Optimum beamforming for pre-FFT OFDM adaptive antenna array

It is well-known that Orthogonal Frequency Division Multiplexing (OFDM) scheme is robust to frequency selective fading in wireless channels. However, once delayed signals beyond a guard interval are introduced in a channel with large delay spread, inter-symbol 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 receiver, for suppressing such delayed signals. We analytically derive the optimum weights for the beamformer based on the Maximum Signal-to-Noise-and-Interference power Ratio (Max-SNIR) and the Minimum Mean Square Error (MMSE) criteria, respectively. Computer simulation results show its good performance even in a channel where Directions of Arrival (DoAs) of arriving waves are randomly determined.