Aperture averaging effects on the probability density of irradiance fluctuations in moderate-to-strong turbulence

Abstract

The lognormal (LN) and gamma-gamma (GG) distributions are compared to simulated and experimental data of the irradiance fluctuations of a Gaussian beam wave propagating through the atmosphere along a horizontal path, near the ground, in the moderate-to-strong turbulence regime. Irradiance data were collected simultaneously at three receiving apertures of different sizes. Atmospheric parameters were inferred from the measurements and scintillation theory and were used to develop the parameters for the theoretical probability density functions. Numerical simulations were produced with the same C n 2 value as the experimental data. Aperture-averaging effects were investigated by comparing the irradiance distributions for the three apertures at two different values of the structure parameter C n 2 , and, hence, different values of the coherence radius 0 . For the moderate-to-strong fluctuation regime, the GG distribution provides a good fit to the irradiance fluctuations collected by finite-sized apertures that are significantly smaller than 0 . For apertures larger than or equal to 0 , the irradiance fluctuations appear to be LN distributed.

Approximation to the Sum of Two Correlated Gamma-Gamma Variates and its Applications in Free-Space Optical Communications

“…We consider the strong turbulence regime with the turbulence strength parameter C 2 n = 6.5 × 10 −14 m −2/3 , the inner and outer scales of turbulence of l 0 = 6.1 mm and L 0 = 1.3 m, respectively. Also, a diverging Gaussian beam is considered at the transmitter at λ = 1550 nm with the beam waist W 0 = 1.59 cm and the curvature radius of the phase front of F 0 = −69.9 m. These parameters correspond to the experimental works reported in [2].…”

“…In fact, small-scale intensity fluctuations originate mostly from the turbulent cells smaller than the first Fresnel zone F = L/k (with k = 2π/λ being the wave number) or the transverse coherence radius ρ 0 , whichever smaller. Here, we have F = 2.22 cm and ρ 0 = 1.41 cm in case (1), and F = 3.51 cm and ρ 0 = 0.83 cm in case (2). As a result, if the two apertures are separated at least 1.41 cm and 0.83 cm apart (that can easily be attained in practice), in cases (1) and (2), respectively, we can effectively assume uncorrelated small-scale fluctuations and neglect ρ Y .…”

“…One of the problems that an FSO system is facing to is the atmospheric turbulence which induces signal fading at the receiver under clear weather conditions [1]. To mitigate the adverse effect of optical scintillation, one solution is aperture averaging which is efficient when the diameter of the receiver lens is larger than the coherence radius ρ 0 of the optical wave [2], [3]. However, at relatively large link distances, on the order of several kilometers, ρ 0 becomes too large and the required aperture size for efficient fading reduction necessitates too expensive optical devices and components.…”

Contrasting space-time schemes for MIMO FSO systems with non-coherent modulation

“…We consider a diverging Gaussian beam at λ = 1550 nm of beam waist W 0 = 1.59 cm and the curvature radius of the phase front of F 0 = −69.9 m. Concerning the channel turbulence, we set C and the inner and outer scales of turbulence of l 0 = 6.1 mm and L 0 = 1.3 m. These parameters correspond to the experimental works reported in [3]. The link distance is given as L = 5 km for which the Rytov variance σ 2 R = 24.7.…”

“…In near-ground FSO systems, one of the principal channel impairments is the atmospheric turbulence which induces intensity fluctuations at the receiver that can considerably degrade the system performance [2]. One solution for fading mitigation is aperture averaging [3]. However, when working at long link distances, the required collecting lens for efficient fading reduction becomes too large [4].…”