Wireless Optical Communications Through the Turbulent Atmosphere: A Review (original) (raw)
IEEE Photonics Technology Letters, 2016
A wireless optical communication (WOC) system is considered under S-distributed atmospheric turbulence. The S-distribution incorporates the line-of-sight (LOS), scattered and not coupled to LOS, and scattered but coupled to LOS (SCLOS) components of the propagating laser beam. The inclusion of SCLOS component provides more generality in modeling wider range of turbulence. The statistics of the instantaneous received signal-to-noise ratio (SNR) are derived for S-distributed WOC channel. Using the derived statistics, the outage and error performances of considered WOC system are investigated. Specifically, the new expressions for the outage probability and average bit error rate (BER) are obtained for various binary signalling techniques. Finally, the ergodic capacity performance is evaluated using numerical methods.
Free-Space Laser Communications IX
Chair, Department Committee on Graduate Students I would like to express my deepest gratitude to my advisor, Vincent W.S. Chan, for his guidance, support, and patience. He taught me that asking the right question is just as important as the answer. His exceptional engineering intuition has always pushed me to look beyond the equations to understand things at a physical level. Vincent has always led by example, showing integrity and honesty-when it's easy to do so and when it's difficult to do so. I would like to thank Steve Prutzer for his encouragement and unequivocal support. It is because of his mentorship that I arrived at MIT prepared for the research ahead. I thank my thesis committee, Eric Swanson and Jeff Shapiro, for many useful comments and discussions. While both contributed to the thesis as a whole, Eric contributed much to the engineering while Jeff kept me firmly rooted in the physics. The friendship and administrative support offered by Donna Beaudry is much appreciated.
Studies of the Effects of Atmospheric Turbulence on Free Space Optical Communications
2005
Even after several decades of study, inconsistencies remain in the application of atmospheric turbulence theories to experimental systems, and the demonstration of acceptable correlations with experimental results. This dissertation shows a flexible empirical approach for improving link performance through image analysis of intensity scintillation patterns coupled with frame aperture averaging on a free space optical (FSO) communication link. Aperture averaging is the effect of the receiver size on the power variance seen at the receiver. A receiver must be large enough to collect sufficient power and reduce scintillation effects at a given range, but must also be of practical size. An imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links will be presented. Weak and intermediate turbulence results will be shown for an 863 meter link at the University of Maryland. Atmospheric turbulence has a significant impact on the quality of a laser beam propagating through the atmosphere over long distances. Turbulence causes intensity scintillation and beam wander from propagation through turbulent eddies of varying sizes and refractive index. This can severely impair the operation of target designation and FSO communications systems. A new geometrical model to assess the effects of turbulence on laser beam propagation in such applications will be presented. The atmosphere along the laser beam propagation path is modeled as a spatial distribution of spherical bubbles with refractive index discontinuity statistically distributed according to various models. For each statistical representation of the atmosphere, the path of rays is analyzed using geometrical optics. These Monte Carlo techniques can assess beam wander, phase shifts and aperture averaging effects at the receiver. An effective C n 2 can be determined by correlating beam wander behavior with the path length. In addition, efficient computational techniques have been developed for various correlation functions that are important in assessing the effects of turbulence. The Monte Carlo simulations are compared with the predictions of wave theory. This is the first report to present weak and intermediate turbulence results using an efficient imaging technique. It is also the first report to geometrically simulate aperture averaging.
A Feasibility Study of Optical Wireless Communication under Turbulent Conditions
2019
Free Space Optical Communication is one of the most emanate form of high data rates transmission. FSO communication system has the ability to fulfill the craving demand of high data rate users. It has the potential for both indoor and outdoor communication links. But there are multiple factors which demean the performance of FSO communication link. The optical communication link is highly affected by the Scattering, Scintillation, Refraction and Turbulence which degrade the execution of optical link in outdoor environment. FSO can be intensively employed as back-hall communication link to join data communication terminals due to its ease of deployment, license free long range operation, high bit rates, full duplex operation, protocol transparency and immunity to electromagnetic interference. The optical radiations which travel through atmosphere spread out and suffer from propagation losses. Some part of radiation is absorbed while other is scattered in the form of optical losses. T...
Data transmission is a vital aspect in any communication system. It characterizes the speed, data reliability, etc. of the data. Radio waves are the conventional mode of transmitting data. However radio waves have longer wavelength and hence shorter bandwidth which causes interference in transmission. The radio waves are replaced by lasers, which have shorter wavelength and hence longer wavelength. Laser communication is further enhanced to free space optical communication. However free space optical communication has also its drawbacks when it comes under atmospheric turbulence e.g.; fog, snow, wind, etc. The photon (data) can absorbed or deviate in atmosphere and hence will not reach the destination (receiving antenna). In this paper we will employ various methods that will overcome these drawbacks. Depending upon the density of fog the laser will either refract or absorb. With high density the laser is going to be completely absorbed by the fog and hence it will not reach the receiving antenna. In such case we will use Franz–Keldysh effect, where we will apply electric field in a direction perpendicular to the modulated light beam which changes the band gap energy (thus the photon energy of an absorption edge). In another method we will use Plano convex lens and multiple receiving antennas, here laser will be deviated from the affected area using Plano convex lens and will be directed towards another antenna whose path would be unaffected. The methods are easy to implement and will avoid the loss of photon (data) in atmospheric turbulences.
