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Indoor optical wireless systems provide an attractive alternative for realizing next generation wireless local area networks (WLANs). In this paper, the performance of diffuse optical wireless systems, employing space time block coding (STBC) techniques, is numerically investigated, accurately taking into account, the indoor channel impulse response and the characteristics of ambient light and thermal noises at the receiver. Discrete multitone modulation (DMT) is used to mitigate the effect of intersymbol interference due to the channel's impulse response. The performance of STBC systems, employing two transmit elements, is compared against single input/ single output (SISO) and maximum ratio combining (MRC) systems operating with the same total optical transmitter power. It is shown that STBC techniques can be used to increase the capacity of diffuse optical wireless systems, improve their coverage and decrease the required optical power at the transmitter. These results demonstrate the usefulness of multiple input multiple output (MIMO) techniques in the realization of optical WLANs.