High-Order Intensity Modulations for OSTBC in Free-Space Optical MIMO Communications

Recently, Simon-Vilnrotter and Premaratne-Zheng modified the binary pulse-position modulation (PPM) to support free-space optical (FSO) multi-input multi-output (MIMO) systems with orthogonal space-time block coding (OSTBC) that requires the use of negative symbols. In this paper, we extend the investigation to optical OSTBC schemes with high-order intensity modulation. A necessary and sufficient condition for the high-order intensity modulation to maintain the orthogonality of the applied OSTBC is derived, and four Q-ary intensity modulation schemes satisfying this condition are proposed. In asynchronous FSO MIMO channels in which the signals from different transmitters arrive at the receiver with timing misalignment, the proposed modulation schemes can be used with shift-OSTBC to mitigate inter-symbol interference (ISI) and provide diversity gain.