A closed-form bit-error rate (BER) expression is derived for free-space optical (FSO) communication systems with circle polarization shift keying and spatial diversity receivers in the gamma-gamma (GG) distribution fading channel. This model can predict the performance without the need of lengthy simulation runs. The performance can be analyzed by some system parameters such as atmospheric conditions, link length, communication wavelength, receiver aperture size, and number of spatial diversity receivers. Numerical results demonstrate the influence of the above parameters on the FSO systems and show quantitatively the differences in behavior among various different parameters.