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Intersymbol Interference Due to the Atmospheric Turbulence for Free-Space Optical Communication System
Abstract
Weak atmospheric turbulence causes pulse broadening of an optical signal. In this article, we drive the pulse broadening as a function of turbulence strength by temporal moments directly. We derive pulse broadening dependency on turbulence strength and propagation distance when an optical signal propagates through atmosphere. This pulse broadening induces intersymbol interference (ISI) between adjacent pulses. Based on this derived results, we develop some observation patterns using a practical example.
... Consequently, the ISI of the transmitted signal through the atmospheric turbulent medium is defined as [12] m m = exp n− H ...
In this paper, the performance of a geostationary (GEO) satellite free-space optical (FSO) communications system is assessed under different turbulent regimes. Such systems are extremely vulnerable to intrinsically random weather conditions, which in turn affect the transmitted signal both in amplitude and phase. In clear weather, optical links are mostly suffered from large-scale fading, while in moderate to strong turbulence due to heavy rain, foggy weather, or thick clouds, the optical link between the satellite and optical ground station (OGS) will affect the signal by large-scale and small-scale fading. This is due to the multipath propagation of the laser beam owing to pulse broadening, which leads to the bandwidth limitation of the link. To overcome this problem, we first analyze the temporal beam spreading and bandwidth limitation of satellite-to-earth downlink communication systems, and then we benefit from the optical orthogonal frequency-division multiplexing (OFDM) scheme to reduce the effect of inter-symbol interference (ISI) on the performance of the system. Results demonstrate that the bit error rate (BER) performance decreases at higher bit rate and zenith angles. Also, we investigate the BER performance for three schemes including pulse amplitude modulation (PAM), phase-shift keying (PSK), and quadrature amplitude modulation (QAM). The results show that QAM has better BER performance in comparison with other schemes. In addition, the error vector magnitude (EVM) of the system is determined under different modulation schemes and various orders of QAM modulation.
... Further, the capacity of FSO communication system has been successfully increased in recent years. In particular, an optical time division multiplexing system operating at 1.28 Tbit/s data transmission over a single-mode channel has been established [1]. According to [2], through free-space optical wireless systems, up to 2.5 Gbit/s of data, voice and video communications can be transmitted. ...
Background: In this paper, the impact of the dispersion effect, due to atmospheric pressure and temperature, on NRZ-OOK terrestrial free-space optical transmission system is investigated. An expression for the dispersion parameter in FSO atmospheric channel is derived. Results: The results show that the variation of the refractive index along the transmission path induces fluctuations of group velocity dispersion of the optical pulse resulting in broadening of the pulse duration. Simulation results show that at a propagation distance of 7.5 km, the broadening ratio for input pulse duration of 300 fs is approximately 2.39. Further, at a propagation distance of 7.5 km, the remaining fraction of energy is approximately 40 % for a 300 fs input pulse duration. However, by increasing the transmitter input power, the effect of dispersion could be reduced. Namely, for a reference BER of 10-9 , the maximum distance that it could be achieved is about 1. 461 km for an input power of 1 mW, while it is about 2.694 km for an input power of 4 mW.
... The FSO link operated stably for 5 hours showing greater reliability under poor visibility conditions. A detailed analysis of dispersion effect for FSO systems at 1.55 μm has been reported [49][50][51]. Most of the literature on FSO links employing QCLs were designed for short range testing purpose only. ...
The temporal broadening of optical pulses in satellite-earth laser communication systems. The expression of turbulence-induced pulse width broadening was given, in which the turbulent atmosphere index-of-refraction structure constant, propagation distance, zenith angle and original pulse width were considered. Based on the measured datum of atmospheric parameters, the refractive index in 1.55μm was derived and the relationship of dispersion-induced pulse broadening to the spectral width, propagation distance, zenith angle, original pulse width and the wavelength of optical signal was obtained. The simulation results show that the turbulence-induced pulse broadening can be neglected for the pulse width greater than 0.5 ps, however, the dispersion-induced pulse broadening can be neglected for the pulse width larger than 20 ps. The received pulse width and maximum achievable bandwidth are calculated in this paper under certain conditions.
In this paper, we proposed a dual-enhanced core photonic crystal fiber (PCF) with high birefringence and ultra-high negative dispersion for dispersion compensation in a polarization maintained optical system. Using finite difference time domain (FDTD) method, we presented dispersion compensating PCF (DC-PCF) with negative dispersion between −1650 ps nm−1 km−1 and −2305 ps nm−1 km−1 in C-band and particularly −2108 ps nm−1 km−1 in λ = 1.55 μm wavelength. By this method, we can compensate dispersion in 124 km long span of a conventional single mode fiber (SMF) by 1 km-long of the DC-PCF at λ = 1.55 μm wavelength. Moreover, fundamental mode of the proposed PCF can induce birefringence about 3.5 × 10−3 at 1.55 μm wavelength.
