  The performance analysis of free space optical communication (FSO) systems
using relays and spatial diversity at the transmitter end is presented in this
paper. The impact of atmospheric turbulence and attenuation caused by different
weather conditions and geometric losses has also been taken into account. The
effect of turbulence is modeled over a log-normal probability density function.
We present the exact closed form expressions of bit-error rate (BER) for M-ary
quadrature amplitude modulation (M-QAM). The FSO system link performance is
compared for on-off keying, M-ary pulse amplitude modulation and M-QAM
modulation techniques. For relay based free space optical communication
systems, M-QAM is proved to be superior than other systems considering the same
spectral efficiency for each system. A significant performance enhancement in
terms of BER analysis and SNR gains is shown for multi-hop MISO FSO system.
