Space-time coded MPSK coherent MIMO FSO systems in gamma-gamma turbulence

Coherent free-space optical communication employing M-ary phase-shift keying is investigated for space-time coded multiple-input multiple-output (MIMO) systems over Gamma-Gamma turbulence channels. The proposed space-time coded system offers an efficient utilization of coherent free-space optical channel capacity by exploiting the degree of freedom at both the transmitter and receiver ends. Such a system design is particularly suitable for turbulence channels. Novel closed-form error rate expressions are presented for the proposed MIMO system using a series expansion approach. In addition, asymptotic symbol-error rate and truncation error analyses are performed. It is shown that our proposed system is a viable solution for coherent optical MIMO applications and achieves the full diversity benefits of MIMO links. Numerical results further demonstrate the high accuracy of the derived error rate solutions.