Free-space Optical Communication Links Research Articles

A Bidirectional WDM-PON Free Space Optical (FSO) System for Fronthaul 5 G C-RAN Networks

High-speed cellular technologies require low-latency and high-capacity optical networks. The Centralized Radio Access Network (C-RAN) architecture offers a cost-effective solution for mobile network deployment. To maximize flexibility and minimize deployment costs of fronthaul networks, we propose a hybrid bidirectional fronthaul C-RAN topology based on Wavelength Division Multiplexing (WDM) passive optical networks (PONs) and free space optical communication (FSO). The wavelength reuse scheme utilized here relies on reflective semiconductor optical amplifiers (RSOAs) to reduce cost and increase capacity. The system was demonstrated for 20 Gbps 16 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> quadrature amplitude modulation (16-QAM) intensity-modulated orthogonal frequency-division multiplexing (OFDM) downstream signals and 5 Gbps On-off keying (OOK) upstream signals, respectively. A Gamma-Gamma channel model is used to demonstrate optical signal transmission over an FSO link. The bit error rate (BER) results indicate that the hybrid WDM-PON-FSO based fronthaul architecture could achieve 320 Gbps over 20 km of single-mode fiber (SMF) and 700 m free space transmission.

Experimental Machine Learning Approach for Optical Turbulence and FSO Outage Performance Modeling

A laser beam propagating in the free space suffers numerous degradation effects. In the context of free space optical communications (FSOCs), this results in reduced availability of the link. This study provides a comprehensive comparison between six machine learning (ML) regression algorithms for modeling the refractive index structure parameter (Cn2). A single neural network (ANN), a random forest (RF), a decision tree (DT), a gradient boosting regressor (GBR), a k-nearest neighbors (KNN) and a deep neural network (DNN) model are applied to estimate Cn2 from experimentally measured macroscopic meteorological parameters obtained from several devices installed at the Naval Postgraduate School (NPS) campus over a period of 11 months. The data set was divided into four quarters and the performance of each algorithm in every quarter was determined based on the R2 and the RMSE metric. The corresponding RMSE were 0.091 for ANN, 0.064 for RF, 0.075 for GBR, 0.073 for KNN, 0.083 for DT and 0.085 for DNN. The second part of the study investigated the influence of atmospheric turbulence in the availability of a notional FSOC link, by calculating the outage probability (Pout) assuming a gamma gamma (GG) modeled turbulent channel. A threshold value of 99% availability was assumed for the link to be functional. A DNN classification algorithm was then developed to model the link status (On-Off) based on the previously mentioned meteorological parameters.

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.

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.

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.

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.

A Bidirectional Wavelength Division Multiplexed (WDM) Free Space Optical Communication (FSO) System for Deployment in Data Center Networks (DCNs).

Data centers are crucial to the growth of cloud computing. Next-generation data center networks (DCNs) will rely heavily on optical technology. Here, we have investigated a bidirectional wavelength-division-multiplexed (WDM) free space optical communication (FSO) system for deployment in optical wireless DCNs. The system was evaluated for symmetric 10 Gbps 16-quadrature amplitude modulation (16-QAM) intensity-modulated orthogonal frequency-division multiplexing (OFDM) downstream signals and 10 Gbps on-off keying (OOK) upstream signals, respectively. The transmission of optical signals over an FSO link is demonstrated using a gamma-gamma channel model. According to the bit error rate (BER) results obtained for each WDM signal, the bidirectional WDM-FSO transmission could achieve 320 Gbps over 1000 m free space transmission length. The results show that the proposed FSO topology offers an excellent alternative to fiber-based optical interconnects in DCNs, allowing for high data rate bidirectional transmission.

Performance of Integrated Ground-Air-Space FSO Links Over Various Turbulent Environments

We analyze the outage probability of integrated ground-air-space free-space optical (FSO) communication links for different atmospheric turbulence channel models. The performance of ground to high altitude platforms (HAPS), HAPS to geostationay Earth orbit (GEO) satellite, HAPS to HAPS and ground to GEO satellite is investigated for various link configurations (downlink, horizontal, and uplink) and parameters such as zenith angle, channel state, horizontal and vertical deviations, altitude, beam waist, receiver aperture diameter, wind speed, and visibility. The atmospheric attenuation (the effects of fog, cloud and volcanic activities), atmospheric turbulence, angle of arrival (AOA) fluctuations and pointing error are included in the considered model. Closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) are obtained for Lognormal, Gamma and exponentiated Weibull distributed channel models. The benefit of using HAPS is shown by comparison of outage probabilities for direct ground to GEO satellite and HAPS assisted ground to GEO satellite links.

Harvesting Planck radiation for free-space optical communications in the long-wave infrared band.

