As the wide application of wavelength division multiplexting (WDM), polarization division multiplexing (PDM) and high-order modulation techniques, the transmission capacity over single mode fiber is rapidly reaching its Shannon limit. The only known way to overcome this limit is space-division multiplexing (SDM), which causes a significant resurgence of research interest in the multimode fibers. The multiple-input multiple-output (MIMO) technique based on multimode fiber is a new area to answer explosive growth demand for transmission capacity. Implementations of optical MIMO, coherent optical MIMO (COMIMO) systems and highlight mode group division multiplexing (MGDM) are studied and compared. Meanwhile, some problems existing in the multimode fiber MIMO system are discussed to provide a help for further research.