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It is well known that orthogonal frequency-division multiplexing (OFDM) is robust to frequency-selective fading in wireless channels due to the exploitation of a guard interval that is inserted at the beginning of each OFDM symbol. However, once delayed signals beyond the guard interval are introduced in a channel with a large delay spread, intersymbol interference causes a severe degradation in the transmission performance. In this paper, we propose a novel pre-fast Fourier transform (FFT) OFDM adaptive antenna array, which requires only one FFT processor at a receiver, for suppressing such delayed signals. We derive the optimum weight set for beamformers based on the maximum signal-to-noise-and-interference power ratio (Max-SNIR) and the minimum mean square error (mmse) criteria, respectively. In addition, we propose a novel mmse-criterion-based commutative optimization scheme, which is more robust to the estimation error of the channel state information. Furthermore, we show the equivalence between the Max-SNIR-criterion-based scheme and the proposed commutative optimization scheme. Computer simulation results show its good performance even in channels where directions of arrival of arriving waves are randomly determined.
Date of Publication: July 2004