A Computationally Efficient Numerical Simulation for Generating Atmospheric Optical Scintillations

Abstract

Atmospheric optical communication has been receiving considerable attention recently for use in high data rate wireless links (Juarez et al., 2006; Zhu & Kahn, 2002). Considering their narrow beamwidths and lack of licensing requirements as compared to microwave systems, atmospheric optical systems are appropriate candidates for secure, high data rate, cost-effective, wide bandwidth communications. Furthermore, atmospheric free space optical (FSO) communications are less susceptible to the radio interference than radio-wireless communications. Thus, FSO communication systems represent a promising alternative to solve the last mile problem, above all in densely populated urban areas. Then, applications that could benefit from optical communication systems are those that have platforms with limited weight and space, require very high data links and must operate in an environment where fiber optic links are not practical. Also, there has been a lot of interest over the years in the possibility of using optical transmitters for satellite communications (Nugent et al., 2009). This chapter is focused on how to model the propagation of laser beams through the atmosphere. In particular, it is concerned with line-of-sight propagation problems, i.e., the receiver is in full view of the transmitter. This concern is referred to situations where if there were no atmosphere and the waves were propagating in a vacuum, then the level of irradiance that a receiver would observe from the transmitter would be constant in time, with a value determined by the transmitter geometry plus vacuum diffraction effects. Nevertheless, propagation through the turbulent atmosphere involves situations where a laser beam is propagating through the clear atmosphere but where very small changes in the refractive index are present too. These small changes in refractive index, which are typically on the order of 10−6, are related primarily to the small variations in temperature (on the order of 0.1-1◦C), which are produced by the turbulent motion of the atmosphere. Clearly, fluctuations in pressure of the atmosphere also induces in refractive index irregularities. Thus, the introduction of the atmosphere between source and receiver, and its inherent random refractive index variations, can lead to power losses at the receiver and eventually it produces spatial and temporal fluctuations in the received irradiance, i.e. turbulence-induced signal power fading (Andrews & Phillips, 1998); but this random variations in atmospheric refractive index along the optical path also produces fluctuations in other wave parameters such as phase, angle of arrival and frequency. Such fluctuations can produce an increase in the 7