Performance of laser communication uplinks and downlinks in the presence of pointing errors and atmospheric distortions

Abstract

Performance of laser communication links between ground terminals, both fixed and mobile, and satellites is generally limited by several factors. Continuous movement of the communicating platforms, complemented by mechanical vibrations, is the main cause of pointing errors. In addition, atmospheric turbulence causes changes of the refractive index along the propagation path, thus creating wavefront distortions of the optical beam resulting in spatio-temporal redistribution of the received energy. The total effect of these phenomena leads to an increased bit-error probability under adverse operation conditions. This paper presents a combined approach to the analysis of a laser link in the presence of pointing errors and turbulence effects, and their contribution to the increased bit-error rates (BER). Analysis of both uplink and downlink communication is performed in the simulation environment. Two distinct approaches to wavefront distortion modeling are used for these scenarios. In uplink propagation the beam is distorted in the initial transition through the atmosphere, and then it travels over a long distance in free space, where even more self-interference occurs. In downlink communication the effects of distortion are only observed during the final transition through the atmosphere, and; therefore, are less severe. Communication performance under different conditions is assessed in terms of the bit-error rate as a function of the pointing error variance and the scintillation index.