Bit error rate expressions for OOK, PPM, DPPM, PAPM and DAPPM have been derived for Earth-to-satellite
and satellite-to-Earth optical links. Threshold detection for OOK, DPPM, DAPPM and optimal detection for
PPM and PAPM have been employed. The numerical results have been compared. It is found that for uplink:
the performance of PPM is the best followed by that of DPPM, PAPM, OOK and DAPPM and for downlink the
performance of PPM is the best followed by DPPM, OOK, DAPPM and PAPM. The difference in performance
between the schemes is not much for uplink as compared to downlink. Since the bandwidth requirement of PPM
is the largest, more bandwidth efficient schemes can be used at the cost of slightly higher transmitted power in
the case of uplink. This study will be useful for modulation scheme selection for optical satellite links.
Atmospheric turbulence can cause significant performance degradation in free space optical (FSO) communication system. Major effects on the beam such as beam wander and beam scintillation degrades the performance at the receiver. We investigate the impact of diversity technique and aperture averaging technique on the performance of FSO system. Performance evaluation is made in terms of average bit error rate and scintillation index.