The free-space optical-channel model used in this case study takes into account the effects of weak turbulence and background radiation on the free-space transmission of an optical signal. The model is based on the work presented in .
The model treats attenuation (specified as a negative number) as a combination of geometric and environmental contributions, the latter of which is expected to vary statistically due to weak turbulence. Weak turbulence is expected to lead to a stochastic variation. Because of the long-time scales involved in this variation, the model treats additional attenuation as a statistical parameter with a normal distribution.
Finally, background radiation can be added to the signal via the specification of the total background radiation power, its center wavelength, its bandwidth, and the desired numerical resolution.
Major impairments over FSO links are atmospheric attenuation and scatter phenomena that vary widely from one micrometeorological area to another. Included here are scintillation, scattering, beam spread, and beam wander [3-4]. Scintillation is best defined as the temporal and spatial variations in light intensity caused by atmospheric turbulence. Such turbulence is caused by wind and temperature gradients that create pockets of air with rapidly varying densities and therefore fast-changing indices of optical refraction. The signal attenuation caused by scintillation effect depends on the time of day and can vary orders of magnitude during a hot day. It drastically increases with distance and can impact the BER performance (burst errors) on a milliseconds timescale. The scintillation is characterized by a scintillation index parameter.
The accurate statistical models for signal fading due to the atmospheric turbulence were developed based on lognormal and gamma-gamma probability density functions [3-4]. It was shown that in the limit of weak turbulence the fade statistics based on lognormal distribution provides an acceptable agreement with measurement data and can be applied to estimate the link margin and availability.
This example employs OptSim compound component for FSO channel model developed under weak turbulence approximation. The signal attenuation in this model is based on the FSO range equation that combines attenuation and geometrical aspects to calculate the received optical power as function of range and receiver aperture size . The range equation expresses the received signal in terms of transmitted signal, receiver aperture area, beam divergence angle, combined transmitter receiver optical efficiency, optical power of background radiation, link range, and environmental attenuation. According to lognormal model, the logarithm of signal intensity is a Gaussian random variable. Hence, the signal attenuation is a Gaussian random variable as well. If values for mean intensity and scintillation index are known from either measurements or theoretical calculations, then we can derive mean additional attenuation and scintillation index. For more details on the mathematical treatment of the channel model, please refer to OptSim models’ reference.
The FSO compound component is shown in Figure 2. It consists of an optical attenuator to model geometrical and additional attenuation, and an optical white noise generator to add the background radiation to received signal.