Post and pre-FFT beamforming in an OFDM system

In this paper, both post-FFT and pre-FFT beamforming in an OFDM system are studied. A maximum signal-to-noise ratio (max-SNR) post-FFT beamformer and a switched-beam pre-FFT beamformer are proposed. For each subcarrier, the max-SNR post-FFT beamformer combines multiple-antenna outputs and maximizes the beamformer output SNR based on estimated one-tap channel coefficient per antenna instead of computing the inversion of the sample matrix (SMI) across antennas. Without the drawback of SMI in estimating the sample matrix, it provides robust BER performance and always performs better than the pre-FFT beamformer. When the complexity of beamforming in the frequency domain is of concern, a pre-FFT switched-beam beamformer is proposed. By appropriate design, the proposed pre-FFT switched-beam beamformer performs very well with little performance degradation but further reduced complexity as opposed to pre-FFT SMI beamformers.