Diversity for Mitigating Channel Effects

Abstract

There is a need to compensate for the power fluctuations in free space optical (FSO) communication links due to the turbulence and other atmospheric phenomena. In order to overcome atmospheric turbulence induced fades, several techniques have been proposed. These include: spatial transmitter/receiver diversity, adaptive beam forming based on the wave front phase error measurement and the setting of the opposite phase aberration on the beam by a deformable mirror, wavelength diversity, multiple-beam communication and novel modulation techniques. In this chapter, the performance of FSO systems with spatial and time diversities is presented. To illustrate the diversity techniques, only the subcarrier intensity modulation scheme will be considered. The diversity techniques outlined can however, be extended to other modulation schemes without any loss of generality. The spatial diversity analysis is based on the linear combining techniques: equal gain combining, maximum ratio combining and selection combining. Inter-symbol interference is not being considered since terrestrial FSO links are basically a line-of-sight technology with the negligible delay spread. The chapter also outlines experimental results for verification of the diversity schemes for FSO links. To ensure link availability at all weather conditions the chapter also presents measurement campaign for an outdoor FSO/radio frequency link.