Optical spatial diversity for FSO communications

The performance of the terrestrial free-space optical (FSO) communication is adversely affected by atmospheric conditions such as fog, rain, dust particles, and smoke. Even in clear atmospheric conditions, the FSO link experiences severe fading due to the turbulence effect. Turbulence is induced by random variations in refractive index due to temperature and pressure fluctuations along the FSO beam propagation. Turbulence can introduce deep fading (20 to 30 dB); as a result, the outage probability increases significantly. In order to mitigate the channel fading, the receiver or transmitter diversities or a combined transmitter and receiver diversity (known as multiple-input multiple-output (MIMO) techniques) are often considered along with other techniques such as modulation, temporal and wavelength diversities. In this chapter, the FSO link performance in the presence of the turbulence fading using transmitter and receiver diversity is discussed.