Free Space Optical (FSO) communication is the transmission of data using optical radiation through unguided medium. It is a promising technology to obtain broad band connections. Operating in unlicensed frequency spectrum, low maintenance cost, flexibility, supporting high data rate are some of the advantages of FSO technology. Though FSO communication has many advantages over RF transmission, its performance is limited due to atmospheric turbulence and severe weather conditions. So the effect of atmospheric turbulence and bad weather conditions has to be mitigated to get the optimal operation of FSO. Many have been worked on mitigating these atmospheric effects using different techniques like spatial diversity, modulation and channel coding. In this research work the BER performance of FSO systems employing MIMO and PSK Subcarrier Intensity Modulation (SIM) under gamma-gamma scintillation channel model (GGSCM) was analyzed. The analysis was done by varying MIMO size, refractive index structure constant (C 2 n), link range and transmitter power of the system. By varying any one of the listed parameters and fixing the others, comparative analysis of BER performance of BPSK/DPSK-SIM employed MIMO over SISO, estimation of range/coverage increment with acceptable BER by employing MIMO system over SISO and power reduction by using MIMO over SISO to achieve a specific link range with acceptable BER was done. For a 1.25 Gb/s FSO system under GGSCM, a Min.logBER of -16.77, -38.50 and -81.67 was found using 2x2 MIMO, 4x4 MIMO and 8x8 MIMO respectively at a link range of 1.2km, C 2 n =5*10 -15 m -2/3 and optical power Pt = 0dBm considering clear sky weather. Similarly, at 7km link distance, C 2 n=5*10 -17 m -2/3 and optical power P t= 30dBm , a Min.log BER of -5.51, -16.45,-47.94 and -131.37 was achieved using SISO, 2x2 MIMO, 4x4 MIMO and 8x8 MIMO respectively under clear air weather. Considering an acceptable BER of 10 -9 (Min.logBER of -9) or less for optical communication systems, we can achieve a link range of 2km using 8x8 MIMO with Min.logBER of -9.49, 0.8km using 4x4 MIMO with Min.logBER -9.63 ,0.4km using 2x2 MIMO with Min.logBER -9.95 at Pt=0dBm and C 2 n=10 -13 m -2/3 . With similar specifications (Pt=0dBm and C 2 n=10 -13 m -2/3 ) the link range is limited to 0.2km to achieve an acceptable BER value using SISO (Min.logBER= -16). Similarly, a better BER performance was achieved for 1.25Gb/s BPSK-SIM under haze and moderate rain conditions by using MIMO as compared to SISO. The BER performances of 10Gb/s BPSK/DPSK-SIM and 1.25Gb/s DPSK-SIM were also analyzed and all of these systems, BER was improved by employing MIMO when compared to SISO system. Key words: FSO, BER, MIMO, SIM, Gamma -Gamma Scintillation Channel Model (GGSCM), refractive index structure constant (C 2 n)