This paper investigates the performance of free-space optical (FSO) communication systems under the influence of atmospheric loss, turbulence and pointing error. Considering the limiting Rytov-based scintillation theory associated with the simplified field models, the Gaussian-beam wave model is introduced to characterize the propagation properties of the optical signal through random turbulent medium. The bit-error rate (BER) and average channel capacity are evaluated, taking into account the effects of aperture averaging and beam width. In general, larger receiving apertures enable lower BER attainment with smaller signal-to-noise ratio (SNR), while beam width optimization is a feasible approach in promoting capacity enhancement.