Characterization of Atmospheric Turbulence Effects and their Mitigation Using Wavelet-Based Signal Processing

In free space optical communication links, the turbulent atmospheric channel causes fluctuations in both intensity and phase of the transmitted optical signal which degrades their performance. In order to make free space optical communications commercially viable, there is a need for characterizing the atmospheric effects on the received signal statistics, and possibly correcting for the signal degradation using physical optics principles and advanced real-time signal processing techniques. As part of this work, we have constructed an experimental free space optical link over 100 meters so as to characterize the effects of inner-scale and outer-scale turbulence effects, and used it as the platform for carrying out the above objectives. A key aspect of our work is the experimental determination of the individual contribution of scintillations and beam wandering to the received signal variance. We have proceeded to correlate such effects with atmospheric parameters with the aim of building a model through which the channel properties may be estimated based on a measurement of the atmospheric parameters. Such analysis has been carried out over a 24 hour period to include a full day-night cycle and also to consider the effect of temperature. In this paper, we have also investigated methods of reducing the atmospheric effects such as scintillations and beam wandering. Specifically, we have demonstrated reduced scintillation-related variance by aperture averaging technique, and reduced beam wandering-related variance using wavelet based signal processing. Through these techniques, we have demonstrated bit error rate (BER) reduction by a factor of 138 compared to the original signal for our experimental free space optical link.