Rate Averaging in Free Space Optics Systems Using Incoherent Sources

Abstract

Effect of the information rate on the scintillation index is examined for free space optical (FSO) broadband access applications that use spatially incoherent sources. For this purpose, intensity fluctuations are formulated indicating the effect of the rate on the scintillation index in the presence of the atmospheric turbulence. The bandwidth of modulation of the incoherent source is taken to be much smaller than the carrier frequency, i.e., narrowband approximation is employed. Rate averaging factor for spatially incoherent source is derived as to represent the averaging in weak atmospheric turbulence due to rate of modulation of the intensity. It is found that the scintillations decrease as the rate of transmission through atmospheric turbulence increases. This decrease is independent of the carrier wavelength of the FSO system but depends on the outer scale of turbulence. Up to 10 Gbps, the decrease is negligible for realistic outer scale values. When extremely large eddies are present in the formation of turbulence, rate can be effective in the reduction of the scintillations even at rates up to 10 Gbps. In the limit when the information rate is taken as zero, our results correctly reduce to the known scintillations for spatially incoherent monochromatic excitation.