Optical Wireless Communication Systems (OWCSs) are becoming more popular each day, especially after numerous mobile applications are being employed within the concept of Internet of Things (IoT). OWCSs are largely used in both terrestrial and non-terrestrial environments, like underwater, air, and space scenarios. Due to the large applicability of OWCS, it represents one of the main candidate technologies for the future 6G wireless communication systems. Naturally, this market trend forces the system designers to reach the best performance in their designs, as well as optimize the cost. In this survey paper, we intend to provide information to the researchers working in this field on the statistical models adopted in OWCS, the methods and techniques used to improve their performances, mainly in outdoor environment like air, space, and underwater. In this respect, the background on theoretical aspects of OWCS, together with their benefits, limitations and challenges are presented. Performance improvement techniques employed in OWCSs, such as power increase, partial coherence, beamforming, aperture averaging, spatial diversity, and intelligent reflecting surfaces, are also introduced. Finally, we discuss the open challenges that researchers are still facing, together with future directions on next steps for a large-scale adoption of OWCS.

The Scintillation index of a higher-order laser beam in turbulent biological tissue is formulated and evaluated. Behaviors of the scintillation indices of various higher-order beams against the tissue turbulence parameters of the strength coefficient of the refractive index fluctuations, fractal dimension, characteristic length of heterogeneity, small length-scale factor, and the source size, tissue length, and wavelength are examined. Fluctuations in the intensity are also investigated when various types of tissues, such as the intestinal epithelium (mouse), liver parenchyma (mouse), and upper dermis (human), are excited by different higher-order laser beams.

The receiver spatial diversity techniques are employed in underwater optical wireless communication (OWC) systems to mitigate oceanic turbulence, improving the bit error rate performance. In this paper, we consider an OWC system employing a binary phase-shift keying (BPSK) modulated Gaussian beam at the transmitter and employing receiver spatial diversity at the receiver. The techniques for receiver spatial diversity systems considered in the study are selection combining (SC), equal gain combining (EGC), and the maximum ratio combining (MRC). The bit error rate (BER) performance of the OWC system operating in weak oceanic turbulence is investigated by calculating the Gaussian beam’s turbulence-induced scintillation index and the received optical intensity. It is found that the receiver spatial diversity techniques, especially EGC and MRC, are very effective for reducing the BER of an OWC system in weak oceanic turbulence. Furthermore, the BER performance of the underwater OWC system sees an improvement with an increase in the number of photodetectors or a decrease in the level of oceanic turbulence. Moreover, an improvement in the photodetector responsivity or a reduction in the system's noise factor contributes to achieving a favorable BER performance.

The spatial diversity and beam shaping are key approaches to mitigating the effects of atmospheric turbulence on laser beam propagation through free space. This study investigates the propagation characteristics of an annular beam array in atmospheric turbulence for uplink satellite links using the extended Huygens-Fresnel principle and the Rytov method. Analytical derivations are presented for various optical parameters, including received intensity, kurtosis parameter, beam footprint size, Strehl ratio, beam wander, scintillation index, and bit error rate (BER), to evaluate the beam performance under turbulence conditions. The findings indicate that converting a Gaussian beam into an annular shape enhances performance, with further improvements observed as the number of beams increases.

In underwater optical vertical link medium, based on the extended Huygens-Fresnel principle, multimode laser beam field correlation is derived and evaluated analytically in the Atlantic Ocean at high latitude and high latitude- low latitudes. With the depth of seawater, the coherence length of a spherical wave operating in the underwater turbulent medium is demonstrated for the range of 0-4000 m. By utilizing the coherence length varying with parameters such as the rate of dissipation of turbulent kinetic energy per unit mass of fluid ε, the rate of dissipation of the mean squared temperature χ_T and nondimensional representing the relative strength of temperature and salinity fluctuations ω, which depend on depth, the field correlation is examined in detail for single modes and multimode. Their variations are exhibited. Our results indicate clearly that as the mode increases, field correlation gets better.

The study examines the average transmittance of Gaussian beams passing through various biological tissues, taking into account the impact of turbulence, absorption, and scattering. The extended Huygens-Fresnel technique, which utilizes the power spectrum of turbulent biological tissues, is applied to determine the optical intensity at the observation point. Additionally, there are tabulated absorption and scattering coefficients available for the application of the Beer-Lambert law, facilitating the calculation of optical light attenuation in biological tissues. Examining the impact of turbulence, as well as absorption and scattering-induced attenuation on the Gaussian beam's propagation, the changes in transmittance are documented across different tissue parameters.

We utilize the Huygens-Fresnel principle to derive the mutual coherence function (MCF) for a vortex beam, which is the main focus of our investigation. Then, we examine the intensity and modulus of the complex degree of coherence (DOC) characteristics of vortex beams in atmospheric turbulence. Our results indicate that as the topological charge increases, the intensity distribution of the vortex beam becomes less affected by atmospheric turbulence. However, the modulus of the complex DOC decreases.