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In this paper, we propose two orthogonal frequency division multiplexing (OFDM)-based generalized spatial modulation (GSM) techniques, i.e., frequency domain GSM (FD-GSM) and time domain GSM (TD-GSM), for indoor multiple-input multiple-output optical wireless communication (MIMO-OWC) systems. Specifically, FD-GSM selects a subset of parallel OFDM modulators to transmit the same quadrature amplitude modulation (QAM) constellation symbol in the subcarrier level, while TD-GSM selects a subset of light-emitting diode (LED) transmitters to transmit the same time domain sample after OFDM modulation. Compared with the existing OFDM-based spatial modulation (SM) scheme, OFDM-based GSM can provide additional transmit diversity. For both FD-GSM and TD-GSM, maximum-likelihood (ML) detectors are employed to recover the spatial and constellation symbols. Moreover, TD-GSM usually requires a secondary direct current (DC) bias to work properly. The simulation results show that in a typical indoor 4 x 4 MIMO-OWC system, OFDM-based TD-GSM outperforms OFDM-based FD-GSM by adding a proper secondary DC bias.