We demonstrate a free-space optical communication link with an optical transmitter that harvests naturally occurring Planck radiation from a warm body and modulates the emitted intensity. The transmitter exploits an electro-thermo-optic effect in a multilayer graphene device that electrically controls the surface emissivity of the device resulting in control of the intensity of the emitted Planck radiation. We design an amplitude-modulated optical communication scheme and provide a link budget for communications data rate and range based on our experimental electro-optic characterization of the transmitter. Finally, we present an experimental demonstration achieving error-free communications at 100 bits per second over laboratory scales.

Orbital angular momentum for turbulence mitigation in long free-space optical communication links

The ability of laser beams carrying orbital angular momentum to reduce the scintillation level of Gaussian beam propagating in atmospheric turbulence is numerically investigated. The effect of topological charge, beam waist, operating wavelength and also the receiver radius on the scintillation indices is examined in detail. It is shown that the Gaussian beam carrying orbital angular momentum has a priority over a fundamental Gaussian beam in communication links longer than 2.5 Km. It is clear that Gaussian beams carrying orbital angular momentum in the cases when having a small beam waist and operating at a high wavelength are able seriously to reduce the scintillation index. Accordingly, the presented results will be significant for designing optical communication links that use laser beams carrying orbital angular momentum.

Wavelet denoising approach in long-distance optical communications.

For free-space optical communication links, the light spot collected by the photodetector at the receiving terminal is not an ideal light spot that is affected by atmospheric turbulence. The light spot collected by the photodetector will also be accompanied by various noises. More importantly, the presence of all noise will bring errors to acquire the light spot's center. As a result, the tracking error can affect the stability of the optoelectronic tracking system. Therefore, it is necessary to remove noise from the collected images. The method of removing noise needs to be effective, but it cannot bring a large amount of calculation to affect the real-time performance. The calculation amount of wavelet transform is small, and the effect of noise removal is better, which can focus on local details with arbitrary expansion coefficients. An improved wavelet denoising method is proposed. The long-distance verification experiment (11.16 km) verified the effectiveness of this approach, compared with the traditional method. Furthermore, to the best of our knowledge, this new approach would be beneficial for the design of optical communication systems.

Evaluating the performance of free space optical communication (FSOC) system under tropical weather conditions in India

SummaryThe 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.

Performance enhancement of multi channel multi beam FSO communication link with the application of Reed Solomon codes

The rising user needs for high speed applications and services have necessitated the requirement of a communication link that can provide the data rate comparable to optical fiber. Free Space Optical (FSO) Communication is an admired means of communication from past few years. To achieve seamless and error free transmission at high speed for heterogeneous multimedia applications at desired bit rates, novel advancements in optical communication technology are being explored along with FSO. However, several constraints such as atmospheric turbulence, uneven terrain, dynamic climatic conditions cause several nonlinearities which limit the connectivity. In this work, a multi channel multi beam FSO communication system by incorporating Reed Solomon (RS) Codes has been proposed at 30 Gbps. The Modified Gamma-Gamma model is being applied for channel turbulences. The application of Forward Error Correction (FEC) coding in the designed system offers ease of error detection and correction with improved performance at higher data rate for implementation over longer link lengths.The system performance has been evaluated for (i) Number of errors corrected, (ii) Bit Error Rate (BER), (iii) Received power, (iv) Geometric path losses and (v) Signal to Noise Ratio (SNR). At an acceptable BER of 10–9, the RS (255, 127, 129) encoded Four channel Four beam FSO communication link is effectively implemented for a distance of 3500 m with − 32.5 dBm received low optical power and 25 dB SNR.

Analog transmission of time-frequency signal in atmospheric turbulence environment.

The high-precision time-frequency transfer of the optical atomic clock signals over ground-to-satellite and terrestrial free-space laser paths is of great significance in the fields of fundamental and applied sciences. However, the phase noises caused by atmospheric turbulence severely degrade the measurement precision. In this paper, a new method to simulate the transmission of time-frequency signal propagating through atmospheric turbulence is investigated. An analog transmission system comparable to the practical out-field link has been demonstrated, which can provide a deep insight into the phase distortion of time-frequency signal of free-space optical communication links.

Performance Analysis of Dual-Beam Free Space Optical Communication Link under Dust and Rain Conditions

