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Data parallelization by means of optical multiple-input multiple-output (MIMO) transmission over dispersive multimode fiber (MMF), with a high degree of modal coupling but not accounting for intermodal dispersion, is investigated by developing an analytical model for direct detection of MMF MIMO frequency-flat transmission with mutually incoherent sources. The MIMO channel performance is derived in terms of a new formulation of a channel matrix for modal group powers accounting, for the first time, for modal coupling. For fixed aggregate signaling rate and power budget, for uncoded bit streams, increasing the number of output detectors improves the bit error ratio (BER)-similarly to wireless MIMO. However, contrary to wireless MIMO, increasing the number of input ports actually yields a BER penalty, which is traceable to the quadratic nature of photodetection. We finally establish the feasibility of enhancing the aggregate bit rate using multiple inputs in the case that the individual single-input-single-output channels are band limited, e.g., given optical data sources each at 2 Gb/s, it is possible to attain a 12-Gb/s signaling rate over several hundreds of meters of MMF at 10-10 BER, by utilizing six such inputs into the MIMO system, while incurring just a small average power penalty of approximately 2 dB/channel. The current model assumes strong intermodal coupling and neglects ISI influence over distances of up to hundreds of meters at gigabit rates, providing a first step in the optical MIMO analysis. On the other hand, similar scenario is practically met for shorter distances (up to 100 m) for the novel types of plastic optical fibers.