  Transmit power control, as in the mobile wireless channels, can enable a
robust and spectrally efficient communication through atmospheric turbulence in
terrestrial free-space optical (FSO) channels. With optical bandwidths in
excess of several GHz and eye safety regulations limiting the transmit optical
power, the per hertz signal-to-noise ratio (SNR) in terrestrial FSO systems can
possibly become limited, especially true for future high-bandwidth and
long-haul applications. Hence, power control becomes significant in terrestrial
FSO systems. However, a comprehensive study of dynamic power adaptation in the
existing FSO systems is lacking in the literature. In this paper, we
investigate FSO communication systems capable of beam power control with
heterodyne detection and direct detection based receivers operating under shot
noise-limited conditions. Under these considerations, we derive unified exact
and asymptotic capacity formulas for the Gamma-Gamma turbulence channels with
and without pointing errors; these novel closed-form capacity expressions
provide new insights into the impact of varying turbulence conditions and
pointing errors. Further, the numerical results highlight the intricate
relations of atmospheric turbulence and pointing error parameters in typical
terrestrial FSO channel setting. A concrete assessment of the impact of the key
channel parameters on the capacity performances of the aforementioned FSO
systems is performed revealing several novel and interesting insights.
