Since a deep and long-term fading in optical intensity results in considerable burst errors in the data, a terrestrial free-space optical (FSO) system has to be designed with consideration of a frequency characteristic of optical scintillation to achieve high quality wireless services over the link. In designing a terrestrial FSO link, we had better design the system considering variations caused by some slow time-varying parameters. This paper proposes a Butterworth-type spectral model of optical scintillation to design a terrestrial FSO link, which enables us to estimate the power spectral density of optical scintillation in a current optical wireless channel when time zone and weather parameters, such as temperature and rainfall intensity, are given. The spectral parameters of optical scintillation, cut-off frequency, and spectral slope are estimated from the data obtained in the experiment, and then their dependencies on time zone, temperature, and rainfall intensity are examined.
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