  Recent advances in small-scale unmanned aerial vehicles (UAVs) have opened up
new horizons for establishing UAV-based free-space optical (FSO) links.
However, FSO technology requires precise beam alignment while random
fluctuations of hovering UAVs can induce beam misalignment and angle-of-arrival
(AoA) fluctuations. For an FSO link to a UAV, we consider a quadrant detector
array for optical beam tracking and study the effect of random hovering
fluctuations of the UAV on the performance of the tracking method, and based on
the degree of instabilities for the UAV, the optimum size of the detectors for
minimizing the tracking error is found. Furthermore, for optimal detection of
On - Off keying symbols, the receiver requires instantaneous channel fading
coefficients. We propose a blind method to estimate the channel coefficients,
i.e., without using any pilot symbols, to increase link bandwidth efficiency.
To evaluate the performance of the considered system, closed-form expressions
of tracking error and bit-error rate are derived. Moreover, Monte-Carlo
simulation is carried out to corroborate the accuracy of the derived analytical
expressions.
