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Orthogonal frequency division multiplexing (OFDM) offers high data rates and robust performance in frequency selective channels by link adaptation utilizing information about the channel quality. Combined with multiple-input multiple-output (MIMO) technique it provides improved link reliability and increased data rate. A crucial parameter required for adaptive transmission is the signal-to-noise ratio (SNR). In this paper, we propose a novel SNR estimation algorithm for wireless 2times2 MIMO OFDM systems based on the reuse of the synchronization preamble. The periodic structure of the preamble is utilized for the computationally efficient estimation of average SNR and SNRs per subcarrier, based on the second-order moments of received preamble samples. The performance of the proposed algorithm is assessed for channels with different levels of frequency selectivity. Simulation results show robust performance of proposed algorithm even in strong frequency selective channels.