Wyniki wyszukiwania - Biblioteka Nauki

1

Variable antennas positions solution to reduce pointing errors due to wind speed and temperature coupled effects during free space optical link using matrix Rician pointing error model

100%

Teuma Mbezi M. , Eke S. , Som I. H. J. , Mouangue R. M.

|

tom Vol. 53, nr 3

393--406

EN

Pointing errors (PE) during free space optical (FSO) transmission can be caused by laser beam wander due to thermal and wind dynamic instability. The aim of this work is to study the coupled effects of temperature and wind speed on PE using matrix Rician pointing error (MRPE) model; then show how variable antennas height can reduce PE due to wind speed and temperature coupled effects. To achieve this purposes, average PE expression was established using MRPE model. Then considering a Gaussian beam wave and Monin–Obukhov similarity functions for the structure parameters of temperature, explicit relationship was established between average PE, temperature and wind speed. It comes out of this study that under dynamic turbulence, one can appropriately modify temperature to reduce PE due to dynamic instability and reciprocally. Depending on turbulence large cells or frozen turbulence eddies distribution, PE can be reduced by appropriately modified antennas height or the distance between transmitter and receiver. That is why this work suggests to install variable or dynamic antennas (rather than fixed ones) which could intelligently modify its positions according to laser beam wander created by atmospheric turbulence.

2

An approach to investigating the feasibility of free-space optical communication technology deployment under scintillation effects

100%

Mahmood A. S.

|

tom Vol. 31, No. 3

art. no. e147037

EN

Local weather conditions have an impact on the availability of free-space optical (FSO) communication. The variation in meteorological parameters, such as temperature, humidity, and wind speed, leads to variations of the refractive index along the transmission path. These refractive index inhomogeneities produced by atmospheric turbulence induce optical turbulence which is responsible for random fluctuations in the intensity of the laser beam that carries the signal (irradiance) called scintillations that can significantly degrade the performance of FSO systems. This paper aims to investigate the feasibility of deploying FSO communication technology under scintillation effects in any urban region and atmospheric environment. To achieve that, firstly by utilizing the Hufnagel-Vally day with the Sadot and Kopeika models together, the scintillation strength for a specified region, Sulaimani City in north-eastern Iraq as an example, has been estimated through the calculation of the refractive index structure parameter (Cn²) over a period of 10 years and it was found to be at the strong turbulence level. Secondly, from the same estimated parameter, the scintillation attenuation of the signal carrying the laser beam intensity can be calculated to investigate the feasibility of FSO communication using Optysistem-7 software. The optimal link distance for northeastern Iraq (Sulaimani City) has been found to be within the limit of about 5.5 km. Analysing the max. Q-factor, bit-error rate and signal to noise ratio for an average of 120 months between 2013-2022 assessed the best and worst seasons for FSO.

3

Coherence characterization of partially coherent flat-topped beam propagating through atmospheric turbulence

100%

Kashani F. D. , Alavinejad M. , Ghafary B.

|

2009

|

tom Vol. 39, nr 2

429-440

EN

We study the change in the degree of coherence of a partially coherent flat-topped (PCFT) beam propagating through atmospheric turbulence. It is shown analytically that with a fixed set of source parameters and under a particular atmospheric turbulence model, a PCFT beam propagating through atmospheric turbulence reaches its maximum value of coherence after propagating a particular distance, and the effective width of the spectral degree of coherence also has its maximum value. This phenomenon is independent of the turbulence model used. We also study the effects of beam width values, the structure constant of turbulent media and the degree of coherence on effective width of spectral degree of coherence. The results are illustrated by numerically calculated curves.

4

Twisted electromagnetic sinc Schell-model beam and its transmission in a turbulent atmosphere

100%

Zhang C. , Fu W.

|

tom Vol. 54, nr 1

15--29

EN

We present the twisted electromagnetic sinc-correlation Schell-model (EM TSSM) beam as an extension of the cylindrical sinc Schell-model beam and analyze the necessary source parameter conditions to generate a physically viable beam. Furthermore, we thoroughly investigate the propagation properties of the EM TSSM beam in atmospheric turbulence using the extended Huygens–Fresnel integral, explicitly focusing on spectral intensity, degree of polarization (DOP), and degree of coherence (DOC). It shows that the twisted phase has a noticeable impact on the intensity profiles of these beams, causing them to exhibit rotation and self-splitting while still maintaining their shape in free space. Moreover, during propagation through a turbulent atmosphere, it exhibits self-combining properties over a long range and recovers the plat-topped distribution. Compared with the sinc Schell-model beam without the twisted phase, the DOP distribution of such a beam can rotate around its distribution center. As these beams propagate through turbulent atmospheres, they can self-heal their DOP distribution within specific ranges affected by atmospheric turbulence. A twist factor causes non-unidirectional rotation of the DOC distribution in free space. The DOC gradually transforms from multi-strip profiles into a Gaussian-like distribution. Furthermore, the beam parameters play a crucial role in shaping the DOC. The results will be useful in optical trapping and optical communication.

