A simple and robust hybrid power transmit diversity in free-space optical communications

Spatial diversity techniques are now widely adapted in most wireless radio-frequency communication systems to increase robustness against signal fading, and also attract a considerable amount of attention in free-space optical (FSO) communications. On downlink, user terminal cost and power consumption considerations indeed favor transmit diversity over receive diversity, and orthogonal space-time block code (OSTBC) appears to be most well-known transmit diversity scheme. However, OSTBC loses 3 dB in terms of total transmit power with respect to ideal hybrid power transmit diversity technique which requires perfect channel state information at the transmitter. In this paper, we present a simple scheme to recover the power loss using partial feedback for FSO links with intensity modulation and direct detection over log-normal atmospheric turbulence-induced fading channel. We also consider the deployment of other conventional transmit diversity techniques including equal power transmit diversity and switching transmit diversity as benchmarks. Simulation results show that most of the power loss is restored, and the proposed scheme performs very close to ideal performance outperforming other techniques.