In this paper, the performance of quad-APD spatial tracking loop (STL)in the presence of scintillation is studied for a frozen atmosphere turbulent model for free-space communications (FSO). The atmospheric turbulence responsible for the beam scintillation is assumed to follow the weak turbulence model, described by Rytov approximation, which in turn suggests log-normal statistics for the received optical signal intensity. It is assumed that pointing error in large part is due to atmospheric wander and that the correlation time of the beam wander is comparable to the correlation time of the amplitude variations due to amplitude scintillation. Provided recently reported estimation strategies, a new model for spatial tracking is proposed where estimates of channel coefficient are used to adjust the gain of the tracking loop, enabling an adaptive bandwidth adjustment in the presence of correlated fading. The performance of the proposed loop as well as that of the standard tracking loop are assessed and compared via simulation in terms of the mean square tracking error (MSTE).
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