5

Study on self-repairing and non-diffraction of Airy beams in slant atmospheric turbulence

88%

Li Y.-Q. , Wang L.-G.

|

tom Vol. 48, nr 3

435--447

EN

The Airy beams propagation in atmospheric turbulence along a slant path was simulated numerically, based on the split-step Fourier method. Also, the self-repairing and non-diffraction characteristics of Airy beams were investigated and compared with beams propagation on a horizontal path. The effects of parameters including zenith angle, propagation distance, radii of Gaussian aperture and turbulence intensity on the two characteristics of beams were revealed. Additionally, the two characteristics of the Airy beam were compared with those of a Bessel–Gauss beam. The results showed that the two beams obscured by Gaussian apertures can be repaired after propagating some distance along a slant path. However, the non-diffraction characteristic of an Airy beam was stronger than that of a Bessel–Gauss beam and the amplitude attenuation rate of the Bessel–Gauss beam was greater than that of the Airy beam in the process of self-repairing. Results obtained can provide a theoretical basis for an outdoor experiment as well as theoretical guidance for various practical applications including laser communications, laser warning systems, and remote sensing.

6

Characteristics of C2µ derived from ultrasonic anemometer in an urban boundary layer

88%

Al-Jiboori M. H. , Jaber S. H.

|

2019

|

tom Vol. 28, No. 1

14--24

EN

Fast-response observations of three components of wind and air temperature have been applied to calculate the refractive index function coefficient (C2µ ), which is needed to describe optical wave propagation in a turbulent medium. These were measured by 3D ultrasonic anemometer installed on the roof of the building of Atmospheric Science Department which is 19 m above ground level. Refractive index function coefficient was calculated for various periods of three seasons: winter, spring and summer.Diurnal variations of (C2µ) have been made at the surface layer for these seasons. The results show that high values ofmean (C2µ) occurred during the day time more than at night, also they occurred more in summer than in winter and spring. The results of (Cµ2) found to change with atmospheric stability, whereas they inversely decrease under unstable conditions, approximately constant at neutral cases, and increase under stable conditions. Values of (C2µ) on average appears to be lower during the rainy and foggy weather cases compared to those of clear sky.

7

Performance analysis of bit error rate for free space optical communication with tip-tilt compensation based on gamma-gamma distribution

88%

Wu H. , Yan H. , Li X.

|

2009

|

tom Vol. 39, nr 3

533-545

EN

In this paper, the gamma - gamma probability distribution is used to model turbulent channels. The bit error rate (BER) performance of free space optical (FSO) communication systems employing on-off keying (OOK) or subcarrier binary phase-shift keying (BPSK) modulation format is derived. A tip-tilt adaptive optics system is also incorporated with a FSO system using the above modulation formats. The tip-tilt compensation can alleviate effects of atmospheric turbulence and thereby improve the BER performance. The improvement is different for different turbulence strengths and modulation formats. In addition, the BER performance of communication systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation with or without tip-tilt compensation.

8

Effect of Atmospheric Turbulence on the Striking Accuracy of Guided Bombs

75%

Grajewski K. , Kowaleczko G. , Pietraszek M.

|

tom Vol. 11, Nr 1 (39)

31--48

EN

This paper presents the test results of a simulation of an air-to-surface guided bomb drop in a turbulent atmosphere. The guided bomb was developed from a practice bomb built and upgraded by the Air Force Institute of Technology. The paper presents the test results of a numerical simulation of an air-to-surface guided bomb drop ran in a proprietary software environment. The numerical simulation inputs included aerodynamic characteristics calculated with PRODAS software and verified by wind tunnel tests. The stochastic components of atmospheric turbulence were simulated with a stochastic process model proposed by Shinozuki. Examples of the guided bomb drop simulation results are given in the paper. The effect of atmospheric turbulence parameters, i.e. standard deviation, σ and turbulence scale, Lw on the striking accuracy and ground impact scatter, are also shown.

PL

W artykule przedstawiono wyniki przeprowadzonych symulacji zrzutu bomby w turbulentnej atmosferze. Bomba ta powstała na bazie bomby ćwiczebnej zbudowanej i zmodernizowanej w Instytucie Technicznym Wojsk Lotniczych. W artykule przedstawiono wyniki numerycznej symulacji z wykorzystaniem autorskiego oprogramowania. W obliczeniach użyto charakterystyki aerodynamiczne wyliczone programem PRODAS, które weryfikowano badaniami w tunelu aerodynamicznym. Do opisu składowej turbulencji wykorzystano model procesów stochastycznych zaproponowany przez Shinozukiego. Przedstawiono przykładowe wyniki symulacji zrzutu bomby. Pokazano wpływ parametrów turbulencji atmosfery - odchylenia standardowego i skali turbulencji σ, Lw, na celność i rozrzut.

9

FSO system performance analysis based on novel Gamma–Chi-square irradiance PDF model

75%

Todorović J. , Spalević P. , Panić S. , Milosavljević B. , Gligorijević M.

|

tom Vol. 51, nr 3

335--348

EN

In order to provide a novel analytically traceable free space optics (FSO) channel model for describing turbulence based irradiance fluctuations, following basic scintillation theory principles, we have derived closed-form expression for probability density function (PDF) of a new statistical Gamma–Chi-square model. Further, capitalizing on provided model, error performances of FSO system over on-off keying (OOK) transmission scheme both in the presence of atmospheric turbulence and misalignment fading (pointing error) is investigated. For both cases, the average bit error rate (ABER) at the receiving side of the system is determined in an analytically closed form. The results are graphically presented in order to analyze the impact of different levels of turbulence, as well as other relevant parameters, on the quality of the received signal in the OOK modulated FSO system.