Performance of Amplify-and-Forward relaying MIMO free-space optical systems over weak atmospheric turbulence channels

In this paper, Amplify-and-Forward (AF) relaying multiple-input multiple-output (MIMO) technology is proposed to extend the coverage of free-space optical communication (FSO) systems using subcarrier amplitude modulation (SC-QAM) over weak atmospheric turbulence channels modelled by log-normal distribution. Unlike previous studies on FSO systems without relaying stations which are suitable for short-range transmissions, whereas relaying stations are used for long-range transmissions. Closed-form expressions for average symbol error rate (ASER) of MIMO/FSO and SISO/FSO systems taking into account the effects of the number of relay stations and SC-QAM signals are formulated over weak atmospheric turbulence channels. The numerical results show that by using AF stations, the transmitted power is reduced accordingly to the amplifier gain therefore the transmission distance can be extended. ASER performance of MIMO/FSO systems is degraded compared with that of SISO/FSO systems at the same transmission distance. Moreover, simulation results of AF relaying MIMO/FSO performance over weak atmospheric turbulence show that the closed-form expression can provide a precision for evaluating ASER of such systems.