ABSTRACT
To design, implement and operate efficient optical communication systems, it is imperative that the characteristics of the channel are well understood. Characterization of a communication channel is performed by its channel impulse response, which is then used to analyse and combat the effects of channel distortions. A considerable amount of work has been published on the channel characterization, covering both experimental measurement and computer modelling of indoor and outdoor systems. The power penalties directly associated with the channel may be separated into two factors, these being optical path loss and multipath dispersion. Two types of configurations are considered in an optical wireless channel as outlined in Chapter 1. For directed LOS and tracked configurations, reflections do not need to be taken into consideration, and consequently the path loss is easily calculated from knowledge of the transmitter beam divergence, receiver size and separation distance. However, a non-LOS configuration, also known as diffuse systems (mainly used in indoor environment), uses reflections off the room surfaces and furniture. These reflections could be seen as unwanted signals or multipath distortions which make the prediction of the path loss more complex. A number of propagation models (ceiling bounce, Hayasaka–Ito and spherical) for LOS and non-LOS are introduced in this chapter. The artificial light interference that affects the link performance is also outlined in this chapter. The atmospheric outdoor channel is a very complex and dynamic environment that can affect characteristics of the propagating optical beam, thus resulting in optical losses and turbulence-induced amplitude and phase fluctuation. There are a number of models to characterize the statistical nature of the atmospheric channel, and these will be discussed in this chapter. A practical test bed for investigating the atmospheric effect on the free space optics link, as well as measured data is also covered in this chapter.
