In this study, the performance of underwater optical wireless communication links is investigated by taking into account turbulence, absorption and scattering effects. Weak turbulent channel is modeled using log-normal distribution while moderate and strong turbulence channels are modeled using gamma-gamma distribution. Rytov variance of Gaussian beam is derived analytically for oceanic turbulence optical power spectrum. Subsequently, scintillation index is calculated using the computed Rytov variance. Moreover, the closed-form expression of bit-error-rate (BER) for underwater wireless optical communication (UWOC) systems using intensity-modulated/direct detection (IM/DD) implementation and on-off-keying (OOK) modulation scheme is obtained. Results show that the performance of wireless optical communication link between two platforms in underwater medium is degraded significantly due to turbulence, absorption and scattering. In fact, as the turbulence level increases, its effect becomes quantitatively comparable to those of absorption and scattering effects. The variation of both scintillation index and BER performance are presented for various underwater medium and communication system parameters, such as chlorophyll concentration, average temperature, average salinity concentration, temperature and dissipation rates, wavelength, link length and receiver aperture size. Optical network and internet of underwater things (IoUT) applications, which are growing day by day and requiring high data rates, will benefit from the results of this study.

In this paper, the outage probability of maritime free-space optical communication system is analyzed. Maritime turbulence channel is modeled by Lognormal distribution. Channel model includes the combined effect of turbulence, pointing error, angle-of-arrival (AOA) fluctuations, and attenuation. Pointing error and AOA fluctuations are assumed to be Beckmann and Rayleigh distributed, respectively. Turbulence power spectrum is considered to present the Kolmogorov characteristics and Rytov variance of a propagating Gaussian beam is obtained. The probability density function, cumulative distribution function and outage probability of maritime communication channel are obtained analytically. Outage performance of a maritime communication link is given depending on various parameters of the maritime environment, the Gaussian beam, and the communication system.

This paper investigates the performance of high altitude platforms (HAPS) based free-space optical (FSO) communication links including HAPS-to-ground station (downlink), ground-to-HAPS (uplink) and HAPS-to-HAPS (horizontal link) communications. The effects of  attenuation loss, atmospheric turbulence, pointing error and angle-of-arrival (AOA) are taken into account. Also, the application of adaptive optics correction, one of the most effective turbulence mitigation techniques, is analyzed using the Zernike polynomials representation. Closed-form expressions are obtained for probability density function (PDF), cumulative distribution function (CDF), Rytov variance, adaptive optics filter function and outage probability mainly in terms of Meijer’s G function when both no adaptive optics correction is used and adaptive optics correction is applied. Some selected results are presented depending on the various  parameters such as the HAPS altitude, the ratio of vertical and horizontal deviations, beam waist, Zenith angle, height of ground station, receiver aperture diameter, channel state threshold and wind speed. The performance improvement with adaptive optics correction is investigated by removing different Zernike modes. We show that the downlik performance outperforms the uplink, the performance of the horizontal link sharply increases above certain altitude and communication links benefit from the adaptive optics correction up to a certain level in terms of performance improvement.

This study evaluates the effect of application of spatial diversity technique, namely multiple-input multiple-output (MIMO), on the outage performance of free-space optical (FSO) communication systems operating in atmospheric turbulent medium for both slant path (downlink and uplink) and horizontal links. FSO communication links cover ground-to-high altitude platform system (HAPS), HAPS-to-ground, and HAPS-to-HAPS configurations. System model includes the combined effect of attenuation (resulting from absorption and scattering), angle-of-arrival (AOA) fluctuations, atmospheric turbulence, and pointing error. Atmospheric turbulent channel is characterized by Gamma-Gamma distribution. The Hoyt and Rayleigh distributions are used to model pointing error and AOA fluctuations, respectively. The performance of MIMO FSO communication links is presented depending on the various environmental and system parameters. Results show that the MIMO application in both transmitter and/or receiver can be a prominent alternative concerning the performance improvement for FSO communication links.

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.