In this paper, pulse broadening caused by atmospheric dispersion is analyzed, which is the key factor in high-speed and long-distance free space laser communication systems according to our simulation. The measured datum of atmospheric parameters, including atmospheric pressure and atmospheric temperature of the desert in Xinjiang, China, is used in the analysis of pulses broadening. The pulse broadening caused by atmospheric dispersion is very significant when pulsewidth is on the order of 5 ps, and pulse broadening is negligible if the pulsewidth is larger than 20 ps. With the increment of optical wavelength, atmospheric dispersion-induced pulse broadening is mitigated. The received pulsewidth is dominated mainly by dispersion-induced pulse broadening at small duty ratio of original signal, and original pulsewidth at large duty ratio of original signal respectively. By selecting proper duty ratio of optical pulse, the received pulsewidth can be minimized and the system bit rate can be maximized.
El núcleo central de esta Tesis ha sido el estudio y modelado del fenómeno físico de la turbulencia atmosférica y su impacto sobre toda señal óptica que se propague desde un transmisor hasta un receptor en un ambiente atmosférico. Como en la atmósfera se van a producir condiciones que favorecen el intercambio de masas de aire a distintas temperaturas, se van a formar un conjunto de células turbulentas distribuidas por todo el vano de propagación, cuya existencia provoca fluctuaciones en la irradiancia de la señal óptica captada por un receptor. Este fenómeno se conoce como centelleo atmosférico y consigue degradar de forma notable las prestaciones que se pueden extraer del sistema. Con el objetivo de modelar esas fluctuaciones de irradiancia, se ha tomado como punto de partida todo el trabajo desarrollado por Kolmogorov en la primera mitad de siglo, se ha aproximado dicho espectro atmosférico por un perfil gaussiano y, retomando la filosofía de los modelos de Jakes y Clarke aplicados en radiofrecuencia, se consigue, finalmente, generar una secuencia de centelleo atmosférico que modela esas fluctuaciones de irradiancia entre un transmisor y un receptor. Sin embargo, cuando intervienen varios receptores en el sistema óptico, la peculiaridad del medio atmosférico turbulento exige una interrelación entre estadísticos temporales y estadísticos espaciales entre distintas secuencias de centelleo, efecto conocido como turbulencia congelada. Se propone modelar este efecto a partir de la implementación de un modelo autorregresivo cuyo núcleo central es un sistema de descomposición matricial alternativo basado en el algoritmo de Schur que permite analizar cualquier situación que se plantee. Además, en el caso en el que la matriz a factorizar presente autovalores negativos, se propone calcular su aproximante positivo para solventar tal problema. Por otra parte, se ha conseguido modelar el efecto de un tiempo de vida finito de las células turbulentas relacionándolo con un parámetro físico denominado tasa de disipación de la energía turbulenta. Así, se rompe, de forma controlada, la uniformidad derivada del esquema de turbulencia congelada mediante la inclusión de un modelo de estadísticos separables espacio-temporalmente, que finalmente permita que las secuencias de centelleo que se generen sigan manteniendo la relación espacial adecuada, pero rompiendo parcialmente su correlación temporal, tanto como indique la tasa de disipación de la energía turbulenta. Posteriormente, se ha constatado que la relación de potencia óptica de pico a promedio funciona en estos ambientes ópticos atmosféricos, como una adecuada figura de mérito que permite anticipar las prestaciones cualitativas que ofrece una técnica de señalización en comparación a otra. De esta forma, maximizando tal figura de mérito, se obtienen mejores prestaciones del sistema óptico, indicando una forma de actuar en el sistema para mejorar los resultados del mismo. Finalmente, se han elaborado algunos resultados analíticos para constatar el suelo de ruido en estos sistemas, obtenido por la presencia de un medio turbulento.
Atmospheric turbulence produces scintillation at an optical receiver, which leads to fading of the received signal. This fading affects the bit-error-rate (BER) of a digital signal in a way that depends on the depth of the fade, the decision threshold at the receiver, and the average signal-to-noise ratio. The degree of fading can be dramatically reduced by the use of a time-delayed diversity technique, which involves retransmission of the data stream after a short delay, and resynchronization of the received data streams.
The problem of propagation of pulsed beam waves in an atmosphere with turbulence, rain, or fog is investigated using the temporal moments method. Exact analytical expressions are derived for the mean arrival time and the mean square pulse width for the pulsed beam after it has propagated through the turbulent atmosphere. The results are valid even for strong scattering conditions. Numerical examples indicate that at optical frequencies, the effects of the atmospheric turbulence on the pulses are small except perhaps for picosecond pulses. However, the effects of clouds on optical pulsed beams can be quite severe. At microwave frequencies, scattering by rain may have very large effects on the pulses.
