Wireless Optical Telecommunications

Chapter 5

Propagation in the Atmosphere

“Look at the light and consider its beauty.

Closes and opens the eyes quickly:

what you see is already passed

and what you saw is gone

Leonardo da Vinci,

Codex Forster,

1493–1505

5.1. Introduction

Laser beams, used to operate free-space optical (FSO) links, involve the transmission of an optical signal (visible or infrared) in the atmosphere. They interact with various components (molecules, aerosols, etc.) of the propagation medium. This interaction is at the origin of many phenomena such as absorption, diffusion, refraction, and scintillation. The only limitation known is heavy fog and they cannot cover distances more than a few kilometers. They are therefore suitable for the construction of networks between nearby buildings. The laser beams generally used have low power; and the environmental impact is negligible.

One of the challenges to take up is a better understanding of atmospheric effects on propagation in this frequency spectrum, to better optimize wireless broadband communications systems, and to evaluate their performance. It is a prerequisite to test communication equipments.

Atmospheric effects on propagation, such as absorption and molecular and aerosol diffusion, scintillation due to the change in the air index under the effect of temperature variation, hydrometeors attenuation (rain, snow), and their different models (Kruse and Kim, Bataille, Al Naboulsi, Carbonneau, etc.) are presented and confronted against experimental ...