Average capacity for PCB propagation in atmospheric turbulence on Earth-space path

Abstract

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.