In this paper, an optical wireless multiple-input multiple-output communication system employingudintensity-modulation direct-detection is considered. The performance of direct current offset space-timeudblock codes (DC-STBC) is studied in terms of pairwise error probability (PEP). It is shown that amongudthe class of DC-STBCs, the worst case PEP corresponding to the minimum distance between twoudcodewords is minimized by repetition coding (RC), under both electrical and optical individual powerudconstraints. It follows that among all DC-STBCs, RC is optimal in terms of worst-case PEP for staticudchannels and also for varying channels under any turbulence statistics. This result agrees with previouslyudpublished numerical results showing the superiority of RC in such systems. It also agrees with previouslyudpublished analytic results on this topic under log-normal turbulence and further extends it to arbitraryudturbulence statistics. This shows the redundancy of the time-dimension of the DC-STBC in this system.udThis result is further extended to sum power constraints with static and turbulent channels, where it isudalso shown that the time dimension is redundant, and the optimal DC-STBC has a spatial beamformingudstructure. Numerical results are provided to demonstrate the difference in performance for systems withuddifferent numbers of receiving apertures and different throughput.
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