Abstract: The unambiguous area in delay-Doppler plane is one of basic parameters of the radar, which determines the unambiguous range, the range-folding, the unambiguous Doppler and the Doppler-frequency aliasing properties. In this paper, for coherent (multiple-input multiple-output MIMO) radar using N frequency waveforms, the unambiguous Doppler is proved to be reduced, and in special conditions to be 1/N of the (Single-input multiple-output SIMO) radar unambiguous Doppler, accordingly, the unambiguous area of coherent MIMO radar is reduced to be 1/N of the SIMO radar unambiguous area. The simulation results display the unambiguous area reduction phenomenon. Furthermore, the paper advances waveforms designing techniques for expanding the unambiguous area of coherent MIMO radar to equal the area of the SIMO radar. The simulation results demonstrate the expansion of the unambiguous area.