Optics Communications, 2010
In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, affecting link performance. Influence of Kolmogorov and non-Kolmogorov turbulence statistics on laser communication links are analyzed for different propagation scenarios, and effects of different turbulence spectrum models on optical communication links are presented. Statistical estimates of the communication parameters such as the probability of fade (miss) exceeding a threshold dB level, the mean number of fades, and BER are derived and examples provided. The presented quantitative data suggest that the non-Kolmogorov turbulence effects on lasercom channels are more severe in many situations and need to be taken into account in wireless optical communication. Non-Kolmogorov turbulence is especially important for elevations above the boundary layer as well as for even low elevation paths over water.
Atmospheric Turbulence Effect on Free Space Optical Communications
This work discusses the effect of atmospheric turbulence on wavelengths transmission in free space. The Rytov variance calculated for plane and spherical waves, were Rytov variance of spherical waves less than from plane waves, on the other hand in spherical waves the values of wavelength 1550 nm less than from the other wavelengths. Scintillation attenuation was calculated depends on Rytov approximation for wavelengths (1550, 850, 633, 532) nm, scintillation attenuation values of the wavelength 1550 nm less than from the other wavelengths. The wavelength 1550nm has a good signal to noise ratio(S/N) from the other wavelengths. The distance between the transmitter and receiver links was (0-1000) m, we take into account the refractive index structure parameter at different turbulence (10 -16 low, 10 -15 medium, 10 -14 high) m -2/3 for all calculations.
Performance Improvement Techniques for Optical Wireless Link in Presence of Atmospheric Turbulence
Atmospheric turbulence can cause significant performance degradation in free space optical (FSO) communication system. Major effects on the beam such as beam wander and beam scintillation degrades the performance at the receiver. We investigate the impact of diversity technique and aperture averaging technique on the performance of FSO system. Performance evaluation is made in terms of average bit error rate and scintillation index.
Study of strong turbulence effects for optical wireless links
Laser Communication and Propagation through the Atmosphere and Oceans, 2012
Strong turbulence measurements that are taken using real time optical wireless experimental setups are valuable when studying the effects of turbulence regimes on a propagating optical beam. In any kind of FSO system, for us to know the strength of the turbulence thus the refractive index structure constant, is beneficial for having an optimum bandwidth of communication. Even if the FSO Link is placed very well-high-above the ground just to have weak enough turbulence effects, there can be severe atmospheric conditions that can change the turbulence regime. Having a successful theory that will cover all regimes will give us the chance of directly processing the image in existing or using an additional hardware thus deciding on the optimum bandwidth of the communication line at firsthand. For this purpose, Strong Turbulence data has been collected using an outdoor optical wireless setup placed about 85 centimeters above the ground with an acceptable declination and a path length of about 250 meters inducing strong turbulence to the propagating beam. Variations of turbulence strength estimation methods as well as frame image analysis techniques are then been applied to the experimental data in order to study the effects of different parameters on the result. Such strong turbulence data is compared with existing weak and intermediate turbulence data. Aperture Averaging Factor for different turbulence regimes is also investigated.
Journal of Physics: Conference Series, 2011
In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, affecting link performance. Most theoretical treatments have been described by Kolmogorov's power spectral density model through weak turbulence with constant wind speed. However, several experiments showed that Kolmogorov theory is sometimes incomplete to describe atmospheric turbulence properly, especially through the strong turbulence with variable wind speed, which is known to contribute significantly to the turbulence in the atmosphere. We present an optical turbulence model that incorporates into variable wind speed instead of constant value, a non-Kolmogorov power spectrum that uses a generalized exponent instead of constant standard exponent value 11/3, and a generalized amplitude factor instead of constant value 0.033. The free space optical communication performance for a Gaussian beam wave of scintillation index, mean signal-to-noise ratio, and mean bit error rate , have been derived by extended Rytov theory in non-Kolmogorov strong turbulence. And then the influence of wind speed variations on free space optical communication performance has been analyzed under different atmospheric turbulence intensities. The results suggest that the effects of wind speed variation through non-Kolmogorov turbulence on communication performance are more severe in many situations and need to be taken into account in free space optical communication. It is anticipated that this work is helpful to the investigations of free space optical communication performance considering wind speed under severe weather condition in the strong atmospheric turbulence.