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This work presents an adaptive frequency-domain channel estimator (FD-CE) for equalization of space-time block code multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems in time-varying frequency-selective fading. The proposed adaptive FD-CE ensures the channel estimation accuracy in each set of four MIMO-OFDM symbols. Performance evaluation shows that the proposed method achieved a 10% packet error rate of 64 quadrature amplitude modulation (QAM) at 29.5 dB SNR under 120 km/h (Doppler shift is 266 Hz) in 4 times 4 MIMO-OFDM systems. To decrease complexity, the rich feature of Alamouti-like matrix is exploited to derive an efficient very large-scale integration (VLSI) solution. Finally, this adaptive FD-CE using an in-house 0.13-mu m CMOS library occupies an area of 3 times 3.1 mm2, and the 4 times 4 MIMO-OFDM modem consumes about 62.8 mW at 1.2 V supply voltage.