Nowadays, the search for a distribution capable of modeling the probability density function (PDF) of irradiance data under all conditions of atmospheric turbulence in the presence of aperture averaging still continues. Here, a family of PDFs alternative to the widely accepted Log-Normal and Gamma-Gamma distributions is proposed to model the PDF of the received optical power in free-space optical communications, namely, the Weibull and the exponentiated Weibull (EW) distribution. Particularly, it is shown how the proposed EW distribution offers an excellent fit to simulation and experimental data under all aperture averaging conditions, under weak and moderate turbulence conditions, as well as for point-like apertures. Another very attractive property of these distributions is the simple closed form expression of their respective PDF and cumulative distribution function.
Abstract. The annual and seasonal variability of aerosol optical properties observed by means of a Raman lidar over Northeastern Spain has been assessed. The lidar representativeness has first been checked against sun-photometer measurements in terms of aerosol optical thickness. Then the annual cycle and the seasonal variability of the planetary boundary layer aerosol optical thickness and its fraction compared to the columnar optical thickness, the lidar ratio, the backscatter-relatedÅngström exponent and the planetary boundary layer height are analyzed and discussed. Winter and summer mean profiles of extinction, backscatter and lidar ratio retrieved with the Raman algorithm are presented. The analysis shows the impact of most of the natural events (Saharan dust intrusions, wildfires, etc.) and meteorological situations (summer anticyclonic situation, the formation of the Iberian thermal low, winter long-range transport from North Europe and/or North America, re-circulation flows, etc.) occurring in the Barcelona area. A detailed study of a special event including a combined intrusion of Saharan dust and biomass-burning particles proves the suitability of combining the retrieval of aerosol optical properties from Raman and pure elastic lidar measurements to discriminate spatially different types of aerosols and to follow their spatial and temporal evolution.
In this paper, a packet-level forward error correction (FEC) coding technique and pre-distortion adaptive optics (AO) technology are applied to a digital transmission scheme for optical feeder links in a geostationary Earth orbit (GEO) satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency (RF) front-end and the optical front-end, as well as the digital signal processor. The system is designed to cater for Terabit/s highthroughput satellite (HTS) applications. The performance of the digital transmission scheme is evaluated in the forward and return links. The turbulent atmospheric optical channel is modeled for different optical ground station (OGS) altitudes. It is shown that fade mitigation techniques such as packet-level FEC coding and pre-distortion AO in the forward link, as well as large-aperture OGS telescope in the return link, are essential to close the link budget of a Terabit/s satellite communication system. Prepared using satauth.cls [Version: 2010/05/13 v2.00] 2 S. DIMITROV ET AL.Index-of-refraction turbulence (IRT) [3] is known to degrade the achievable data rates of FSO communication links [4]. For uplinks above 30 • elevation, the IRT falls within the weak turbulence regime [5], where the probability density function (PDF) of the irradiance at the receiver plane can be modeled by a log-normal distribution [6]. Moreover, the correlated fading events represent a lowpass process defined by the Greenwood frequency [7]. Regarding site selection and deployment, an OGS at sea level may suffer from strong influence of the turbulence, since the atmosphere is denser at lower altitudes. Astronomical sites are generally located on mountain tops, but these locations are usually isolated, and Terabit/s infrastructure needs to be developed. In this paper, 300 m, 900 m and 1500 m are chosen as examples of possible OGS altitudes.Adaptive optics (AO) is a technology used for correction of the incoming optical wavefront phase which is distorted by the turbulent atmosphere [8]. In ground-to-GEO FSO links, the scale size of the phase distortions is much larger than the feasible satellite telescope aperture size [3], and therefore, the spatial phase at the satellite receiving aperture is rather flat. Nevertheless, in an uplink scenario the traveling wavefront is initially perturbed by the atmosphere, and then it propagates through free space up to the GEO satellite. Thus, at the satellite plane the received wavefront profile differs from that of a pure diffracted laser beam. These distortions can be mitigated by applying pre-distortion AO (PAO) techniques to the outgoing optical wave at the ground station, using a beacon reference that senses the distortions introduced by the turbulent atmosphere.The simplest PAO correction method is tilt compensation of the first three spatial modes of the wavefront phase information [9]. It uses pointing by tracking, where a downlink beacon from the satellite is used a...
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