Pulse propagation in a random medium is studied through the calculation of the two-frequency mutual coherence function. An exact integral representation is formulated for the two-frequency mutual coherence function of a Gaussian beam pulse propagating in a weakly fluctuating random medium. Based on the modified von Karman spectrum for refractive-index fluctuations, an analytic approximation to the integral representation is presented and compared with exact numerical results.
Pulse propagation in a random medium is determined by the two-frequency mutual coherence function which satisfies a parabolic equation. In the past, numerical solutions of this equation have been reported for the plane wave case. An exact analytical solution for the plane wave case has also been reported for a Gaussian spectrum of refractive-index fluctuations. Using the same approximation, an exact analytic solution for the more general case of an incident beam wave is presented. The solution so obtained is used to study the propagation characteristics of the beam wave mutual coherence function at a single frequency as well as at two frequencies. Simple expressions are obtained which qualitatively describe the decollimating and defocusing effects of turbulence on a propagating beam wave. The time variation of the received pulse shape, on and away from the beam axis, is studied when the medium is excited with a delta function input. The results are presented for both collimated and focused beams.
The three basic atmospheric propagation effects, absorption, scattering, and turbulence, are reviewed. A simulation approach is then developed to determine signal fade probability distributions on heterodyne-detected satellite links which operate through naturally occurring atmospheric turbulence. The calculations are performed on both angle-tracked and nonangle-tracked downlinks, and on uplinks, with and without adaptive optics. Turbulence-induced degradations in communication performance are determined using signal fade probability distributions, and it is shown that the average signal fade can be a poor measure of the performance degradation.
An analytic expression is derived for the long-term temporal broadening (fluctuations of arrival time) of a collimated space-time Gaussian pulse propagating along a horizontal path through weak optical turbulence. General results are presented for nominal parameter values characterizing laser communication through the atmosphere. Specific examples are calculated for both upper-atmosphere and ground-level cross links. It is shown that, for upper-atmosphere cross links, pulses shorter than 100 fs have considerable broadening, whereas at ground level, broadening is predicted in pulses as long as 1 ps.
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Pulse propagation in a random medium is determined by the two‐frequency mutual coherence function which satisfies a parabolic equation. In the past, approximate or numerical solutions of this equation have been reported. This paper presents an exact analytical solution for a plane wave case when the random medium is approximated by A(0) − A(ρ) ∝ ρ2. Analytic expressions for two‐frequency mutual coherence function, pulse shape, temporal and angular spectra are presented in universal forms, and should be applicable to a large number of practical problems.
In this paper, analytic expressions for the temporal broadening of narrowband space-time Gaussian pulses propagating in weak optical turbulence are derived for both near and far fields. General results are presented for nominal parameter values characterizing laser communication through the atmosphere. Specific examples are calculated for both upper atmosphere UAV-UAV cross-links and uplink/downlink satellite communication paths. It is shown that for both the near and far fields, pulses on the order of 10 - 20 femtoseconds can broaden by more than 100% whereas pulses greater than 500 femtoseconds have negligible broadening.
Atmospheric turbulence produces scintillation at an optical receiver, which leads to fading of the received signal. This fading affects the bit-error-rate (BER) of a digital signal in a way that depends on the depth of the fade, the decision threshold at the receiver, and the average signal-to-noise ratio. The degree of fading can be dramatically reduced by the use of a time-delayed diversity technique, which involves retransmission of the data stream after a short delay, and resynchronization of the received data streams.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Since publication of the first edition of this text in 1998, there have been several new, important developments in the theory of beam wave propagation through a random medium, which have been incorporated into this second edition. Also new to this edition are models for the scintillation index under moderate-to-strong irradiance fluctuations; models for aperture averaging based on ABCD ray matrices; beam wander and its effects on scintillation; theory of partial coherence of the source; models of rough targets for ladar applications; phase fluctuations; analysis of other beam shapes; plus expanded analysis of free-space optical communication systems and imaging systems. © 2005 The Society of Photo-Optical Instrumentation Engineers. All rights reserved.
The theory of plane wave pulse propagation through a random medium, under the forward-scattering assumption is presented. Since pulse propagation characteristics are determined by two-frequency mutual coherence function Gamma , a set of normalized curves is given for Gamma for different propagation parameters (operating frequency, propagation distance, turbulence strength or density of scatterers, etc.). From the curves one can obtain the coherence bandwidth of a wave for a variety of situations. A received pulse form due to an input delta function is given in a normalized form which is applicable to the whole range of strong fluctuation. The results are applied to optical pulse propagation in dense clouds. It is shown that the high data rate optical pulse communication through clouds may be limited due to a narrow coherence bandwidth of the order of megahertz. A good agreement between the theoretical prediction and the available experimental data has been demonstrated for both the received pulse shapes and the pulse durations of an optical pulse in clouds.
Propagation of pulsed beam waves through turbulence, cloud, rain, or fog This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the ICC 2007 proceedings
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