Improvement of Ground to Satellite FSO Link Performance Using Transmit Diversity in Weak Atmospheric Turbulence

The performance of ground to satellite free space optical （FSO） communication link is degraded due to the presence of atmospheric turbulence that causes fluctuations in both intensity and phase of the received optical signal. In this paper, bit error rate （BER） performance of FSO link using transmit diversity is investigated for coherent （sub-carrier BPSK and sub-carrier QPSK） and non-coherent （OOK and Q-PPM） modulation schemes in weak atmospheric turbulence channel. It is seen that multiple transmitting antennae help to overcome the turbulence induced fading effect. Further, sub-carrier BPSK modulation outperforms the other modulation schemes in terms of minimum SNR requirement for a given BER. Among non-coherent modulation schemes, Q-PPM performs better than OOK. However, as the level of Q in Q-PPM increases, its performance degrades and it becomes worse than OOK for certain values of atmospheric turbulence. Also, the effect of correlation among different transmitting antennae beams is analyzed on the link performance.