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Accompanying the journey for higher spectral efficiency in mobile wireless communications, this paper studies multiple-input-multiple-output (MIMO) transmission in a unified framework that consolidates the phenomena responsible for frequency selectivity and time selectivity, i.e., delay overspreading and Doppler overspreading, respectively. Starting with a baseband description of the linear time-varying MIMO system, a novel approach using continuous, discrete, and hybrid linear operators leverages the process of channel orthonormalization and model discretization. Notably, this process leads to an optimal time-varying semiorthonormal matrix matched filter (the ORTHO-TS-MMF). Monte Carlo simulations test the error performance for typical wireless MIMO realizations. In particular, they unveil that combining the ORTHO-TS-MMF with optimal linear detection takes advantage of both delay and Doppler diversities, significantly reducing the symbol error probability.