Wireless Optical Telecommunications

Chapter 4

Propagation Model

“Definition of free space propagation: In a homogeneous space comparable to the vacuum, the radiated energy of an isotropic source propagates at light speed and uniformly spread over the surface of a sphere whose radius increases with time.”Fundamentals of Radio Propagation,Jacques Deygout, Eyrolles, Paris, 1994

4.1. Introduction

The aim of this chapter is to give a description of a baseband wireless optical link. First, we compare the modeling of the system to a radio system. Then, we describe the disruptive noises and define the electrical signal-to-noise ratio (SNR). Finally, an analysis of the diffuse channel is performed and different calculation models are presented.

4.2. Baseband equivalent model

In this section, the radio propagation principles are first remembered and compared to those of free-space optical ones. Finally, a baseband free-space optical link model is presented.

4.2.1. Radio propagation model

We can define a real signal f (t), located in a narrow frequency band around a carrier frequency f₀. When the frequency band occupied by f(t) is small compared to f₀, the signal can be called narrowband signal or passband signal. It can be written in terms of its complex envelope in the following form:

Equation 4.1. Narrowband signal (1st representation)

with:

where f₀ is the carrier frequency, a_f(t) is the complex envelope of the signal, ...