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Orthogonal frequency-division multiplexing (OFDM) has been selected as the basis for the new IEEE 802.11a standard for high-speed wireless local area networks (WLANs). We consider doubling the data rate of the IEEE 802.11a system by using a multi-input multi-output (MIMO) system with two transmit and two receive antennas. We propose a preamble design for this MIMO system that is backward compatible with its single-input single-output (SISO) counterpart as specified by the IEEE 802.11a standard. Based on this preamble design, we devise a sequential method for the estimation of the carrier frequency offset (CFO), symbol timing, and channel response. We also provide a simple soft-detector to obtain the soft-information for the Viterbi decoder. Both the sequential parameter estimation method and the soft-detector are ideally suited for real-time implementations. The effectiveness of our methods is demonstrated via numerical examples.