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In this paper, we propose a novel open-loop autorate fallback algorithm (i.e., the autorate fallback for high-throughput (ARFHT) algorithm) for the emerging high-throughput (HT) IEEE 802.11n wireless networks. ARFHT extends the legacy link adaptation algorithms for single-input-single-output (SISO) wireless networks to be applicable in the context of multiple-input-multiple-output (MIMO)-based 802.11n wireless networks. It adapts the MIMO mode in terms of spatial multiplexing and spatial diversity, which are the two fundamental characteristics of MIMO technology. It also modifies the link estimation and probing behavior of legacy SISO algorithms. The combined adaptation to the appropriate MIMO mode and the modulation coding scheme together achieves more efficient channel utilization. We will present in detail the design guidelines and key ideas for the ARFHT algorithm. A comprehensive simulation study using ns-2 demonstrates that ARFHT achieves excellent throughput and packet error-rate performance in diverse environments and is highly responsive to time-varying link conditions with minimum computational complexity and protocol overhead.