The performance of free-space optical (FSO) communication suffers from atmospheric turbulence, which mainly results in scintillation and phase fluctuations to optical signal. In this paper, we study the bit error rate (BER) performance of coherent FSO system employing phase compensation and spatial diversity. Based on weak fluctuation theory, the amplitude fading and phase fluctuation induced by atmospheric turbulence are assumed to follow log-normal model and Gaussian distribution, respectively. Distinct from previous works, a new closed-form expression for signal fading at the receiver is derived. Then the impact of various compensating parameters is examined on the BER performance of the system, including the normalized receiver aperture diameter and the number of phase compensation modes. The optimal compensation parameters are found showing that the FSO coherent system can obtain the minimum BER. Furthermore, we explore the potential of spatial diversity, particularly receiver diversity to further enhance the system performance. Numerical results show that the addition of phase compensation and diversity techniques can largely reduce the impact of atmospheric turbulence.
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