Acquisition time for laser uplink communication to space-borne satellite using transmit diversity in atmospheric turbulence

A laser beam propagating through a random medium such as atmosphere will experience irradiance fluctuations due to the presence of turbulence. The turbulence induced fading can be circumvented by implementing transmit diversity which involves the deployment of multiple laser transmitters at the ground station. In this paper, performance of laser uplink communication with transmit diversity in atmospheric turbulence channels has been analyzed in terms of acquisition time for coherent (sub-carrier BPSK and QPSK) and noncoherent (OOK and Q-PPM) modulation schemes. In the analysis, both uncorrelated and correlated uplink laser beams are considered. It has been observed that transmit diversity decreases the acquisition time in all the modulation schemes irrespective of turbulence strength. Sub-carrier BPSK gives the least acquisition time to acquire the space borne satellite among all the modulation schemes. In case of non-coherent modulation schemes, OOK gives slightly better results than 2-PPM but there is an improvement in the acquisition time of Q-PPM as the value of Q increases.