Scintillation loss in free-space optic IM/DD systems

Abstract

When designing free-space optics systems, one key issue is to assess the impact of scintillations and to find an appropriate link margin to cope with atmospheric fading. Huge effort is spent to find mathematical models to describe laser beam propagation through the atmosphere. However, these models are quite cumbersome to use for the communications engineer. On the other hand, there are empirical models that try to describe the influence of scintillations and other system parameters in a simple and easy to use manner. Nevertheless, they are empirical and not based on theory. This paper is intended to close the gap between mathematical theory and empirical models. Therefore, a simple yet accurate receiver model is introduced. Based on turbulence theory and using the recently proposed convolution method assuming independent sub-aperture intensities, probability distributions of the received power are derived. Aperture averaging as well as multiple transmitter systems can be described this way. Power penalties are found by numerically calculating the resulting bit error probabilities for varying mean values of received power. Finally a model for appropriate link margins under different atmospheric conditions, taking transmitter diversity and aperture averaging into account, is derived and compared to empirical models.