Free space optical (FSO) communication has become an enduring and well-established communication technology in the last few decades with several advantages of high data rate, enormous bandwidth, low power consumption, transportable technology without right of way, and inherently secure line of a sight communication system. The invisible, intensity-modulated signal of light propagated through the air and detected on the receiver side experiences attenuation because of uneven distribution of rain droplets, suspended dust aerosol particles, and the droplet size distribution of fog particles in atmospheric layer degrade performance of FSO communication link. The ever-increasing demand for high data rate has quest for an innovative research for a communication link. In this paper, the performance of a dual-beam FSO communication link is evaluated under rain and dust as attenuation conditions. The system parameters, such as link distance transmitted and received power, link distance, diameter of transmitter and receiver aperture, and divergence angle, are optimized for a metropolitan FSO communication link. Dual-beam FSO communication signal propagated through an estimated attenuation level at 30 dBm transmitted power for link distance up to 2.5 km. The optical power splitter and power combiners are used in the simulation to estimate different channel parameters without the MIMO technique. The information signal of the 10 Gbps data rate is internally modulated using the NRZ generator, externally modulated by the Mach-Zehnder, and an optical signal transmitted through a dual-beam optical spectrum frequency of 193.1 THz using power splitter technique apart from each other. A comprehensive analysis is performed to design and assess robust optical communication systems through efficiency parameters such as received power, optical signal-to-noise ratio (OSNR), bit error rate (BER), and Q-factors. Results show that received optical power is a weather-dependent variable that shows a decreasing pattern as weather attenuation increases. Likewise, Q-factor and OSNR show similar decreasing trend with introduction of rain and dust as attenuator; however, BER increases in presence of attenuation.

On the Placement and Sustainability of Drone FSO Backhaul Relays

We consider free-space optical (FSO) communication links for the backhaul connectivity of small cells (SCs) where a UAV with an FSO apparatus can serve as a backhaul relay node. We demonstrate how such drone relay stations (DRSs) can be deployed in a high-rise urban area in order to provide FSO line-of-sight (LOS) links that are unobstructed by buildings. Also, in our solution we consider the case where solar panels are mounted on DRSs such that placing the DRS in a sunny location is prioritized, and we show the gain in terms of number of required trips to recharge the UAV.

Characterization of fog-induced attenuation for optimizing optical propagation links in Nigeria

Fog-Induced Attenuation (FIA) is crucial in assessing the performance of Free-Space Optics (FSO) systems for optimal results. The goal of this work is to characterize FIA seasonally and yearly at the selected optical wavelengths over selected locations in Nigeria. FIA has been analyzed using five (5) years (2012–2016) visibility measurements data for Abuja, Ikeja, Jos, Port Harcourt, Ilorin, Makurdi, and Sokoto locations in Nigeria. The visibility re-analysis data were obtained from achieves of the European Center for Medium-Range Weather Forecasts (ECMWF).over the selected locations. The characterization has been based on three different frequencies and their respective optical wavelengths (850 nm/352 THz, 1300 nm/230 THz, and 1550 nm/193 THz). The sensitivity of the FIA to cloud cover rate (information on visibility range) and the comprehensive cumulative distribution rate at the selected frequencies were also examined. The results show that a minimal FIA of about 0.4 dB/km can be achieved at 1550 nm/193 THz for optimal link planning and design of FSO communication links in Nigeria. However, attenuation of 0.25 dB/km can be achieved at link availability of 99.99 % for FSO transmission in the commercial city of Lagos (Ikeja) at the same 1550 nm/193 THz under bad weather conditions. However, due to additional dust in the region, FIA can extend up to 0.6 dB/km in Sokoto (arid region) under the same conditions. The variation pattern in the arid region and the associated ones will require a high-power laser with special mitigation techniques to improve the chances of link availability for optimal performance.

Hybrid OAM-Amplitude multiplexing and demultiplexing of incoherent optical states

We propose multiplexing and demultiplexing of Orbital Angular Momentum (OAM) states realized with incoherent beams to increase the effective transmission area in a free space optical communication link. The overlapped states of the beams have been effectively distinguished using a hybrid OAM-Amplitude scheme exploiting only a small portion of the entire wavefront at the receiver. The method has been theoretically formalized and the transmission experimentally tested up to 1 Mbit/s. The proposed method is suitable for high density transmission in long-distance free space optical communication systems.

Performance Improvement of the Free-Space Optical Communication Link Using Spatial Diversity Reception-Assisted OFDM Signals

Orthogonal frequency division multiplexing (OFDM) technology is presented for use in free-space optical (FSO) communications accompanied by the spatial diversity reception. Using quadrature phase shift keying (QPSK) modulation formats, the OFDM signals show robustness to support high spectral efficiency and compatibility with the spatial diversity reception to improve receiver sensitivity. Compared with the single-carrier QPSK signal, the OFDM-QPSK signal with 64 sub-carriers can reduce the BER from 2.87 × 10−3 to 2.98 × 10−4 at the SNR of 6 dB. Using a two-aperture spatial diversity reception with OFDM, the BER can be reduced from 2.45 × 10−3 of a single receiver to 6.10 × 10−4 under moderate turbulence conditions. Under strong turbulence, the BER of the single receiver is 2.14 × 10−2. It can be improved to 1.16 × 10−3 by using four-aperture receivers, and even 6.87 × 10−4 by using six-aperture receivers. The optimized aperture number should be selected according to channel conditions.