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We investigate the error performance of an earth-to-satellite free space optical uplink using transmitter spatial diversity in presence of turbulence and weather conditions, using gamma–gamma distribution and Beer–Lambert law, respectively, for on–off keying (OOK),  M -ary pulse position modulation ( M -PPM) and  M -ary differential PPM ( M -DPPM) schemes. Weather conditions such as moderate, light and thin fog cause additional degradation, while dense or thick fog and clouds may lead to link failure. The bit error rate reduces with increase in the number of transmitters for all the schemes. However, beyond a certain number of transmitters, the reduction becomes marginal. Diversity gain remains almost constant for various weather conditions but increases with increase in ground-level turbulence or zenith angle. Further, the number of transmitters required to improve the performance to a desired level is less for  M -PPM scheme than M-DPPM and OOK schemes.

Transmitter Spatial Diversity for FSO Uplink in Presence of Atmospheric Turbulence and Weather Conditions for Different IM Schemes