In order to analyze the effect of atmospheric turbulence along Earth-space path on the performance of laser
communication system, the change of Kolmogorov spectrum with altitude is taken into account on the basis of the
relation of turbulence structure constant with altitude. The scintillation index under the condition of the different
coherent degree for optical source are discussed from weak to strong turbulence regime for a partially coherent Gaussian
beam (PCB) propagating through turbulent atmosphere. The relation of the scintillation index with elevation is analyzed
on Earth-space path. The expressions for the Log-normal and the Gamma-Gamma turbulence channel of irradiance
fluctuations are given. The mathematical expression for the evaluation of the average capacity is shown for a
Kolmogorov turbulent atmosphere channel. The average capacity of a PCB is calculated under the condition of different
coherent length, aperture, on the horizontal and Earth-space path, respectively. The results of scintillation index show
that the difference among plane, spherical, and beam wave are obvious from weak to moderate turbulence regime, at
strong regime, the results tend to saturation. The degradation of optical source coherence causes scintillation depression
at weak turbulence regime. At moderate regime, the scintillation is little increase. The effect of source partially coherence
on scintillation is disappear at stronger regime. Based on the Log-normal model in weak turbulent fade channel and the
Gamma-Gamma model from weak to strong turbulence fade channel, the average capacities for different initial Gaussian
beam radius, partially coherent degree beam are estimated, respectively. The results show that at weak turbulence, the
difference between the capacities by Log-normal and Gamma-Gamma distribution is small, and from moderate to strong
regime, the difference is gradually increase. At weak regime, the average capacity of partially coherent beam is greater
than the coherent beam. On the contrary, at stronger regime, the average capacity of coherent beam is slightly greater
than the partially coherent. At saturation regime, the capacity is not improved by means of partially coherent optical
source. Hence, it is shown that in weak turbulence, the suppression of scintillation index and the improvement of average
capacity is effective by means of partially coherent source.
Based on the theory of optical wave propagation in the slant path and the ITU-R turbulence structure constant model
which is dependent on altitude, the on-axis scintillation index of the flat-topped Gaussian beam at the receiver plane in
slant path turbulence was given by using Kolmogorov atmospheric turbulence power spectrum model. The influences of
the link altitudes, atmospheric refractive index structure constant C<sub>0</sub> at the ground，the source size and the beam order on scintillation index of the flat-topped Gaussian beam are discussed in detail. The result shows that the scintillation index
increased first and then decreased with the increase of the beam order. The advantage of a flat-topped Gaussian beam
over a single Gaussian beam is restricted to small source sizes, which is consistent with the case of the horizontal path.
To find the average bit error rate under weak slant path turbulence, the log-normal distribution model of the intensity
fluctuation was used. The influence of beam order and source size on BER was discussed. The result indicates that the
smaller sized flat-topped Gaussian beam will bring average bit error rate advantage over the same size Gaussian beam.
Our results correctly reduce to the result of the horizontal path with atmospheric structure constant fixed.
The propagation characteristic of a Gaussian beam through turbulent atmosphere have been studied in the past several
years. The main advantage of Gaussian beam wave model is that the infinite plant wave and a spherical wave are being
included. Non-Kolmogorov spectrum can describe generalized turbulent atmosphere environment. The propagation
properties of Gaussian beam propagating through the turbulent atmosphere described by non-Kolmogorov spectrum are
studied on slant path. The scintillation index is analyzed with the Gaussian beam of different turbulent strength, zenith
angle, φ , and the spectral exponent, α, of non-Kolmogorov, respectively. The effect of the turbulent structure constant
on the ground on sicntillation is notable. The scintillation index reduces remarkably with zenith angle and structure
parameter decrease. At weak turbulenc, scintillation index increases as spectral exponent decreases. The bit error rate
(BER) of a Gaussian beam propagating in non-Kolmogorov atmospheric turbulence channel is estimated on Earth-space
slant path. By comparing the effect of the spectral power with the structure constant on BER at moderate and strong
turbulence, the effect of the spectral power change on BER is small. With turbulence weakening , at the order of
10<sup>-15</sup>m<sup>-2/3</sup> , the relative effect of the spectral power on BER is gradually increase. Hence, at the small structure constant
on the ground, or weak turbulence, the effect of the turbulent spectral power on BER is required to take into account.
Analysis of log-amplitude scintillation due to troposphere clear-air turbulent and meteorological parameters variation is shown at microwaves (MW) on slant paths, based on ITU-R turbulence atmosphere structure parameter, temperature and relative humidity along vertical path. Comparisons of Karasawa model based on the data obtained from a low-elevation microwave propagation experiment and ITU-R Recommendation model prediction results with evaluation results are shown and discussed. The results show that although the relative humidity effected on optical refractive index of a cell is not principally element at optical wave band, at microwave and millimeter-wave band, the relative humidity and temperature is the major factor impacted on log-amplitude scintillation. Hence, the variations of temperature and relative humidity with height, which can be obtained by experiment (or test) and weather observation method is important for low-elevation satellite communication and microwave remote sensing. A atmosphere structure constant C<sub>n</sub><sup>2</sup> model, which varies with height, is presented based on ITU-R and Karasawa amplitude scintillation model, existing ITU-R C<sub>n</sub><sup>2</sup> model for optical and meteorologic measured relative humidity and temperature data, at 10~30GHz. In this C<sub>n</sub><sup>2</sup> model it is considered that relative humidity and temperature varies with height. The log-amplitude scintillation deviation calculated in terms of the C<sub>n</sub><sup>2</sup> model based on humidity and temperature vertical profile compare with values predicted by means of ITU-R and Ortgies model applied to Italsat channels. It is emerged that the calculation results based on the C<sub>n</sub><sup>2</sup> model agree almost with prediction results by ITU-R and Ortgies model at 10~30GHz and there is an advantage that relative humidity and temperature varied with height has be considered in the C<sub>n</sub><sup>2</sup> model. Therefore, it is shown that the C<sub>n</sub><sup>2</sup> model is usable and is more practical.
Introduction of satellite services using higher frequency bands such as Ku- and Ka-band requires the characterization of tropospheric propagation factors that are normally considered negligible at lower frequency bands. Clouds scattering and attenuation are considered one such factor. Clouds are present during a large fraction of an average year and cloud scattering and attenuation, together with gaseous absorption, will determine the system performance under non-rainy conditions. In this paper, macro- and micro-physical properties of clouds over Xi'an region are discussed based on meteorologic observation data. The theory and mechanism of backscattering for clouds are analyzed. According to statistics parameters of the clouds over Xi'an region, the quantity calculations of the backscattering cross sections for various reflection mechanism of cloud are dealt with. The results calculated show that the major contributions for the backscattering cross section come possibly from clear-air and humidity turbulence and also from distributions of cloud particles, especially, at higher frequencies. These quantification calculation results on the basis of Xi'an typical clouds properties confirm previous results and it is significance for satellite communication systems, particularly, low availability satellite links and active and passive remote sensing, etc.