among all locations considered in this work. Consequently, the proposed system design is capable of supporting an FSOC link of 5 km, for delivering a bit error rate (BER) of the order of 10−6 in the case of Hyderabad. On the flip side, the average rainfall intensities are maximum for the coastal region of Mumbai, and the specific attenuation coefficient is maximum with a value of 4.08 dB/km. Due to this, the maximum possible FSOC range for Mumbai gets restricted to 3.5 km only, ensuring BER of the order of 10−6. Likewise, the proposed FSOC system has performed superior for inland regions of India in comparison with coastal areas of India under the worst rainfall scenario as well. [ABSTRACT FROM AUTHOR]

FSO is one of the fast-growing fields towards the next generation of communication technology. FSO facilitates the requirement of high data rate. On the other hand, the impairments like, atmospheric turbulence and other nonlinear effects of optical link limits the overall system performance. To mitigate the effect of these impairments, different diversity combining schemes are implemented by different researchers. Also, it is noticeable that MGF based approach is more tractable compare to that of PDF based approach. However, performance analysis in terms of error rate and capacity under Equal Gain Combining (EGC) diversity scheme for Double Generalized Gamma fading distribution using MGF is missing from the literature of FSO. Therefore, this paper presents closed form expressions of Average Capacity (AC) and Symbol Error Rate (SER) of M-PSK and M-QAM for Double Generalized fading distribution under EGC scheme. Further, the effect of different turbulence conditions such as weak, moderate and strong turbulences is presented. Furthermore, respective AC and SER are used to deduce those over Double Weibull and Gamma-gamma as special cases of Double Generalized fading distribution. Increase in the number of diversity branches improve AC and SER of the system. Simulated results show perfect agreement with those available in the literature. [ABSTRACT FROM AUTHOR]

For the fronthaul link in the fifth-generation (5G) wireless networks, optical fiber is one of the most widely used medium, owing to its large bandwidth and low transmission loss. However, in some scenarios, especially the dense urban areas, the installation of fiber optic cables is costly and difficult to establish. Alternatively, the free space optical (FSO) link can be installed easily and spans complex terrain. In this context, we propose and experimentally demonstrate a hybrid FSO/Wireless transmission system based on bandpass delta-sigma modulator and real-time coherent transponder. Advantage of the proposed technique is twofold. Firstly, coherent detection is applied to improve the system sensitivity compared to conventional direct detection. Secondly, delta-sigma modulation (DSM) is utilized to greatly simplify the wireless transmitters and to enhance the bandwidth flexibility. Over a 50-m FSO and 1-m wireless link, a 64-QAM wireless signal at 1 GHz with 500 MHz bandwidth is successfully transmitted. With 16 wavelength division multiplexing channels, the total throughput of the system is up to 24 Gb/s. Meanwhile, the error vector magnitude (EVM) of all channels is around 2.68% and 6.51%, which are demonstrated after FSO and FSO/Wireless transmission, respectively. The results meet the requirement of the third-generation partnership project release 15 with EVM value of 8% for 64-QAM. Moreover, sensitivity of the real-time coherent receiver is lower than −48 dBm under the 7% forward error correction limit with bit error rate of 3.8e-3.

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding.