  Free-space optical communication is emerging as a low-power, low-cost, and
high data rate alternative to radio-frequency communication in short-to
medium-range applications. However, it requires a close-to-line-of-sight link
between the transmitter and the receiver. This paper proposes a robust $\cHi$
control law for free-space optical (FSO) beam pointing error systems under
controlled weak turbulence conditions. The objective is to maintain the
transmitter-receiver line, which means the center of the optical beam as close
as possible to the center of the receiving aperture within a prescribed
disturbance attenuation level. First, we derive an augmented nonlinear
discrete-time model for pointing error loss due to misalignment caused by weak
atmospheric turbulence. We then investigate the $\cHi$-norm optimization
problem that guarantees the closed-loop pointing error is stable and ensures
the prescribed weak disturbance attenuation. Furthermore, we evaluate the
closed-loop outage probability error and bit error rate (BER) that quantify the
free-space optical communication performance in fading channels. Finally, the
paper concludes with a numerical simulation of the proposed approach to the FSO
link's error performance.
